Emerging trends in photodegradation of petrochemical wastes: a review

Environ Sci Pollut Res DOI 10.1007/s11356-016-7373-y REVIEW ARTICLE Emerging trends in photodegradation of petrochemical wastes: a review Pardeep Singh 1,2 & Ankita Ojha 1 & Anwesha Borthakur 3 & Rishikesh Singh 4 & D. Lahiry 5 & Dhanesh Tiwary 1 & Pradeep Kumar Mishra 6 Received: 16 May 2016 / Accepted: 1 August 2016 # Springer-Verlag Berlin Heidelberg 2016 Abstract Various human activities like mining and extraction of mineral oils have been used for the modernization of society and well-beings. However, the by-products such as petrochemical wastes generated from such industries are carcinogenic and toxic, which had increased environmental pollution and risks to human health several folds. Various methods such as physical, chemical and biological methods have been used to degrade these pollutants from wastewater. Advance oxidation processes (AOPs) are evolving techniques for efficient sequestration of chemically stable and less biodegradable organic pollutants. In the present review, photocatalytic degradation of petrochemical wastes containing monoaromatic and poly-aromatic hydrocarbons has been studied using various heterogeneous photocatalysts (such as TiO2, ZnO and CdS. The present article seeks to offer a scientific and technical overview of the current trend in the use of the photocatalyst for remediation and degradation of petrochemical waste depending upon the recent advances in photodegradation of petrochemical research using bibliometric analysis. We further outlined the effect of various heterogeneous catalysts and their ecotoxicity, various degradation pathways of petrochemical wastes, the key regulatory parameters and the reactors used. A critical analysis of the available literature revealed that TiO2 is widely reported in the degradation processes along with other semiconductors/nanomaterials in visible and UV light irradiation. Further, various degradation studies have been carried out at laboratory scale in the presence of UV light. However, further elaborative research is needed for successful application of the laboratory scale techniques to pilot-scale operation and to develop environmental friendly catalysts which support the sustainable treatment technology with the Bzero concept^ of industrial wastewater. Nevertheless, there is a need to develop more effective methods which consume less energy and are more efficient in pilot scale for the demineralization of pollutant. Responsible editor: Suresh Pillai Keywords Petrochemical waste . Photocatalysts . Heterogeneous catalysts . Ecotoxicity . Harmful effect * Pardeep Singh psingh.rs.apc@itbhu.ac.in 1 Department of Chemistry, Indian Institute of Technology (IIT-BHU), Varanasi 221005, India 2 Department of Environmental Studies, PGDAV College, University of Delhi, New Delhi 110068, India 3 Centre for Studies in Science Policy, Jawaharlal Nehru University (JNU), New Delhi 110067, India 4 Institute of Environment and Sustainable Development (IESD), Banaras Hindu University, Varanasi 221005, India 5 Rajghat Education Centre, KFI, Varanasi 221005, India 6 Department of Chemical Engineering and Technology, Indian Institute of Technology (IIT-BHU), Varanasi 221005, India Abbreviations H3PW12O40 MWNT ZSM HMS CNT SNTZS BTEX LTS LFS PANI VOC Phosphotungstic acid Multi-walled nanotubes Zeolite socony mobil Hexagonal mesoporous silicate Carbon nanotubes Supported nano-titania ZSM silicate Benzene-toluene-ethylbenzene-xylene Low temperature sol–gel Liquid flame spraying Polyaniline Volatile organic compounds Environ Sci Pollut Res HPW SnO2 TiO2 AgBr Bi2O3 SiO2 ZrO2 CCVD Nd(NO3)3.6H2O Pd Zr La(NO3)3.6H2O Ti(O-Bu)4 SrCO3 CeO2 TTIP CdS RF Ag4V2O7/ Ag3VO4 Short form for H3PW12O40 Tin oxide Titania Silver bromide Bismuth trioxide Silica Zirconia Catalytic chemical vapour deposition Neodymium(III) nitrate hexahydrate Palladium Zirconium Lanthanum(III) nitrate hexahydrate Tetra-n-butyl titanium Strontium carbonate Ceria Titanium isopropoxide Cadmium sulphide Radiofrequency Silver anadates Introduction Mining and petrochemical industries are instrumental in the economic development of a significant number of countries worldwide and the products of these industries are considered major boons to the modern society (Li 2016; Liew et al. 2014; Raufflet et al. 2014). However, the wastes generated from the activities of these industries are toxic, carcinogenic and recalcitrant, posing serious threats to the human health and the environment (Chen et al. 2015a; Mechhoud et al. 2016; Suarez et al. 2007; Yang et al. 2015). Thus, wastes from these industries have been classified as ‘hazardous’ which do not have any further use (Rovira et al. 2014). There is a constant increase in the pollution concerns associated with various petrochemical compounds and their by-products in the form of water, air and soil pollution (Álvarez et al. 2016; Chakraborty et al. 2015; Freije 2015; Haghollahi et al. 2016; Hu et al. 2013; Park et al. 2002; Wake 2005; Zhong and Zhu 2013; Zolfaghari et al. 2016). For instance, several studies have reported that pollution of soil-water interface due to the release of hydrocarbons into the environment is a major public health apprehension (Freije 2015; Dórea 2006). It has also been documented that about 90 % of these hydrocarbons released into the water are in the form of potentially perilous benzene-tolueneethylene-xylene (BTEX) compounds (Janbandhu and Fulekar 2011; Souza et al. 2014). These BTEX pollutants, in surface and ground water, generally originate from leakage of petroleum storage tanks, spills over at production wells, refineries, pipelines, and storage and distribution terminals (Bonvicini 2014; Boopathy 2004; Hazrati et al. 2015; Karadag et al. 2016; Lan and Minh 2013; Louis et al. 2016; Rattanajongjitrakorn and Prueksasit 2014). Further, the emission of carcinogenic volatile organic pollutants (VOCs) has become a nightmare for environmentalists and chemical engineers as it results in health ailments, such as acute and chronic respiratory effects, neurological toxicity, lung cancer, fatigue, headaches, and depression (Brodzik et al. 2014; Chen et al. 2015b, 2016; Gałęzowska et al. 2016; Huang et al. 2014; Li et al. 2016b; Madureira et al. 2016; Saeaw and Thepanondh 2015). Among VOCs, the most serious threats are posed by BTEX compounds (Yaws 1991) which have a plethora of emitting sources (Truc and Oanh 2007). Long-time exposure to these compounds even at a very low concentration causes a series of chronic effects (Madureira et al. 2016). Various studies have shown that benzene is the most toxic of all the petrochemical pollutants and has negative health impacts, such as leukaemia, liver cancer, damage to bone marrow, necrosis, oedema, haemorrhage, fibrosis (Bahadar et al. 2014; Edokpolo et al. 2015; Hosseinzadeh and Moosavi-Movahedi 2016; Melo et al. 2007; Wen et al. 2016). Adverse effects of hydrocarbon pollutants on health of living beings have been studied by many researchers which are presented later in more detail. Various physical, chemical and biological methods have been adopted to degrade BTEX and other petrochemical waste products from the effluents of various industries. Physical methods such as gravity separation, adsorption membrane separation, reverse osmosis (RO), ultra-filtration (UF), micro-filtration (MF) and nanofiltration (NF) (Cui et al. 2016; El-Naas et al. 2009; Hosseinzadeh et al. 2015; Ji 2015; Jou and Huang 2003; Padaki et al. 2015; Takht Ravanchi et al. 2009) are increasingly being applied for treating oil-laden waste water, but these methods have certain limitations, like production of large volumes of sludge, high cost of equipments, and high operating costs (Zhao et al. 2013; Diya’uddeen et al. 2011). Chemical methods like precipitation, electrochemical processes, and Fenton process are applied for petrochemical waste treatment which produce low quantity of sludge but consume large quantities of chemicals, require skilled manpower and have high operation and maintenance costs (Akizuki et al. 2014; Anirudhan and Ramachandran 2014; El-Naas et al. 2014b; Hu et al. 2015; Srichandan et al. 2015; Sharma and Rangaiah 2014; Zhou et al. 2014). Biological methods used to treat petrochemical wastes, include activated sludge, trickling filters, sequencing batch reactors, chemo-state reactors, biological aerated filters, bioremediation, and bio-augmentation (Das and Kumar 2016; Diya’uddeen et al. 2011; Farhadin et al. 2008; Joseph and Joseph 2009; Mrayyan and Battikhi 2005; Riser-Roberts 1998; Srichandan et al. 2015; Wang et al. 2016c). Advance oxidation processes (AOPs), such as photocatalytic degradation and ultrasonication (Levchuk et al. 2014; Petronella et al. 2016; Szreniawa-Sztajnert et al. 2013) are evolving as promising techniques for efficient sequestration of chemically stable and less biodegradable organic pollutants (Aljuboury et al. 2015; Bokare and Choi 2014; Hasan et al. Environ Sci Pollut Res 2012; Oller et al. 2011; Shahidi et al. 2015). AOPs involve production of hydroxyl radical which attacks organic compounds and converts them into less toxic products like CO2, H2O and other inorganic salts (Oturan 2014; Neyens and Baeyens 2003; Cooper et al. 1993; Martínez-Huitle et al. 1993; Tedder and Pohland 1993). Photocatalytic degradation of these organic pollutants has drawn much research attention (Karunakaran 2005; Karunakaran and Senthilvelan 2003; Li et al. 2013; Li et al. 2012; Lin et al. 2012; Manzetti et al. 2014; Qi et al. 2016; Romão and Mul 2016). In most photocatalytic degradation processes, titanium dioxide (TiO2) powder, with its strong oxidizing tendency, acts as a photocatalyst (Barreca et al. 2015; De León et al. 2015; Kubo et al. 2007; Lee et al. 2014; Su et al. 2016). Further, low cost and non-toxicity adds to its value and usage in broad manufacturing industries. However, its applicability and efficiency for practical use is still moderate (Han et al. 2007; Haque 2007). The photocatalytic activity of the catalysts gets escalated many times in the presence of UV radiation (Mahmoodi et al. 2011). However, the process becomes economical if sunlight is used instead of ultraviolet light for excitation of photocatalyst (Carbajo et al. 2016; Fenoll et al. 2012; Foletto et al. 2013; Liu et al. 2015; Vinoth et al. 2016; Wang et al. 2016a). Various other light sources, such as light emitting diodes (LEDs), have also been used as light source for the excitation of heterogeneous catalyst (Dai et al. 2014; Ge et al. 2015; Xiong and Hu 2016). However, studies dealing with a detailed photodegradation and ecotoxicological effects on the by-product are lacking. The present review attempts to address the treatment of various organic pollutants related petrochemical wastes in addition to their structure and chemical properties of various pollutants (as shown in Table 1 and Fig. 1) using photocatalytic degradation processes. Further, various adverse effects and ecotoxicity of petroleum hydrocarbon on human and environmental health were also addressed. An endeavour has also been undertaken to understand different aspects of these rapid, Table 1 Various adverse effects of petrochemical waste Occupational or non-occupational exposure to BTEX compounds and other hydrocarbons is extremely harmful for living organisms, including human beings (Abdel-Shafy and Mansour 2016; Adams and Marquez 1983; Bahadar et al. 2014; Barhoumi et al. 2016; Caselli et al. 2010; Edokpolo et al. 2015; Fan et al., 2014a, b; Kim et al. 2013; Li et al. 2015a; Li et al. 2014; Park and Park 2010; Poli et al. 2016; Scheepers et al. 2012). Mazzeo et al. (2010) reported the cellular damages in the Allium cepa test system caused by BTEX. Benzene is considered to be the most carcinogenic to human beings, causing various disorders like leukaemia, lymphoma, chromosomal breakage and interference with their segregation, etc. (Antonio and Georgina 2014; Andreoli et al. 2015; Bird et al. 2010 Palma and Manno 2007). According to Chen et al. (2008), various types of DNA damages were observed in human lymphocytes on being exposed to hydrocarbons like BTEX and methyl tert-butyl ether (MTBE). Pariselli et al. (2008) studied the effect of toluene and benzene in air mixtures on human lung cells and found variable effects of both the pollutants. For example, the presence of benzene and toluene in mixture causes irreparable severe DNA damage; however, no effect on its glutathione redox status (Wen et al. 2016). Toluene is reported to cause genetic damage and oxidative stress (Angela et al. 2012; Kodavanti et al. 2011, 2015; Moro et al. 2012; Singh et al. 2009). Sole toluene (in concentration of 0.25 ppm) causes DNA damage to cells which get repaired within 24 h after the treatment (Pariselli et al. 2009). However, sole benzene did not induce DNA damage, even under different conditions, but it leads to decrease in the glutathione ratio. Hydrocarbon compounds are genotoxic to human as well as plant cells. Genotoxicity is the property of chemical agents Physical and chemical properties of few petrochemical pollutants (Adapted and modified from Van Agerten et al. 1998) Name Molecular formula Density Molecular weight (g mol−1) (kg L−1) Benzene C6H6 78.1 92.1 106.2 106.2 106.2 106.2 94.11 93.13 128.17 Toluene effective and energy-efficient photocatalytic degradation methods of petrochemical wastes. C7H8 Ethylbenzene C8H10 ortho-Xylene C8H10 meta-Xylene C8H10 para-Xylene C8H10 Phenol C6H6O Aniline C6H5NH2 Naphthalene C10H8 Melting point Boiling point Vapour Tm (°C) Tb (°C) pressure (kPa) Aqueous solubility (mg L−1) 0.878 5.5 80.01 10.13 1780 547 2.13 0.867 0.867 0.880 0.864 0.860 1.07 1.0217 1.0253 −95.0 −95.0 −25.0 −48.0 13.0 40.5 −6.3 78.2 110.8 136.2 144.4 139.0 138.4 181.7 184.13 217.97 2.93 0.93 0.67 0.80 0.87 0.053 0.079 0.00864 515 152 175 200 198 83,000 36,000 31.7 669 588 496 699 709 0.04 0.1925 46.6 2.65 3.20 2.95 3.20 3.18 1.46 1.9 3.29 Henry’s Law constant (Pa m3/mol) Log Kow (−) Environ Sci Pollut Res Fig. 1 Structure of various petrochemicals that damages the genetic information within a cell, causing mutations and leading to cancer (Albuquerque et al. 2016; Benford et al. 2010; Borska et al. 2010; Goswami et al. 2016; Kim et al. 2013; Li et al. 2016a; Nakata et al. 2014; Oliveira et al. 2016; Wang et al. 2015). Singh et al. (2007) studied how hydrocarbons cause genotoxicity and apoptosis in Drosophila melanogaster. They observed increased apoptotic markers and genotoxicity in a concentration and timedependent manner in organisms exposed to benzene, toluene or xylene. They also observed that cytochrome P450 activity increased significantly in larvae exposed to test chemicals, but this did not happen in the presence of 3′,4′dimethoxyflavone, a known aryl hydrocarbon receptor (AHR) blocker. Polycyclic aromatic hydrocarbons (PAHs) are environmental pollutants having detrimental effects on living beings up to the genetic level (Boström et al. 2002). These compounds easily get absorbed in the gastrointestinal tract of mammals and distributed in different body parts due to their high lipid solubility (Cerniglia 1984). Further, they get oxidized in the presence of the cytochrome P450-mediated mixed function oxidase system, resulting into formation of epoxides or phenols. Further degradation of these compounds resulted into formation of sulphates, glucoronides or glutathione conjugates. Moreover, epoxides may also get metabolized into dihydrodiols, which leads to the formation of either diol-epoxides or conjugation to form soluble detoxification products after their oxidation (Stegeman et al. 2001). Naphthalene is a common micropollutant in potable water. It covalently binds to molecules in kidney, liver and lung tissues (LeFevre et al. 2012; Liu et al. 2010; Wang et al. 2015; Wincent et al. 2015). It is also an inhibitor of mitochondrial respiration. Acute naphthalene poisoning in humans can lead to nephrotoxicity and haemolytic anaemia. In addition, it also causes dermal and ophthalmological changes on prolonged exposure (Falahatpisheh et al. 2001). Phenanthrene is a mild allergen and a photosensitizer of human skin. It is also mutagenic to bacterial systems under specific conditions (Mastrangela et al. 1997). It induces sister chromatid exchanges and inhibits gap junctional intercellular communication (Weis et al. 1998). Most of the hydrocarbons also affect aquatic organisms adversely on metabolic activities (Akhbarizadeh et al. 2016; Arias et al. 2009; Beg et al. 2015; Booij et al. 2016; Houde et al. 2011; Gu et al. 2016; Karacık et al. 2009; Li et al. 2015b; Perhar and Arhonditsis 2014; Perrichon et al. 2015; Van Geest et al. 2011; Xia et al. 2015). Thus, the degradation of various petrochemical wastes is imperative for the conservation of human as well as environmental health. In the present review, we made an effort to collate the information available for the degradation of photocatalysts and the possible mechanisms involved. Environ Sci Pollut Res Scope of the review and bibliometric analysis of the related literature This review focuses on heterogeneous photocatalytic degradation of petrochemical wastes using inorganic semiconductors as catalyst. A bibliometric analysis of the available literature on heterogeneous catalysis of petrochemical wastes has been carried out and presented in Figs. 2 and 3. An exponential increase in the number of publications on the topic has been observed during the recent past. This signifies that problems associated with petrochemical contamination have intensified over the years, which drew due attention of the research community. The bibliometric analysis also revealed that researches on this topic are not country-specific and are not confined to only a few particular research communities. It has been observed that significant number of researchers representing a wide range of countries across the globe is engaged in scientific activities related to this field. Such research attentions illustrate the intensity of the crisis associated with environmental contamination caused by petrochemical wastes on a global scale. It also shows the popularity of photocatalytic remediation approach among the global research community working in this field. Advanced oxidative processes AOPs are evolving as promising techniques towards efficient sequestration of chemically stable and less biodegradable organic pollutants (Hu and Long 2016; Ma et al. 2014; Misra 2015; Mohapatra et al. 2014; Parilti and Atkin 2010; Serpone et al. 2010). These oxidation processes are proved to be another alternative for degradation of petrochemical wastes and are regarded as environmental clean-up technologies which are both efficient and cost effective (Chen et al. 2015a; Ramteke and Gogate 2015; Saritha et al. 2007; Shahidi et al. 700 Number of publicaƟon 600 500 400 300 200 0 China India Spain United States Japan South Korea France Australia Germany Italy Iran Greece Taiwan Hong Kong Brazil United Kingdom Portugal Mexico Canada Switzerland Singapore Saudi Arabia Malaysia 100 Fig. 2 Countries of origin of the publications sampled in the background study of the literature available in the field of photocatalytic degradation of petrochemical waste 2015; Tisa et al. 2014; Vaferi et al. 2014; Zangeneh et al. 2014). AOPs are dependent on generation of highly reactive radical species, mainly the hydroxyl radical (OH°), using various sources of energy. This hydroxyl radical attacks organic compounds and causes their mineralization into CO2, H2O and other inorganic salts (Karci et al. 2013; Xiao et al. 2016). This mechanism is depicted in Fig. 4. The series of reactions that are involved in generation of radical species involve the absorption of radiation quanta by titania and generation of electron-hole pair. The hole breaks the H2O into H+ and OH. radical and electron generates oxygen radical as shown in Fig. 5. Active radical species generated are OH·, HO2· and O2·. These radicals have tendency to oxidize the organic moieties. The most attractive feature of AOPs is that they can degrade a wide range of organic pollutants with no selectivity (Xiao et al. 2016). ðiÞTiO2 Organic contaminants → ðiiÞ O2 ðiiiÞ hυÊEg Intermediates→CO2 þH2 O ð1Þ Thus, AOPs, an economically feasible processes, lead to complete mineralization and production of lesser quantities of sludge from the degradation of various organic contaminants (Akpan and Hamid 2009; Nath et al. 2016; Rajeshwar et al. 2008; Wang et al. 1999). Photocatalytic degradation of petrochemical wastes Among AOPs, heterogeneous photocatalytic oxidation, which involves the acceleration of photoreaction in the presence of a semiconductor catalyst, has proved to be of utmost interest due to its efficiency in degrading recalcitrant organic compounds. Developed in the 1970s, heterogeneous photocatalytic oxidation has been provided considerable attention by the scientific research community. During the past two decades, numerous studies have been carried out on application of heterogeneous photocatalytic oxidation processes with an aim to decompose and mineralize recalcitrant organic compounds. Photocatalysts are the class of compounds which generate electron-hole pairs on coming in contact with or on absorption of light quanta leading to chemical transformation of substrate that comes in contact with them and regenerate them (Jo and Tayade 2016). These substances are invariably semiconductors. Semiconductor heterogeneous photocatalysis has enormous potential to treat organic contaminants in water and air. Many semiconductors have been studied for the degradation of petrochemical compounds. Several semiconductors (TiO2, ZnO, Fe2O3, CdS, ZnS) can act as photocatalysts (synthesis processes of various other semiconductor used for Environ Sci Pollut Res Fig. 3 An approximate evolution of literature available on photocatalytic degradation of petrochemical waste 250 Number of Publication 200 150 100 50 2015 2014 2013 2012 2011 2010 2009 2008 2007 2006 2005 2004 2003 2002 2001 2000 1999 1998 1997 1996 1995 1994 1993 1992 1991 1990 1987 0 year degradation are tabulated in Table 2). However, TiO2 has been most widely and universally studied because of its high reactivity, reduced toxicity, chemical stability, lower costs, its ability to break down organic pollutants and resulting in complete mineralization (Ahmed et al. 2010; Carraro et al. 2014; Hao et al., 2015a, b; Zhu et al. 2010). It shows stability even at low pH. Photocatalytic and hydrophilic properties of TiO2 make it close to an ideal catalyst. Fujishima and Honda (1972) showed the possibility of water splitting in a photoelectrochemical cell containing an inert cathode and rutile titania anode. The applications of titania photoelectrolysis has since been greatly focused for environmental pollution abatement efforts including water and wastewater treatment (Ahmed et al. 2010; Ren et al. 2015a, b; Sood et al. 2015). In addition to these applications, various other applications of TiO2 as photocatalyst for the degradation of various environmental pollutants are discussed later in this paper. The general mechanisms involved for degradation of petrochemicals (organic compounds) by titania nanomaterialbased photocatalysts are shown in Fig. 6. It revealed that in the presence of OH. radicals, the phenolic compound gets converted into polyphenolic products, which further gets Fig. 4 Mechanism of photocatalytic degradation (adapted from Jo and Tayade 2014) converted to aldehydes and carboxylic acids by oxidation and finally gets mineralized to CO2 and H2O. TiO2 as photocatalysts Titania, also known as titanium dioxide (TiO2), is a naturally occurring oxide of transition metal family. It is the most commonly used catalyst in photocatalytic degradation. It has been widely explored than other heterogeneous photocatalysts due to various electrochemical properties, e.g. low cost, chemical stability, non-toxicity, resistance to photo-induced reaction and degrade a wide range of organic pollutants. (Abdullah et al. 2016; Nasir et al. 2016). It is used in various industries, including cosmetics, medicines, textiles, electronics, pharmaceutics, and environmental remediation (Feizi et al. 2012). The applications of titania photoelectrolysis found wider focus for water and wastewater treatment from different environmental pollutants after Fujishima and Honda pioneered work on the possibility of water splitting in an inert cathode and rutile titania anode constituting photoelectrochemical cell. Several studies (Antoniou and Dionysiou 2007; Armelao et al. 2007; Awitor et al. 2008; Bai et al. 2012; Bagheri et al. 2015; Banerjee et al. Environ Sci Pollut Res Fig. 5 Schematic photophysical and photochemical processes over photon-activated semiconductor cluster (p) photogeneration of electron/hole pair, (q) surface recombination, (r) recombination in the bulk, (s) diffusion of acceptor and reduction on the surface (adapted from Gaya and Abdullah 2008) 2015; Carp et al. 2004; Fujishima et al. 2000; Fittipaldi et al. 2011; Fujishima and Zhang 2006; Fujishima et al. 2008; Gondal and Sayeed 2008; Galenda et al. 2014; Lan et al. 2013; Nag et al. 2008; Ortega-Liébana et al. 2012, Ochando-Pulido and Stoller 2015; Quagliarini et al. 2012; Reddy et al. 2015; Ravelli et al. 2010; Sampaio et al. 2013; Singh et al. 2013; Smits et al. 2013; Thompson and Yates 2006; Verbruggen 2015) have been conducted for photodegradation of various pollutants from water using the photocatalytic properties of TiO2, which are outlined in Table 3. ZnO-based photocatalysts ZnO semiconductors-based photocatalysts are widely used for degradation of a wide range of organic pollutant released from the petrochemical waste (e.g., benzene, xylene, phenol, hexane, naphthalene present in water, air and soil (Barreca et al. 2007; Ferrari-Lima et al. 2015a, b; Gondal and Sayeed 2007; Hernandez-Garcia et al. 2016; Silva et al. 2014). However, several factors regulate the photocatalytic degradation efficiency of a catalyst. For example, major factors influencing the catalysts photodegradation efficiency can be their structure, particle size, dispersibility, band gap and the hydroxyl density on the surface (Feng et al. 2014; Pardeshi and Patil 2009; Xu et al. 2009a, b). ZnO-based catalysts have high sensitivity to light and greater band gap (3.37 eV), thus have ability to absorb in a wider range of solar spectrum than other photocatalysts like TiO2 (Pardeshi and Patil 2009). The greater band gap of ZnO helps in better oxidation-reduction activities (Saucedo-Lucero and Arriaga 2013). The photoelectrons and holes are usually generated from ZnO semiconductor material under ultraviolet (UV) light which can be used for degradation of organic pollutants (Malekshoar et al. 2014). Further, this composite displayed high rate of degradation efficiency along with other materials. In addition, the studies revealed better efficiency and electron mobility by ZnO than TiO2 for photocatalytic degradation of various organic pollutants (Saucedo-Lucero and Arriaga 2013) (Malekshoar et al. 2014; Mikami et al. 2016). Silva et al. (2014) developed the highly active gold nanoparticles-loaded ZnO (Au/ZnO) photocatalysts under solar radiation for the mineralization of phenol. Structural differences of ZnO materials are depicted in Fig. 7. They found higher degradation efficiency by Au/ZnO composite photocatalysts than the bare ZnO, possibly due to improved electron sink or light harvesting in the presence of gold particles. However, single-phase semiconductors having fast recombination of the photogenerated electron-hole pairs may lower the photocatalytic efficiency of ZnO-based catalysts (Liu et al. 2013b; Prabha and Lathasree 2014; Yu et al. 2012a, b). CdS-based photocatalysts CdS is also one of the well-studied photocatalysts used for the degradation of organic compounds. It has narrow band gap (2.4 eV) which helps in its greater activity and better degradation potential of various environmental pollutants (Wang et al. 2016b). In addition, it forms heterojunction composite with other photoactive materials which further improves its photocatalytic activities (Fan et al. 2016). Moreover, CdS has been extensively studied for photodegradation of petrochemical waste along with other semiconductors like TiO2, AgCl, ZnO as heterogeneous photocatalytic nanocomposites (Li et al. 2015c). Composite of the CdS with other semiconductor has higher efficiency Environ Sci Pollut Res Table 2 Synthesis processes and calcinations temperature of various catalysts used for degradation of petrochemical wastes Catalyst Synthesis processes Calcinations temperature (°C) Reference H3PW12O40/TiO2/palygorskite composite Al/TiO2 Sol–gel method, dispersed HPW 350 (Ma et al. 2015) TiO2 and Al/TiO2 of different w/w % used for catalyst synthesis by conventional sol–gel method Doping La3+ or Nd3+ onto TiO2 by sol–gel method. Ti(O-Bu)4 and Ln3+ La(NO3)3.6H2O or Nd(NO3)3 6H2O as precursors CCVD method in fluidised bed reactor on a Fe/Al2O3 catalyst Sol–gel method. Precursor solutions for film coating were prepared using tetra-n-butyl titanate Sol–gel method 500 (Lee et al. 2003) 500 (Li et al. 2005) 400 (Wang et al. 2005) 550 (Zhang et al. 2006) 540 (Zou et al. 2006) Ln3+–TiO2 MWNT-TiO2 La-doped TiO2 TiO2–SiO2 TiO2/Sr7CeO4 TiO2 CNT-doped TiO2 SrCO3 and CeO2 were mixed, finely ground for 1 h and fired 500 at 1273 K in ceramic crucible for 4 h in air. Finally, the Sr2CeO4 were obtained by thermal and sol–gel method Sol–gel method 600 NA (Zhong et al. 2007a, b) (Selli et al. 2008) Hydroxide ZnSn(OH)6 CNT–TiO2 composites were prepared from alkoxide precursors using a modified sol–gel method Hydrothermal method (Chen et al. 2009) NA (Fu et al. 2009) TiO2/BaAl2O4:Eu2+,Dy3+ Sol–gel method 500 (Li et al. 2009) Graphite-supported TiO2 Ultrasonication method 400 (Palmisano et al. 2009) Pd/TiO2 Pore impregnating method using Pd(NO3)2 solution 500 (Zhong et al. 2009) S, C, SnO2-codoped TiO2 400 (Chen et al. 2010a, b) TiO2/ZSM-5/silica gel (SNTZS) One pot reaction of TTIP, SnCl4 and thiourea, followed by ageing for 2 days Sol–gel method 600 (Zainudin et al. 2010) Ag-CNT/TiO2 NA 400 (Zhang et al. 2010) TiO2/CexZr1-xO2 Pore impregnating method using TiO2 sol 600 (Zhong et al. 2010) Ti-HMS Sol–gel method NA (Zhuang et al. 2010) BiVO4/TiO2 Sol–gel and hydrothermal method (180 °C) 500 (Hu et al. 2011) Pt/TiO2 A pure TiO2 sample was prepared by calcining the sol–gel-derived TiO2 xerogel at 350 °C for 3 h Sol–gel method followed by annealing at different temperatures Flame CVD method TiO2 TiO2 nanoparticles Palygorskite and SnO2–TiO2 (Li et al. 2011) 500 °C, 700 °C and 900 °C NA (Lin et al. 2011) (Xie et al. 2011) 300 (Zhang et al. 2011) Ag–AgBr–TiO2 SnO2–TiO2 oxides attached on the surface of palygorskite by in situ sol–gel technique Simple deposition–precipitation method 500 (Zhang et al. 2011) Zr-doped TiO2/SiO2 Sol–gel method 500 (Kim et al. 2012) Fe/TiO2 500 (Liu et al. 2012a) CdS-sensitized TiO2 film coated on fibreglass cloth BiPO4 catalysts Sol–gel method, calcination and chemical reductive deposition of Fe Sol gel treatment method 400 (Liu et al. 2012b) Hydrothermal method 750 (Long et al. 2012) Pt-loaded TiO2/ZrO2 Homogeneous co-precipitation method 450 (Ren et al. 2012) W-doped TiO2 Solvothermal method 450 (Sangkhun et al. 2012) CNT/Ce-TiO2 Synthesized via modified sol–gel method 450 (Shaari et al. 2012) Mg-ferrite/hematite/PANI nanospheres Chemisorption method 650 (Shen et al. 2012) TiO2 with Fe2O3 clusters Chemisorption method 400 (Sun et al. 2012) Au/ZnO Fabrication via wet chemical method (Yu et al. 2012a, b) Electrophoretically deposited TiO2 nanoparticles N-doped TiO2 Electrophoretic deposition of titanium dioxide nanoparticles (Banitaba et al. 2013) Solvothermal method using different nitrogen sources 350 (He et al. 2013) Environ Sci Pollut Res Table 2 (continued) Catalyst Synthesis processes Calcinations temperature (°C) Reference Perlite granules coated with In-doped TiO2 Controlled sol–gel method 450 (Hinojosa-Reyes et al. 2013) Ca2Nb2O7 Facile template-free hydrothermal method for the first time 160 (Liang et al. 2013) TiO2/SiO2 nanocomposites Simple reflux-stir method in different ratios of TiO2-SiO2 NA (Liu et al. 2013a) (CM)-n-TiO2 Sol–gel method 500 (Shaban et al. 2013) TiO2/SiC Synthesized by ball-milling and screen printing technique 550 (Zou et al. 2013) Pd-deposited TiO2 Sol–gel dip-coating method 300 (Kim et al. 2014) N–H–TiO2 600 (Li et al. 2014b) NA (Maury-Ramirez et al. 2014) V–N-codoped TiO2 thin film Hydrothermal synthesis followed by a thermal treatment in NH3 and H2 atmospheres Low temperature sol–gel (LTS) and liquid flame spraying (LFS), to develop autoclaved aerated concrete with air-purifying properties RF-magnetron sputtering 500 (Patel et al. 2014) ZnO, TiO2 and ZnO-TiO2 composites Sol–gel method 500 N-doped mixed TiO2 and ZnO Sol–gel method (Prabha and Lathasree 2014) (Ferrari-Lima et al. 2015a, b) 800 (Hao et al. 2015a, b) 500 (Jansson et al. 2015) 1350 (Kushwaha et al. 2015) 600 (Ren et al. 2015a, b) TiO2 coatings concrete TiO2 doped with CeO2 and supported on Facile co-precipitation method SiO2 Incipient wet impregnation method using an acidic TiO2 sol Zeolite–TiO2 hybrid composites precursor and five different zeolites Conventional melt-quench technique and subsequently subTiO2 microcrystallized glass plate jected to controlled heat treatment at an appropriate temperature to growanatase TiO2 microcrystals in the glass matrix Prepared from TiOSO4, SiO2 sol and Bi(NO3)3 via TiO2/SiO2/Bi2O3 co-precipitation method Fabricated via a facile one-pot solvothermal method MnFe2O4 hollow nanospheres NA (Shen et al. 2015) V2O5/TiO2 catalysts 500 (Wang et al. 2015) Fig. 6 Photocatalytic degradation of phenol in wastewater (adapted from Guo et al. 2006) Traditional impregnation method Environ Sci Pollut Res Table 3 TiO2 and other nanoparticles used as catalysts for petrochemicals degradation Catalyst Targeted compound Carbon/nitrogen-doped TiO2 V2O5/TiO2 TiO2 nanoparticles MnFe2O4 Degradation efficiency Light source Reference Phenol 64 (after 30 min) UV (Abdullah et al. 2016) 1,3,5-Trichlorobenzene chlorinated benzenes Gaseous benzene Gaseous benzene 60 58 – – UV (Wang et al. 2015) UV Visible (Wang and Wu 2015a, b) (Shen et al. 2015) TiO2/SiO2/Bi2O3 Benzene 96 (after 7 h) UV-visible (Ren et al. 2015a, b) TiO2 modified by transition metals Gaseous benzene 58 (in 20 min) (Huang et al. 2015) TiO2 nanoparticles doped with CeO2 and supported on SiO2 N-doped mixed TiO2 and ZnO Phenol Vacuum ultraviolet (VUV) Visible (Hao et al. 2015a, b) Visible (Ferrari-Lima et al. 2015a, b) Carbon-doped TiO2 nanoparticles wrapped with nanographene Nano-ZnO, TiO2 and ZnO–TiO2 composite Phenol Visible (Yu et al. 2014) Phenol 70–80 (Prabha and Lathasree, 2014) N–H–TiO2 photocatalyst by annealing in NH3 and H2 Pd-deposited TiO2 fil Benzene 100 (within 3 h) UV light irradiation and direct sun light Visible light irradiation (Li et al. 2014) Gaseous toluene 94 UV254 + 185 nm (Kim et al. 2014) TiO2/SiC nanocomposite fil Toluene 100 UV LED (Zou et al. 2013) TiO2/SiO2 Benzene 92.3 Mercury lamp (Liu et al. 2013a) Ca2Nb2O7 nanopolyhedra and TiO2 Benzene UV (Liang et al. 2013) Perlite granules coated with In-doped TiO2 Ethylbenzene UV (Hinojosa-Reyes et al, 2013) N-doped TiO2 Benzene UV (He et al. 2013) Au/ZnO nanocomposites Benzene UV (Yu et al. 2012a, b) Mg-ferrite/hematite/PANI nanospheres Benzene 62 and 52 Visible (Shen et al. 2012) CNT/Ce-TiO2 Phenol 94 UV (Shaari et al. 2012) 100 BTX W-doped TiO2 BTEX Degussa P25 TiO2 Phenol Pt-TiO2/Ce-MnOx Benzene Pt-loaded TiO2/ZrO2 80 (in 120 min) Visible (Sangkhun et al. 2012) UV (Royaee et al. 2012) (Ren et al., 2012a) 90 Thermo photo (Aarthi et al. 2007) UV (Long et al. 2012) BiPO4 catalysts Benzene CdS-sensitized TiO2 fil Benzene 92.8 UV (Liu et al. 2012a) Fe/TiO2 2,4-Dichlorophenol 97 (in 2 h) UV (Liu et al. 2012b) TiO2-based catalyst BTEX UV (Korologos et al. 2012) TiO2-based catalyst BTEX UV (Korologos et al. 2012) Zirconium-doped TiO2/SiO2 Toluene and xylene UV (Kim et al. 2012) Ag–AgBr–TiO2 Benzene UV-visible (Zhang et al. 2013) SnO2–TiO2 Phenol UV (Zhang et al. 2011) TiO2 Benzene UV (Xie et al. 2011) TiO2 Phenol UV (Lin et al. 2011) Pt/TiO2 Benzene 86 UV (Li et al. 2011) BiVO4/TiO2 Benzene 92 Visible (λ > 450 nm) (Hu et al. 2011) Sun light 99.8 Micro-nanosized TiO2 Pyrene 78.3 TiO2 Benzene 60–70 Ti-HMS Benzene UV (4 W) TiO2 nanoballs Gaseous benzene UV TiO2/CexZr1-xO2 Benzene (Chang et al. 2011) (Bui et al. 2011) (Zhuang et al. 2010) (Aarthi et al. 2007) (Zhong et al. 2010) Environ Sci Pollut Res Table 3 (continued) Catalyst Targeted compound Degradation efficiency Nano-TiO2/ZSM-5/silica gel (SNTZS) Phenol 90 CNT in a TiO2 matrix Ag4V2O7 and Ag3VO4 phases Benzene aqueous phase Benzene S, C, SnO2-codoped TiO2 Phenol Pd/TiO2 Benzene Light source UV (Zainudin et al. 2010) near-UV to visible (Silva and Faria 2010) (Chen et al. 2010) Visible (Chen et al. 2010) UV. (Zhong et al. 2009) (Palmisano et al. 2009) 48 60 Reference Graphite-supported TiO2 4-Nitrophenol UV TiO2/BaAl2O4:Eu2+,Dy3+ Benzene UV (Li et al. 2009) Hydroxide ZnSn(OH)6 Benzene UV (under 254 nm) (Fu et al. 2009) CNT-doped TiO2 electrodes Phenol Visible (Chen et al. 2009) TiO2 catalyst UV (Bougheloum and Messalhi 2009) TiO2 Phenol, chlorobenzene and toluene in aqueous medium 1,4-Dichlorobenzene UV (Selli et al. 2008) TiO2-based materials Propene and benzene UV (Bouazza et al. 2008) TiO2/Sr2CeO4 Benzene over UV (Zhong et al. 2007a) TiO2/Sr2CeO Benzene 58 (Zhong et al. 2007b) TiO2 on perlite granules Phenol UV (Hosseini et al. 2007) TiO2–SiO2 catalyst Toluene UV (Zou et al. 2006) La-doped TiO2 fil Benzene UV (Zhang et al. 2006) MWNT-TiO2 Phenol Ln3+–TiO2 VOC BTEX TiO2 catalyst Toluene Al/TiO2 nanometre Benzene H3PW12O40/TiO2/palygorskite Benzene for the photo-degradation of organic pollutants (Fan et al. 2016; Gupta and Pal 2014; Hernández-García et al. 2015; Kim and Kan 2015; Meng et al. 2012; Wang et al. 2011, 2016b; Wilson et al. 2012; Zhu et al. 2012). It has almost similar band structure and other photocatalytic properties as that to TiO2; thus, CdS/TiO2 composite has also been extensively studied in several photodegradation studies (Kim and Kan 2015). Generally, CdS get exited under visible light irradiation which limits its application in this spectral region. The dual advantage of such composites (CdS/TiO2) involves a mechanism of photodegradation under which the photoexcited electron in the conduction band of CdS get transferred to the conduction band of TiO2 leaving behind a hole in the valence bond of CdS. Thus, it helps in photodegradation under visible range as well as improves the organic pollutant degradation efficiency of the composite material (Hernández-García et al. 2015). Dong et al. (2015) synthesized the TiO2 (thickness 3.5–40 nm) coated on CdS nanorods surface under mild conditions (shown in Fig. 8). It enhances the photo-activity as compared to the bare TiO2 and CdS photocatalysts. 95 Visible (Wang et al. 2005) UV (Li et al. 2005) 46 UV (Jeong et al. 2004) 60 UV (Lee et al. 2003) (Ma et al. 2015) Factors affecting the photo-degradation performances Catalyst loading Various scientific studies and reports show that the amount of catalyst plays a vital role in the degradation of organic pollutants, mostly in aqueous phase. The amount of catalyst used in the degradation processes is directly proportional to the overall degradation rate of organic compound (Akpan and Hameed 2009; Jain and Srivastava 2008; Diyauddeen et al. 2011; Prabha and Lathasree 2014). However, increasing concentration of catalysts beyond certain limits would not result in any significant change in the rate of degradation processes (Alhakimi et al. 2003), and even decreases light penetration and photoactivated volume by increasing the turbidity of the solution (Akyol et al. 2004). It is also postulated that the excessive use of catalyst causes light scattering and a screening effect, thus, reduces the specific activity. In this case catalyst surface becomes unavailable for Environ Sci Pollut Res Fig. 7 TEM (a), (c), (e) and (g), STEM (b), (d) and (f) and HRTEM (h) micrographs of Au/ ZnO-niddle like (a) and (b), Au/ ZnO rods (c) and (d), Au/ZnOflowers like (e) and (f), and Au/ ZnO-t (prepared by solid state thermal processes) (g) and (h), respectively. Adapted from (Silva et al. 2014) with permissions photon absorption and pollutant adsorption which reduces the reaction rate (Gao et al. 2007; Gaya and Abdullah 2008). Similar phenomena have also been reported in several other studies (Mehrotra et al. 2003; Heredia et al. 2001; Saien et al. 2003; Saien and Soleymani 2007; Yatmaz et al. 2004). However, in the case of TiO2, its excessive use as a catalyst causes the shielding effect. TiO2 * þ TiO2 → TiO2 # þTiO2 ð2Þ Here, TiO2* is the active species which adsorbs on surface, while TiO2# is the deactivated form (Harmann 1995; Tayade et al. 2009). Therefore, to avoid the use of excess catalyst, it is imperative to optimize the amount of catalysts that is actually needed to obtain the required degradation rate (Tayade et al. 2009). pH of the solution The effect of pH of the solution on photocatalytic degradation of organic pollutants and their adsorption on the catalyst surface has been studied by many investigators (Akpan and Hameed 2009; Wang and Ku 2007). pH is an important parameter because of the fact that different effluents need to be treated at different pH values. Further, pH regulates the surface charge properties of the photocatalyst and size of the aggregates they form. For example, the surface of TiO2 remains positively Environ Sci Pollut Res Fig. 8 TEM images of (a) CdS and (b)–(f) CdS@TiO2 nanorods; the inset is corresponding HRTEM image of CdS (Adapted from Dong et al. (2015) charged in acidic medium (pH less than 6.9) and negatively charged in alkaline medium (pH greater than 6.9) due to its amphoteric nature. et al. 2011; Fathinia and Khataee 2015; Mansilla et al. 2006; Saien and Shahrezaei 2012). TiOH þ Hþ →TiOH2 þ ð3Þ Concentration and nature of pollutants TiOH þ OH‐ →TiO‐ þH2 O ð4Þ The effect of the concentration and nature of organic pollutants on the rate of photocatalytic degradation has been widely studied (Malekshoar et al. 2014). The organic compounds which have greater tendency to bind to the catalyst surface, Natarajan et al. (2011) studied the effect of pH in dye degradation and found that when the pH of the solution is decreased from 6.5 to 2.9, the percentage of degradation and decolonization decreases too. This must be due to the acidic solution retraining the adsorption of dye. Also, when the pH value of the solution is increased from 6.5 to 9.8, it is observed that up to pH 0.8 of the solution, there is an increase in the percentage of degradation and decolorization. This is because of the fact that in alkaline medium, the surface of photocatalyst possesses much negative charge (Tayade et al. 2009). As shown in the Fig. 9, the pH has strong effect on the toluene degradation. Sharma and Lee (2016) studied the effect of pH on toluene degradation and found the enhanced degradation on acidic pH 0.5 (Fig. 9). It is due to the surface properties of catalyst TiO2 and metal oxides-doped carbon sphere. Thus, optimizing it for maximum efficiency is essential. Nevertheless, the process of optimization of the pH value is difficult because it is related to the ionization state of the catalyst surface as well as on the pollutant (Diya’uddeen Fig. 9 The effect of solution pH on the catalytic removal of toluene under visible light. Concentration of toluene = 50 mg/L, amount of composite material = 0.05 g, reaction volume = 100 mL, reaction time = 4 h, temperature = 30 °C. (Adapted from Sharma and lee (2016) with permission) Environ Sci Pollut Res such as aromatic hydrocarbons, are more prone to be oxidized (Pardeshi and Patil 2009). However, it majorly depends on substituent group of these organic pollutants. Further, the high concentration of pollutants in aqueous medium saturates the catalyst surface, which reduces photonic efficiency, and thus, ultimately causes deactivation of photocatalysts (Lee et al. 2016; Prabha and Lathasree 2014). Therefore, it can be concluded that photocatalytic degradation of organics (aromatic and non-aromatic) depends on the substitution group. Light/radiation source for photocatalysis Photocatalytic degradation techniques require high-energy sources. UV radiation and sunlight–both can be used for photocatalytic process; however, UV radiation is mostly used. Various recent studies have been conducted on LEDs as light source for photodegradation of organic compounds, mainly in aqueous and air medium (Chen et al. 2007; Daniel and Gutz 2007; Tayade et al. 2009; Wan-Kuen Jo et al. 2014). Photocatalysis reactions depend mainly on the radiation absorption by catalysts, which, in turn, relies on the intensity of light. Generally, an increase in light intensity increases the rate of degradation of organic pollutants. This is due to an increase in photon flux of electrons in conduction band (Vohra and Tanaka 2002). The photon flux enhances the degradation due to favourable collision chances between photon and activated centre. Various other studies also support that an increase in intensity of light leads to an increase in the degradation rate of organic pollutants (Karunakaran and Senthilvelan 2005; Qamar et al. 2006). The nature or form of light does not affect the pathway of reaction (Gaya and Abdullah 2008). The economy of photocatalytic degradation can be enhanced further if sunlight is used instead of UV and LEDs for excitation of photocatalysts (Rajeshwar et al. 2008). Only 5 % of the total sunlight energy is sufficient for photosensitization. Energy losses due to various factors like light reflection, transmission and loss in the form of heat must be anticipated in photocatalytic processes. These limitations are some of the challenges in the application of TiO 2 as a photocatalyst for the degradation of organic compounds (Gaya and Abdullah 2008). reactors can be divided into concentrating or nonconcentrating reactors. Both the reactor types possess certain advantages and disadvantages. Photocatalytic reactors with immobilized TiO2 are those in which catalyst is fixed to support via physical surface forces or chemical bonds. These reactors extend the benefit of not requiring catalyst recovery and permit the continuous use of the photocatalysts. Figure 10 shows one such reactor. Hybrid photocatalytic membrane reactors have been developed to achieve the purpose of downstream separation of photocatalyst (Guo et al. 2015; Molinari et al. 2015a, b; Molinari et al. 2014; Molinari et al. 2013; Mozia 2010; Mozia et al. 2014; Ong et al. 2014). The photocatalytic membrane reactors can be generalized in two categories–irradiation of the membrane module and irradiation of feed tank containing photocatalyst in suspension. Various membranes such as microfiltration, ultrafiltration and nanofiltration membranes may be used for this purpose depending on the requirements of the treated water quality. Membrane photoreactors appear to be a promising alternative to conventional photoreactors, and more research in this area can assist to overcome some of the problems faced with the use of conventional reactors (Khan et al. 2015; Kim et al. 2014; Motamed Dashliborun et al. 2013; Mozia et al. 2014; Mozia et al. 2013; Nasir et al. 2016; Szymański et al. 2015). Eco-toxicity Various studies report that the intermediates are sometimes more toxic than the original compound. Intermediates or end products of photocatalytic degradation may be toxic to various organisms in the environment. Therefore, besides the advancement in the materials as the catalysts for degradation Reactors used in photocatalytic degradation Photocatalytic reactors can be classified based on the deployed state of the photocatalyst, i.e., by whether it is suspended or attached. Photocatalytic reactors can use either UV or solar radiation. Solar photocatalytic reactors have been of great interest for the photo-oxidation of organic contaminants in water (Petit et al. 2007; Sarkar et al. 2015; Sia et al. 2015; Vincent et al. 2009; Yao and Kuo 2015; Wang and Ku 2003; Romero et al. 1999; Bahnemann 2004). Such kind of Fig. 10 Reactor used for photocatalytic degradation using LEDs (Adapted from Jo and Tayade 2014) Environ Sci Pollut Res of persistent organic pollutant from water and air, their ecotoxicity should be considered too. Various studies supported that TiO2 increases the production of various crops and also improves essential element content in plant tissue by increasing peroxidase, catalyse and nitrate reductase activity in plant tissues and enhancing their chlorophyll content (Feizi et al. 2012; Hruby et al. 2002). Adam et al. (2015) studied the effect of ZnO and CuO metal oxide nanoparticles on species sensitivity distributions and found that the ZnO nanoparticles, bulk material and zinc salt cause comparable toxicity whereas CuO nanoparticles are more toxic than bulk material but less toxic than copper salt. A significant number of studies have also proved the toxicity of ZnO and CuO nanoparticles to aquatic organisms. The acute t o x i c i t y o f Z n O n a n o p a r t i c l e s ( L ( E ) C 5 0 , 0 . 0 5– 1000 mg L−1) had already been studied for bacteria, algae, protozoa, nematodes, crustaceans and fishes (Aruoja et al. 2009; Bai et al. 2010; Franklin et al. 2007; Heinlaan et al. 2008; Mortimer et al. 2010; Wang et al. 2009; Wiench et al. 2009; Zhu et al. 2008). CuO nanoparticles (E(L)C50, 0.05– 569 mg L−1) have also been observed to be toxic (Aruoja et al. 2009; Heinlaan et al. 2008; Jo et al. 2012; Mortimer et al. 2010; Sovova et al. 2009). The toxicity of titania on microalgae and yeast has been studied by Al-Awady et al. (2015) and Adam et al. (2015). Phototoxic nano-TiO2 has yielded puzzling results because of its both positive and negative effects ranging from strong toxicity to positive effects (Song et al. 2013a). A wide range of studies (such as Clément et al. 2013; Dehkourdi and Mosavi 2013; Feizi et al. 2012, 2013; Song et al. 2013a) also support the positive effects of these nanoparticle-based photocatalysts on the plants due to their particle size and crystalline structure. They increase the germination rate in various plants up to certain concentration (Singh et al. 2016). However, several researchers (such as García et al. 2011; Mushtaq 2011; Jośko and Oleszczuk 2013; Song et al. 2013b) studied the negative effects of nano-TiO2 particles on plants which include inhibition of root growth in few plants. However, wider exploration of the effects of photocatalysts and the end products of photocatalytic degradation on plants and soil ecology is still lacking (Lin and Xing 2007). for efficient degradation of monocyclic and polycyclic petrochemical wastes. These catalysts are used in the photocatalytic processes since there is a need to reduce the toxic effect of the catalysts. The available literature revealed that TiO2, along with other semiconductors/nanomaterials, has been widely studied for the degradation processes in visible and UV light irradiation. In addition, most of the degradation studies were carried out at laboratory scale under UV light. The uses of alternative light sources such as LEDs are promising initiatives for reduction in power consumption. It is imperative to develop more efficient techniques in which solar energy is used for photocatalysis processes. Presently, only 5 % of solar radiation is used for catalysis processes. We also focussed on the ecotoxicity aspect of nanocatalysts. Various studies reported that the intermediates are sometimes more toxic than the original compound. Therefore, intermediates or end products of photocatalytic degradation may prove harmful to various organisms in the environment. Thus, besides the advancement in the materials as the catalysts for degradation of persistent organic pollutant from water and air, their eco-toxicity should also be considered. It is essential to develop environmentfriendly photocatalysts or hybrid processes and models which produce non-toxic/less toxic intermediates for wide applicability. Acknowledgments The authors thankfully acknowledge Indian Institute of Technology (Banaras Hindu University) Varanasi, and University Grant Commission , New Delhi for providing financial support as fellowship. Compliance with ethical standards There are no conflicts of interest among authors. There is no involvement of human or animal cell in this work. All the co-authors have seen the final manuscript and agreed with the submission to the journal. The manuscript has not been published elsewhere. The article is not consideration under any other journal in full or in part. The data or figures have been used with permission. 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