A Critical Review of the Advances and Current Status of the Application of Adsorption in the Remediation of Micropollutants and Dyes Through the Use of Emerging Bio-Based Nanocomposites

Abstract

The demand for drinking water is a reality that plagues modern society and will worsen in the coming decades. Factors such as climate change, population growth, and intense, often disorderly urbanization are expected to limit the availability of this essential resource for life. With this justification, several technologies involving water remediation/purification have been improved to increase energy efficiency. One key approach involves the use of residual biomass derived from biological sources as adsorbents with valuable properties. This line of research supports waste management, and the materials are easily obtainable, especially on a large scale, with low costs and negligible secondary environmental impacts. In the early 2000s, it was demonstrated that these materials possess functional groups (amino, hydroxyl, and carboxyl) that are favorable for attracting certain pollutants that are present in wastewater. Generally, the unmodified precursor material has properties that are not favorable for adsorption, such as limited adsorption capacity, low mechanical resistance, and unstable surface chemistry. Therefore, there has been a strong investment in studies aimed at developing methodologies to produce bio-based materials with high properties supported by mathematical models aimed at water purification. This critical review describes the modifications, functionalization, and production of bio-based materials aimed at remediating wastewater via the adsorption process. Their use involves the elimination of organic pollutants, water/oil separation, the removal of micropollutants, and membrane filtration. The properties of bio-based materials from biopolymers and their synthesis methodologies are analyzed, with a focus on water remediation. Finally, the challenges and future perspectives are highlighted, highlighting the relevance of this group of adsorbents in minimizing the challenges and limitations present in the field of water purification and providing new, innovative solutions.

1. Introduction

Common industrial processes in virtually all countries include leather tanning, electroplating, pesticide development, battery and paper production, and mining. These processes release a significant volume of contaminants into the final effluent. The volume of contaminants has increased due to population growth and rapid urbanization and modernization [1]. The aforementioned aspects have increased the need for technologies and materials with improved capabilities to remediate contaminated water [2,3,4,5,6]. In an ideal scenario, the material should have a high affinity for various pollutants, ensuring high removal; potential for reuse over several cycles while maintaining stable removal; low cost; and high flexibility in pore adjustment [7,8]. Bio-based materials are materials that are made entirely or partially from biological resources rather than fossil-based raw materials. They are not necessarily biodegradable or compostable. Bio-based materials should be adopted when they are better for the environment than their fossil-based equivalents. Biodegradable materials biodegrade under certain conditions at the end of their useful life. They should be used when they are intended to be released into an open environment where they cannot be easily recovered, for example, in agricultural mulch films, biolubricants for wind turbines, and fishing equipment. Bio-based materials have the advantage of being optimizable for various properties such as porosity, mechanical strength, diffusion, surface area, and hydrophobicity, highlighting their high potential for water purification. The increase in industrialization has contributed to the discharge of heavy metals, dyes, pharmaceutical residues, and other contaminants into different water compartments [9]. In addition, more than one contaminant may be present in an organic compound. For example, lead is present in the azo dye Ponceau red, a synthetic molecule produced from tar, and has a high potential to cause cancer in the rectum, stomach, and colon, as well as skin allergies. Water compartments contaminated with dyes generally have high concentrations of chemicals, suspended particles, and a high chemical oxygen demand. This is justified by the fact that the textile industry uses more than eight thousand different chemicals to produce approximately three thousand six hundred synthetic dyes [10]. Given the damage caused throughout the human system, these chemicals can be considered a serious threat, mainly due to mutagenic and carcinogenic behaviors [11]. Therefore, proper management before disposal is essential. Chemicals and dyes must be removed using materials with highly efficient adsorptive properties [12]. Contamination by oil spills can cause eye damage and skin irritation, and in more severe cases, damage to the respiratory system and neurological changes are observed. In the long term, the possible damage caused by oil is still limited; however, cleaning in a short period is essential.

Due to the high demand for industrial and drinking water, wastewater treatment processes have become essential in recent years. These processes involve photocatalysis/ultraviolet (UV) photolysis [13], complexation [14], electrodialysis [15], precipitation [16], activated carbon adsorption [17,18], ion exchange [19], reverse osmosis [20], membrane filtration [21], and advanced oxidation processes [22]. Clean water can be generated sustainably through desalination and wastewater processing [23]. Most current studies involve improving effluent purification systems and developing new technologies that are cost-effective and highly energy-efficient. The main examples in the field of new advanced materials are artificial water channels and fluorinated oligoamide nanorings [24], protein nanofibrils [25], carbon nanotubes [26], covalent organic frameworks [27], graphene [28], and metal–organic frameworks [29]. When considering large-scale applications, these materials have high production costs, and some of them involve more complex synthesis routes. Presenting high abundance in nature and potential for use as natural raw materials for the synthesis of bio-based nanofibers, biopolymers have begun to attract considerable attention from the scientific community. In addition, they have good properties such as being hydrophilic, thus avoiding the generation of incrustations, in addition to being chemically and mechanically stable and moldable with other chemical groups. Thus, biopolymers have begun to be analyzed as adsorbents for a wide range of recalcitrant pollutants present in wastewater [30]. The most studied polymers in this regard include alginic acid, cellulose, cellulosic biopolymer, chitosan, polylactic acid, and hemicellulose [31].

Cellulose and chitosan are the two groups that have been most commonly used in processes such as water/oil separation, membrane filtration, and micropollutant removal due to their favorable properties, including high biocompatibility, non-toxicity, disinfection potential, and high adsorptive capacity [32]. When compared to conventional materials (mesoporous silica, metal oxides, zeolites, activated carbon, carbon-based nanomaterials, metal sulfides, and clays), bio-based materials have several advantages [33]. The positive aspects include their easy biodegradation, natural and highly abundant occurrence, efficiency and low cost in terms of energy consumption, selective adsorption capacity for certain impurities such as metal ions. Therefore, these materials are excellent candidates for water adsorption and filtration applications [34]. Despite the positive aspects, bio-based materials still have limitations that need to be addressed. Their synthesis cost cannot be reduced to a value considered favorable for the industry, as this would limit their adsorptive efficiency in addition to the effective exploration of materials with high accessibility (lignocellulosic materials). The inherent properties of bio-based materials need to adapt to the various target adsorbates. In this regard, surface modifications and synthesis routes still need to be discovered [35]. Important criteria and concepts involving the study of these materials should focus on the recyclability of biomaterials, the nature of contaminants, possible efficient applications, kinetics, green synthesis techniques, and removal mechanisms [36].

This study critically analyzes bio-based nanomaterials such as cellulose, chitosan, and various plant-based nanofibers, focusing on their potential for the remediation of organic pollutants, pharmaceutical toxins, and heavy metals. In the case of micropollutants, conventional processes (chemical precipitation, adsorption, flocculation, membrane filtration, and coagulation) can be used to remove them. Natural nanofibers can be produced via phase inversion, chemical modification, or electrospinning. The great advantage of their use is their low cost, efficiency, and high reproducibility [37]. Using other chemical groups for the functionalization of plant-based materials makes it possible to increase their adsorptive properties, which is a widely discussed and analyzed topic. The most commonly used bio-based materials in the water treatment field and various synthesis techniques for bio-based nanofibers, as well as the adsorptive behavior and surface changes of these materials, are aspects analyzed in depth during this investigation. Regarding bio-based materials, a broad overview of their structural characteristics and their relationship with adsorption capacity is provided. The latest studies involving the most current designs for bio-based materials aimed at water purification are highlighted. Various functionalization routes and their limitations and advantages are analyzed. Finally, the limitations encountered when working with bio-based materials to obtain high capacities are confronted. Therefore, this study aims to provide a consolidated theoretical basis, supporting future attempts to build more sophisticated adsorbents and eventual large-scale use.

2. Materials and Methods

The use of petrochemicals and fossil fuel derivatives has considerable harmful effects, raising concerns in the scientific community about these contaminants. It is estimated that by 2050, the manufacture of synthetic polymers will use about twenty percent of fossil fuel reserves [33,38,39]. In this regard, it is essential to migrate from hydrocarbon energy to the use of a more sustainable renewable fuel to find substitutes for petroleum-based products. The extraction of fossil-based materials is generally associated with environmental degradation. Therefore, replacing them with bio-based raw materials is a viable alternative. Biopolymers can be produced from renewable resources through three routes. The first corresponds to extraction and fractionation, which occur with sodium alginate, cellulose, chitin, starch, protein, and lignin [40]. The second corresponds to the polymerization of monomers that are derived from renewable biomass, including polylactic acid [41]. And the third is direct production carried out by microorganisms, for example, in the case of polyhydroxyalkanoates [42]. These biopolymers have functional groups (ester, hydroxyl, acetal, amino, and carboxyl) that enable varied physical and chemical properties (Figure 1) [43]. The commercial application of bio-based materials in water purification is promising, especially in the last ten years, since a wide range of materials (powders, fibers, particles, membranes, foams, and hydrogels) has been developed through various synthesis and modification routes.

2.1. Polysaccharides

Polysaccharides, such as agarose, alginate, cellulose, chitosan, pectin, and starch, are widely studied biopolymers due to their abundance, biodegradability, and potential applications in water remediation. These materials exhibit a variety of functional groups, including hydroxyl, carboxyl, and amino groups, which enhance their adsorption capacity and chemical reactivity [44,45,46,47]. Agarose, derived from red algae, forms stable hydrogels that can be used in composite materials for dye and heavy metal removal [44]. Alginate, obtained from brown algae, has a high affinity for metal ions and radionuclides due to its carboxyl functional groups, which enable ionic crosslinking and surface functionalization [45]. Cellulose, the most abundant natural polymer, has been widely explored for membrane filtration and adsorbent applications, with modifications such as esterification and amination improving its efficiency in water treatment [48]. Chitosan, a deacetylated derivative of chitin, possesses unique chelating properties and electrostatic interactions that facilitate the adsorption of various organic and inorganic pollutants, although its mechanical limitations often require chemical modifications [46]. Pectin, a plant-derived polysaccharide, has been investigated for its ability to form complexes with metal ions and organic pollutants, making it a promising candidate for the stabilization of nanoparticles in environmental applications [47]. Starch, composed of amylose and amylopectin, is a cost-effective and renewable adsorbent, but its native form requires chemical modifications such as grafting and oxidation to enhance its adsorption potential [49]. The diverse properties of these polysaccharides make them highly valuable in developing sustainable water treatment solutions. Each of the mentioned polysaccharides is addressed in the following sections.

2.1.1. Agarose

Agarose and 3,6-anhydro-L-galactose are synthesized by red algae. Agarose is characterized by being a natural polymer formed by D-galactose. The hydrogen atoms present in its structure can create bonds with other hydrogen atoms present both in its structure and in water molecules. This is possible due to the presence of OH throughout its chemical structure [44]. The molecular weight can vary between 80 and 140 kilodaltons. A study showed that hydrogels manufactured from agarose presented hysteretic behavior, high stability, and easy control potential [50]. The final result was a translucent solution, which occurred when the polymer molecules dispersed and assumed a random spiral shape. Using temperatures ranging from 30 to 40 °C, a firm and organized gel structure in the form of a double helix was obtained due to the occurrence of bonds established between the hydrogen atoms present in the agarose molecules. Current studies state that in the next five years, the global agarose market will show linear growth, whereas by 2028, economists predict that the global market will be worth around USD 99.35 million, where the annual increase is expected to occur with a compound annual growth rate of 2.97% [51]. One study synthesized an agar/graphene oxide hydrogel to remove cadmium metal and methyl violet dye from water [52]. The isothermal data of the adsorption of the metal and the dye were best represented by the Sips model, presenting high values of statistical coefficients and adsorption capacities more compatible with the values obtained experimentally. The maximum capacities were 76 and 11.7 mg g^–1 for the dye and the metal, respectively. In the case of kinetic studies, the pseudo-second order model was the one that best represented both systems. Reuse and desorption studies showed that the adsorbent could be used for up to three cycles for both adsorbates, maintaining a good removal rate.

A lyophilized hydrogel named graphene oxide–agar biocomposite presented a three-dimensional porous morphology [53]. The material was used to remove the dye safranin-O and the drug chloroquine, where the isotherms presented a best fit to the Sips model for the dye and the Freundlich heterogeneous surface model for the drug; the statistical coefficient values were above 0.98 when obtained from studies conducted in batches. The Fick diffusion equation and driving force models were used to model the kinetic data. In the case of competitive adsorption with multiple components, both showed a reduction in adsorption of approximately 10 mg g^–1 in the presence of both components in the same solution. This offers evidence that both adsorbates competed for the adsorptive sites present on the hydrogel surface. The individual maximum capacity values were 41 and 31 mg g^–1 for safranin-O and chloroquine, respectively. In experiments conducted in a fixed bed on a laboratory scale, the capacities were 63 mg g^–1 (chloroquine) and 100 mg g^–1 (safranin-O) [53]. In another study carried out by the same authors, hydrogels were synthesized using agar and graphene oxides, and interactions of the polymer with graphene oxide were evidenced, generating a three-dimensional irregular material [54]. The adjusted pH showed a lower dependence on adsorption capacity in adsorption studies using the dyes malachite green and basic fuchsin. When increasing the pH, a linear increase in the adsorption capacity was observed for the dye methylene blue, while the increase for the dye Nile blue A was more prominent up to a pH of 8, after which a reduction was observed. The isotherms were best represented by the Freundlich model for the systems containing the dyes methylene blue, basic fuchsin, and malachite green. The system containing the dye Nile blue A was best represented by the Sips isotherm. The best fits of the kinetic data for both dyes were obtained using the Fickian diffusion, quasi-dynamic flux, and lattice diffusion flux equations. The capacities in unit solutions for each dye were 79.5, 58.2, 38.1, and 224.4 mg g^–1 for methylene blue, malachite green, brilliant blue FCF, and Nile blue, respectively. When analyzing the effectiveness of an agar and graphene oxide hydrogel column, good results were observed regarding the selective separation of colors from the synthetic textile effluent by the adsorbent [54].

2.1.2. Alginate

Brown algae make it possible to obtain a natural polysaccharide called alginate, which is a natural resource in the synthesis of bio-based materials. Alginate is composed of units, namely, an S unit that makes up L-guluronic acid, an M unit that makes up D-mannuronic acid, GMGMGMGM blocks present in the form of heteropolymeric sequences, and MMMM or GGGG present in homopolymeric sequences. Due to the high presence of hydroxyl and carboxyl groups, the polymer has demonstrated high affinity for heavy metals and various radionuclides [45]. To increase its mechanical strength and stability, researchers began to further study the issue of the synthesis of alginate derivatives. This interest increased the number of studies on surface functionalization. It is worth mentioning here that environmentally friendly processes are the most accepted and studied. Studies indicate that, in general, alginate can be functionalized in two ways, either chemically or physically. Chemical functionalization, including the processes of phosphorylation, amination, copolymerization, and esterification, enables the formation of covalent bonds between the functional groups and the alginate [55]. The physical method of functionalization is performed through mixed or ionic crosslinking, which provides highly weak bonds such as coordination, van der Waals, electrostatic, and hydrogen bonds. Because these interactions are weak, they can be reversible, which is the opposite behavior of the chemical functionalization mentioned above [56]. To enhance the removal of uranium radionuclides and improve adsorbent properties such as thermal, mechanical, and radiation resistance, researchers used grafting and crosslinking to create calcium alginate spheres [57]. The spheres were modified with SiO₂ nanoparticles, which were trapped and functionalized in solutions containing uranium for later removal (Figure 2). The modification of the nanoparticles was carried out by the addition of -SH groups on the surface of the alginate–calcium gel spheres, a pioneering study in the area. The weight percentages of 3-mercaptopropyltrimethoxysilane (3-MPT) and silicon dioxide were used as a function to determine the ability of the spheres to adsorb uranium ions. For example, at thirty percent weight of 3-MPT and five percent weight of silicon dioxide (relative to the weight of alginate), the alginate/silicon dioxide/3-MPT/acidic group-converted polyacrylonitrile beads demonstrated 3.6 times greater adsorption capacity when compared to the unmodified beads, which contained only alginate and acidic group-converted polyacrylonitrile. Therefore, radionucleotide metal ions can be effectively removed using a compound containing silicon dioxide, alginate, 3-mercaptopropyltrimethoxysilane and acidic group-converted polyacrylonitrile. In addition to silicon dioxide, which increases the nanoparticles’ resistance to alginate, other functional groups besides -SH and amidoxime can also be applied in the synthesis step in order to increase the adsorption of the metal. These new synthesis routes may support future progress in the field of radionucleotide remediation.

An alginate fiber was integrated with graphene oxide, iron oxide, and metal–organic frameworks to develop a dispersive magnetic solid-phase extraction adsorbent [58]. The extraction of phthalate ester was improved by combining graphene oxide with the metal–organic frameworks, which was facilitated by hydrophobic interactions and hydrogen bonding. The most commonly used cation during the synthesis of alginate hydrogels is Ca²⁺ because this chemical element, through coordinated interactions, generates a three-dimensional network with the G-block regions. A study synthesized activated carbon from algae; the material was activated with iron (III) chloride and then crosslinked with calcium chloride, generating an alginate hydrogel composite [59]. The composite showed plasticizing properties and was able to efficiently remove the cationic textile dyes methylene blue and bisphenol A, present in a singular form in synthetic aqueous solutions. Therefore, it is possible to infer that alginate can be applied as an adsorbent in long-term analyses. It is also observed that new investigations are analyzing its adsorptive properties against other contaminants.

2.1.3. Cellulose

The most widespread source of cellulose in nature is lignocellulosic biomass. This naturally occurring biopolymer has 1,4 glycosidic bonds in its linear structure that connect the -D-glucopyranose units. Cellulose can vary in its degree of polymerization from 100 to more than 10,000, depending on the plant species [48]. Three hydroxyl groups are present in an individual manner on each carbon (2, 3, and 6) within the anhydroglucose cellulose unit. Hydroxyls can form inter- and intramolecular hydrogen bonds in the internal part of macromolecules. These interactions are important since they provide crystallinity and rigidity to the structure [60]. In the case of cellulose as an adsorbent, it is favorable when it has firm and consistent pores, a small size, and a basic structure with numerous sites distributed over its surface [61]. The abundance of hydroxyl groups in its structure has led to numerous studies exploring the potential of cellulose as an adsorbent for water contaminants [62,63]. Cellulose modification can occur through two primary classes of techniques: The first involves the attachment of functional groups directly to the hydroxyls of cellulose (direct attachment); this group includes amination [46], sulfation [64], esterification [65], etherification [66], silylation [67], and carbonylation [68]. The second class corresponds to radical polymerization, ring-opening polymerization, and free-radical polymerization, all of which are controlled to graft monomers onto cellulose chains, which can occur in both heterogeneous and homogeneous solutions [69]. The chemical compounds of cellulose that are modified help with water remediation by eliminating metal ions and other chemicals that are harmful to nature and humans [70].

Cellulose nanocrystals that can be obtained from cotton or wood have a high aspect ratio, good mechanical properties, low density, and high hydrophilicity and are therefore classified as good nanostructures for membrane processes in the area of environmental engineering and ecological sciences. The removal of cellulose at the nanoscale increases in antifouling potential of cellulose nanocrystal membranes; this process helps to remove flaws that are related to the layer-shaped structure. A study developed nanofiltration membranes comprising thin films composed of cellulose nanocrystals, which sought to exhibit high resistance to chlorine and for use under high water flow [71]. The modified membrane showed a greater salt removal capacity when compared to the unmodified membrane. The removal of Na₂SO₄ and MgSO₄ was 96 and 98%, respectively; a higher water penetration flux (106.9 L m⁻² h⁻¹) was also observed in the membrane with cellulose nanocrystals compared to that in the thin-film membrane without cellulose nanocrystals. In another study, a polyamide layer was grafted with cellulose nanocrystals in the synthesis step of a thin-film composite membrane to increase the antifouling separation performance (Figure 3) [72]. The penetration of the membrane with nanocrystals was 60% higher when containing only 0.02% by weight of cellulose nanocrystals in a comparative analysis with the polyamide membrane without cellulose nanocrystals. The modified thin-film composite membrane also showed exceptional permeability and hydrophilicity, demonstrating the viability of its use in water purification and desalination. Finally, these membranes also showed greater resistance to the occurrence of fouling and greater recovery capabilities when compared with commercial membranes.

2.1.4. Chitosan

The partial deacetylation process that occurs in chitin is the main mechanism that makes it possible to produce chitosan. It is worth noting that it is the only polysaccharide that occurs naturally and has a positive charge (cationic polysaccharide). The free amino and hydroxyl groups along the chitosan chain present a highly effective chelating behavior. In addition, it is also possible to observe adsorption by electrostatic interactions with various organic pollutants, generally those with opposite charges (anionic), and other heavy metals. Despite these advantages, materials based on this polysaccharide have limitations that hinder their large-scale application. Among the negative points, it is possible to highlight their low mechanical resistance, acid solubility, and low adsorption capacity for some pollutants [73]. Seeking to increase the capacity and chemical resistance, in recent years, researchers have developed some modifications in the chitosan chain, such as acetylation, carboxymethylation, grafting, and crosslinking. A study used a layer-by-layer self-assembly design to create a heterostructured chitosan multilayer membrane; this sieve was modeled in layers using a wooden structure [74]. Used to purify water, the sieve showed a high level of van der Waals forces and hydrophobic behavior; thus the biomimetic membrane was able to efficiently remove several pollutants. It was also possible to observe that the multilayer membrane constantly presented high recycling rates, removing several oil droplets and corroborating its durability [74]. In the case of the adsorption of heavy metals and dyes on the surface of chitosan-based materials, it has been observed that the process can be improved by impregnating the chitosan matrices with nanoparticles; this is mainly due to the increase in the specific surface area. A study constructed graphene oxide aerogel microspheres by combining freeze-drying and electrospraying, where the microchannel designs were in a radial position together with honeycomb spider webs [75]. A chitosan nanofiber membrane showed high adsorption capacities for various organic pollutants (methyl orange, methylene blue, rhodamine B, and eosin Y) and micropollutants through a variety of interactions between the adsorbates and the adsorbent surface, such as chelation, coordination, conjugation, and complexation [76]. With a penetration flux of 1533.2 L m⁻² h⁻¹, it efficiently filtered both metal ions and anionic and cationic dyes present in pure water. No volatile organic compounds were used in the synthesis of the nanofiber membrane; therefore, both sustainable development and green chemistry are supported in this process. This study supports the creation of an economical and efficient method for the development of high-tech membranes for water purification.

2.1.5. Pectin

Pectin, a naturally occurring linear polysaccharide, can be extracted from the cell walls of higher plant organisms. The advantages of this polysaccharide include its anionic, non-toxic nature, high molecular weight, biocompatibility, and high flexibility. Pectin is present in the intercellular area and is one of the main constituents of the cell wall. In recent years, its use has expanded, mainly in the biotechnology and medical sectors. Due to its complexity, its composition can vary and also depends on the isolation conditions used and its source. Its main composition is based on chains containing d-galacturonic acid molecules, which are linked via 1,4 glycosidic bonds that connect with some neutral sugars, which are distributed along the sides of the chain [47]. Carboxylic acids can occur naturally, such as methyl esters, while others can react with ammonia to produce new carboxamide groups. In the presence of metal ions, the carboxamide groups can form complexes and can be reduced to nanoparticles without the need to add hazardous stabilizers or other reducing chemicals. In this sense, polysaccharide materials can be stabilized in palladium nanoparticles, the most environmentally friendly and easy-to-produce forms being palladium/starch, palladium/chitosan, and palladium/gelatin, generating organically recyclable nanocatalysts. The hybrid inclusion of inorganic and organic nanocomposites allows for versatility, supporting the great interest in these materials [77]. A pectin/agar composite was synthesized, presenting a highly active surface area, strong covalent bonding, and high-temperature durability, and was used efficiently as a stabilizer. After this step, under environmentally preferable and safe conditions, palladium nanoparticles were first synthesized by the reduction of palladium ions (in situ) and subsequently immobilized on the membrane surface. During the reduction process, the new biopolymer composites showed high catalytic activity, corroborating their use as a stabilizer for noble metal nanoparticles. The reduction of O-nitroaniline allowed for the production of 1,2-benzenediamine; reuse/recycling experiments confirmed a reduction in the reaction yield of almost 27% after eight consecutive cycles of use of the nanocomposite. Nanospheres containing titanium oxide and pectin (hybrids) were synthesized and used as nanoadsorbents in the remediation of cationic heavy metals present in water [78]. The pH had a strong influence on the removal of metals; in the case of lead and copper, a pH of 5 was considered ideal to favor adsorption; in the case of cadmium, a pH of 6 was the most suitable; and for zinc, conditions close to neutrality were the most favorable for its capture. The metals with the best removal values on the surface of the hybrid nanospheres were lead and copper, with maximum capacities of 0.83 and 0.68 mmol g⁻¹, respectively.

2.2. Microbial and Plant Sources

Biopolymers derived from microbial and plant sources have gained significant attention due to their sustainability, biodegradability, and potential for environmental applications. Microbial biopolymers, such as polyhydroxyalkanoates (PHAs), are synthesized by bacteria as intracellular carbon and energy storage compounds, exhibiting properties similar to conventional plastics but with superior biodegradability [79]. The composition of PHAs can be tailored through fermentation conditions, making them suitable for applications ranging from packaging to wastewater treatment [80]. Similarly, polylactic acid (PLA), derived from plant-based starch sources such as corn and sugarcane, has emerged as a promising biopolymer due to its biocompatibility and ability to degrade under controlled conditions [81]. PLA-based materials can be processed into membranes and porous structures, enhancing their applications in water purification and filtration [82]. Additionally, lignin, a complex aromatic polymer obtained from plant biomass, has been explored for its adsorptive properties, particularly in removing heavy metals and organic pollutants from water [83]. Lignin-based materials can be modified through grafting and crosslinking to improve their stability and efficiency as biosorbents [84]. These microbial and plant-derived biopolymers provide eco-friendly alternatives to synthetic materials, aligning with global sustainability efforts while offering versatile applications in environmental remediation and wastewater treatment.

2.2.1. Lignin

Lignin is a naturally occurring biopolymer with a complex heterogeneous chain. Its chemical structure is formed by three different types of monomeric phenolic substructures (p-hydroxyphenyl H units, guaiacyl G units, and syringyl S units), these units originating from monolignols, coniferyl alcohol, sinapyl alcohol, and coumaryl alcohol [83]. The hydrophilicity, functionality, and reactivity that encompass the chemical and physical properties of lignin are determined by the existence of functional molecules that are present within the macromolecules, which may include carboxylics, phenolic hydroxyls, methoxyl aliphatic hydroxyls, and also carbonyl groups [85]. As an alternative to more expensive and commercially used sorbents, scientists have begun to explore biosorbents containing lignin. Their synthesis involves procedures such as hybridization, crosslinking, copolymerization, and grafting. A study developed various types of lignin as adsorbents through the use of fundamental poly(acrylic acid) as tentacles and lignin as grafts [84]. The branched chain of poly(acrylic acid) presented greater density, and this corresponded to greater selectivity and adsorption; these results corroborated the values predicted by a nonlinear binary model supported by phenomenological theory and Pearson’s correlation analysis. The literature describes theoretical foundations that aid in the synthesis of new lignin-based biosorbents, which present connections in their structure through grafting in binary pollutants. As a final result, it is possible to observe high adsorption capacities. A chemoselective crosslinker (cyanuric chloride) was used to chemically bond amine-functionalized magnetic nanoparticles to lignin [86]. The adsorbent, which was synthesized via a temperature-controlled synthetic approach, demonstrated high stability, high selectivity, and excellent regeneration potential for lead removal. Due to their rapid recovery capacity, which increases the sustainability of the process, the use of lignin-based magnetic materials has also been analyzed. A foldable lignin-based material was used to develop a nanotrap to inhibit bacterial growth and aid in the capture of positively charged metal ions [86]. Interactions through a robust bond between the functionalized lignin surface and the metal ions allowed the nanotrap to maintain high efficiency. In the case of a lignin-based nanotrap loaded with silver ions, it showed strong antimicrobial properties against S. aureus and E. coli species, with removal values of 99.7 and 99.6%, respectively [86]. The Mannich procedure with triethylenetetramine was used to treat lignin to remove As(V) ions present in the water. Lignin activated at 298 K and a pH of 9 showed the best silver removal ability, exhibiting better selectivity when compared to chromium and phosphorus ions. In general, other studies also show that lignin-based biosorbents offer promising uses at an industrial scale [87].

2.2.2. Polyhydroxyalkanoates

Polyhydroxyalkanoates are manufactured by microorganisms and are characterized by being a type of aliphatic hydrophobic polyester. Their properties and positive aspects are similar to those of typical plastics. In addition, they have the advantage of exhibiting superior biodegradability and biocompatibility. Once exposed to the natural environment, polyhydroxyalkanoates can easily be broken down into harmless compounds, which is why they are considered important emerging biopolymers that can replace traditional petrochemical plastics. Two classes of polyhydroxyalkanoates have recently been discovered: short-chain polyhydroxyalkanoates, which contain three to five carbon atoms in each monomer, and medium-length polyhydroxyalkanoates, which contain six to fourteen carbon atoms in each monomer [79]. Changing the fermentation stage, voltage, and feed allows for the simple manipulation of the composition of polyhydroxyalkanoates. As a result, polyhydroxyalkanoates have become necessary and useful in several applications, since they can be elastomeric or rigid [88]. In order to improve environmental and economic sustainability in water remediation, researchers proposed a method to remove polyhydroxyalkanoates from a contaminated mixed microbial culture [80]. The authors assumed that scaling the process from laboratory to industrial scale was promising, and so the production of mixed microbial cultures (polyhydroxyalkanoates) was transformed into a pilot-scale study. Municipal solid waste and sewage sludge mixtures were used to study the generation of polyhydroxyalkanoates as a high-value bio-based final material at the pilot scale [89]. The three-step mixed microbial culture method was used in production. One of the advantages is that it accelerates the conversion of urban waste into polyhydroxyalkanoates using a single technology, significantly reducing the volume of final waste that is discarded.

A combined process using biotransformation and thermochemistry enabled the extraction of crotonic acid and polyhydroxyalkanoates obtained from anaerobically digested sludge; this process maximized the use of chemical energy and the final sludge volume [90]. An interdisciplinary process, consisting of four steps, as shown in Figure 4, determined the real potential of the residue for the generation of platform chemicals and biopolymers. The evaluation included the combination of two toolboxes, one related to biological conversions and the other based on thermochemical treatments. Figure 4B shows the experiment designed at the laboratory scale with bacterial fermentation. The chemical oxygen demand (chemical energy of the raw material) must be transferred to volatile fatty acids, which are excellent substrates since they promote adaptation and growth in mixed microbial cultures, especially during famine and abundance cycles, in order to investigate the generation of polyhydroxyalkanoates in the mixed microbial culture system.

The acidogenic fermentation studies showed that the hydrothermal carbonization stage used the same anaerobically digested sludge as the acidogenic inoculum under varying temperatures of 150, 200, and 250 °C. Over time, the duration of the hydrothermal carbonization fermentation was measured using large-scale semi-continuous hydrothermal carbonization in sludge without liquid–solid separation at a temperature of 200 °C. This was selected in order to maximize process reliability and volatile fatty acid yield. It is worth noting that less harsh conditions generally cost less energy and are easier to establish. The method addressed aims to eliminate energy and material losses, in addition to reducing the volume of waste. Thus, the anaerobically digested sludge is managed in order to obtain high-value-added chemical products and commodities. The widespread use of polyhydroxyalkanoates and their mass production are hindered by high manufacturing costs. Therefore, it is essential to study other possible sources of raw materials that are easily accessible and low in cost to enable the easy synthesis of polyhydroxyalkanoates. Finally, polyhydroxyalkanoates need to have positive aspects such as high hydrophilicity and machinability in order to meet all their applicability demands.

2.2.3. Polylactic Acid

Bio-based natural resources such as sugarcane, corn, and cassava are starchy raw materials and are a source of polylactic acid, which is a commercially promising biopolymer. High-purity lactic acid is generated through fermentation carried out by bacteria [81]. Lactic acid is important because it serves as a starting point for the chemical route for the production of polylactic acid, which is generated from renewable and abundant raw materials such as starch and has a predetermined molecular weight. Without generating waste, polylactic acid can be completely decomposed by microorganisms after its use in the natural environment, thus being effectively recycled during the process [91]. For porous polylactic acid, some methods are available, such as 3D printing, thermally induced phase separation, freeze-drying, electrospinning, and foaming. Through in situ oxidative polymerization, researchers obtained electrospun polylactic acid nanofibers doped with chloride and coated with polyaniline. Weak physical interactions, such as electrostatic interactions, between the adsorbate and the adsorbent surface were induced by combining porous polylactic acid with the conductive polyaniline coating, enabling the adsorption process carried out by membranes [82]. The synthesis of porous polylactic acid materials has limitations and challenges, such as preventing the porous structure from collapsing (which would lead to cracking and shrinkage), optimizing process parameters, and selecting the ideal solvent. The use of supercritical carbon dioxide during the drying step of porous material synthesis can also prevent the occurrence of fragmentation and shrinkage, in which the original structure and high open porosity are preserved. A permeability of up to 73% was achieved in porous polylactic acid materials dried with supercritical carbon dioxide, supporting their efficient use in wastewater remediation. Depending on the solvent used and the drying method, the morphology and structure of polylactic acid can be adjusted [92]. The use of polylactic acid in the treatment of contaminated water is widespread in the literature; however, most studies are limited to effluents containing dyes and oils. Therefore, in the future, new highly porous polylactic acid materials should also be used for disinfection in waters contaminated with bacteria, surfactants, and pesticides.

2.2.4. Starch

A common and highly adaptable polymer is starch, which has gained increasing studies due to its usefulness in the chemical, nutritional, and pharmaceutical industries. The polysaccharides amylose and amylopectin are present in starch [49]. Amylose corresponds to approximately 20 to 30% of starch, and it has a linear chain of D-glucose elements, where the majority (99%) have 1,4 glycosidic bonds, and the minority (1%) have -1,6 glycosidic bonds; this linear chain is insoluble in water. The amylopectin chain is about three times larger in size than amylose; thus, its molecular weight varies from one to two million daltons [93,94]. The fraction corresponding to amylopectin refers to the water-soluble part of the starch and is the largest fraction, approximately 80% [95]. Biodegradability, high availability, low cost, and biocompatibility are the main positive aspects of starch. A negative point is that its natural form does not have a high adsorption potential, which is mainly related to the lack of strategic functional groups such as ester, amino, or carboxyl groups, which are important for establishing different interactions with various micropollutants and organic compounds. In order to increase its usefulness, some techniques have been used to alter its chemical structure, such as esterification, irradiation, oxidation, and grafting [96]. (3-Chloro-2-hydroxypropyl trimethyl) ammonium chloride was reacted with starch to form ammonium clusters containing cationic starch, which were used to produce starch nanocomposites and adsorbents for cationic dyes [97]. A maximum capacity of 205 mg g^–1 was observed for the adsorption of nitrate groups present in wastewater on the surface of quaternary starch. The adsorbent also showed high reuse potential, since after eight cycles it maintained a removal rate of 78%. The literature describes several studies on the adsorption of dyes and heavy metals by the surface of modified starch; however, the removal of phenols and ammonia, which are toxic and are frequently detected in wastewater, is still insufficient. Therefore, it is recommended to invest in research on developing new methods of starch modification to increase its affinity for these two toxic chemical components [98].

2.3. Animal-Derived Biopolymers

Gelatin

Tendon, skin, cartilage, and bone are connective tissues that contain collagen protein in their composition. When the partial hydrolysis of collagen occurs, gelatin is generated, which is a biopolymer that is highly soluble in water. The biopolymer produced has functional groups in its structure such as -OH, -NH₂, and COOH, which can act as adsorptive sites since they can undergo polar or ionic interactions with pollutants present in wastewater. The limited use of gelatin in adsorption processes is mainly due to its low resistance in contact with water and its rapid degradation. One way to improve these properties is with the use of nanomaterials such as carbon nanotubes, titanium dioxide, manganese dioxide, and iron (III) oxide. In addition, chemical surface modification also makes it possible to add strategic groups that aim to increase removal through the addition of various functional groups in the gelation molecular chains [99]. Gelatin–xylan hydrogels were synthesized using ethylene glycol diglycidyl ether. The high removal of methylene blue dye and shear thinning behavior were exhibited by the hydrogels. Another aspect is that the gel showed high stability against high temperature and stress variations without losing its crosslinked structural shape. Carboxylic acid-treated multi-walled carbon nanotubes and magnetic iron oxide nanoparticles were used to remove cationic and anionic dyes [100]. Nitrates and phosphates were efficiently removed on the surface of a hybrid composite of amine-grafted magnetic gelatin [101]. A surface complexation process plus electrostatic interactions was the mechanism responsible for the adsorption of PO₄³⁻ and NO₃⁻ on the surface of the gelatin hybrid composite. The combination of freeze-drying, polyethyleneimine crosslinking, and titanium dioxide enabled the development of a multifunctional organic–inorganic composite aerogel based on gelatin [102]. The hierarchical porous structure combined with a super amphiphilic surface enabled the aerogels to have high selectivity for water/oil separation in liquid and oil/water combinations. High water purification potential has also been observed in gelation-based adsorbent composites. Solutions containing multicomponent and pharmaceutical compounds have been treated using gelation composites with high mechanical strength as adsorbents.

3. Synthesis Strategies to Manufacture Bio-Based Materials

Due to the advancement of processes to synthesize nanomaterials and thin films, a variety of new bio-based materials have been made available in the databases of scientific literature. This study focuses on the most current methodologies that present high applicability and efficiency in the synthesis stage of obtaining bio-based materials. Chemical modification, electrospinning, phase inversion, and molecular imprinting are among some of the processes detailed in Figure 5 used to obtain these materials; this section also discusses the positive and negative aspects of each strategy.

3.1. Molecular Imprinting

Synthetic polymers began to be designed with the insertion of recognition sites in order to capture certain target molecules with a high degree of specificity, and this became possible due to the emergence of molecular imprinting. In the case of monomers and functional crosslinkers, researchers use a model molecule for copolymerization, generating bio-based materials. To extract the model groups from the polymer networks, new cavities must be generated that act as recognition sites. These cavities must have chemical functional groups, configurations, dimensions, and structures similar to those of the model molecule. Thus, it is possible to recognize the original models and selectively reconnect them when faced with a mixture of closely correlated compounds. Due to advantages such as ease of fabrication, physical robustness, high stability, excellent selectivity, and low cost, a variety of molecularly imprinted polymers have been synthesized and used as adsorbents for the efficient separation of pollutants such as metal ions and other carcinogenic organics present in liquid effluents [103,104]. Due to the efforts of various research groups, significant progress has been made in the application of bio-based molecularly imprinted polymers in the area of water remediation. New positive aspects have been reported in different materials, such as low-cost synthesis, easy fabrication methods, no toxicity, high affinity, and selective adsorption. One study synthesized a photoresponsive cellulose-based imprinted adsorbent through atom transfer radical polymerization that was initiated on the surface of the solid [105]. The high adsorption specificity allowed it to achieve a capacity of 11 mg g^–1 against the removal of the herbicide 2.4-D. The molecularly imprinted photoresponsive cellulose-based intelligent material could be reused for several desorption cycles, exhibiting high stability and compatibility with the target molecule. Regeneration was possible by exposure to ultraviolet light, where the cellulose adsorbent incorporated an azobenzene functional monomer.

Due to its high concentration of amino groups and hydroxyl radicals, chitosan is also a polysaccharide widely used in molecular imprinting. These groups allow for interactions with various contaminants. Researchers sought to selectively remove cadmium through the use of a chitosan polymer functionalized with double-surface-imprinted acrylamide [106]. The substrate used was chitosan, the mold was cadmium metal, the mimetic mold was 4-hydroxybenzoic acid, and the crosslinker was epichlorohydrin. The authors analyzed the adsorptive potential of the imprinted acrylamide-functionalized chitosan-based polymer against the adsorbates salicylic acid and cadmium; the maximum capacities were 45.7 and 53.4 mg g^–1, respectively [106]. Chitosan was used to develop ion-imprinted cryocomposites using an innovative method called ice modeling and ion imprinting. The monomers used were acrylamide and N,N′-methylenebisacrylamide, the crosslinker was glutaraldehyde, and the template ion was copper [107]. In the chitosan matrix, the metal ions, hydroxyl radicals, and amino groups underwent a chelating reaction, which is why the chitosan-based ion-imprinted cryocomposites had a high capacity to remove copper and high specificity [107]. With the correct recognition spaces (cavities) at the microscopic level, bio-based molecularly imprinted polymers are adsorbents with high potential for use in the treatment of real wastewater, mainly due to the large number of bonds formed between the individual recognition sites.

3.2. Phase Inversion

The manufacture of polymeric filtration membranes uses phase inversion on a large scale because this method has the advantages of high versatility, low maintenance requirements, cost-effectiveness, and easy adaptation. In this method, polymer solutions that are initially homogeneous are transitioned to a solid phase. Spinning fibers and flat soles can be synthesized from hollow membranes with asymmetric fibers. There are several methods for phase inversion; among the most common are non-solvent-induced phase separation, vapor-induced phase separation, and thermally induced phase separation. Some factors determine the interactions (physical or chemical) between the environment and the membrane; among them are the temperature of the bath used in the casting stage, the polymer concentration, the interaction with the solvent, and the additives used. Two structures and two morphologies can be produced in this membrane synthesis method; the structural parts can be categorized as sponges or finger-like structures. When demixing occurs immediately, the formation of macrovoid or finger-like structures is observed, while the sponge shape occurs when demixing is delayed. In the case of mechanical resistance, it is observed that the sponge-shaped structure is more resistant than the finger-shaped one because the micropores have solid walls around them, corroborating the generation of an entire thin active layer. However, the flow resistance is lower because these pores present less distortion. Therefore, it is necessary to use a support that has a finger sublayer in addition to the final layer of spongy material, which enables the synthesis of highly efficient filtration membranes [92,108]. The phase inversion method makes it possible to produce a large quantity of cellulosic membranes. Several separation techniques (reverse osmosis, ultrafiltration, microfiltration, and nanofiltration) have started to use cellulose acetate [109]. In water purification and desalination processes, asymmetric cellulose acetate membranes have the advantage of retaining salt and exhibiting high porosity; these pores can accommodate various adsorbate molecules. Cellulose acetate was also used in the synthesis of composite films, which were subjected to phase inversion under pressure at room temperature and in the presence of water vapor [110]. The composite film showed an efficiency of 99% and a separation flux of 667 L m⁻² h⁻¹, thus separating the water/oil emulsions while being stabilized with the use of a surfactant with a manometric size. The powerful destructive activity of the film against organic molecules was possible thanks to the photocatalytic activity of titanium dioxide under ultraviolet irradiation, being an encouraging result for the domestic and industrial water treatment sectors [110].

The synthesis of membranes by phase inversion has made it possible to replace toxic solvents with more correct and sustainable materials. This group includes some solvents such as ionic liquids [111], dimethylisosorbide [112], triethylphosphate [113], and dimethyl sulfoxide [114]. The application of bio-based green solvents (γ-valerolactone and glycerol derivatives) and non-solvents with induced phase separation makes it possible to synthesize permeable membranes. The use of γ-valerolactone in the synthesis of membranes with small pore sizes enables their application to nanofiltration, ultrafiltration, and microfiltration [115]. The use of the green solvents 2-methyltetrahydrofuran and methyl lactate allowed for the synthesis of a cellulose acetate nanofiltration membrane [116]. The highly permeable membrane presented a porous morphology formed of finger-shaped macrovoids when using cellulose acetate at low concentrations. By increasing the acetate concentration in the casting solution, an increase in the removal of rhodamine B dye by 68% and a reduction in permeability (from 32 to 2.4 L m⁻² h⁻¹ bar⁻¹) were observed. Highly selective and specific separation, permeation flux, and antifouling are some of the properties for which membrane efficiency can be optimized using nanomaterials in the phase inversion step for the subsequent synthesis of nanofiltration membranes with bio-based materials. A deficiency in acid–base tolerance and a lack of thermal stability are among the limitations present in organic nanoparticles; to mitigate these negative aspects, it is recommended to control the structure of organic nanoparticles. Loose nanofiltration membranes have as a limitation a tendency to agglomerate, and biopolymer matrices exhibit incompatibility with inorganic nanoparticles, leading to the generation of imperfections and defects at the bipolymer interface. To develop reliable, high-performance, and advanced hybrid inorganic loose nanofiltration membranes, it is necessary to improve the distribution of inorganic nanomaterials, modify the structure and surface morphology, and increase the existing correlation between biopolymers and inorganic nanoparticles. Nanofiltration membranes have their selectivity and permeability compromised due to the reasons mentioned above. For this reason, several researchers are focusing their efforts on the synthesis of innovative materials that present a matrix with high permeation flux without negatively affecting the accuracy of the separation.

3.3. Electrospinning

The production of nanofibers with good property/structure ratios in the range of 5 to 500 nm (submicron) in large volumes and at a low cost is feasible due to electrospinning. Electrospun nanomaterials have shown great potential for filtration in water remediation due to their favorable properties such as high stability, good permeability in water, and high surface area. With a high-voltage field in a viscoelastic polymer solution, electrospun polymer nanofibers are produced via uniaxial stretching and elongation. The four components of this step are a spinneret, a source that will supply a high voltage, a grounded metal collector, and a syringe pump. The release of a voltage between the grounded collector and the tip of the needle occurs in the Taylor cone, where, during the electrospinning process, the polymer solution is transformed from a spherical shape to a conical shape. The electrostatic repulsion present in the surface charges will be greater than the surface tension of the droplet; this is possible when the intensity of the electric field is extremely strong. The fluid jet will be ejected toward the grounded collection; this is performed by the Taylor cone present at the tip of the needle. When the electric field strength is small, the surface tension of the droplet will be greater than the electrostatic repulsion of the charges. Researchers have managed to generate non-woven membranes under conditions where the solvent was rapidly evaporated and the jet curved and stretched rapidly [117,118]. These new synthesized nanofibrous membranes can be applied as filters or as adsorbents in separation technologies such as nanofiltration, microfiltration, and ultrafiltration due to their properties such as multichannel architectures, densely interrelated pores, and permeability greater than 90%.

Using electrospinning, researchers synthesized a polyvinylpyrrolidone/polyvinyl alcohol/chitosan membrane to adsorb heavy metals and organic pollutants present in the liquid phase [119]. With an average diameter of 160 nm and a water permeability of 4518.91 L m⁻² h⁻¹ bar⁻¹, the new membrane fabricated under ideal conditions showed a consistent and biocompatible structure and high permeability. The maximum removal values for the dyes methylene blue and malachite green and for the metals lead, cadmium, copper, and nickel were 70, 94, 90, 83.3, 84, and 80%, respectively. Due to the good structure of highly open pores and high surface area, it can be said that the electrospun cellulose acetate membrane is a viable material for water filtration applications. With an antifouling surface structure, researchers fabricated a hydrophilic cellulose nanofibrous membrane using the electrospinning technique [120]. Cellulose acetate fibers tend not to organize themselves equally, so it is possible to have fibers preferentially oriented in the diagonal position, in the horizontal position, and also in the vertical position, where the sizes can vary from 0.5 to 1 m, leading to the possibility of the high degradation of colored molecules with values above 80% and reaching 99% [120]. Due to their chelation capacity, high hydrophilicity, and good biodegradability, electrospun membranes have become a good alternative in the remediation of heavy metals [121]. An advanced filter medium can be established with the use of electrospun nanofiber membranes since they have antifouling capabilities, low operating pressure, improved reduction, and high flux. Electrospinning also has negative points, such as inapplicability to all biopolymers, needle clogging, a lack of stability in membrane structure, the need for a high electric field, low productivity, and long operating time. In order to overcome these limitations, an electrospinning process was developed without the need for the use of needles. In continuous systems without needles for membrane synthesis, it is possible to obtain high productivity and high levels of safety. Although large-scale manufacturing is in the development and research stage, it is possible to observe the existence of medium-sized manufacturers. Therefore, the newest generation of filtration materials corresponds to electrospun nanofibrous membranes, which have advantages that support the establishment of cutting-edge and viable filtration systems [122].

3.4. Chemical Modification

Altering the structure of biopolymers through chemical modification results in a change in order to produce new derivative products that present improved or unique chemical and physical properties. When chemical modifiers bind to the substrate by covalent bonds, they increase chemical stability. Chemically modifying the membrane by increasing hydrophilicity promotes an increase in water permeability in addition to reducing the impacts of fouling. Researchers used (common) A4 printer paper and transformed it into (multifunctional) cellulose membranes through chemical modification [123]. The A4 paper was filled with calcium carbonate, and the solvent used was hydrochloric acid. The second step consisted of adding alkoxide groups along the cellulose fiber. After chemical treatment, the authors also altered the hydroxyl groups in the cellulose network using trichlorooctylsilane. With a simple and low-cost procedure, chemical modification made it possible to use A4 paper as a cellulose-based membrane with multiple functions. The Mannich reaction consists of the electrophilic substitution of an amine by active hydrogens, producing aminated lignin as the final product. The chelation of iron (III) together with amine using the Mannich method was performed to synthesize lignin-based adsorbent materials to remediate phosphates in water [124]. For a concentration of 5 mg L^–1 of adsorbate, the new adsorbents showed a removal above 90%, where the mechanism was highly dependent on phosphate and iron complexation. This study shows that bio-based materials can be constructed economically and realistically in order to treat waters containing low phosphate concentrations, supporting a reduction in the eutrophication process. Chitosan was used to synthesize a highly porous aerogel barrier to recover drinking water from a ship dismantling yard with an oil spill. The process used genipin as a crosslinking agent [125]. The steps required to develop the superhydrophilic aerogel chitosan membrane with an agarose-based inner wall coating are illustrated in Figure 6. A naturally occurring resource with a high abundance of chitosan is the shells of crustaceans such as crabs, lobsters, and shrimp. The functionality of chitosan in the -NH₂ group allows for chemical crosslinking. After this step, its application can occur in a variety of ways (physical and chemical changes) precisely because it is highly fibrous in nature. A robust scaffold-like structure makes it possible to transform chitosan by regulating the degree of crosslinking. Acting as a gelling agent, agarose promotes the synthesis of a porous aerogel membrane. The agent also contributes to the improvement of the hydrophilic property, which occurs through the interaction with chitosan via hydrogen bonds during the lyophilization step. According to the authors at the temperature of 80 °C, genipin can instantly crosslink chitosan. In the case of using agarose, it produces a highly stable gel that interacts with the chitosan hydroxyls through H-bonds. This allows the gel to also be applicable at room temperature. With a flow rate of 600 L m⁻² h⁻¹ bar⁻¹ and using ultrapure water (>99%), it can be produced by altering the chitosan aerogel and modifying the genipin crosslinking step into a structure similar to a rigid scaffold and a column [125]. The spatial conformation, quantity and location of the groups used to modify the biopolymer are strongly related to the chemical and physical properties of bio-based materials. To further elucidate the link between function and structure, it is important to conduct more studies involving modified biopolymers. In addition, current procedures need to optimize the modification conditions, increasing the stability and structure of bio-based materials with a high degree of precision.

4. Green Synthesis and Its Benefits Compared to Chemical Synthesis

One advantage of green synthesis is that bio-based products are highly accessible and abundant, which can significantly reduce costs [126]. This reduction can be supported by the use of biocompatible and natural reducing and coating agents, both available in the field of biology. Waste materials from both industry and agriculture can be used to produce nanoparticles. In the case of chemical synthesis, it requires the use of expensive and often hazardous chemical agents that generate potential damage to the environment [127]. One projection predicts that iron oxides, iron nanoparticles, and commercial activated carbon are fifty times more expensive than green-synthesized iron-based nanomaterials. The main method to reduce this cost is the application of bio-based materials in the synthesis step. The replacement of chemical preparation with the synthesis of plant-based materials has been favored by several studies, highlighting rapid synthesis kinetics [128]. Biomimetic green synthesis illustrates the interface between inorganic compounds and biomolecules and other implicit biological processes. The synthesized nanoparticles have a well-defined size and geometry, as well as high chemical purity, which is possible due to the well-defined chemical compositions, structures, and properties present in the biomolecules [129].

The two most common methods for synthesizing nanoparticles are top-down and bottom-up. While the bottom-up approach assembles tiny building blocks to create a huge structure (nanoparticle development), the top-down approach breaks down the bulk material by crushing, milling, and machining. Reducing agents are typically used in chemical and physical procedures to create nanoparticles by reacting with the precursor material [130]. But as a result of these techniques, byproducts that are extremely poisonous to humans and potentially detrimental to the environment are produced. Synthesizing such NMs that result in less harmful residues or byproducts during production is therefore quite interesting. Since greener reducing agents are non-toxic and produce fewer health hazards such as genotoxicity, lung toxicity, and carcinogenicity, it is clear why the remediation process should switch to them [130]. Because biological compounds contain bioactive components, they have lower toxicity and higher stability. There is currently hope for sustainable development and better environmental health thanks to bio-based NMs. The development of advantageous and ecologically friendly NMs with fewer negative environmental consequences is the result of recent advancements in bio-based nanotechnology [131]. The utilization of microorganisms such as bacteria, fungi, algae, [132] and plant materials was reported. The biological approach makes use of materials like peptides, proteins, polysaccharides, enzymes, and agricultural waste [133]. These naturally occurring compounds are recognized to be effective reducing, capping, and stabilizing agents, which leads to the reduction of metal ions during NM synthesis. A few key factors, including cost, energy consumption, and environmental friendliness, must be taken into account before adopting a greener technique. For biologically produced metal sulfide and metal oxide forms of NMs, natural or biological materials are consequently regarded as effective sources [133]. When paired with environmental sciences, these NMs offer a simple method for water purification [134]. Additionally, nanotechnology has surfaced in agriculture in the form of biosensors, nanopesticides, nanofertilizers, and intelligent drug delivery systems [135]. The utilization of microorganisms, including fungi, bacteria, and algae, is one method of creating bio-based NMs from biological sources. Furthermore, plant-based NMs produced using natural fibers like chitosan or chitin [136], cellulose [137], and lignin [135] serve as nanocarriers for the effective delivery and control of medications and fertilizers and are widely employed in pest control (nanopesticides). Heavy metals can also be absorbed using agricultural waste as biosorbents. Various biomasses of agricultural waste, including sugarcane bagasse (SCB), banana pseudostem (BTPT), and wasted mushroom substrate (SMS), are used to create nanofibers, nanopapers, and nanofilms [138].

One of the issues with synthesizing NMs is using environmentally acceptable methods that do not hurt the environment and using fewer harmful chemicals. Numerous species, including yeasts, actinomycetes, fungi, and bacteria, contribute to the synthesis of functionally active NMs. Bio-based nanomaterials are frequently seen as less hazardous because of their natural origin. However, because cotton dust contains cellulosic components, some textile workers suffered from respiratory illnesses in the middle of the century. According to studies, mesothelioma and other severe lung conditions may be brought on by these cellulosic fibers. Nanoscale materials can accumulate inside the cell and overcome biological barriers because of their small size. More interactions between the cell and its surroundings are made possible by the large surface area. Increased interaction with the biological environment due to this huge surface area may also lead to a rise in reactive oxygen species [139]. This makes it necessary to address issues pertaining to environmental and human safety. Before applying bio-based NMs, several factors are taken into account during testing, including genotoxicity (mutations can cause cancer cells to progress), cytotoxicity, oxidative stress, immunological activity, bacterial lipopolysaccharides or lipopolysaccharides, and endotoxins (cause inflammation). A study based on risk assessment related to the nano-life cycle was carried out to investigate the safety aspects of employing lignin-coated cellulose nanocrystals (L-CNCs) and lignin-coated cellulose nanofibrils (L-CNFs). According to various tests, the nanofibrils were relatively less hazardous [140].

The production of NMs presents issues related to social flexibility, financial viability, environmental sustainability, and the availability of local resources [141]. Similar to bio-based NMs, the effective extraction of bioresources, toxicity, commercialization, purification, bioavailability, and appropriate methods for processing bio-nanomaterials are all issues that come with good and efficient methodologies. Furthermore, two significant obstacles to broader adoption are cost-effectiveness and energy consumption. At the industrial level, researchers and businesspeople should work together to achieve exact efficiency in large-scale production. Manufacturing firms must take a proactive approach to safety in order to prevent safety issues from impeding commercialization. Regulations on the safer use of chemicals, the expansion of commercial products, and testing methods for bio-based NMs have been established by regulatory bodies in many nations [142]. Furthermore, the primary objective of creating effective NMs is to preserve their stability. Various factors may affect stability, depending on the type of nanomaterial that is manufactured. Sulfuric acid-synthesized CNCs can have excellent thermal stability [143]. Similar to this, other macromolecules found in the plant itself, such as polyphenols, organic acids, and alkaloids, can stabilize biological metal nanoparticles derived from plants. An investigation demonstrated how a thin layer of macromolecules stabilizes nanostructures (AuNPs, AgNPs) in an Ocimum leaf extract [144]. The hydrolysis of sulfuric acid is currently the foundation of most production because it is cheap and recyclable. Furthermore, several less expensive CNC production processes that use oxidation techniques and conditions, like a pre-extraction reaction in hot water, are acknowledged as effective biorefinery process output steps. As a cost-effective method, one study proposed using dicarboxylic acid hydrolysis to prepare CNFs and CNCs together [145]. Bacterial cellulose synthesis is very expensive and time-consuming. Consequently, in order to somewhat lower the cost, cheaper raw materials (biomass from plant waste) must be used [146]. Finally, because NMs exceeding the limitations can be dangerous to both persons and the environment, attempts to synthesize unique and original NMs might only be possible with enhanced information regarding safety, toxicity, and stability before applying these materials to the natural environment. Before using NMs, all necessary safety precautions must be taken. To create effective bio-based NMs for sustainable use and overcome impending obstacles with new developments, an in-depth scientific understanding is needed.

5. Advantages of Remediation via Materials of Biological Origin

Industrialization has made environmental remediation one of the most important pillars of sustainability. Natural degradation is hindered by the large volume of continuous discharge of various synthetic chemical products, which means that large volumes of recalcitrant and persistent contaminants remain in the environment, often causing irreparable damage [147]. Any remediation process involves concentrating contaminants, removing them from water compartments, and subsequently treating them to ensure their elimination. In the case of the adsorption capacity of each adsorbent, it can be improved via interactions of both a physical and chemical nature. Physical processes generally encompass most of the processes since they have no specificity and require low energy, being reversible. Chemical processes require high energy and their reversal is more complex, which limits the desorption and reuse of the adsorbent [148,149,150,151,152]. Non-biological processes have been used to produce several adsorbents and enable their subsequent use in the remediation of environmental contaminants. Examples of materials that have been extensively studied include mesoporous silica [153], metal–organic frameworks [154], resins [155], and natural mineral clays [156]. The advancement of the application of these adsorbents on a large scale often faces environmental and monetary issues. Macroeconomic analyses carried out by professionals indicate that the synthesis of these traditional materials is costly in terms of both resources and labor [157]. The pyrolysis stage makes it possible to increase the textural properties of the material by increasing adsorption; however, it is limited due to generating hazardous gases and corrosive products. Moreover, the generation of secondary contaminants and costs are even higher in the rejuvenation and post-adsorption stages. As an ecological and renewable alternative, biomaterials based on lignocellulosic biomass are an excellent replacement for these conventional materials. Their use in an integrated and multidisciplinary manner can promote remediation and highly efficient adsorption [158]. Agricultural residues, microbial biomass, chitosan, and biochar have been used to remove persistent organic pollutants and heavy metals, presenting the advantage of being renewable and non-toxic to the environment [159,160,161,162,163]. Studies involving the engineering of these biomaterials have enabled advances in recent years that have allowed for a reduction in the risk of secondary pollution (leaching of organic compounds), as well as high contaminant degradation values and high adsorption capacities. Avoiding the traditional approach to effluent treatment allows for the use of modern and biodegradable materials to remove/degrade adsorbates in the same system, saving time and money [164].

6. Bio-Based Materials: Functionalization Methods

In addition to thermal processing, modifications to the biopolymer structure can create more advanced materials capable of remediating a wider range of pollutants, thereby enhancing their cleaning capabilities [165]. Materials engineering with adsorptive properties has focused its efforts on the synthesis of 3D materials and, through biomaterial design, increasing the specific surface area (

Table 1. Wastewater remediation through the application of bio-based functionalized materials.

Bio-Based Materials	Synthesis Method	Primary Component	Contaminant	Removal Time (min) and Efficiency (%)t	Mechanism of Removal	Reference
Soda–lignin gels extracted from Nypa fruiticans	Chemical method	Lignin	Pb2+	55/96.34	Electrostatic interaction	[166]
Lignin-based carbon/ZnO hybrid nanocomposite	Chemical method	Lignin	Rhodamine B	50/79.2	Electrostatic interaction, H-bonds	[167]
TiO2/pyrolytic carbon from Kraft lignin, 31	X	Lignin	MB, rhodamine B	x/85 and 88	x	[168]
NPs@Fe3O4-lignin	Chemical method	Lignin	Cr(VI)	24/100	x	[169]
Lignin-based flocculant (LBF) combined with polyaluminum chloride (acrylamide and dimethyl diallyl ammonium chloride monomers)	X	Lignin	Disperse yellow	60/93.58	x	[170]
Hydroxypropyl sulfonated lignin	Chemical method	Lignin	Blue 79	x/94.27	x	[171]
Polypyrrole/lignin–graphite felt	Chemical method	Lignin	Acid orange	20/92.55	Electrostatic interaction	[172]
Chitosan/carboxymethyl cellulose	Cross-linked	Cellulose	Cu2+ and Pb2+	x/>99	Electrostatic interaction	[173]
Cu-MCC (microcrystalline cellulose)	Grafting	Cellulose	Prometryn	x/98.46	Electrostatic interaction	[174]
α-Fe2O3 magnetic NPs on immobilized Bacillus sp.	x	Cellulose	Atrazine	x/90	Microbial enzyme and surface activity	[175]
Fe3O4-ECH-CS (chitosan) on immobilized S. cerevisiae	x	Chitosan	Atrazine	x/88	Binding to intracellular space and surface hydroxy and amine groups of chitosan Reductive dechlorination and chlorination by Fe3O4	[176]
Sugarcane cellulose-based composite hydrogel enhanced by g-C3N4 nanosheets	Grafting	Cellulose	MB	45/99	Electrostatic interaction	[177]
Fe3O4-based starch–poly(acrylic acid)	X	Cellulose	Cu(II), Pb(II), CR, and methylene violet	x/95, 88, 93 and 93	Spontaneous and chemical	[178]
Cellulose aerogel nanocomposites	x	Cellulose	MB	60/99	Electrostatic interaction	[179]
Lignin-derived porous carbon	Casting method	Lignin	Orange II	24/98	Electrostatic interaction	[180]
Succinic anhydride-functionalized CNCs	Grafting	Cellulose	Cr(III) Cr(VI)	x/94.84, 98.33%	Electrostatic interaction	[181]
CoMn2O4 NPs on lignin supported on fibrous phosphosilicate, 10 (diameter of the NPs of CoMn2O4)	Grafting method	Lignin	4-Nitrophenol	55/99.8	X	[182]
Fe2O3/lignocellulose nanocomposite	x	Lignocellulose	Nitrate	15/97.68	X	[183]
2,2,6,6-Tetramethyl-1-piperidinyloxy-oxidized cellulose nanofibers	Grafting	Cellulose	Copper	x/100	Electrostatic interaction	[184]
Esterified cellulose nanofibers/graphene oxide	Chemical treatments (esterification)	Cellulose	Ciprofloxacin and ofloxacin (antibiotic)	240/97 and 96, respectively	Electrostatic interaction, hydrogen bonding, and π–π interactions	[185]
Crosslinked carboxymethyl cellulose-grafted carboxymethyl polyvinyl alcohol	Casting method	Cellulose	Copper	240/95	Ion exchange	[186]
Polyvinyl alcohol blend/cellulose nanofibers	In situ	Cellulose	Anionic and cationic dyes	x/84	Electrostatic interaction	[187]
Tin(IV) sulfide/carbonized loofah	Scalable method	Loofah plant	Chrome	120/99	Photocatalytic and physical adsorption	[188]
Sodium styrene sulfonate gels/carboxymethyl cellulose	Radiation grafting	Cellulose	Lead, chromium, iron, and manganese	x/41, 68, 33, and 35, respectively	Electrostatic attraction	[189]
Poly(ethylenimine)-functionalized cellulose microcrystals	Ion exchange followed by amine functionalization and oxidation	Cellulose	Poly-fluoroalkyl substances	120/85	Electrostatic interaction	[190]
Lignin sulfonate	Grafting method	Lignin	Malachite green dye	240/97	Electrostatic interaction	[126]
Crosslinked cellulose nanocrystals	Freeze-drying	Cellulose	Methylene blue dye	x/86	Electrostatic attraction	[191]
Lanthanum hydroxide/poly(ethyleneimine)-grafted alkali lignin	Facile fabrication	Lignin	Phosphate	60/95	Surface precipitation and ligand exchange	[165]
Carboxycellulose nanofibers	Nitro-oxidation	Cellulose	Cadmium	x/84	Electrostatic interaction	[192]

). Bio-based materials have as their main positive features environmental friendliness, biodegradability, renewability, and a high cost–benefit relationship. The most advanced materials developed in the last 5 years have demonstrated optimal properties such as improved separation, adsorption, flocculation, chelation, and complexation [165]. The increase in adsorption capacity is due to the presence of reactive carboxymethyl groups present in functional materials based on alginate, lignin, and cellulose, also improving the chelation potential, solubility, and chemical reactivity.

Here, we primarily concentrate on the recently documented adaptable and successful methods for SBM preparation in the water treatment industry. Phase inversion, molecular imprinting, chemical modification, and electrospinning are among the fabrication techniques that are outlined in this section. Each technique’s advantages and disadvantages are also discussed. The process of altering biopolymer structures chemically to produce biopolymer derivatives with enhanced or unique physicochemical characteristics is known as chemical modification. In order to covalently attach the biopolymer to the substrate and improve chemical stability, a chemical modifier is added. By greatly enhancing the water permeability of water treatment membranes, the high hydrophilicity produced by chemical modification may lessen the fouling effects on the membrane. Through straightforward chemical modification, a study team created multifunctional cellulose membranes from commercially available A4 printing paper. First, they added alkoxide functional groups to the cellulose fibers and eliminated calcium carbonate fillers from the A4 paper using an HCl solution. Trichlorooctylsilane (COS) was selected for the alteration of hydroxyl groups on the cellulose surface based on the acid–base-treated A4 membrane. Following effective water/oil separation, the acid–base-treated and COS-treated A4 membranes were used for origami-assisted oil absorption [123]. The ease of use and low cost of the chemicals are the benefits of this chemical modification technology, which also offers the potential for the future usage of commercial A4 paper as a user-modifiable multifunctional cellulose-based membrane. The drawback is that for large-scale operations, these membranes’ long-term stability may present challenges.

Aminated lignin is produced when an amine attacks active hydrogen through an electrophilic substitution process known as the Mannich reaction. Researchers created new lignin-based adsorbents for the highly effective removal of low-concentration phosphate from water by treating lignin with triethylenetetramine (TETA) via the Mannich reaction and then chelating Fe(III) onto the aminated lignin [124]. The materials that were obtained had a clearance effectiveness of over 90% for phosphate (5 mg L^–1). The complexation between Fe(III) and phosphate in adsorbents provided the basis for the lignin-based adsorbent’s adsorption mechanism. The study provides a practical and affordable approach to the creation of low-cost sustainable bio-based materials (SBMs) that can eliminate low concentrations of phosphate from wastewater and stop water bodies from becoming eutrophic [124]. To create a highly porous chitosan-based aerogel membrane for superior water recovery from shipbreaking oil spills, researchers employed genipin as a crosslinking agent. By managing the genipin crosslinking process, the chitosan-based aerogel was made into a stiff scaffold-like structure and column that produced high-quality water (>99% purity) at a flow rate of 600 L m^–2 h^–1 bar^–1 [125]. The position, degree, and spatial conformation of the substituents on the modified biopolymers have a direct impact on the physicochemical characteristics of SBMs. Therefore, in order to better understand the structure–function link, researchers must carry out more thorough investigations on modified biopolymers. Furthermore, the current methods for chemically altering SBMs still require improvement, and identifying the ideal modification conditions is crucial for more accurately describing the stability and structure of SBMs.

A scalable and economical method for creating submicron-sized (5–500 nm) polymer nanofibers with superior structure–property relationships is electrospinning. Because of their high porosity, huge surface area, good stability, and high water permeability, electrospun polymer nanofibers exhibit significant promise as filtration materials in water treatment. The uniaxial stretching of a viscoelastic polymer solution under a high-voltage field is the basis for the creation of electrospun polymer nanofibers. In a standard electrospinning configuration, a high-voltage power source, a grounded metal collector, a spinneret, and a syringe pump make up its four parts. A voltage is delivered between the needle tip and the grounded collector during the electrospinning process, causing the polymer solution to distort a spherical droplet into a conical shape. The charged fluid jet is expelled from the Taylor cone on the needle tip onto the grounded collector when the electric field is beyond the threshold voltage at which the electrostatic repulsion force of the surface charges surpasses the surface tension of the droplet. During this time, the jet’s stretching and bending distortion creates non-woven membranes while the solvent rapidly evaporates [193]. Because of their high porosity (>90%), highly interconnected pores, and multichannel structures, these nanofibrous membranes, which are made up of randomly overlapping nanofibers, can be used as adsorbent materials or to separate filter membranes like MF, UF, and NF (micro-, ultra-, and nanofiltration, respectively) membranes [194]. Because of their huge specific surface area and open, interconnected pore structure, electrospun cellulose acetate (CA) membranes have shown promise in water filtration. For membrane applications, a research team created electrospun cellulose nanofibers with hydrophilia and an antifouling surface. Random mats were produced by the CA fibers’ preference for vertical, horizontal, and diagonal alignment. The nanofiber membranes demonstrated strong dye rejection (80–99%) and 20–56% rejection for particles measuring 0.5–2.0 µm [120]. To effectively remove ammonium from the water system, scientists created an electrospun cellulose acetate (CA) nanofibrous network that immobilizes a bacterial strain that oxidizes ammonia [195]. Furthermore, because of their superior metal chelating capabilities, high hydrophilicity, and biodegradability, electrospun chitosan (CS) membranes have been extensively employed for metal ion removal. By adding poly(glycidyl methacrylate) (PGMA) and polyethyleneimine (PEI) to the electrospun chitosan membrane’s surface, scientists created a CS-PGMA-PEI electrospun membrane, which enhanced both the electrospinning of chitosan nanofibers and the adsorption capacity of heavy metal ions [196].

Promising options for advanced filter media are electrospun nanofibrous membranes, which have outstanding qualities such as low operating pressure, high flux, good rejection, and antifouling ability. Needle clogging, lengthy processes, high electric field requirements, limited productivity, a lack of structural stability, and inapplicability to all polymers are some of the drawbacks of electrospinning technology. To overcome these restrictions, needle-free electrospinning technology has recently been created [122,197,198]. High safety and productivity in continuous systems are provided by needle-free electrospinning for the production of nanofibrous membranes. Although there are a few medium-sized manufacturing enterprises, electrospun nanofibrous membranes are still in the development stage for industrial scaling up. Electrospun nanofibrous membranes, the newest generation of filtration materials, offer several benefits that will make it easier to create commercial filtration systems in the future.

The most flexible method for creating polymer filtration membranes at the moment is phase inversion, which is the controlled inversion of an originally homogeneous polymer solution from a liquid phase to a solid phase. Spun fibers and flat sheets can be used to create asymmetric and hollow fiber membranes. Phase inversion methods can be classified as non-solvent-induced phase separation, thermally induced phase separation, or vapor-induced phase separation based on the many phases and factors that drive phase separation. Several variables, including bath temperature, solvent interaction, polymer concentration, and casting solution additives, affect the membrane’s structure, characteristics, and chemical interaction. Two kinds of membrane morphologies can be produced by phase inversion: sponge-like and finger-like structures. A finger-like structure or macrovoid is typically formed by rapid demixing, whereas a sponge-like structure is formed by delayed demixing [199]. The spongy structure has superior mechanical strength over the finger-shaped structure and is made up of tiny pores encircling dense walls. This allows for the construction of a full thin active layer. On the other hand, the figure-shaped pores show the lowest flow resistance since they have the smallest tortuosity. Therefore, for the creation of high-performance filtration membranes, the ideal support with a spongy thin layer deposited on the surface of the finger-shaped sublayer is crucial [200].

The phase inversion approach has been used to manufacture several cellulose membranes. The most important industrial cellulose ester, cellulose acetate (CA), has enormous potential for use as a membrane in separation processes. CA has so far been employed in a number of separation procedures, including RO, MF, UF, and NF [109]. High water permeability and salt retention are two beneficial properties of asymmetric CA membranes that make them useful for desalination or water treatment. Water vapor-induced phase inversion was used to create CA-based composite films at room temperature and standard pressure. With a separation flux and separation efficiency of 667 L m^–2 h^–1 and 99%, the resulting composite film demonstrated a micro–nano hierarchical structure and was capable of efficiently separating nano-sized surfactant-stabilized water-in-oil emulsions [110]. More importantly, TiO₂ photocatalytic activity demonstrated outstanding photocatalytic destruction of organic molecules under UV irradiation, indicating considerable promise for water treatment in everyday life and business. The addition of nanomaterials during the phase inversion process when creating SBM nanofiltration membranes might enhance the membranes’ performance in terms of permeation flux, antifouling, and highly selective separation; however, several issues have to be resolved. The first is that organic nanoparticles have inadequate acid–base tolerance and poor heat stability. Therefore, controlling the structure of organic nanomaterials is required to make up for these drawbacks. The second issue is that loose nanofiltration membranes are limited by inorganic nanoparticles’ low compatibility with the biopolymer matrix and propensity for agglomeration, which leads to the creation of defects at the nanofiller/bipolymer interface. A key approach to creating high-performance inorganic hybrid loose nanofiltration membranes for various separation systems is to modify the surface and the distribution of inorganic nanomaterials and improve the interaction between inorganic nanoparticles and biopolymers. Lastly, the trade-off between permeability and selectivity is currently a problem for loose nanofiltration membranes. The primary issue that researchers are concentrating on is the design and development of sophisticated matrix materials that allow for large permeation flux without sacrificing separation precision.

Last but not least, molecular imprinting is a new technique that produces precise target molecule recognition sites in artificial polymers. In the presence of template molecules, functional monomers and crosslinkers can copolymerize to create molecularly imprinted polymers (MIPs). The highly crosslinked polymer matrix forms recognition cavities complementary to the template in size, shape, and chemical functionality after the template molecules are extracted from the polymer networks using solvent extraction. These cavities can selectively identify and rebind the original templates from a mixture of closely related compounds [201]. MIPs have been extensively developed as adsorbents for the selective and effective separation or removal of organic contaminants and hazardous metal ions in water purification and wastewater treatment because of their high selectivity, physical robustness, high stability, low cost, and ease of synthesis [103,202,203]. Because of their various benefits, including strong affinity, non-toxicity, low cost, easy manufacturing, and good selective adsorption, bio-based polymeric MIPs have recently gained popularity in the water treatment industry. To create a photoresponsive cellulose-based smart-printed adsorbent that can selectively adsorb common pesticides, researchers employed surface-initiated atom transfer radical polymerization. With a binding capacity of 11 mg g^–1, PR-Cell-MIP demonstrated outstanding stability, reusability, and great adsorption selectivity for the target 2,4-D. When an azobenzene (Azo) functional monomer was added, the resulting cellulose adsorbent was readily regenerable by exposure to UV light [105]. Because of its many amino and hydroxyl groups, which show exceptional affinity for a variety of chemicals through a variety of particular interactions, chitosan is one of the most commonly utilized natural polymers in molecular imprinting technology. To create a dual-surface-imprinted acrylamide-functionalized chitosan-based polymer for the selective removal of salicylic acid and heavy metals in aqueous solutions, researchers utilized chitosan as a substrate, epichlorohydrin as a crosslinker, cadmium as a template, and 4-hydroxybenzoic acid as a mimetic template. The acrylamide-functionalized chitosan-based polymer’s highest adsorption capabilities for salicylic acid and heavy metal were 45.77 and 53.42 mg g^–1, respectively [106].

7. Use of Bio-Based Materials as Adsorbents

Adsorption technology has been used in the last 20 years, with considerable growth in the number of studies aimed at developing new adsorbent materials to remediate contaminants present in industrial effluents [204,205]. This process consists of accommodating pollutants, also called adsorbates, on the surface of a solid, called an adsorbent, which in turn must present good adsorptive properties to support varied chemical and physical interactions with the adsorbate molecules [18,206,207]. The format of these materials can be powder, pellets, or grains; generally, the powder format is the most common. Biological materials in a natural form have the advantage of being less costly in the synthesis stage; however, unfortunately, they have a low surface area and few pores, which makes it difficult to obtain viable and sufficient capacity values to meet the existing legislation in each country. In water treatment, the application of several commercial adsorbents is observed, a classic example being activated carbon, which has porosity and good surface area, enabling remediation against a wide range of pollutants (substances that contribute to bad taste, undesirable odors, and organic compounds) [208]. Other commercial adsorbents include clay minerals, zeolites, and silica gel. Studies show that these materials can remove several adsorbates, from heavy metals, dyes, and pharmacological residues to phenolic compounds [75]. Conventional adsorbents have limitations, the most prominent being the cost of large-scale implementation, which limits their translocation from the laboratory to the industrial field. Because these materials are expensive to synthesize, desorption studies for subsequent reuse are highly acceptable and viable. However, the use of additional chemical reagents that are classified as eluents can often generate secondary pollution, further increasing operational costs. In this regard, thermal regeneration would be a good alternative. However, many studies describe that this method does not allow for good regeneration, causing the material to lose its removal capabilities in the second cycle [209,210]. Therefore, it is recommended that new reuse processes be created that allow the material to be reused for several consecutive cycles while maintaining high removal and that are safe to avoid the use of toxic reagents.

In the last ten years, significant progress has been observed in the field of bionanocomposites as adsorbents [211]. Bionanocomposites are hybrid materials that combine nanoparticles with biopolymers to increase adsorption capacity. The functional capabilities and high surface area of nanoparticles are combined with biopolymers, which are highly renewable and biocompatible with these composites. Because they are effective, easy to produce, and inexpensive, researchers became interested in clay and chitosan nanocomposites, which can achieve removals of up to 99% in solutions contaminated with dyes, metals, and other hazardous chemicals [212]. A study by Bhattacharyya [213] used starch and graphene oxide composites to adsorb methylene blue, achieving 90% removal. The structure of the composite resembles plywood containing nanocages. This was possible due to the combination of graphene oxide nanosheets organized in long polysaccharide chains. This structural arrangement is very important for adsorption since it increases the potential to accommodate dye molecules along the surface. The known mechanisms between graphene oxides and cationic dyes are established via electrostatic interactions, while the aromatic part of the organic molecule and the delocalized electron system of graphene oxide suggest stacking interactions [214]. In the case of heavy metals, they interact with graphene oxide through complexation interactions. In the remediation of contaminated waters, graphene oxides containing chitosan present excellent mechanical qualities and high chemical stability. As a biopolymer with positive charges due to hydroxyl and amino groups, chitosan can efficiently coagulate contaminants. Limitations of its use include low efficiency at low pollutant concentrations, pH adjustment, and high synthesis costs, which limit its large-scale application. Recent studies have focused on the adsorption remediation of heavy metals; thus, there is a need for studies that analyze competitive adsorption and the presence of complex mixtures of adsorbates [215]. The development of three-dimensional porous graphene oxide–maize amylopectin composites enabled the effective remediation of inorganic and organic compounds present in a liquid phase. In another study, Congo red and rhodamine B dyes were removed with high efficiency using a crosslinked graphene oxide–chitosan–poly(vinyl alcohol) sponge [216]. The authors inferred that the 3D structure of the composite sponge and the biopolymeric sponge enabled interactions with the aromatic rings present in the structures of both dyes.

A biopolymer and a geopolymer underwent a fusion that enabled better adsorption of the metals cobalt and nickel present in water [217]. The Grewia biopolymer from the Tiliaceae family and the lateritic geopolymer showed synergistic behavior, acting favorably to achieve a removal above 80% for both metals. The geopolymer synthesized from lateritic clay is rich in silica, aluminum, and iron oxide, while compounds such as arabinose, glucuronic acid, glucose, xylose, and mannose are found in the Grewia plant biopolymer. The lateritic clay geopolymer contains kaolinite platelets in its structure, while the Grewia biopolymer has several open holes of different sizes in its structure; due to this, it presented better adsorption results compared to those of the other material. In-depth analyses were performed with graphene oxide with chitosan, and it was observed that the cationic and biodegradable biopolymer obtained good removal results due to its coagulation capacity. Its extensive use is hindered by its high synthesis cost, low efficiency at low pollutant concentrations, and the need for pH adjustment. The scientific articles cataloged in this review provide useful insights regarding the adsorption capacities of various pollutants of different chitosan–graphene oxide composites [213,214].

7.1. Use of Bio-Based Materials in Dye Adsorption

With their recyclability potential, good sorption capacity, three-dimensional network, and high surface area, bio-based materials have come to be considered highly efficient adsorbents for the removal of hazardous pollutants. With their high efficiency and low synthesis cost, studies have focused mainly on the removal of dyes, which are toxic and can cause allergies, skin irritation, and, in more serious cases, DNA damage with the occurrence of mutations. The processes that can be used to remediate these organic molecules include biodegradation, reverse osmosis, coagulation, adsorption, and oxidation. Existing studies mainly report the biodegradation of the dye without negative impacts or the generation of harmful byproducts. The materials are designed so that interactions occur via electrostatic contact with their ionic charges. Ultrapure chitosan nanofibrous membranes were manufactured via electrospinning and were efficient in removing the azido blue-113 dye, reaching a maximum capacity of 1338 mg g^–1 [218]. The ideal conditions for the morphology and structure of the nanofiber were a reduction in the nanofiber diameter, promoting a high capacity. Biodegradability, biocompatibility, and hydrophilicity were increased by the electrospinning of polyamide-6 with chitosan. Response surface methods were used to select the ideal parameters, which reached a maximum filtration of 95 and 96% for the dyes polar yellow GN and solophenyl red 3BL, respectively. Casting a chitosan/polyvinyl alcohol composite into silicon dioxide nanoparticles aimed to increase fiber stability, making it more efficient in the long term, increasing the surface area, reducing the degradation rate, and increasing compaction resistance, thus ensuring a high electrostatic affinity between the fibers and the anionic dyes [219]. Thus, the removal of direct red 23 dye was 98% at a flow rate of 1711 L m⁻² h⁻¹. In another investigation, chitosan/polyvinyl alcohol nanofibers were used to remediate dyes, where removal was favored under acidic conditions due to reduced protonation [220]. In the case of desorption, it occurred under alkaline conditions with release efficiencies of up to 90%. The ethylenediamine functionalization of chitosan fibers also enabled the recovery of dyes; in this case, the low boiling point of ethylenediamine enabled a reduction in the degradation temperature due to functionalization. In another study, fiber recovery was also possible under alkaline conditions using anionic and cationic dyes, showing high removal of methylene blue and low removal of coccinum because adsorption was favorable for the protonation of anionic dyes [221]. In order to increase their density, the chitosan hydrogels were coated, and the fibers were electrospun. The bond energies of different compounds vary; for example, the oxygen–carbon bond and alcoholic ester and hydroxyl groups have a bond energy of 289.3 eV and hydrocarbons have an energy of 284.32 eV. Various alcohols and carbonyl hydroxyls are present in materials containing added cellulose multiwalled nanotubes; the removal of these materials from dyes can reach 90% using a high flow rate (150.7 L m⁻² h⁻¹) and an operating pressure of 0.5 MPa for a molecular mass greater than 600 g mol⁻¹ [221].

The hydrophilicity of montmorillonite was used to synthesize clay-based nanofibers. The process occurred via a thermally induced gel–sol transition, and the material was used to remove dyes [222]. The removal rate of the basic blue 41 maxima dye was 95% at a compaction strength of 0.4 bar and a water flow rate of 1765 L m⁻² h⁻¹, as when a 2% weight of carbon nanotubes added, the reusability potential and resistance to fouling were increased [222]. Thermally induced phase separation was used to create sponges using tetrahydrofuran in an acetic acid and water solvent system [223]. Although tetrahydrofuran is not classified as the best solvent, it played a great role in this study since it opened spaces for a porous phase to be created and was thus able to generate an interconnected and porous structure in the sponge. The highest adsorption capacity of 604 mg g^–1 was obtained for the sample containing 0.5 wt% chitosan tempered at a temperature of 196 °C and with acetic acid/tetrahydrofuran/water at a proportion of 1:20:79 [223].

Hydrogen bonding and physical entanglement enabled the fabrication of water-resistant hydrogels [224]. The carboxyl groups and high surface area allowed the aerogels obtained from cellulose nanofibrils via freeze–thaw synthesis to remove approximately 92% of malachite green dye. The authors used the ratio of 5/10 mg mL⁻¹ of malachite green to aerogel w/v, and the removal kinetics were fast, and the organic compound was released instantly when the ionic strength was increased to 200 mM NaCl. Aerogels of cellulose nanofibrils and graphene oxide were modified by adding iron (III) ions. Through the Fenton oxidation process, methylene blue dye was rapidly degraded by the presence of H₂O₂. The loaded fibers were able to remove the dye at a rate of 30%, a result superior to that of the unloaded fibers [225]. Chemical crosslinking was performed with photocatalytic nanoparticles of titanium dioxide–NH₂ on graphene–cellulose oxide nanofibers. This process increased the degradation of dyes [226]. Active oxidation with hydrogen peroxide occurred when titanium dioxide was exposed to ultraviolet light, which accelerated the degradation of methylene blue and indigo carmine. The efficiency of the process was impacted by the parameters of initial dye concentration, process duration, pH, and power density [226]. The conical shape allowed it to be hydrophilic on the outside and hydrophobic on the inside, thus favoring the oligosaccharide cyclodextrin. The hollow cavity allowed for excellent mechanical strength and improved adsorption; this was made possible via the incorporation of B-cyclodextrin polymer with polycaprolactone during the electrospinning step [227]. Desorption studies confirmed a good reuse capacity, since after eight consecutive cycles, removal was 78% for a capacity of 24.1 mg g⁻¹.

Rhodamine B and crystal violet dyes were successfully adsorbed using dialdehyde starch nanocrystals coated with graphene oxide nanosheets [228]. The maximum capacities obtained were 539 and 318 mg g^–1 for rhodamine B and crystal violet dyes, respectively. Interactions between organomontmorillonites and a chitosan-based polyelectrolyte were driven by dye and enabled rapid and effective adsorption on the surface of an organoclay/chitosan hybrid composite [229]. The ratio of the organoclay/chitosan hybrid was 90/10; thus removal was complete for a solution containing various hydrolyzed anionic dyes [229]. Magnetic nanoparticles encapsulated in pyrolyzed chitosan and chitosan can be used to extract polycyclic aromatic hydrocarbons present in wastewater. Anthracene and naphthalene were effectively removed using a chitosan-based polymer [230]. An alginate-based hydrogel was used to decompose and degrade several organic contaminants via a photocatalytic process and adsorption [231]. The highest removal rates of the alginate-based hydrogel were for the dyes methylene blue, malachite green, and crystal violet, being 1610.3, 993.2, and 3000 mg g^–1, respectively [231]. Epichlorohydrin, 3-chloro-2-hydroxypropyl trimethylammonium chloride, and cornstarch were used to synthesize a crosslinked cationic starch [232]. The maximum capacity achieved against the solution containing reactive golden yellow dye was 208.7 mg g^–1 obtained at a temperature of 308 K. Chlorosulfonic acid-based vinyl acetate was functionalized via the crosslinking of polyvinyl alcohol in starch; this process created a material named starch-g-poly(vinyl sulfate)@magnetic nanoparticle membranes. The material showed excellent reuse potential, since after five cycles, it maintained removal rates for methylene blue and malachite green dyes above 90%, with maximum capacities of 567 and 621 mg g^–1 for green and methylene blue, respectively.

Table 2. Remediation of organic molecules through the use of bio-based composites.

Adsorbate	Adsorbent (Bio-Based Composites)	Synthesis Method	pH/Contact Time (min)	Adsorption Capacity (mg g−1)	Efficiency (%)	Reference
Brilliant cresyl blue	Copolymerized acrylic acid hydroxyethyl methacrylate sodium alginate	Free-radical polymerization	x/100	x	94	[233]
Direct red 80	Polyvinyl alcohol chitosan	Electrospinning	2.1/110	151	X	[220]
Solophenyl red	Chitosan/polyamide-6	Electrospinning	3/x	x	91	[234]
Methylene blue	USM–chitin	Ultrasonication	10/x	26.69	x	[235]
Congo red	Chitin	Sonoenzymolysis	6/45	232	x	[236]
Eriochrome black T	Chitin	Extraction	5/30	167.31	x	[237]
Methylene blue	Chitin microspheres	Sol–gel	8.4/24	46	x	[238]
Crystal violet	Chitin/ZSM-5	Hydrothermal	7.5/x	1217.3	x	[239]
Basic fuchsin	Zeolite/chitin	Hydrothermal	9/36	237.5	x	[240]
Direct blue 71	Chitin/lignin	Mechanical milling	2.4–8.4/45	9.3	x	[241]
Crystal violet	Chitin and psyllium	Freeze-drying	8/25	227.11	x	[242]
Methyl orange	Chitin/CS-g-PAM	Extraction	5/x	x	61	[243]
Acid blue 25	PEI–chitin	Surface modification	2/67	177.32	x	[243]
Indigo carmine	Graphene oxide/cellulose acetate nanofibers	Electrospinning	2/150	x	99	[226]
Direct red 80	Chitosan composite/silica/polyvinyl alcohol	Electrospinning	2/110	322	X	[244]
Methylene blue	Starch-doped Fe2O3 nanostructures	Co-precipitation	7/120	x	X	[245]
Eosin and ethidium bromide	Gelatin- and cellulose-based hydrogel	Grafting of poly(acrylic acid)	10/30	x	X	[246]
Congo red and titan yellow	Magnetic lignin	x	7/180	198 and 192, respectively	x	[247]
Congo red	Chitosan–gelatin hydrogel loaded with ZnO	Grafting of acrylamide	10/480	x	x	[248]
Methylene blue	Bacterial cellulose@CdS nanocomposite	“Anchoring–reacting–forming” pathway	x/180	x	77	[249]
Methylene blue	Cassava starch (hydrogel)	x	x/2700	2000	x	[250]
Methyl orange	Polyvinyl alcohol/zeolite/chitosan	Electrospinning	8/14	153	x	[251]
Reactive black 5	Polyamide/chitosan	Force spinning	1/240	457	x	[252]
Malachite green dye	Hemicellulose	Acetylation and periodate oxidation	6.5/60	456.2	x	[253]
Reactive blue 19	Cellulose nanocrystals grafted with acryloyloxyethyltrimethyl ammonium chloride	Polymerization	7/x	x	80	[254]
Optilan blue (textile dye)	Starch-coated Fe3O4 nanoparticles	Green synthetic approach	2/x	50	x	[255]
Methyl orange and acid green 25	Cationized starch/silica–sand composite	One-step etherification	5.8 and 6.8, respectively/20	458 and 912, respectively	x	[256]
Eriochrome blue and Congo red	Cellulose	Free-radical polymerization	7/100 and 200, respectively	349.3 and 380, respectively	x	[257]
Methylene blue	Cellulose-based flexible carbon aerogels	Hydrothermal	8/60	x	x	[234]
Methylene blue	Activated carbon composite/starch	Carbonization	10.5/90	x	90	[249]
Blue dye 19	Trimethyl ammonium chloride/cellulose aerogel	Chemical crosslinking and freeze-drying	7/30	160	x	[258]
Rhodamine B	Activated carbon/gelatin composite beads	Facile method	4/45	256.4	x	[259]
Methyl orange	Pectin/Fe3O4 nanoparticles	Co-precipitation	x	x	x	[260]

was prepared to provide an overview of bio-based materials that have been developed and used effectively for dye removal.

7.2. Removal of Micropollutants

Micropollutants are present in textile and pharmaceutical effluents and include heavy metals. A wide range of processes have been used to remove them, preventing their accumulation in water compartments and thus reducing the occurrence of damage to ecosystems and humans, since they may be below the limits acceptable by current legislation. In this sense, bio-based materials have positive aspects, as they present high removal rates, high selectivity, good surface area, and favorable malleability.

Table 3. Remediation of micropollutants by bio-based composites.

Adsorbate	Adsorbent (Bio-Based Composites)	Synthesis Method	pH/Contact Time (min)	Adsorption Capacity (mg g−1)	Efficiency (%)	Reference
Copper	Silk fibroin/cellulose acetate	Electrospinning	x/120	75.9	x	[261]
Copper	Tricarboxylic cellulose nanofiber	x	x/120	92.2	x	[262]
As (V)	Gelatin hydrogels and UV-cured chitosan	Microwave-assisted	6/600	136.7	x	[263]
4-Nitrophenol	Nitrogen and boron co-doped lignin biochar	One-pot carbonization	11/720	x	83	[264]
Fluoxetine	Lignin nanofibers	x	4.5/120	185	x	[265]
Lead	Fe3O4@SiO2/alginate/chitosan hydrogel	x	6/120	234.7	x	[266]
Ibuprofen	FeCl3·4H2O/f-MWCNTs	Solvothermal	7/120	11.8	x	[267]
Arsenic	Starch@γ-Fe2O3	Co-precipitation	9/120	8.67	98	[268]
Lead	Modified pectins and ethylenediamine	x	4/x	x	94	[269]
Lead	Poly(vinyl alcohol-co-ethylene)/nanocellulose	Melt blending extrusion	4/1440	471.5	x	[270]
Cadmium	Zinc oxide–NH2/poly vinylalcohol/chitosan	Electrospinning and cast method	6/240	139.27	x	[271]
Mercury	Lignin–chitosan-functionalized (polyethyleneimine)	Crosslinking	5.5/120	663	x	[272]
Uranium	Phosphorylated chitosan cellulose and phosphate-decorated carboxymethyl	Crosslinking	5/x	977.5	x	[273]
Tetracycline (antibiotic)	PVA/chitosan	Electrospun method	6/18,000	102	x	[274]
Acetophenone	Crosslinked starch polymer	Facile one-pot synthetic route	7/x	180.2	x	[275]
Metformin (Antidiabetic Pharmaceutical)	Microcrystalline cellulose graphene oxide	Chemical oxidation cum exfoliation, acid hydrolysis	4.5–8.5/x	132.1	x	[276]
Immunoglobulin G	Alginate protein cryogel beads	Extrusion dripping	5/90	175	x	[277]
Amoxicillin (antibiotic)	TiO2-supported chitosan scaffolds	Sol–gel transition	7/180	x	50	[278]
Ciprofloxacin (antibiotic)	Graphene oxide/sodium alginate composite beads	Magnetic stirring	4/2880	86.12	x	[279]
Sulfamethazine	CMC-stabilized nano zero-valent iron, starch	Reduction	5–9/60	x	83	[280]
Cadmium and lead	Polyethyleneimine/chitosan	x	7/x	321 and 341, respectively		[281]
Mercury	Thiolated-spherical nanocellulose	Acid hydrolysis	5.6/20	98.6	x	[282]
As(III) and As(V) (arsenic)	Binary oxide Mn-Fe/starch	Co-precipitation and redox reaction	7/120	284.6 and 160.6, respectively	x	[273]
Mercury	Magnetic starch (composite adsorbent)	Co-precipitation	4–7/110	324.4	x	[283]
Cadmium and lead	Sodium alginate/lignin/graphene nanocomposite	Hydrothermal polymerization	6/90	79.8 and 226.2, respectively	x	[284]
Nickel	TiO2–hemicellulose–chitosan	Polymerization, sol–gel method, and self-assembly	4/x	370.4	x	[285]
Tetracycline (antibiotic)	3D alginate-based MOF hydrogel	One-step	8/x	364.8	x	[286]

summarizes the performance of several materials developed in recent years for the remediation of micropollutants.

The primary goal of adsorption research has always been the effective removal of micropollutants. However, in recent years, researchers have also focused on economic aspects, aiming to develop materials that can be scaled up with low production costs. Even at low concentrations, micropollutants can be selectively adsorbed; in this respect, biopolymers are the most commonly used materials of plant origin. Therefore, bio-based materials serve as an alternative in order to recycle and separate these adsorbates, preventing occurrence of secondary contamination. A maximum arsenic removal capacity of 30.8 mg g^–1 was achieved with a pure chitosan nanofiber adsorbent; good performance was dependent on pH, with the best removal in acidic conditions [287]. When doping chitosan nanofibers with iron, an increase in the surface area was observed, increasing the selectivity of the metal. The authors also stated that groups with carbon–carbon and carbon–nitrogen bonds were essential for arsenic remediation [287]. The immersion of epichlorohydrin together with the crosslinking of glutaraldehyde led an amine to be grafted onto the surface of nanofiber/chitosan membranes, after which copper ions were removed via electrospinning, as shown in Figure 7A,B. Diethylenetriamine incorporated amine groups, thus increasing the stability of the nanofibers, presenting a weight loss of only 6% after 24 h. This process may have been caused by the synthesis of amines and imines, while weight loss occurred at different temperatures. Silver nanofibers exhibited high chelation with copper due to the large number of active nitrogen groups; thus, the capacity reached 166.7 mg g⁻¹, indicating the high adsorptive performance of the material [288].

Films with nanofibrous characteristics were formed from cellulose-based nanocrystals and functionalized with chitosan/thiols/poly(vinyl alcohol). They were used to remediate lead and copper and showed high mechanical stability [289]. The films were recovered, maintaining an efficiency of 90% for both metals. The high selectivity of the thiol groups for mercury ions together with the large surface area, led to an improvement in removal, reaching a value of 86% for cellulose nanocrystals, a value much higher than that of unmodified nanocellulose. In another study, bio-based silk fibroin was used to hybridize a cellulose acetate nanofibrous membrane. The good removal of metal ions was due to the binding of the COOH group, which is electronegative, to the amino group, leading to the generation of a covalent coordination bond with copper. The presence of 20% cellulose acetate in the crosslinked fibers enable the recovery of the process to a capacity of 76 mg g^–1 after one hour [290]. Cellulose nanofibers modified by the addition of MnO₂ and carboxyl groups showed a high surface area, leading to an increase in their adsorptive capacity for copper ions, reaching a capacity of 399 mg g⁻¹ [291]. The synthesis of nanofibers was performed through the use of pulp with low chitosan and cellulose contents. The pulp was disintegrated for subsequent oxidation with TEMPO in order to increase the COO- groups that interacted with the target molecule to be adsorbed. An oxidation level of 26% and the pH value of 6.2 enabled the maximum removal. Desorption studies confirmed that after five cycles, the bio-based nanofibers could be reused with a recovery of 90% of their initial mass; these performance values are similar to those obtained for commercial montmorillonite nanoclay materials. Citric acid and chitosan were used to functionalize the carboxyl groups present in carbon-based polyacrylonitrile nanofibers [292]. Through hydrothermal carbonization, peroxidation, and pretreatment, stability was obtained via the addition of new chemical groups to the fibers. The addition of citric acid in 0.5 g increments led to a linear increase in capacity; however, above 1 g of citric acid, the capacity began to decrease. The removal efficiency decreased by 10% after five desorption cycles, going from 80% to 70%. The removal of both lead and cadmium was significantly increased by using oxolane-2,5-dione in the surface functionalization step of the bio-based materials. Functionalization improved the adsorptive performance of the material without compromising the reuse/desorption step [292].

Chromium ions are frequently released into water streams due to their large-scale release, mainly in the effluents of textile dyeing plants. Therefore, researchers began to study adsorbent materials with high selectivity for the metal. Quaternary ammonium was grafted onto the surface of cellulose nanofiber aerogels, which had a capacity of 17.6 mg g^–1, a value similar to that of quaternary ammonium anion exchangers, which are commercially available to remediate chromium metal [292]. Polyurethane (synthetic polymeric material) was used as a binder for carboxymethylated cellulose nanofibrils, allowing the fibrils to interrelate alternately and freely with the adsorbate and avoiding agglomeration, favoring adsorption. The combination of polyurethane with nanofibril/carboxymethyl cellulose aerogels gave the material high malleability and strength; therefore, the failure/deformation ratio increased, confirming interfacial contacts between the fibrils and the matrix [292]. Polyurethane with nanofibril/carboxymethyl cellulose aerogels showed removal rates of 61, 95, and 40% for the remediation of the metals cadmium, lead, and copper, respectively. These performance values were much higher than those obtained with pure polyurethane, evidencing that the chemical groups of NH₂ present in the polyurethane were fundamental for the adsorptive process. The electrospun chitosan nanomaterial were supported using a coiled spur module, which also generated a significant improvement in the volume ratio and increased the membrane surface area compared to the packing density [293]. The arrangement and input flux density of the chitosan-based nanofiber were proportional to the removal values of heavy metals. The nanofibers showed a retention rate above 90%, a value compatible with those obtained by conventional nanofiltration membranes; this retention was observed at a flow rate of 0.9 L h⁻¹. The specific removal of chromium in the presence of other metals showed a desorption peak after 2 min of the process; the same occurred for a wide range of pH values (alkaline and acidic), indicating that the material can be efficiently regenerated [293]. Also, in the remediation of heavy metals, the complexed permutation of chitosan/poly(ethylene oxide) was performed, where chitosan was intended for chelation and poly(ethylene oxide) for improving the mixture, and each polymer fiber provided a combinatorial effect with unique properties [294]. The exchange of positive hydrogens for positive sodium ions prevented the generation of chromic acid and the possible forms of electrostatic interactions of OH²⁺ and NH³⁺ (ionized molecules) with chromium. The remediation of chromium from water was 90% for a concentration of 50 mg L^–1 and a capacity of 208 mg g^–1, showing that the bio-based materials were effective [294]. In another study, the same group of scientists used polyethylene glycol, polyvinyl alcohol, and chitosan to synthesize nanofibrous membranes containing multiple layers [295]. The bio-based materials integrated into an Fe₃O₄ nanomaterial showed improved thermomechanical properties and increased water resistance. The number of active sites that were used to remove chromium and lead metals increased as the amount of bio-based material increased, providing new sites with the potential to chelate the metals. Thus, the maximum capacities were 509 and 525 mg g^–1 for chromium and lead metals, respectively. Several consecutive desorption cycles confirmed the reuse potential of the fabricated membranes [295].

A thermally induced phase separation method at low temperatures was also analyzed for the fabrication of chitosan nanofibers. The development of holes was induced by the use of ethanol, acetic acid, and water, where the nanofibers were generated using high concentrations of chitosan [295]. The lack of chelation of heavy metals by the amine groups present in the nanofiber was overcome with the use of ethylenediaminetetraacetic acid. After six cycles of washing with water, the copper removal rate was maintained at 90%, evidencing that the synthesis method had a remarkable recyclability potential and increased the productivity of the nanofibers. Functional groups such as aromatic rings, alcohols, and ethers were present in electrospun lignin nanofibers, where they were responsible for the removal of over 70% of pharmaceutical residues (fluoxetine) present in water [265]. The interactions responsible for this removal were hydrogen bonds, stacking, electrostatic interactions, and van der Waals forces. Glutaraldehyde was used for chemical crosslinking, which enabled the excellent removal of the tetracycline drug from the surface of the chitosan/polyvinyl alcohol nanofibers [274]. By adjusting the polyvinyl acetate/chitosan ratio to 25:75, the maximum capacity obtained was 102 mg g^–1. The pollutants diclofenac and bisphenol A were degraded using ultraviolet light through titanium dioxide nanoparticles, which were immobilized in electrospun gum karaya/polyvinyl alcohol nanofibers. The hydrophobicity and adsorption capacity of titanium dioxide membranes were optimized when they were treated with methane plasma. This was due to the creation of water droplets on their surface, leading to reductions of 18 and 20% in bisphenol A and diclofenac levels, respectively. This reaction was possible due to the generation of free radicals in response to ultraviolet light [274]. Hydrophobicity allowed water to accumulate in the cellulose nanofibers that were treated with trichloro(heptadecafluorodecyl)silane [296]. This characteristic was important for the process of extracting oil from water. Generally, nanofiber membranes modified via this process presented a separation of 99% for oil present in water when using gravity. Increasing penetration fluxes and hydrophobicity for a variety of oil types resulted in 95% separation performance using polydopamine and -cyclodextrins on poly(lactic acid) nanofibers and silver nanoparticles.

7.3. Contamination via Oil Spills

Due to numerous incidents of environmental contamination caused by leaks and discharges of crude oil, there has been increasing interest in research on oil/water separation. For example, the catastrophic environmental accident that occurred in 2010 due to the Deepwater Horizon oil spill was responsible for the release of five million barrels of crude oil directly into the Gulf of Mexico [297]. Oil and gas production areas generate large volumes of muddy sewage barrels daily, which has encouraged the synthesis of new materials with cheap and environmentally friendly oil adsorption properties for future applications in these areas affected by spills. Bio-based materials are a viable choice for oil/water separation precisely because they are safe, cheap, and efficient. Bio-based materials synthesized for this purpose exhibit hydrophilic/hydrophobic interface structures and have been studied by several study groups, as shown in

Table 4. Oil adsorption on the surface of bio-based composites.

Oil	Bio-Based Materials	Synthesis Method	Hydrophobic Modification	Adsorption Capacity (mg g−1)	Efficiency (%)	Reference
Hydraulic oil, kerosene, and toluene	Blends of poly(vinyl alcohol) (nanoparticles) with starch or chitosan	Emulsion polymerization	x	22.7, 39.3, and 48.7, respectively	X	[301]
MC252 oil	Polyvinylpyrrolidone-coated magnetite nanoparticles	Hydrothermal method	x	X	100	[302]
Castor oil	Chitosan foam/polyurethane	Polymerization	Ricinoleic acid and chitosan	267.2	X	[303]
Chloroform, n-heptane, cyclohexane, and toluene	Chitosan–silica hybrid	Sol–gel encapsulation	3-(triethoxysilyl)propyl isocyanate, tetraethyl orthosilicate	X	90	[304]
Organic solvent, gasoline	Modified activated carbon aerogel	Polymer coating	Polydimethylsiloxane	12.31	x	[305]
Crude oil	Polyvinyl alcohol and cellulose nanofibril	Freeze-drying and emulsification	Span-80	140	x	[140]
Insoluble oil	Bio-based material (CH-PAA-T)	Thermal crosslinking	Polyacrylic acid and chitosan	990.1	x	[306]
Marine diesel	Cellulose nanocrystals	Deep eutectic solvents	Oxalic acid dihydrate and choline chloride	X	x	[307]
Marine diesel	Carboxymethyl chitosan	Partial carboxymethylation	Monochloroacetic acid	X	x	[308]
Marine diesel	Oleoyl carboxymethyl chitosan of sodium salt	Acylation and carboxymethylation	Oleoyl chloride	X	85	[309]
Wastewater from oil extraction	Carboxymethyl chitosan-oleoyl-H	Acylation and carboxymethylation	Oleoyl chloride	X	95	[310]

. A fluorosilane–azobenzene compound was manufactured via the modification of the cellulose–dopamine surface via grafting, where the isomerization of UV radiation impacted the hydrophilic surface of the cellulose-based substance [140]. Biomaterials can adsorb oil present in water and may also have high resistance to fungi, bacteria, and organic pollutants, making this group of materials interesting for these processes. Cotton was used to develop a super-wetting material with a contact angle with water of >150° (superhydrophobicity) and oil-loving properties (superoleophilicity), which was used to separate emulsions (water/oil) with a contact angle of 5° with oil [298]. The infiltration fluxes present in the material were very high compared to those in the water-in-oil emulsion; these were reported after a filtration step that occurred via external pressure and gravity [299]. Because they are recyclable and have high durability under aggressive chemical conditions, super-wetting agents are a viable and practical option for large-scale use in the industrial sector in the separation step of oil/water emulsions. Cellulose aerogels were impregnated with epoxidized soybean oil in order to absorb the oil [300]. After hydrophobicity adjustment, the water contact angle in the cellulose aerogels increased to 132°, and even after 30 cycles, the aerogel maintained 90% removal, evidencing its potential for oil/water separation in the face of oil spill contamination. Current studies have analyzed the potential of bio-based materials in oil/water separation processes and focused on surface modifications of hydrophobic and hydrophilic interfaces, aiming to improve performance.

7.4. Membrane Filtration

Due to its advantages, including fuel efficiency, size screening capacity, and favorable working conditions, the membrane separation process has become an established and effective method in wastewater treatment. Bio-based membranes with nanoparticles have high potential for water filtration because they are sustainable and have high mechanical capacities, low ecological effects, and high volume-to-surface ratios. Bio-based membranes often use polymer matrices such as poly(vinylidene fluoride), poly(vinyl alcohol), and poly(sulfone ether) to optimize properties such as biofouling, tensile strength, permeability, and selectivity [311]. Oxidative cellulose nanofibers and graphene oxide nanocolloid biohybrids were fabricated using vacuum filtration, and the cellulose nanofibers were treated with TEMPO to create self-assembled membranes for water remediation [312]. In graphene oxide nanocolloids, copper remediation occurred at joints and nodes, increasing flexibility, water flux, hydrolytic stability, and mechanical strength, while in oxidative cellulose nanofibers, the large presence of CCOOH groups enabled interactions with the metal. A process that did not use a crosslinker and fabricated a nanofiber membrane containing an ultrathin graphene oxide barrier layer at a high water flow rate (18 124 L m⁻² h⁻¹ bar⁻¹) was established at an infiltration rate of 90 L m⁻² h⁻¹ bar⁻¹ for rhodamine B dye [313]. Glycerol and 2-methyl tetrahydrofuran derivatives were applied to fabricate asymmetric cellulose acetate nanofibers based on biological material using a solvent-free induced phase separation process. Increasing the concentration of glycerol, 2-methyltetrahydrofuran, and cellulose acetate increased porosity and performance, reaching a removal rate of over 90% [314]. The main positive characteristic is economic viability; however, these membranes have much lower resistance, reducing their useful life, which limits their use. Future studies should investigate thin-film antifouling and green chemistry materials in order to overcome these limitations.

7.5. Mechanisms Involved in the Remediation of Environmental Pollutants

The mechanisms underlying the removal of organic molecules and heavy metals by nanocomposites are different. Adsorption, ion exchange, precipitation, and other intricate physical and chemical processes are all part of interactions with heavy metals like cadmium [315]. It is commonly known that depending on the type of intermolecular binding force, adsorption can be classified as either physical or chemical [316]. Since the specific surface area of nanoparticles is greater than that of conventional materials, the adsorption process is one of the primary methods for removing cadmium from contaminated soil [317]. Higher cadmium adsorption efficiency is the result of a larger surface area, more adsorption sites, and more functional groups. The most efficient way to immobilize cadmium in polluted soils is to combine heavy metals and phosphates. Metal ions primarily inactivate cadmium in soil using unique chemical adsorption with a large specific surface area [318]. Similarly, Si-O, which has high polarity and many basic groups, makes up the majority of the functional groups in porous ceramic nano-modified materials. Because of their structure and composition, these porous ceramic nanoparticles naturally adsorb positively charged heavy metal ions. The primary method by which iron oxide/chitin nanoparticles remove cadmium over an extended period of incubation is adsorption [319]. Nonspecific adsorption takes the form of exchange states in the soil environment, whereas specific adsorption occurs for crystalline or amorphous oxides and hydroxides in soil clay. Furthermore, selective adsorption occurs at a faster pace than nonspecific adsorption. The main process has been determined to be the adsorption of cadmium ions on the surface of nanoparticles. Ligand complexation is an additional mechanism. Cadmium complexes greatly enhance the immobilization of cadmium in polluted soils because they are comparatively stable in the soil environment. Through electrostatic attraction and particular surface complexations of the inner sphere, ligands, including carboxyl, alcoholic, and phenolic hydroxyl groups, can promote complexation reactions with cadmium [320]. The primary adsorption processes of biochar made from rice bran for the removal of cadmium are complexation and electrostatic interactions. According to related studies, fresh biochar’s oxygen-containing functional groups can react with cadmium to lessen its toxicity. Consequently, the oxygen-containing functional groups and mineral components of biochar immobilize cadmium [321]. The ability of the iron oxide layer to adsorb and complex metal ions on its surface is crucial for the immobilization and removal of pollutants [322].

Numerous studies have demonstrated the various ways that nanomaterials stabilize HMs. It is important to remember that the method by which metals and nanomaterials interact in soil is typically not a single process but rather a combination of several processes. By examining the interaction between nano-hydroxyapatite and heavy metals, it was discovered that surface complexation and ion diffusion in networks constitute the primary stabilizing mechanism of cadmium [323]. Additionally, ion exchange, surface complexation reactions, soil–colloid sorption, and other processes are involved in the immobilization mechanism of combined amendments [324]. Electrostatic action is the primary mechanism by which Cd²⁺ is adsorbed on the surface of soil particles. However, because of their huge surface area and abundance of negative charges, mineral amendments have a strong adsorption and ion exchange capacity for cadmium ions. FeS nanoparticles may remove heavy metals from soil by adsorption, reduction, ion exchange, surface complexation, and co-precipitation, among other processes. Fe²⁺ and S₂⁻ in FeS can act as reducing agents to remove pollutants by giving them electrons [325]. It might be necessary to investigate other mechanisms further. To direct the synthesis of nanomaterials, a thorough understanding of the mechanisms underlying the interactions between nanomaterials and heavy metals in soil is necessary.

Both physical (physisorption) and chemical (chemisorption) mechanisms can be used in the adsorption process of organic contaminants such as dyes, medications, and insecticides. The physical adsorption mechanisms are determined by a rigorous examination of organic contaminants’ adsorption on nanostructured adsorbents. In this instance, weak interactions, including electrostatic interactions, hydrophobic interactions (van der Waals, π–π, and electron donor–acceptor), and hydrogen bonding are mainly responsible for the binding of pollutant molecules to the corresponding adsorbent surfaces [161,326,327]. The nature of persistent organic pollutants has to do with the mechanism. Mechanisms of interaction like electrostatic attraction would not be feasible if the chemical species in the aqueous phase were neutral. This is because the ionic charge of the chemical species needs to be opposite to that of the adsorbate surfaces. The particular functional group that is activated upon ionization determines whether the charge is positive or negative. The pKa value is typically used in the literature to describe this mechanism, which is highly sensitive to pH. Researchers have looked for methods to enhance the adsorption of organic contaminants on different adsorbents in light of this information. The main route employed is the modification of the adsorbent. A variety of modification strategies are employed to improve dye absorption [148,328]. Some researchers have chosen to make a composite of two different types of materials to take advantage of their favorable properties [329], while others have impregnated various metals into an activated carbon matrix to enhance the simultaneous adsorption of multiple organic molecules [330]. There are also cases of a lanthanum-substituted bimetallic magnetic adsorbate in a graphene matrix [331] and the use of nZVI to decorate carbon sub-microspheres [332]. All of these actions are to enhance performance and adjust the absorption process in the chemisorption and physisorption domains.

8. Scalability Challenges

Water matrices in the real world can be very complex, with a range of toxins, dissolved ions, organic debris, and other pollutants. Competing ions and other chemicals can affect the selectivity and adsorption capacity of biopolymer-functionalized adsorbents, hence affecting their efficacy. The performance of biopolymer-based adsorbents in controlled laboratory settings might not be entirely transferred to real-world applications if there are no standardized test procedures that account for the complexity of the actual world. Because they are renewable and biodegradable, biopolymer-based adsorbents are frequently promoted as environmentally friendly alternatives to traditional adsorbents. Assessing their sustainability and suitability for use in current water treatment systems, however, requires a thorough consideration of their entire environmental impact, including the possible leaching of additives or byproducts during the adsorption process. Furthermore, there are possible hazards and difficulties associated with extensive industrial use that should be properly evaluated. These issues mostly pertain to low stability, additive leaching, pollutant release, and environmental impact. The following factors require special consideration for the actual application of biomass and lignocellulose-based adsorbents for the scaling up and industrialization of these biomaterials to enter the high-value-added bio-based adsorbent market. Aspect 1: Biomass, including industrial and municipal waste, should supply stable and sustainable biopolymer resources and feedstocks. Aspect 2: To satisfy the requirements of various biopolymers, economical, ecologically benign, and successful isolation and purification procedures are required. Aspect 3: The intricacy of employing biopolymer-based adsorbents in a system with several pollutants necessitates significant and reliable improvements to bolster the efficacy of this strategy. Aspect 4: Functionalizing biopolymers requires straightforward techniques and processes to guarantee high stability, effective removal, and few financial consequences. Aspect 5: Adsorbents based on biopolymers are frequently less stable than synthetic materials and may degrade over time. Their long-term use and capacity may be restricted as a result. Aspect 6: Certain biopolymer adsorbents might not have broad-spectrum decontamination properties and might only be effective against specific kinds of contaminants. Selecting the right biopolymer for each kind of pollution is therefore advised. Aspect 7: Regulators and policymakers may consider funding research projects to create widely accepted testing procedures. Standardized procedures will ensure uniformity in evaluating adsorbent performance and facilitate comparisons between studies. Aspect 8: Stakeholders from government organizations, businesses, and academia can work together to exchange information and skills. This can promote the appropriate use of biopolymer-functionalized adsorbents, foster innovation, and help overcome the difficulties of practical applications. Aspect 9: Regeneration can pose several challenges, including the inability to remove impurities from the adsorbent matrix, the potential for adsorbent structure loss or degradation, and the application of harsh regeneration conditions that may compromise the biopolymer’s stability. Aspect 10: A deeper comprehension of the bio-adsorbate/adsorbent structure relationship can be ensured by a thorough investigation of adsorption processes using theoretical simulations.

Researchers, decision-makers, and stakeholders must work together to overcome these issues. Accurately evaluating the effectiveness of biopolymer-based adsorbents requires the establishment of standardized test procedures that mimic actual operating circumstances. Furthermore, life cycle evaluation, scalability assessment, and long-term stability studies can provide information about the environmental effects and practical usability of materials. Researchers and practitioners can make better decisions about incorporating biopolymer-functionalized adsorbents into efficient and sustainable water decontamination procedures by taking these drawbacks into account. Legislators and professionals working in the water treatment industry may want to consider incorporating these biomaterials into current treatment procedures or creating fresh approaches that capitalize on their special adsorption qualities. Professionals in the field can also investigate these natural, renewable resources as environmentally friendly substitutes that lessen their influence on the environment and lessen the need for adsorbents made from fossil fuels.

The green transformation of economic structures and the expansion of the global economy are facilitated by the manufacture of materials from plentiful renewable resources. In the last few decades, research and industry have concentrated on creating ecologically safe and sustainable substitutes for petroleum-based polymeric materials, which have historically been widely utilized in water treatment applications. Bio-based materials have many advantages over synthetic polymers, including abundant active functional groups, biocompatibility, biodegradability, and a variety of sources. The limitations of conventional single-function water treatment materials and the possibility of secondary pollution in water bodies can be successfully avoided if we can fully utilize bio-based materials and create high-performance water treatment materials. However, bio-based materials generally have poor mechanical properties, are brittle, and are challenging to produce. They can be improved through sophisticated production and modification techniques that can greatly enhance their qualities, making them more appropriate for particular industrial applications. The state of the art in the creation, alteration, and use of bio-based materials for water treatment has been covered in this review. To create and customize the structure and characteristics of bio-based materials for the removal of pollutants like metals and dyes from water, a variety of manufacturing procedures are discussed in the review. Bio-based materials can eliminate contaminants and enable a green and ecologically friendly water treatment business, according to a thorough literature assessment. Balancing mechanical and functional qualities is a difficulty for bio-based materials. Therefore, more research is needed to determine the removal effectiveness and adsorption methods of functionalized bio-based materials for realistic large-scale systems. To design and develop advanced bio-based materials for water treatment, more focus should be placed on methods for fabricating bio-based materials through chemical modification, electrospinning, phase switching, and molecular imprinting. This will lead to broad market prospects and possible commercialization.

The development of a sustainable and circular bioeconomy makes the utilization of alternative biomass resources such as lignocellulose, crop wastes, or organic waste in closed-loop methods a strategic objective. The true sustainability and viability of technologies that depend on non-food feedstocks or circularity principles, however, are often questioned. A review study that examined 173 life cycle analyses concluded that even though there are not enough scientific publications to be regarded as representative of a particular industry or commercial technology, the variety of approaches discussed offers a thorough picture of the sustainability issues associated with enabling technologies that support innovative and more effective uses of biomass [333]. The findings imply that no unambiguously optimal technology exists that provides benefits in every aspect of sustainability. Economic, social, and environmental results depend on several methodological assumptions, including system architecture, production scale, and geographic and temporal scope. The variety of outcomes and underlying methodologies can be partially explained by the complexity of bio-based systems. To consistently handle sustainable trade-offs, there should be more harmonization between the life cycle assessment, life cycle cost, and social life cycle assessment methodologies, ideally taking into account the effects of end-of-life and land use change. Furthermore, outside variables like crude oil pricing, agricultural regulations, waste laws, trade agreements, and supply chain standards affect how competitive bio-based industries are. These aspects should be taken into account in strategies that go beyond the technological system. A technology must be both economically and environmentally advantageous to be considered vital. The actual potential for reuse and recyclability of waste streams and byproducts ultimately determines the technical and budgetary limitations associated with closing loops. Furthermore, in order to establish interconnected and lucrative networks, the waste treatment and industrial sectors must coordinate for the successful implementation of cradle-to-cradle systems. In this way, industrial symbiosis can increase technical viability while lowering transaction, transportation, and waste treatment costs, all of which are often paid for by the general public. This would also entail altering how consumers view recycled goods and decreasing the quality requirements for the raw materials used by recycling operations. To create a trustworthy instrument that improves the market position of more sustainable products and informs consumers and public purchasers, it is necessary to incorporate stakeholder perspectives into the life cycle assessment framework. In order to promote investment, guarantee raw material access, and boost consumer acceptance of a circular bioeconomy, ambitious and well-thought-out policy combinations must be put into place concurrently.

9. Conclusions and Future Perspective

Natural resources based on biopolymers, such as polysaccharides, are likely to inspire further research into the synthesis of diverse derivative products. Positive aspects, including the potential for a wide range of applications, low cytotoxicity, adaptability, renewability, non-toxicity, and high abundance, have made biopolymers the object of studies by several research groups. Materials based on carbon, Fe₃O₄, titanium dioxide, zinc oxide, palladium, copper, and silver supported by carbon nanotubes and graphene oxides (biopolymers) are among the most efficient and successful processes in the area of nanotechnology. In addition, the polysaccharides described are renewable bioresources and excellent supports as substitutes in the synthesis of nanocatalysts. Nanomaterials based on biopolymers or natural materials (such as conventional adsorbents) require further studies to optimize the synthesis stage, improving their useful life and practicality. Using natural substances such as biopolymers in nanomaterials is feasible and can support the coagulation and flocculation of solid compounds. In the case of polysaccharides (pectin, chitosan, cellulose, and starch), they are promising and can compete in terms of efficiency, significant specificity, chemical stability, physicochemical attributes, reactivity, and sustainability for the removal of metals, dyes, and aromatic compounds. Wastewater treatment has become a priority as concerns about the well-being of the population and the quality of the environment have become the focus of government authorities, especially in developed countries. This is also related to large losses in the public health sector caused by environmental contamination. The application of nanostructured materials and their constant evolution have great future potential to increase the efficiency of remediation of water generated by industries, supporting the lower use of resources when compared to traditional processes. Further research is needed to try to overcome the limitations involving the lack of competitiveness compared to conventional processes and the inflexibility of engineering processes. The use of traditional methodologies in the manufacture of bio-based nanomaterials has the positive aspects of being harmless to the environment, simple, easy, and multidisciplinary, being used in various applications. Resource management and ecological remediation have prospects for advances in the area of effluent treatment, mainly because they are economical and ecological and present good quantitative performance results. The next investigations in the area of natural nanomaterials based on biopolymers should focus on a low-cost method that can be scaled for the synthesis and subsequent use of these materials in water purification. Finally, a wider range of biowaste nanomaterials should be analyzed; natural supports such as montmorillonite, zeolites, and clays should be used in the production of nanomembranes; agricultural and animal waste should be used to develop new biopolymer-based nanomaterials; ecological processes should be considered in order to increase nanocatalytic efficiency and reduce the cost of production; and, finally, nanoparticles with magnetic properties based on polysaccharides should be synthesized to remediate sewage and water.