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Review

Adsorption of Polycyclic Aromatic Hydrocarbons from Wastewater Using Iron Oxide Nanomaterials Recovered from Acid Mine Water: A Review

by
Tumelo M. Mogashane
1,*,
Johannes P. Maree
2,* and
Lebohang Mokoena
1
1
Analytical Chemistry Division, Mintek, Private Bag X3015, Randburg 2125, South Africa
2
Institute for Nanotechnology and Water Sustainability (iNanoWS), College of Science, Engineering and Technology, University of South Africa, Private Bag X6, Florida Science Campus, Johannesburg 1709, South Africa
*
Authors to whom correspondence should be addressed.
Minerals 2024, 14(8), 826; https://doi.org/10.3390/min14080826
Submission received: 9 July 2024 / Revised: 28 July 2024 / Accepted: 13 August 2024 / Published: 14 August 2024
(This article belongs to the Special Issue Acid Mine Drainage: A Challenge or an Opportunity?)
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Abstract

:
Polycyclic aromatic hydrocarbons (PAHs) are a group of organic pollutants known for their persistence and potential carcinogenicity. Effective removal techniques are required since their presence in wastewater poses serious threats to human health and the environment. In this review study, iron oxide nanomaterials (IONs), a by-product of mining operations, recovered from acid mine water are used to investigate the adsorption of PAHs from wastewater. The mechanisms of PAH adsorption onto IONs are investigated, with a focus on the effects of concentration, temperature, and pH on adsorption efficiency. The better performance, affordability, and reusable nature of IONs are demonstrated by comparative studies with alternative adsorbents such as activated carbon. Economic and environmental ramifications highlight the benefits of employing recovered materials, while case studies and real-world applications show how effective IONs are in removing PAHs in the real world. This review concludes by discussing potential future developments in synthesis processes, areas for more research, and emerging trends in nanomaterial-based adsorption. This research intends to contribute to the development of more effective and sustainable wastewater treatment technologies by offering a thorough assessment of the present and future potential of employing IONs for PAH removal from wastewater.

1. Introduction

Polycyclic aromatic hydrocarbons are a class of organic chemicals composed of numerous aromatic rings fused together [1,2]. The main sources of PAHs are incomplete combustion of organic resources like wood, biomass, and fossil fuels; they can also come from industrial operations, automobile emissions, and natural sources like wildfires [3,4,5]. PAHs are important environmental contaminants because of their persistence and bioaccumulative nature [6]. Groundwater, air, and soil can become permanently contaminated by PAHs due to their lengthy half-lives in the environment [7]. For PAHs with relatively low molecular weights in landscaping materials containing 1–2 percent organic matter, half-lives vary from 1.5 to 4.4 weeks. The half-lives of materials with 13% and 56% organic composition vary from 2.5 to 52 weeks [8].
PAHs have a propensity to adsorb onto sediments in aquatic environments, where they can endanger benthic creatures and make their way up the food chain to affect humans and other higher trophic levels [9,10]. Regarding health, PAHs are recognized for their ability to cause mutagenesis and cancer [11]. Long-term PAH exposure has been connected to a number of malignancies, including cancers of the bladder, skin, and lungs [12,13]. The primary ways of being exposed are through skin contact, breathing in polluted air, and consuming tainted food or water [14,15]. Furthermore, PAHs can lead to skin irritations, chronic health concerns, and respiratory problems, which emphasizes the urgent need for efficient remediation techniques [16,17].
It is essential to eliminate PAHs from wastewater since they are hazardous to human health and the environment [18,19]. Because PAHs are hydrophobic and have a propensity to bind with organic debris, wastewater treatment plants sometimes find it difficult to entirely eradicate them [20,21]. When PAHs are not effectively removed during the wastewater treatment process, they are released into natural water bodies, worsening threats to the environment [22].
By limiting exposure to these dangerous substances, the efficient removal of PAHs from wastewater not only contributes to the reduction of environmental pollution but also protects human health [15]. Physical techniques like adsorption, chemical processes like oxidation, and biological breakdown employing microbes are common strategies for removing PAHs from the environment [23]. Among them, adsorption has become more popular because of its effectiveness and ease of use, and different adsorbents are being investigated [24]. Because IONs have a high adsorption capacity and the possibility for regeneration and reuse, using them to remove PAHs offers a viable and sustainable alternative [25]. Specifically, utilizing IONs extracted from acid mine wastewater is a creative and environmentally friendly method [22,26]. These nanoparticles have a high surface area and reactivity, which allow them to efficiently adsorb PAHs from aqueous solutions [24]. Table 1 compares the types and characteristics, as well as the chemical formulas, of the sixteen PAHs.
The purpose of this review is to investigate IONs’ potential for PAH adsorption from wastewater, with a focus on those recovered from acid mine water. This research aims to provide a thorough understanding of these nanomaterials’ efficacy and useful uses in wastewater treatment by investigating their characteristics, recovery techniques, and adsorption mechanisms. We intend to further the development of long-term and practical PAH clean-up strategies with this investigation.

2. Iron Oxide Nanomaterials

A class of minerals called IONs is made up of iron and oxygen, and they can be found in several forms including magnetite (Fe3O4), maghemite (γ-Fe2O3), goethite (FeO(OH)), and hematite (α-Fe2O3). Due to their unique chemical and physical properties, these nanoparticles are highly helpful in environmental remediation processes [24,27,28].

2.1. Magnetite

Because of its huge surface area, remarkable chemical stability, and magnetic characteristics, magnetite is a mixed-valence iron oxide [29]. Because of these properties, it is a great option for adsorption applications, as they facilitate simple separation from aqueous solutions through the use of magnetic fields [30]. It is usually characterized by a stripe of black colour, a metallic to submetallic lustre, and appearances of black to silver grey. Sedimentary, metamorphic, and igneous rocks are among the geological settings in which magnetite can occur [27,31].
This mineral is a main source of iron for the steel industry and one of the main ores utilized in the production of steel. In addition to being used as an iron ore, magnetite’s strong magnetic qualities make it useful in a variety of technological and industrial uses, such as magnetic recording media and catalytic processes in the chemical industry [30,32]. Additionally, glass, pottery, and black paints can be coloured with finely ground magnetite. In addition to its distinct hardness, metallic sheen, and black streak, magnetite is easily recognized by its high attraction to magnets [33]. In terms of the environment, magnetite’s presence in soils and sediments can reveal information about historical temperatures and redox states. In industrial settings, magnetite is essential to the steel and iron sectors, where high-purity iron concentrates are typically produced by crushing, grinding, and magnetic separation. Magnetite is also used as a catalyst in a variety of chemical reactions, in the manufacture of some ceramics, and in the treatment of water to remove arsenic [31,34].

2.2. Maghemite

With increased chemical stability, maghemite is a fully oxidized version of magnetite that nonetheless has identical magnetic characteristics. Applications requiring increased stability and reactivity frequently employ it [34]. It usually has a metallic to dull lustre, a brownish stripe, and colours ranging from reddish-brown to yellowish-brown. In addition to being a by-product of other iron-containing minerals weathering, maghemite is typically formed via the oxidation of magnetite. It is commonly found in oxidized iron ore deposits, sediments, soils, and a variety of industrial by-products, including furnace slags. This mineral is widely distributed and can be found in a variety of geological settings worldwide [34].
Maghemite is a magnetic mineral that finds use in many applications such as magnetic recording media and pigment for coatings and ceramics. Despite being marginally less strong than magnetite, its magnetic characteristics make it useful for both technological and medicinal applications. Because of its magnetic and catalytic qualities, which are employed in a variety of chemical reactions and environmental applications, maghemite is significant in industrial contexts. For example, it is employed as a catalyst in organic synthesis operations and in the removal of pollutants from water systems [34].

2.3. Hematite

Hematite, which stands as the most thermodynamically stable form of iron oxide, exhibits a substantial degree of crystallinity and heat resistance [35]. Although it does not possess magnetism, it shows a high surface area and powerful adsorption capacity, making it suitable for specific adsorption processes [36]. Its reddish streak distinguishes it from other similar-looking minerals. Hematite forms under diverse conditions, such as sedimentary processes where it precipitates from water and metamorphic processes where it transforms from other iron-bearing minerals. It is commonly found in sedimentary rocks like banded iron formations, metamorphic rocks like schist, and as a weathering product in soils [34,36].
This mineral is a key iron ore and has been mined for thousands of years for iron production [37]. Apart from its use in iron ore, hematite is also utilized as a pigment due to its rich red and brown colours [27]. Historically, it has been used in art and decoration, often as red ochre. Its high density and chemical inertness make it useful in various industrial applications, such as radiation shielding material and heavy media separation [27,36]. Overall, hematite is a versatile and abundant mineral, integral to different natural processes and industrial applications, making it a subject of considerable interest in both scientific research and economic activities [35].

2.4. Goethite

Goethite, an iron oxyhydroxide mineral, is a common iron ore that typically has a yellowish-brown to reddish-brown colour [24]. Goethite usually forms under oxidizing conditions as a weathering product of iron-bearing minerals or as a precipitate from hydrothermal fluids [35]. It is often found in soil, sediments, and ore deposits. It is widespread and can be found in a variety of geological environments, including iron-rich sedimentary rocks, laterites, bogs, and hydrothermal deposits. Goethite is a significant source of iron and is extracted for iron production [34]. Its fine particles are employed as pigments due to their vibrant yellow to brown hues, historically in ochre paints [38]. The presence of goethite in sediments and soils can indicate past environmental conditions, such as oxygen levels and pH. In addition to iron extraction, goethite is utilized in various industrial processes, including water treatment and as a precursor for other iron compounds. Goethite is a crucial component in the geochemical cycling of iron and is a vital mineral for comprehending Earth’s geologic and environmental history [38,39]. Table 2 presents a comprehensive analysis of the forms and characteristics of iron oxide nanomaterials, with particular attention to magnetite, maghemite, goethite, and hematite, along with an overview of their benefits and drawbacks.

3. Synthesis of Iron Oxide Nanomaterials from Acid Mine Drainage

3.1. Overview of the Environmental Effects of Acid Mine Drainage

Acid mine drainage (AMD) is a pervasive issue that arises from mining activities, particularly coal and metal mining [46]. It occurs when sulphide minerals, such as pyrite (FeS2), are exposed to water and air during mining operations, triggering a series of chemical reactions that produce sulphuric acid [39,47]. This highly acidic solution then leaches out heavy metals and other harmful substances from the surrounding rocks, resulting in water that is both extremely acidic and rich in dissolved metals and metalloids [48,49].
When pyrite oxidation-related AMD occurs, thallium, a very toxic metal, is discharged into the environment, posing serious health and environmental dangers. In sulphide ores like pyrite, thallium is frequently present in trace amounts [50]. Because of its extreme toxicity, thallium can seriously harm aquatic ecosystems and constitute a health risk to people and wildlife through polluted drinking water and bioaccumulation in the food chain, even at low quantities [51]. Exposure to thallium can cause serious health concerns, such as gastrointestinal upset, cardiovascular problems, and harm to the nervous system. Thallium pollution in AMD need to be addressed immediately in order to reduce their negative impacts on the environment and public health. This can be achieved by strengthening regulatory procedures, implementing remediation plans, and improving monitoring [50,51].
The effect of AMD on the environment is substantial and complex. One of the most immediate and visible consequences is the severe pollution of surface and groundwater [52]. The acidic water can lower the pH of affected water bodies to harmful levels for aquatic life, leading to the death of fish, invertebrates, and plant species [53]. The dissolved metals, such as iron, copper, lead, and arsenic, can accumulate in the tissues of aquatic organisms, causing toxic effects and biomagnification through the food chain. This contamination not only disrupts local ecosystems but also poses significant risks to human health when these metals enter drinking water supplies [35]. Acid mine water contaminates surface and groundwater with high levels of acidity and toxic metals like iron, copper, lead, and mercury, rendering the water unsafe for consumption, irrigation, and aquatic life [54].
The environmental and economic consequences of AMD are significant. The presence of iron hydroxides and other metal precipitates often discolours streams and rivers, creating unattractive and uninhabitable conditions for wildlife and diminishing the recreational value of natural water bodies [55,56]. The expense of remediation and water treatment to mitigate the effects of AMD can be substantial, placing financial strain on local communities, governments, and mining companies [35].
An innovative approach is to utilize waste streams from acid mine water to produce valuable adsorbents by synthesizing iron oxide nanomaterials. Acid mine water, a by-product of mining activities, contains dissolved iron, making it an ideal source for synthesizing iron oxide nanomaterials [35,49,57]. According to a study by Akinwekomi et al. (2017), Fe(II)/Fe(III) can be extracted from AMD and used at 2:1 Fe(III)/(II) ratios to synthesize magnetite. This was explained by the fact that Fe, Al, Mn, and SO42−, among other trace elements, predominate in AMD from South African coal basins. The amount of these chemical species varies according on the host rock, which affects how the resulting AMD is composed [33]. According to published reports, the usual method for synthesizing goethite and hematite is thermo-treatment, which involves calcining the recovered material at temperatures between 80 and 800 °C [34]. Akinwekomi et al.’s (2020) investigation looked into new approaches to AMD beneficiation through the creation of precious minerals with a wide range of industrial uses. They synthesized iron oxides and gypsum (product minerals) in their investigation using actual AMD. In batch reactors, Fe(III) and Fe(II) were recovered by successive precipitation for the synthesis of goethite, hematite, and magnetite. The study demonstrated that AMD can be extracted to produce precious minerals that have a variety of industrial uses. The process can become self-sustaining if the proceeds from the sale of the product minerals are sufficient to cover the operating expenses of the treatment procedure [35].

3.2. Processes for Recovering Iron Oxide from Acid Mine Water

Recovering iron oxide nanomaterials from AMD is an innovative solution that addresses environmental concerns while providing useful materials for adsorption applications [37]. The recovery process typically involves several key steps:
  • Neutralization: the AMD is neutralized using alkaline substances such as lime (Ca(OH)2), sodium carbonate (Na2CO3), and sodium hydroxide (NaOH), which raises the pH, causing dissolved iron to precipitate as iron hydroxides (Fe(OH)3) [58].
  • Precipitation and coagulation: the neutralization process promotes the formation of iron hydroxide flocs, which coagulate and settle out of the solution [59].
  • Filtration and washing: The precipitated iron hydroxides are separated from the treated water through filtration. The solids are then washed to remove impurities and excess neutralizing agents [32].
  • Oxidation: The iron hydroxide precipitates are oxidized to form iron oxide nanomaterials. This can be achieved by the following:
    • Air oxidation: allowing the hydroxides to air-dry and oxidize naturally.
    • Chemical oxidation: using oxidizing agents like hydrogen peroxide (H2O2) to accelerate the conversion [55].
  • Thermal treatment: the oxidized materials are subjected to thermal treatment (calcination) at controlled temperatures to produce the desired phase of iron oxide, such as magnetite, maghemite, or hematite [33].
  • Characterization: the synthesized nanomaterials are characterized using techniques like scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR) to verify their phase, morphology, and functional properties.
A study by Mogashane et al. (2022) examined the recovery of Fe2O3 nanoparticles and clean water from AMD that was rich in iron. The purpose of their research was to assess the efficiency of the reverse osmosis cooling (ROC) process in producing clean water and nanoscale Fe2O3 from AMD. The pH was elevated to 4.5 for the recovery of Al(OH)3 after Fe(OH)3 was recovered. The pH was then increased to 8.5 for the removal of the remaining metals, and desalination was accomplished using reverse osmosis. Fe(OH)3 was thermally processed to produce a pigment that could be used as a nanomaterial because its particles were smaller than 100 nm [39].
The size, surface area, and surface functional groups of IONs are significantly influenced by their preparation methods, which include hydrothermal synthesis, sol–gel, co-precipitation, and thermal breakdown [24,43]. To maximize their reactivity, adsorption capacity, and magnetic characteristics, these factors are essential. Co-precipitation can lead to wider size distributions but is simpler and more scalable. Specific control over size and crystallinity can be achieved through thermal breakdown and hydrothermal synthesis, while different surface groups can be functionalized through sol–gel techniques. As a result, the synthesis process used for a certain application in environmental remediation or catalysis must match the desired nanoparticle characteristics [36].
The durability and efficacy of IONs are significantly impacted by pH, which also affects how the particles interact with pollutants and how their surface charge persists [27]. Improved stability, reduced aggregation, and large surface area for effective adsorption and catalytic activity are all present in nanoparticles at ideal pH values. On the other hand, excessive pH levels may cause the nanoparticles to become unstable, lose their reactivity, or dissolve or change. Iron oxide nanoparticle performance in environmental applications must therefore be maximized, and their long-term stability and efficacy must be ensured, by regulating and maintaining the proper pH [23].
High purity and performance are frequently deemed dependent on the removal of iron from other metals during the preparation of IONs [35]. Nevertheless, the introduction of advantageous characteristics including better magnetic behaviour, elevated catalytic activity, and greater adsorption capacity might boost the effectiveness of nanoparticles in the presence of metal impurities or deliberate inclusions. The presence of these metal inclusions can enhance the overall functionality of the nanoparticles by working as synergistic positive sites for chemical reactions [53]. Due to the possible benefits of metal inclusions, a strategy that does not remove impurities may be more efficient and cost-effective, simplifying the preparation process and lowering production costs [35].
A study by Legodi and Dewaal, 2007, investigated the process of preparing pigment-quality hematite, magnetite, maghemite, and goethite from iron waste from mill scale operations. Their research showed that particular precursors can be used to manufacture various iron oxide pigments from mill scale iron waste. The precipitates of magnetite and goethite were obtained in aqueous media from their respective predecessors. Different red tints of hematite were produced by calcining the precipitated goethite at temperatures ranging between 600 and 900 °C. Magnetite was heated to 200 °C in order to produce maghemite. Typically, they consist of high-surface-area, ultra-small particles (often less than 0.1 mm). The aforementioned pigments (made from mill scale) should exhibit strong tinting strength, excellent hiding power, and effective oil absorption based on these particle qualities [34]. Table 3 presents a comprehensive analysis and comparison of different methods for recovering IONs from acid mine water, emphasizing the benefits and drawbacks for real-world use.
Chemical-based synthesis techniques are the most widely used because of their high yield and cheap cost of manufacturing [59]. According to research by Silva et al. (2019), adding chemical agents to wastewater to increase pH and precipitate dissolved metals in the form of oxides and hydroxides is one way to treat AMD actively [30]. Their research has demonstrated that the iron found in AMD may be used to generate the yellow pigment known as goethite. On the other hand, pigment synthesis may be impeded by the existence of additional metals. In order to recover raw materials from AMD and produce high-quality yellow pigment, they also assessed a number of methods for cleaning iron sludge. Strong and weak bases were used in the studies to induce precipitation, and other metals were extracted from the sludge by centrifugation or by washing and filtering the sludge [59]. The findings demonstrate that the amount of contaminants affected the colour, type, and morphology of the compounds [59]. A typical procedure for recovering iron oxide from acid mine water is displayed in Figure 1.

3.3. Characterization of Recovered Iron Oxide Nanomaterials

Once the iron oxide nanomaterials have been recovered, they undergo characterization to determine their suitability for adsorption applications [39]. The main techniques used in this process consist of XRD to identify the crystalline phases and confirm purity and structural properties, SEM to provide detailed images of the nanomaterials’ morphology, size, and surface structure, transmission electron microscopy (TEM) to offer high-resolution images and further investigate the nanomaterials’ internal structure and size distribution, FTIR to examine the functional groups on the surface of the nanomaterials and their impact on adsorption capabilities, Brunauer–Emmett–Teller (BET) analysis to determine the specific surface area of the nanomaterials, which is crucial for adsorption efficiency, and magnetic characterization using techniques such as a vibrating sample magnetometer (VSM) for magnetite and maghemite to assess magnetic properties, ensuring easy separation from treated water [26,33,43]. Ultimately, these processes enable the effective utilization of IONs recovered from AMD for the adsorption of PAHs from wastewater, providing both environmental remediation and resource recovery. This approach not only addresses the challenge of AMD but also offers a sustainable solution for wastewater treatment [35,39]. A study by Gupta et al. (2016) investigated different forms of IONs. They successfully characterized IONs through XRD and SEM. The XRD spectra were found to be concordant with JCPDS-ICDD data. Goethite and magnetite were found to have crystallite diameters of 9.85 and 14.13 nm, respectively. SEM analysis yielded mean particle sizes of 32.23 and 63.27 nm, respectively. The elemental makeup of the produced IONs was estimated using energy-dispersive X-ray (EDX) spectroscopy [45]. Moreover, using Raman spectroscopy, XRD, surface area calculation, and SEM, the iron oxides were satisfactorily characterized as reported by [34].

4. Mechanisms of PAH Adsorption onto Iron Oxide Nanomaterials

4.1. Adsorption Isotherms and Kinetics

The adsorption of PAHs on iron oxide nanomaterials is commonly assessed through adsorption isotherms and kinetics, which provide insight into the interaction mechanisms and efficiency of the process [22,26]. Adsorption isotherm models, such as Langmuir and Freundlich, offer information on the equilibrium PAH–surface interaction. The Langmuir isotherm assumes monolayer adsorption on a homogenous surface with finite identical sites, implying a constant adsorption capacity [23]. In contrast, the Freundlich isotherm accounts for heterogeneous surfaces and multilayer adsorption, imitating the diversity of binding sites on the nanomaterial surface. These models are essential for predicting adsorption capacity and optimizing material usage [36,63].
Adsorption kinetics describe the rate at which PAHs are adsorbed onto iron oxide nanomaterials [64]. The kinetics are often analysed using pseudo-first-order and pseudo-second-order models [22]. According to the pseudo-first-order model, the number of empty sites that are available for physisorption and the rate at which adsorption sites are occupied are proportionate [65]. The pseudo-second-order model, which provides a better fit for chemisorption processes, postulates that adsorption takes place via an electron exchange or sharing mechanism between the adsorbent and the adsorbate, which involves valence forces. Comprehending these kinetic models facilitates the computation of the equilibration period and possible mechanisms entailed in the adsorption process [23].
A study by Zhang et al. (2024) examined the adsorption of iron oxide and micro/nano carbon black on PAHs (pyrene) and arsenite. Their study examined the adsorption of pyrene and arsenite (As (III)) utilizing micro/nano carbon black and iron oxide under different conditions using a series of static adsorption tests. Their study aimed to ascertain the adsorption kinetics and isotherms of pyrene and As (III) and assessed the influence of co-existing circumstances on the adsorption process using micro/nano carbon black and iron oxide [64]. The micro/nano black carbon demonstrated an adsorption capacity of 283.23 μg/g at 24 h and a pseudo-second-order rate constant of 0.016 mg/(g·h), both of which demonstrated the ease with which pyrene was absorbed. The main adsorption processes involved chemical reactions. The presence of As (III) decreased the adsorption capacity of micro/nano carbon black for pyrene, and this impact grew as the As (III) concentration increased. However, As (III) was hardly absorbed by micro/nano carbon black. In contrast, the adsorption capacity of micro/nano iron oxide for As (III) was improved by the presence of pyrene, and this effect increased with pyrene concentration. However, the micro/nano iron oxide was rarely able to adsorb the pyrene. The study’s conclusions offer recommendations for risk management and environmental clean-up in situations where PAH and As contamination co-exist [64].

4.2. Factors Influencing Adsorption Efficiency

Several factors influence the adsorption efficiency of PAHs onto iron oxide nanomaterials, including pH, temperature, and PAH concentration [36]. The ionization state of PAHs and the surface charge of iron oxide nanoparticles are both strongly impacted by the pH of the solution [66]. Because iron oxide surfaces are typically more positively charged at lower pH values, this can strengthen the electrostatic interaction between the negatively charged PAHs and the adsorbent surface [65]. Extreme pH levels can negatively affect adsorption efficiency by either dissolving iron oxides or protonating PAHs [43,67].
Temperature is another critical factor that influences adsorption capacity. As temperature increases, molecular movement enhances the interaction between PAHs and the adsorbent surface, resulting in improved adsorption capacity [23]. However, very high temperatures can cause desorption or decomposition of the adsorbate, leading to reduced adsorption efficiency [28]. The adsorption process is significantly influenced by the initial concentration of PAHs in wastewater. A larger driving force for mass transfer is provided by higher starting concentrations, which raises the adsorption rates. However, after reaching a certain point, saturation of adsorption sites may occur, causing efficiency to plateau [65].

4.3. Interaction Mechanisms between PAHs and Iron Oxide Surfaces

The interaction mechanisms between PAHs and iron oxide nanomaterials involve various physical and chemical processes, including van der Waals forces, chemisorption, and hydrophobic interactions [24,66]. Van der Waals forces facilitate physical adsorption of PAHs onto the surface of iron oxides, while chemisorption involves the formation of chemical bonds between PAHs and the iron oxide surface, resulting in higher adsorption capacities and stability [68,69,70]. Hydrophobic interactions also play a key role in the adsorption process by driving PAHs to move from the aqueous phase to the relatively more hydrophobic surface of iron oxides [43]. The movement of PAHs from water to the hydrophobic surface of iron oxides is driven by the hydrophobic effect, which reduces the system’s free energy [71,72]. This mechanism is crucial in water-based environments, as it enhances the affinity of PAHs for the iron oxide nanomaterial surface. Surface complexation and π–π interactions also contribute to the adsorption process [65]. Surface complexation involves the binding of PAHs to specific functional groups on the iron oxide surface, forming stable complexes. The π–π interactions occur between the aromatic rings of PAHs and the delocalized π-electrons on the surface of iron oxides, particularly if the surface is modified with aromatic groups [37,71].

5. Comparison of Iron Oxide Nanomaterials with Other Adsorbents

5.1. Performance Comparison with Other Commonly Used Adsorbents

Iron oxide nanomaterials are highly effective in adsorbing PAHs and other contaminants due to their unique properties [43]. Compared to other commonly used adsorbents, such as activated carbon and other metal oxides, iron oxide nanomaterials offer several performance advantages [29,73].
The vast porosity and large surface area of activated carbon are well known for their ability to effectively absorb a wide spectrum of organic contaminants, including PAHs [73]. However, iron oxide nanomaterials offer several advantages over activated carbon [70,74]. The magnetic properties of IONs, particularly magnetite and maghemite, allow for easy separation from treated water using an external magnetic field, reducing the need for additional filtration steps [35]. Moreover, iron oxide nanomaterials can engage in both physical adsorption and chemical interactions, such as surface complexation and π–π interactions, enhancing their overall adsorption efficiency and selectivity for specific contaminants like PAHs [27]. The main drawbacks of employing IONs to remove PAHs are depicted in Figure 2, including their high cost, possible toxicity, aggregation problems, and difficulties with selectivity.
Other metal oxides with photocatalytic qualities, such zinc oxide (ZnO) and titanium dioxide (TiO2), are also frequently utilized for adsorbing organic contaminants [28,37]. Although these materials are effective under specific conditions, they often require ultraviolet (UV) activation to achieve optimal performance [75]. In comparison, iron oxide nanomaterials have a broader range of operating conditions and do not require UV light, giving them more versatility [75]. Additionally, their strong magnetic properties provide a distinct advantage in separation processes that other metal oxides cannot match [37].
Pérez-Gregorio et al. (2010) investigated the removal of PAHs from organic solvents by ash wastes. Their study showed that while wood ashes acquired at higher temperatures (>500 °C) had readily available adsorbent sites for the PAH molecules, ashes acquired at lower temperatures (300 °C) did not demonstrate any sorption of PAHs. For sorption wood ash wastes, a rise in the molecular weight of PAHs has a significant impact. Due to variations in adsorbent sites, the sorption of PAHs increases with decreasing wood ash particle size. In terms of cost, the effectiveness of wood ash wastes compared to activated carbon in eliminating ten PAHs from organic solvents is comparable, making them a viable option for waste disposal [76].

5.2. Cost-Effectiveness and Availability

The cost-effectiveness and abundance of iron oxide nanomaterials are significant advantages. Iron, one of the most abundant elements on earth, makes IONs relatively inexpensive compared to other adsorbents [39]. Furthermore, the recovery of iron oxide nanomaterials from acid mine drainage enhances their cost-effectiveness and addresses an environmental issue [24]. In contrast, the production of high-quality activated carbon often involves complex and energy-intensive processes, making it more expensive. Similarly, the synthesis of other metal oxides can be costly due to the use of rare or expensive raw materials and the need for specialized production techniques [35].

5.3. Regeneration and Reusability

Iron oxide nanoparticles have strong regeneration and reusability qualities, which is one of their main advantages [28]. Following adsorption, a magnetic field can be used to quickly separate these nanoparticles from the wastewater, making recovery simple [28,66]. Then, with little to no loss of adsorption capability, they can be regenerated by a variety of processes like thermal treatment, chemical cleaning, or desorption treatments [33]. The entire operational costs and environmental effect of the treatment process are decreased by this regeneration’s ease. Table 4 presents a summary of the various techniques for the regeneration of IONs, emphasizing their main benefits and drawbacks. Even though activated carbon is quite good at adsorption, regeneration can be difficult. High-temperature thermal treatment is a common step in the regeneration process [77,78]. This step can be energy-intensive and may eventually damage the material, lowering its adsorption capability [75]. Furthermore, in order to avoid secondary pollution, managing used activated carbon needs to be performed carefully. The magnetic separation and regeneration of iron oxide nanoparticles may be more economical than the procedures required to regenerate other metal oxides, such as TiO2 and ZnO, which frequently require UV activation or chemical treatments [28,75]. A thorough comparison of iron oxide nanoparticles and other popular adsorbents is given in Table 5, which also highlights important performance characteristics pertinent to real-world uses in environmental remediation and other domains.

5.4. Advantages of Using IONs for Adsorption Processes

Iron oxide nanomaterials are highly effective in adsorption processes, particularly in the removal of contaminants like PAHs from wastewater [35,46]. Their advantages include a high surface area-to-volume ratio, magnetic properties, chemical and thermal stability, and eco-friendly synthesis [31,43]. The increased surface area allows for more active sites for interaction with contaminants, leading to higher adsorption efficiency [36]. Additionally, iron oxide nanomaterials exhibit strong magnetic properties, which enable easy separation from the aqueous solution using an external magnetic field [38]. This separation process is efficient, rapid, and does not require additional filtration or centrifugation steps. The materials are also known for their excellent chemical and thermal stability, allowing them to withstand a wide range of environmental conditions without significant degradation [53,59]. The synthesis of iron oxide nanomaterials from acid mine water is an environmentally friendly process that mitigates the environmental impact of mining activities and provides a sustainable and low-cost source of high-performance adsorbents. This approach promotes waste valorisation and contributes to circular economy practices [35].
The recovery and employment of iron oxide nanomaterials from acid mine water offer substantial financial advantages [35,39]. Traditional methods of synthesizing nanomaterials can be costly due to the expense of raw materials and energy-intensive processes [43]. On the other hand, utilizing acid mine water as a raw material lowers production costs, making the entire process economically viable [37]. Furthermore, the magnetic properties of these nanomaterials simplify separation and regeneration processes, reducing operational costs [83]. Iron oxide nanomaterials are versatile adsorbents that can effectively remove a wide range of pollutants, including heavy metals, organic pollutants, and PAHs [31,46]. The capability to regenerate and reuse the nanomaterials reduces the need for continuous production and disposal, minimizing environmental impact [37].
The surface chemistry of iron oxide nanomaterials can be customized to improve their interaction with specific contaminants [37]. Surface functionalization with various chemical groups can increase selectivity and adsorption capacity for target toxins like PAHs [25]. These modifications enable the design of highly efficient adsorbents for specific wastewater treatment applications, improving overall treatment performance. After numerous cycles of use, iron oxide nanomaterials can be safely disposed of or recycled. Their non-toxic nature and the ability to undergo further processing or conversion into other valuable materials make them an environmentally friendly option. The disposal or recycling processes for iron oxide nanomaterials are generally less hazardous compared to other adsorbents that may contain harmful chemicals [35].
The processes for synthesizing and employing iron oxide nanomaterials are scalable, making them suitable for both laboratory-scale studies and large-scale industrial applications [37]. The low cost and high efficiency of iron oxide nanoparticles, along with their simple manufacturing and recovery methods, make them easy to integrate into practical wastewater treatment systems [24]. Because of these qualities, IONs are especially well suited for the adsorption of PAHs from wastewater, providing an effective, long-lasting, and financially feasible remedy for environmental clean-up [35]. In addition to comparing these nanomaterials’ performance with that of other adsorbents, this study will go deeper into the mechanics underlying PAH adsorption and look at potential future research areas as well as practical applications. The iron oxide nanoparticles that are utilized to adsorb polycyclic aromatic hydrocarbons from soil and water are summarized in Table 6 along with the number of PAHs, recovery percentage, country, sample matrix, and references.

6. Treatment

6.1. Wastewater Containing PAHs

Due to the hydrophobic and persistent nature of these organic contaminants, treating wastewater-containing PAHs is a difficult procedure. A combination of chemical, biological, and physical techniques is usually used in effective treatment. Because of their large surface area and affinity for organic chemicals, activated carbon, IONs, and biochar are examples of physical approaches that can be used to adsorb PAHs from water [24,73,79]. Chemical therapies, including advanced oxidation processes (AOPs), break down PAHs into less toxic compounds by using potent oxidants like ozone, hydrogen peroxide, or photocatalysis. Utilizing microorganisms capable of metabolizing PAHs and converting them into non-toxic by-products is the biological therapy approach [43]. Bioaugmentation and biostimulation methods can be used to improve this bioremediation. By combining these techniques, removal efficiency can be maximized, and wastewater can be thoroughly treated. To preserve the environment and public health, PAH contamination must be properly addressed, which calls for strict monitoring and cutting-edge treatment techniques [49]. Different adsorbents for removing PAHs from wastewater are displayed in Figure 3.

6.2. Case Studies and Practical Applications

6.2.1. Examples of Lab-Scale and Field-Scale Studies on PAH Removal Using Iron Oxide Nanomaterials

Iron oxide nanoparticles have been shown to be successful at eliminating polycyclic aromatic hydrocarbons from wastewater in a lab- and field-scale investigations [22,26,28,65]. According to these investigations, under regulated settings including pH, temperature, and contact time, IONs like magnetite and maghemite exhibit good adsorption capabilities for distinct PAHs [26]. For example, under ideal conditions, a lab-scale study discovered that magnetite nanoparticles could adsorb over 90% of naphthalene, a prevalent PAH, in a matter of hours [28]. These findings suggest that in a controlled setting, large adsorption efficiencies are possible [85]. Although they are less prevalent, field-scale investigations offer important insights into the real-world uses of iron oxide nanoparticles [19,86,87]. After being added to the contaminated site, iron oxide nanoparticles were observed for several months to see how they performed [63]. The study’s findings, which showed a considerable decrease in PAH concentrations, supported the practical application of these nanomaterials [19,86].
The study by Hassan et al. (2018) investigated the use of green synthesized IONs for the adsorption of pyrene and benzo(a)pyrene micropollutants to remove them from water [19]. Pomegranate peel extract was used to create the green synthesized iron oxide nanoparticles for their investigation, which was conducted at room temperature. The pyrene and benzo(a)pyrene in water were adsorbed by the green synthesized IONs. They examined the factors influencing the adsorption. The total findings demonstrated that IONs’ highest adsorption capabilities for benzo(a)pyrene and pyrene were 0.029 mg g−1 and 2.8 mg g−1, respectively [19]. The thermodynamic analysis revealed that pyrene and benzo(a) pyrene adsorb exothermically. The isotherm and kinetic analyses were conducted. The acquired data demonstrated that the Langmuir isotherm model and a pseudo-second-order mechanism obey the adsorption process. In order to extract the investigated PAHs from artificially contaminated water, the experiment was expanded to include a semi-pilot plant. According to the findings, pyrene and benzo(a)pyrene could be eliminated by IONs at rates of 98.5 and 99%, respectively [19].

6.2.2. Success Stories and Challenges Encountered

The IONs have been successfully applied in a number of cases for adsorption of organic pollutants [19,26,86]. For instance, a pilot experiment in a mining region effectively treated wastewater by using magnetite nanoparticles recovered from acid mine drainage [33,65,68,88]. The experiment showed the dual advantage of addressing acid mine drainage while treating wastewater [89]. Due to this project’s success, other contaminated locations may now consider launching projects along these lines for removing PAHs from wastewater [63]. Even with these successes, there are still difficulties. The synthesis and application procedures’ scalability presents a big obstacle [70]. Even while studies conducted on a lab scale frequently demonstrate high adsorption efficiencies, it might be challenging to scale up the manufacturing of iron oxide nanoparticles and guarantee consistent performance in field applications [19,89]. The possibility of interference from other pollutants found in actual wastewater presents another difficulty [49]. These contaminants may compete with PAHs for adsorption sites, lowering overall effectiveness. In order to make sure that the used nanomaterials do not create any new environmental hazards, it is also essential to thoroughly investigate the long-term stability and potential environmental effects of these materials [49].
A study by Muedi et al. (2022) demonstrated that contaminants can be removed from aqueous solutions using a polycationic Fe/Al di-metal nanostructured composite (PDFe/Al) synthesized from Fe(III) and Al(III) recovered from actual AMD. The aqueous solution’s Congo red dye (CR) was effectively eliminated by the PDFe/Al. When the circumstances were optimal, the PDFe/Al demonstrated a removal effectiveness of ≥99% for CR dye and a remarkable Langmuir adsorption capacity of 411 mg·g−1. The internal strain and deformation caused by the adsorption of CR were seen in the PDFe/Al matrices and interlayers, resulting in a considerable increase in the adsorbent pore surface area and pore volume [88]. According to a regeneration study, the adsorbent may be utilized to absorb CR more than four times. The results of this investigation showed that valuable minerals could be extracted from dangerous and poisonous AMD and showed promise for treating industrial wastewaters [88].

6.2.3. Real-World Applications and Pilot Projects

To prove that employing iron oxide nanoparticles in wastewater treatment is both feasible and effective, real-world applications and pilot projects are essential [49,90]. Few studies have been published on pilot experiments involving highly PAH-contaminated effluent from a chemical production facility [19,49]. Moreover, little work has been carried out on real-world applications concerning the incorporation of iron oxide nanoparticles into an existing wastewater treatment plant’s treatment process [19,91]. This will improve urban wastewater’s ability to remove organic pollutants, such as PAHs. When compared to using only conventional treatment methods, the plant will achieve better removal efficiency for PAHs, demonstrating the efficacy of the integration [91].

7. Environmental and Economic Implications

7.1. Environmental Benefits of Using Recovered Materials

The deployment of iron oxide nanomaterials extracted from acid mine drainage for wastewater treatment presents significant environmental advantages [89]. Acid mine drainage is a significant environmental challenge that causes severe water pollution with acidic water rich in heavy metals [35]. By obtaining iron oxide nanomaterials from AMD, this method not only addresses the contamination issue but also transforms a hazardous waste product into a valuable resource for pollution control [89]. This dual-purpose solution helps mitigate the environmental impact of both AMD and wastewater contaminants such as PAHs [66]. Additionally, the use of these nanomaterials lessens the need for mining new raw materials, thereby conserving natural resources and reducing the ecological footprint linked to material extraction and processing [27,49].

7.2. Economic Analysis of Using Iron Oxide Nanomaterials for Wastewater Treatment

Economically, the application of IONs derived from AMD offers a cost-effective alternative to conventional adsorbents [39]. The raw material expense for these nanomaterials is significantly lower since they are obtained from waste products [19,92]. The process of extracting iron oxide from AMD involves affordable and scalable techniques, such as neutralization and precipitation, which do not require pricey reagents or energy-intensive processes. Furthermore, the magnetic properties of IONs simplify the separation process, decreasing operational expenses connected with filtration and centrifugation [43].
In contrast, the production of activated carbon, a commonly employed adsorbent, entails high-temperature processes and the use of chemical activators, contributing to higher production costs [73]. Similarly, the synthesis of other metal oxides can be costly due to the use of expensive raw materials and complex production methods [39]. Thus, the use of iron oxide nanomaterials not only reduces material costs but also lowers overall operational expenses, making it an economically attractive option for large-scale wastewater treatment applications [46].

7.3. Sustainability and Scalability of This Approach

Utilizing iron oxide nanoparticles that have been recovered from AMD is a naturally scalable and sustainable method [89]. It complies with the circular economy’s tenets by converting a waste by-product into an important component for environmental restoration [35]. This promotes resource efficiency and sustainability in addition to lowering waste and pollution. The very straightforward recovery procedures involved and the widespread availability of AMD bolster this approach’s scalability. To recover iron oxide nanoparticles from AMD, large-scale treatment facilities can be built, offering a consistent supply of adsorbents for the treatment of wastewater [35,83].
Furthermore, iron oxide nanoparticles’ regeneration and reusability contribute to their sustainability [55]. The necessity for constant manufacture and disposal is lessened by the ease with which these nanoparticles can be recycled and reused several times without suffering a considerable loss of adsorption ability [38,49]. With repeated use, this cycle of use reduces environmental effect and eventually lowers costs. The application of iron oxide nanoparticles offers a scalable and workable solution to the problems associated with worldwide water pollution because of its advantages for the environment, economic feasibility, and sustainable practices [27].

7.4. Feasibility

The study by Mogashane, et al. (2023) investigated the ROC process’s economic feasibility for treating acid mine water that contains a lot of iron. Their research was focused on evaluating the viability of an integrated process for the removal of Fe3+, Al3+, and Mn2+ using alkalis such as CaCO3 and Ca(OH)2. The desalination process yielded salt (Na2SO4·10H2O/L H2O) and a minimal brine by means of reverse osmosis and freeze crystallization. Fe(OH)3, which was extracted at pH 3.5, was processed into a pigment, a product that could be sold. Fe(OH)3 was thermally treated to produce a pigment with a particle size of less than 100 nm, which made it suitable for usage as a nanomaterial [46]. In order to conduct their study, a spreadsheet-based model was created to assess the viability of nearly zero waste treatment. Based on information from the literature, a 1 m3/h demonstration plant for neutralization and reverse osmosis and other methods were used. Chemical and electricity costs for feed water amount to ZAR42.39/m3 (chemical costs are ZAR27.46/m3, and electricity costs are ZAR14.93/m3). At a cost of ZAR20/kg, the value of pigment amounts to ZAR96.78/m3 of feed water. Under typical treatment techniques, the cost of disposing of brine and sludge will be ZAR432.99/m3 feed water at a disposal cost of ZAR2 500/t. According to the study, when zero waste disposal is implemented, the ROC process can be employed to prevent paying excessive disposal expenses. A significant amount of the capital redemption and operating expenses can be covered by the value of marketable products, such as pigments and water [46].

8. Future Prospects and Research Directions

8.1. Emerging Trends and Innovations in Nanomaterial-Based Adsorption

A number of new developments and trends in the field of nanomaterial-based adsorption are set to improve the effectiveness of wastewater treatment procedures [73,85]. The creation of multifunctional nanomaterials that combine catalytic degradation and adsorption is one noteworthy trend [25]. These hybrid materials provide a more thorough treatment option by simultaneously degrading pollutants like PAHs into less hazardous compounds and absorbing them [66]. Furthermore, developments in nanotechnology are enabling the creation of nanomaterials with specialized surface characteristics and functional groups that are intended to target and efficiently and selectively adsorb particular pollutants [25,91].
Utilizing IONs in combination with other materials like graphene, carbon nanotubes, or polymeric components to create nanocomposites is another innovative technique [60]. These composites increase total adsorption capacity, mechanical strength, and durability by utilizing the complementary qualities of various materials [63,71]. In addition, there is growing interest in the application of artificial intelligence and machine learning to the design and optimization of nanomaterials [25]. These technologies can direct the synthesis of materials with the best qualities for certain applications and forecast how different nanomaterial combinations would function [25].

8.2. Potential Improvements in Recovery and Synthesis Methods

Iron oxide nanomaterials recovered from AMD can be used to their full potential with a few changes to recovery and production techniques [89]. Optimizing the neutralization and precipitation procedures to raise the production and purity of iron oxide nanoparticles is one possible area for development [35]. By producing nanomaterials with more consistent size, shape, and surface properties, sophisticated procedures like sol–gel and controlled hydrothermal synthesis might improve their adsorption performance [43].
Additionally, the synthesis process can be made more ecologically friendly by adding the ideas of green chemistry into it [49]. For example, the synthesis of IONs can be performed with less harmful chemicals and with a smaller environmental impact if bio-based reducing agents or plant extracts are used. To ensure that these upgraded technologies can be used successfully in large-scale industrial settings, it is also imperative to improve their scalability [25,38].

8.3. Future Research Needs and Directions for Enhancing Adsorption Efficiency and Practical Applicability

In order to improve iron oxide nanoparticles’ adsorption effectiveness and practical utility in wastewater treatment, future research should concentrate on the following areas: Iron oxide nanomaterials’ selectivity and binding affinity for PAHs can be increased by researching different surface modification methods to affix particular functional groups or molecules onto them. To improve the interaction between the adsorbent and the pollutants, this may entail the addition of polymers, organic ligands, or other nanomaterials [26,49,91]. Deep research into the molecular mechanics underlying adsorption can shed light on the ways in which various parameters, including pH, temperature, and the existence of co-contaminants, impact the adsorption process. This understanding can influence the design of stronger and more effective adsorbents [49].
For sustainable wastewater treatment, it is crucial to create efficient techniques for the regeneration and reuse of iron oxide nanoparticles [63]. To recover the adsorption capacity of utilized nanomaterials without causing a major loss of performance, research should investigate a variety of regeneration strategies, such as thermal, chemical, and photonic methods [24,93]. It is important to carry out full-scale and pilot-scale field tests to assess iron oxide nanoparticles’ effectiveness in actual wastewater treatment situations [87]. These studies can aid in identifying real-world difficulties, maximizing operational parameters, and proving the technology’s viability from an economic and environmental standpoint [36]. Conducting thorough life-cycle analyses to analyse the financial and environmental effects of employing iron oxide nanoparticles from production to disposal can assist with pinpointing the regions in need of development and guarantee that the overall process aligns with sustainability goals [39].

9. Conclusions

The persistent pollution of water bodies by PAHs and the harmful effects of acid mine drainage can be addressed with a novel and promising approach: the adsorption of PAHs from wastewater using IONs recovered from acid mine drainage. The present review has elucidated the several beneficial characteristics of iron oxide nanoparticles, including their elevated surface area, robust adsorption capability, facile separation via magnetic properties, and exceptional chemical and thermal durability. Because of these qualities, they are quite successful in removing PAHs from wastewater. By converting a hazardous waste product into a useful resource for environmental restoration, recovering IONs from AMD provides a sustainable and economical method.
Large-scale applications can be made from the comparatively simple synthetic methods for extracting and converting iron from AMD into nanomaterials. This offers a cost-effective supply of high-performance adsorbents in addition to reducing the environmental impact of AMD. According to the reviewed literature, IONs outperform numerous traditional adsorbents in PAH adsorption. Their magnetic qualities improve their usefulness and lower operating expenses by streamlining the recovery and reuse procedures. Using recovered materials also improves the environment and is consistent with sustainable development and the circular economy. Even with these encouraging results, there are still issues and areas that require more study. It is imperative to optimize the synthesis and functionalization of IONs in order to maximize their selectivity and adsorption efficiency for different PAHs.
Further steps towards wider implementation include increasing the recovery and application processes, evaluating the long-term stability and reusability of IONs in practical settings, and carrying out thorough life-cycle assessments to completely comprehend the environmental and financial ramifications. An extremely efficient and environmentally friendly method for adsorbing PAHs from wastewater is to use iron oxide nanoparticles that have been recovered from acid mine water. It solves important environmental challenges by reducing the negative impacts of AMD and cleaning up wastewater tainted with PAHs. This strategy could make a significant contribution to the advancement of wastewater treatment systems that are more inexpensive, ecologically friendly, and efficient.

Author Contributions

Conceptualization, T.M.M. and J.P.M.; methodology, T.M.M.; software, J.P.M.; validation, L.M., T.M.M. and J.P.M.; formal analysis, T.M.M.; investigation, T.M.M.; resources, L.M.; data curation, T.M.M.; writing—original draft preparation, T.M.M.; writing—review and editing, J.P.M.; visualization, L.M.; supervision, J.P.M.; project administration, L.M.; funding acquisition, L.M. All authors have read and agreed to the published version of the manuscript.

Funding

Tumelo M. Mogashane received financial support from Mintek (Analytical Chemistry Division).

Acknowledgments

The authors express their gratitude to Fritz Carlsson for proof reading and extensive editorial input.

Conflicts of Interest

There are no conflicts of interest for the authors to disclose.

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Minerals 14 00826 g001
Figure 1. Schematic diagram for recovery of IONs from acid mine drainage.
Figure 1. Schematic diagram for recovery of IONs from acid mine drainage.
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Figure 2. Limitations of IONs in the removal of PAHs.
Figure 2. Limitations of IONs in the removal of PAHs.
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Figure 3. Various adsorbents for removal of PAHs from wastewater [23].
Figure 3. Various adsorbents for removal of PAHs from wastewater [23].
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Table 1. The 16 United States Environmental Protection Agency (US EPA) priority PAHs, their types, and characteristics [1].
Table 1. The 16 United States Environmental Protection Agency (US EPA) priority PAHs, their types, and characteristics [1].
PAHChemical FormulaNumber of RingsPropertiesSourcesToxicity
NaphthaleneC10H82Volatile, used in mothballs and as a chemical intermediateFossil fuels, combustion of organic matterPossible human carcinogen
AcenaphthyleneC12H83Pale yellow solid, used in dyes and plastics manufacturingIncomplete combustion of fossil fuelsToxic to aquatic life, not classifiable for humans
AcenaphtheneC12H103White solid, used in insecticides and fungicidesCoal tar, oil, wood burningLow acute toxicity, not classifiable for humans
FluoreneC13H103White crystalline solid, used in production of dyes, plastics, and pesticidesFossil fuels, combustion of organic materialsPossible human carcinogen
PhenanthreneC14H103White crystalline solid, used in dyes, drugs, and explosivesCoal tar, crude oil, combustion processesNot classifiable as a human carcinogen
AnthraceneC14H103Used in the production of dyes, plastics, and pesticidesCoal tar, combustion of organic materialsNot classifiable as a human carcinogen
FluorantheneC16H104Pale yellow solid, used in the production of dyes and plasticsIncomplete combustion of fossil fuelsPossible human carcinogen
PyreneC16H104Pale yellow solid, used in the production of dyes and plasticsFossil fuels, combustion of organic materialsPossible human carcinogen
Benzo[a]anthraceneC18H124Yellow solid, used in the production of dyes and pigmentsFossil fuels, incomplete combustionProbable human carcinogen
ChryseneC18H124Yellow solid, used in the production of dyes and pigmentsCoal tar, oil, wood burningProbable human carcinogen
Benzo[b]fluorantheneC20H125Yellow solid, used in researchFossil fuels, incomplete combustionProbable human carcinogen
Benzo[k]fluorantheneC20H125Yellow solid, used in researchFossil fuels, incomplete combustionProbable human carcinogen
Benzo[a]pyreneC20H125Yellow solid, well-studied PAHFossil fuels, combustion of organic materialsKnown human carcinogen
Indeno[1,2,3-cd]pyreneC22H126Yellow solid, used in researchFossil fuels, incomplete combustionProbable human carcinogen
Dibenzo[a,h]anthraceneC22H145Yellow solid, used in researchFossil fuels, incomplete combustionProbable human carcinogen
Benzo[ghi]peryleneC22H126Yellow solid, used in researchFossil fuels, incomplete combustionPossible human carcinogen
Table 2. Comparison of types and properties of iron oxide nanomaterials.
Table 2. Comparison of types and properties of iron oxide nanomaterials.
Iron Oxide NanomaterialChemical FormulaCrystal StructureMagnetic PropertiesChemical PropertiesAdvantagesLimitationsAbility to Remove Organic SubstancesReferences
MagnetiteFe3O4Inverse SpinelStrongly magnetic (ferromagnetic)Fe2+ and Fe3+ present, black in colourHigh magnetic response, easy separation, good conductivityProne to oxidation to maghemite, less stable in acidic conditionsHigh, effective for organic removal due to catalytic properties[40,41]
Maghemiteγ-Fe2O3Cubic (defective spinel)FerrimagneticFe3+ only, brownish red in colourStable over a wide range of pH, good for biomedical applicationsLower magnetic saturation compared to magnetite, more expensiveModerate to high, good for catalytic degradation of organics[42,43]
GoethiteFeO(OH)OrthorhombicWeakly magnetic (antiferromagnetic)Fe3+, yellow to brown in colourHigh surface area, good adsorption capacity for heavy metalsLower magnetic properties, less effective for magnetic separationModerate, effective for adsorption and oxidation of certain organics[42,44,45]
Hematiteα-Fe2O3RhombohedralWeakly magnetic (antiferromagnetic)Fe3+, red in colourHigh stability, corrosion-resistant, good for photocatalysisLow magnetic properties, less effective for magnetic separationModerate, good for photocatalytic degradation of organics[43,44]
Table 3. Comparison of various processes for recovering iron oxide from acid mine water.
Table 3. Comparison of various processes for recovering iron oxide from acid mine water.
ProcessDescriptionAdvantagesLimitationsReferences
NeutralizationAddition of alkaline substances (e.g., lime, limestone) to increase pH and precipitate iron as iron hydroxidesSimple, cost-effective, widely usedGenerates large volumes of sludge, potential for incomplete reactions[35,39]
Precipitation and CoagulationAddition of chemicals (e.g., ferric chloride, aluminium sulphate) to promote aggregation of iron particles for easier removalEffective at low iron concentrations, enhances settlingChemical costs, sludge disposal issues, may require pH adjustment[46,55]
Filtration and WashingPhysical separation of precipitated iron particles through filtration, followed by washing to remove impuritiesProduces relatively pure iron oxide, can be combined with other methodsFilter clogging, requires regular maintenance, may need pre-treatment[47,60]
OxidationOxidation of ferrous iron (Fe2+) to ferric iron (Fe3+) using oxidizing agents (e.g., air, oxygen, chlorine) to facilitate precipitation as iron hydroxideEnhances iron removal efficiency, can be combined with biological methodsRequires controlled conditions, potential for incomplete oxidation[61]
Thermal TreatmentHeating of precipitated iron hydroxides to convert them to more stable iron oxides (e.g., hematite, magnetite)Produces high-purity iron oxides, potential for resource recoveryEnergy-intensive, high operational costs, may release contaminants[33,62]
Table 4. Different methods for regenerating iron oxide nanoparticles [27].
Table 4. Different methods for regenerating iron oxide nanoparticles [27].
Regeneration MethodDescriptionAdvantagesLimitations
Chemical RegenerationUses chemical reagents (e.g., acids, bases) to desorb contaminants and regenerate nanoparticlesEffective for a wide range of contaminants, can be performed at room temperatureUse of chemicals can be hazardous, potential environmental impact
Thermal RegenerationHeating nanoparticles to high temperatures to remove adsorbed contaminantsHigh effectiveness in removing organic contaminants, restores nanoparticle propertiesEnergy-intensive, can cause particle sintering and reduce surface area
Electrochemical RegenerationApplying an electric field to facilitate desorption of contaminantsEnvironmentally friendly, selective desorption, no chemical useRequires specialized equipment, limited to certain types of contaminants
Magnetic SeparationUsing magnetic fields to separate and recover nanoparticles from the treated mediumEfficient and rapid recovery, minimal chemical useMay not fully regenerate adsorption capacity, requires magnetic equipment
Biological RegenerationUsing microorganisms to degrade or transform contaminants on nanoparticle surfacesSustainable and environmentally friendlySlower process, effectiveness depends on contaminant type and microbial activity
Ultrasonic TreatmentApplying ultrasonic waves to agitate and clean nanoparticle surfacesEffective in removing weakly adsorbed contaminants, can be combined with other methodsLimited to specific contaminants, requires specialized equipment
Solvent ExtractionUsing organic solvents to dissolve and remove contaminantsEffective for organic contaminantsUse of organic solvents can be hazardous, solvent disposal issues
Table 5. Comparison of iron oxide nanoparticles with other common adsorbents.
Table 5. Comparison of iron oxide nanoparticles with other common adsorbents.
AdsorbentAdvantagesLimitationsCost-EffectivenessAvailabilityReusabilityReference
Iron Oxide NanoparticlesHigh surface area, high adsorption capacity, selective adsorption of certain contaminants, magnetic properties for easy separationPotential toxicity, high synthesis cost, requires specialized equipmentModerately high initial cost, but efficient in small quantitiesLimited, requires specialized production facilitiesGood, can be regenerated and reused multiple times[26,36]
Activated CarbonHigh surface area, effective for a wide range of contaminants, widely usedHigh cost, non-selective adsorption, regeneration can be challengingHigh, especially for large-scale applicationsWidely available commerciallyLimited, performance decreases after regeneration[65,73,74]
ZeolitesHigh selectivity, good for ion exchange processes, relatively low costLimited by pore size, less effective for large organic moleculesModerate, depends on specific type and applicationReadily availableGood, can be regenerated with proper treatment[72]
BiocharLow cost, sustainable, produced from waste biomass, good adsorption for organics and metalsLower adsorption capacity compared to activated carbon, variable qualityLow to moderate, cost-effective for large-scale useWidely available, can be locally producedModerate, depends on contaminant type and regeneration method[79]
Clay MineralsLow cost, natural availability, good for cation exchangeLow adsorption capacity, limited by specific surface area, possible desorptionVery cost-effective, especially for large volumesReadily available, abundant in natureLimited, often single-use due to low regeneration efficiency[80,81]
Silica GelHigh surface area, good for polar compounds, relatively low costLimited to specific applications, can be affected by humidityModerate, depends on applicationWidely available commerciallyLimited, often not reused due to cost and practicality[82]
Table 6. Some iron oxide nanomaterials used in the adsorption of PAHs.
Table 6. Some iron oxide nanomaterials used in the adsorption of PAHs.
Iron Oxide NanomaterialNo. of PAHs% RecoveryCountrySample MatrixReference
Iron oxide nanoparticles 370%–90%USAWater[26]
Iron oxides1_ChinaWater[64]
Iron oxides1_IndiaSoil[42]
Iron hexacyanoferrate nanoparticles570%–90%IndiaWater, Soil[84]
Iron oxide nanoparticles298.5%–99%EgyptWater[19]
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Mogashane, T.M.; Maree, J.P.; Mokoena, L. Adsorption of Polycyclic Aromatic Hydrocarbons from Wastewater Using Iron Oxide Nanomaterials Recovered from Acid Mine Water: A Review. Minerals 2024, 14, 826. https://doi.org/10.3390/min14080826

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Mogashane TM, Maree JP, Mokoena L. Adsorption of Polycyclic Aromatic Hydrocarbons from Wastewater Using Iron Oxide Nanomaterials Recovered from Acid Mine Water: A Review. Minerals. 2024; 14(8):826. https://doi.org/10.3390/min14080826

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Mogashane, Tumelo M., Johannes P. Maree, and Lebohang Mokoena. 2024. "Adsorption of Polycyclic Aromatic Hydrocarbons from Wastewater Using Iron Oxide Nanomaterials Recovered from Acid Mine Water: A Review" Minerals 14, no. 8: 826. https://doi.org/10.3390/min14080826

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