Detoxification of water and wastewater by advanced oxidation processes

Science of the Total Environment 696 (2019) 133961

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Review

Detoxification of water and wastewater by advanced oxidation processes D. Syam Babu a, Vartika Srivastava a, P.V. Nidheesh b,⁎, M. Suresh Kumar a,b a b

Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India Environmental Impact and Sustainability Division, CSIR-National Environmental Engineering Research Institute, Nagpur, Maharashtra, India

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Detoxification of wastewater by AOPs are detailed. • AOPs are effective to reduce the toxicity of wastewater. • AOPs are also effective to reduce arsenic toxicity.

a r t i c l e

i n f o

Article history: Received 26 June 2019 Received in revised form 16 August 2019 Accepted 16 August 2019 Available online 17 August 2019 Editor: Yifeng Zhang Keywords: Advanced oxidation process Detoxification Arsenic toxicity Bioassay

a b s t r a c t Nowadays there is a continuously increasing attention for the treatment of recalcitrant compounds present in water and wastewater due to their toxicity on both human health and the environment. Advanced oxidation processes (AOPs) are found to be effective for the degradation of recalcitrant compounds by increasing biodegradability and reducing toxicity. The present review focuses on the detoxification aspects of AOPs with special emphasis on arsenic toxicity. Different bioassays employing bacteria, invertebrates, algae, plants, and fish have been critically reviewed in this article as a valuable tool for assessing the toxicity as well as biodegradability of the industrial wastewater post AOP treatment. Various toxicity tests employed during AOP treatment of wastewater with high toxicity revealed that AOPs are effective for reducing their toxicity significantly. These processes are also effective to reduce arsenic toxicity by oxidizing arsenite to arsenate. By-products formed during AOP treatment of wastewater are also found more toxic than its parent compound. Thus, toxicity tests are essential for AOP treated wastewater before its disposal. © 2019 Elsevier B.V. All rights reserved.

⁎ Corresponding author. E-mail address: [email protected] (P.V. Nidheesh).

https://doi.org/10.1016/j.scitotenv.2019.133961 0048-9697/© 2019 Elsevier B.V. All rights reserved.

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D.S. Babu et al. / Science of the Total Environment 696 (2019) 133961

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . Concept of toxicity . . . . . . . . . . . . . . . . . . . . 2.1. Test for finding toxicity . . . . . . . . . . . . . . . 2.1.1. Bacterial bioassays . . . . . . . . . . . . . 2.1.2. Invertebrate bioassays . . . . . . . . . . . 2.1.3. Algae . . . . . . . . . . . . . . . . . . . 2.1.4. Plants . . . . . . . . . . . . . . . . . . . 2.1.5. Fish bioassay. . . . . . . . . . . . . . . . 2.2. Detoxification by AOPs . . . . . . . . . . . . . . . 3. Arsenic toxicity reduction by AOP treatment . . . . . . . . . 3.1. Arsenic toxicity. . . . . . . . . . . . . . . . . . . 3.1.1. Acute toxicity . . . . . . . . . . . . . . . 3.1.2. Chronic toxicity . . . . . . . . . . . . . . 3.1.3. Effects of arsenic on immune cells . . . . . . 3.2. Advanced oxidation methods for treatment of arsenite . 4. Conclusions and future perspectives. . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction Advanced oxidation processes (AOPs) are water and wastewater treatment techniques which utilize in-situ generated hydroxyl or sulfate radicals for the degradation of organic pollutants existing in the aqueous medium. Hydroxyl radical based AOPs are most common and widely accepted due to the superior oxidation potential of hydroxyl radical (E°(•OH/H2O) = 2.8 V), which is just below the oxidation potential of fluorine (Bethi et al., 2016; Brillas et al., 2009; Ghatak, 2014). Sulfate radical based AOPs have also got much attention due to their efficiency to degrade most of the organic pollutants. This is due to the fact that sulphate radicals have higher efficiency than hydroxyl radicals (Nidheesh and Rajan, 2016) due to the life-span of sulfate radicals (30–40 μs) which is greater than hydroxyl radicals (20 ns). Major AOPs and their properties are detailed in Table 1. The complete mineralization of pollutants was observed non-selectively with the help of hydroxyl radicals. Organic pollutants are degraded via dehydrogenation, redox reaction and hydroxylation reactions (Oturan, 2000). AOPs are found to be effective for the degradation of all the ‘dirty dozen’ including aldrin (Bandala et al., 2002; Kurakalva, 2016; Kusvuran and Erbatur, 2004), chlordane (Moradas et al., 2008), DDT (Villa et al., 2010), dieldrin (Kida et al., 2018; Ormad et al., 2010), endrin (Ormad et al., 2010; Riaz and Bilal Butt, 2010), heptachlor (Ormad et al., 2010), hexachlorobenzene (Jin et al., 2012), mirex (Kida et al., 2018), toxaphene (Ikehata et al., 2008), polychlorinated biphenyls (Oturan et al., 2015), polychlorinated dibenzo-p-dioxins (Katsumata et al., 2007; Lee et al., 2009; Vallejo et al., 2013) and polychlorinated dibenzofurans (Vallejo et al., 2015). These processes are also found effective to degrade chlorophenols (Sharma et al., 2013), dyes (Chakma et al., 2015; Guin et al., 2017; Nidheesh et al., 2014; Rokesh et al., 2018), ciprofloxacin (Mondal et al., 2018), carbamazepine (Thanekar et al., 2018), ethyl paraben (Dhaka et al., 2018), salicylic acid (George et al., 2014, 2016), benzene (Ramteke and Gogate, 2015), toluene (Ramteke and Gogate, 2015), naphthalene (Ramteke and Gogate, 2015), o-xylene (Ramteke and Gogate, 2015), amoxicillin (Verma and Haritash, 2019), atenolol (Hussain et al., 2013), 4-chloro-2 nitrophenol (Saritha et al., 2007), bisphenol A (Sharma et al., 2015), rabeprazole-N-oxide (Shankaraiah et al., 2014), 2, 4 dichlorophenol (Dixit et al., 2011), dichlorvos (Patil and Gogate, 2015), 2,4-dinitrophenol (Bagal and Gogate, 2013), imidacloprid (Patil et al., 2014), pyridine (Gosu et al., 2018), ciprofloxacin (Giri and Golder, 2019; Yahya et al., 2014) and diphenylmethanol (Menachery et al., 2018) from aqueous medium. The processes are also effective to treat municipal wastewater

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2 4 4 6 7 7 7 7 7 12 12 12 12 12 13 14 14 14

(Mondal et al., 2019; Singh et al., 2013), textile wastewater (Dinesh et al., 2016; Khatri et al., 2018; Kumar et al., 2018a, 2018b; Nidheesh and Gandhimathi, 2014), tannery wastewater (Sivagami et al., 2018), landfill leachate (Asha et al., 2017; Baiju et al., 2018; Joshi and Gogate, 2019; Laiju et al., 2014; Venu et al., 2014), coke oven wastewater (Sharma and Philip, 2016), alkyd resin wastewater (Kausley et al., 2018), pharmaceutical wastewater (Changotra et al., 2019; Talwar et al., 2018), petroleum refinery effluent (Doltade et al., 2019), composite wastewater (Kumar et al., 2018a, 2018b), slaughterhouse wastewater (Davarnejad and Nasiri, 2017), winery wastewater (Amor et al., 2019) and carwash wastewater (Ganiyu et al., 2018). As compared to other conventional water and wastewater treatment, one of the properties of AOPs is that during the mineralization, the biodegradability of the wastewater enhances with AOP treatment (Nidheesh and Gandhimathi, 2015; Roshini et al., 2017). Detoxification of the wastewater is yet another property of AOPs (Sharma et al., 2018) owing to their ability to degrade highly toxic organic pollutants to a much lesser toxic organic compound. This article reviews the detoxification during the AOP treatment of water and wastewater. Apart from reducing the toxicity of organic pollutants, AOPs are also found to be effective for reducing arsenic toxicity by oxidizing arsenite to arsenate. Arsenic poison is one of the serious problems that the world is facing (Mandal and Suzuki, 2002). Arsenite is more toxic than the arsenate as it is 60 times or more mobile in nature (Vaclavikova et al., 2008), whereas organic arsenic is non-toxic (Gupta and Chatterjee, 2017; Hughes, 2002; Mandal and Suzuki, 2002; Wang and Mulligan, 2006). Most of the studies indicate that arsenic, due to high reducing conditions in subsurface water, exist as arsenite in dominant form which is as much as 50 to 90% of the total arsenic concentration (Ahmad et al., 2018; Guo et al., 2014; Mukherjee and Bhattacharya, 2001; Si et al., 2017; Wei et al., 2018). Researchers are using several methods and process to remove the arsenic contamination and their major focus are on separation techniques includes chemical coagulation, lime-softening, sedimentation, filtration, adsorption and ion exchange methods, which are able to remove b30% arsenite and b60% of arsenate from the aqueous medium (Feenstra and Erkel, 2007). Thus converting the highly toxic trivalent arsenic to less toxic pentavalent arsenic becomes mandatory before proceeding with separation techniques (Bissen et al., 2003; Feenstra and Erkel, 2007; Nidheesh and Singh, 2017; Si et al., 2017; Wang et al., 2014; Zhao et al., 2010). Recent reviews on the application of AOPs for organic pollutant removal as well as the removal of arsenic from water medium by various methods are given in Table 2. Most of the articles are a focus on the

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Table 1 Details of previous articles published on description of major AOPs used for water and wastewater treatment. Si. Process no.

Description of the process

Major reactions

Recent improvements in the process

Recent review articles published on the process

1.

This process generates hydroxyl radical by the reaction between ferrous ion and hydrogen peroxide at pH near to 3.

Fenton reaction: Fe2+ + H2O2 → Fe3+ + OH− + HO• Regeneration of ferrous ion: Fe3+ + H2O2 ⟶ Fe2+ + H+ + HO•2

Heterogeneous Fenton process: Use solid catalyst instead of soluble ferrous ion. Effective at wide pH range.

Homogeneous Fenton process: Nidheesh et al. (2013), Neyens and Baeyens (2003), Pignatello et al. (2006), Babuponnusami and Muthukumar (2014)

Fenton

2.

Anodic oxidation

Hydroxyl radicals are generated by water oxidation in presence of high O2 evolution overvoltage anodes

3.

Electro-Fenton

It is an extended version of Fenton process in which hydrogen peroxide is generated in the electrolytic medium by supplying oxygen at the cathode surface in acidic condition.

Water oxidation at the anode surface: M + H2O ⟶ M (HO•) + H+ + e− M: Anode In-situ generation of hydrogen peroxide: O2 + 2H+ + 2e− ⟶ H2O2 Regeneration of ferrous ion by cathodic reduction of ferric ion: Fe3+ + e− → Fe2

Fenton like process: Use heavy metals Heterogeneous Fenton process: other than ferrous ions Nidheesh (2015), Rahim Pouran et al. Fluidized Fenton process: Iron sludge (2014), Rusevova et al. (2012) formed during the Fenton process is Fluidized bed Fenton process: attached to the carriers provided in fluidized bed reactor. Thus the sludge Garcia-Segura et al. (Garcia-Segura et al. 2016) will act as heterogeneous Fenton catalyst. More efficient anodes like Pt and Panizza and Cerisola (2009), Sirés boron doped diamonds are used et al. (2014), Chaplin (2014), Efficiency of anodes improved by Martínez-Huitle and Brillas (2009), adding suitable substrates Martínez-Huitle and Ferro (2006)

Heterogeneous electro-Fenton processes Peroxicoagulation process: Use iron or stainless steel as anode for in-situ generation of ferrous ion

Electro-Fenton process: Nidheesh and Gandhimathi (2012), Brillas et al. (2009), Nidheesh et al. (2018a, 2018b), Vasudevan and Oturan (2014), Rodrigo et al. (2014), Oturan and Aaron (2014)

Anodic Fenton process: Add hydrogen Nidheesh (2018) peroxide externally to the electrolytic system

+

4.

Photo-catalysis

Generates hydroxyl radicals and other reactive oxygenated species by irradiating UV light over semiconductor catalyst like TiO2, ZnO and ZnS

5.

Photo-Fenton

It is combination of photolysis and Fenton process in hydroxyl radicals are generated by Fenton process as well as degradation of hydrogen peroxide in the presence of UV light

6.

Ozonation and catalytic ozonation

Photo-excitation reaction: S + hν ⟶ S(e− + h+ ) Water ionization: S(h+) + H2O ⟶ H+ + HO• Oxygen ionosorption: S(e−) + O2 ⟶ O•− 2 Superoxide protonation reactions: + O•− 2 +H ⟶ HO•2 HO•2 + S(e−) + H+ ⟶ H2O2 H2O2 + S(e−) ⟶ OH− + HO• S: Semiconductor Photolysis: H2O2 + hν ⟶ 2HO• (λ b 300 nm)

Ferrous ion regeneration: Fe(OH)2+ + hν ⟶ Fe2+ + HO• (λ b 450 nm) Catalytic Organic pollutants can be degraded either by direct attack of ozone or by ozonation hydroxyl radical generated at alkaline (example): Fe2+ + O3 + pH (decomposition of ozone). Unfortunately, in acidic condition, H2O → Fe3+ + formation of hydroxyl radicals is OH− + HO • +

Supporting materials like graphene is used for reducing recombination between hole and electron

Kumar and Devi (2011) Kabra et al. (2004) Nidheesh (2017) Gandhi et al. (2016) Hashimoto et al. (2005)

Heterogeneous photo-Fenton process

Herney-Ramirez et al. (2010) Soon and Hameed (2011) Clarizia et al. (2017) Giannakis et al. (2016)

Solar photo-Fenton process Acceleration of photo-Fenton reactions by using ferrioxalate as mediator

Heterogeneous catalytic ozonation process

Ozonation: Khamparia and Jaspal (2017), Agustina et al. (2005)

Photocatalytic ozonation

Catalytic ozonation: Nawrocki (2013), Legube and Leitner (1999), Nawrocki and Kasprzyk-Hordern

Ozone/H2O2 process

(continued on next page)

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D.S. Babu et al. / Science of the Total Environment 696 (2019) 133961

Table 1 (continued) Si. Process no.

7.

8.

9.

Sonochemical methods

Sono-Fenton

Persulfate/peroxymonosulfate oxidation

10. Zero valent metal/H+/O2

Description of the process

Major reactions

quite less. This disadvantage of ozonation is rectified in catalytic ozonation in which catalyst are able to decompose ozone even in lower pH conditions.

O2

Acoustic cavitation is the main phenomenon responsible for the degradation of organic pollutants. Cavitation of micro-bubbles in water medium generates high temperature and pressure, thus pollutants are removed either by homolytic bond breaking or by the attack of hydroxyl radicals and other reactive oxygen species generated via water dissociation Utilizes the effectiveness of Fenton and sonolysis for pollutant degradation.

Thermal dissociation of water in presence of ultrasound: H2O + ))) → HO • + H•

Sulfate radicals are generated by the decomposition of persulfate or peroxymonosulfate. This reaction is accelerated by the presence of activating agents like heavy metals, UV light, ultrasound and heat.

In acidic medium, zero valent metals like iron and aluminium undergo corrosive oxidation and generate hydrogen peroxide. This hydrogen peroxide further decomposes in the presence of zero valent metal and generates hydroxyl radicals.

Recent improvements in the process

Electroperoxone process: It is a combination of ozonation and electro-Fenton process

Ultrasound/H2O2 process Ultrasound/O3 process Sono-photo-catalysis Sono-electro-chemical process

Fenton reaction

Sono-photo-Fenton process

Thermal dissociation of water Persulfate activation by heavy metal: 2+ S2O2− 8 + Fe ⟶ Fe3+ + SO2− 4 + SO4•−

Sono-electro-Fenton process

Corrosive oxidation of zero valent metal: 2Al0 + 3O2 + 6H+ ⟶ 2Al3+ + 3H2O2 Decomposition of hydrogen peroxide: Al0 + 3H2O2 ⟶ Al3+ + 3OH− + 3HO●

Recent review articles published on the process (2010), Kasprzyk-Hordern et al. (2003) Photocatalytic ozonation: Mehrjouei et al. (2015) Electroperoxone process: Turkay et al. (2017), Wang et al. (2018) Gogate and Pandit (2004a, 2004b) Adewuyi (2005) Joseph et al. (2009) Muthupandian Ashokkumar and Mason (2007) Gogate and Pandit (2004a, 2004b)

Bagal and Gogate (2014) Nidheesh et al. (2013) Eren (2012)

Ike et al. (2018) Activation of persulfate or peroxymonosulfate by heterogeneous Duan et al. (2015) Matzek and Carter (2016) catalysts In-situ electrolytic generation of sulfate radicals by the addition of sulfate ions and in the presence of high oxygen over-voltage potential anodes like boron doped diamond and Pt. ZVM/H+/O2/EDTA process: Addition of EDTA enhances hydrogen peroxide production

Raman and Kanmani (2016) Bokare and Choi (2014) Nidheesh et al. (2018a, 2018b)

ZVM/H+/O2/ultrasound process ZVM/H+/O2/heavy metal process ZVM/H+/O2/Polyoxometalate process: Polyoxometalate addition increase hydrogen peroxide production as well as electron transfer rate Where, ZVM: Zero valent metal

efficiency of AOPs whereas a few of them deal with the pollutant removal by AOPs combined with other processes. To the best of our knowledge, we could not find any article, which deals purely on the toxicity changes during AOP treatment. Similar to this, the review articles on arsenic removal are focused on total arsenic reductions by various methods. A few articles mention the application of AOPs for arsenic removal. Apart from this, this review is not a mere compilation of the different AOPs, however, it critically focuses on the associated challenges as well as the toxicity tests which are employed post AOP treatment. Recent studies reported on this theme are summarized and a special focus has been given to explain the challenges during detoxification by AOPs. 2. Concept of toxicity Unattended discharges of toxic pollutants in the environment pose a great threat to terrestrial as well as the aquatic environment (Brack et al., 2016). As the toxicity generally refers to the level of a toxic chemical to elicit damage to the living organism, assessment of some conventional parameters like total suspended solid, total organic carbon, biochemical oxygen demand, and chemical oxygen demand is not enough for assessing the environmental risk of the pollutant to the

environment. Hence it is imperative to test the toxicity of the wastewaters which are being discharged. AOP in combination with a biological treatment involves the application of toxicity assays for determining the potent biodegradability and effluent toxicity prior to their further biological treatment for complete mineralization. However, during the AOP, the incomplete oxidation of the contaminants sometimes give rise to the intermediates which are often more toxic than the parent compound itself (Kim et al., 2007). Acute toxicity testing is hence often employed during different stages of the AOP treatment utilizing different organisms and exposing them for at least 96 h (United States Environmental Protection Agency, 2002). Moreover, toxicity testing also helps in demonstrating the utility of AOPs in reducing the toxicity of the effluents for their safe disposal in the environment (Oller et al., 2011). 2.1. Test for finding toxicity Toxicity tests/bioassays, as a complex mixture of the different chemicals on the living organisms, are used to assess the toxicity of the effluent. A wide range of living organisms coming under different taxonomic groups have been utilized for this purpose. The different

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Table 2 Recent reviews on application of AOPs for organic pollutant removal as well as removal of arsenic from water medium by various methods. Title of the review article

Contaminants

Process used AOPs

1. Evaluation of advanced oxidation processes for water and wastewater treatment - A critical review

Water and Wastewaters

2. Advanced oxidation processes for the removal of natural organic matter from drinking water sources: A comprehensive review 3. Recent advances in application of UV light-emitting diodes for degrading organic pollutants in water through advanced oxidation processes: A review 4. Electrochemical advanced oxidation processes: A review on their application to synthetic and real wastewaters

Natural organic contaminates

5. A review on advanced oxidation processes for the removal of taste and odor compounds from aqueous media

Taste and odor compounds (Geosmin, Methylisoborneol, Benzothiazoles, Mercaptans, and Sulfide)

6. Combination of Advanced Oxidation Processes and biological treatments for wastewater decontamination-A review

Industrial wastewater (Synthetic and real)

7. Degradation of chlorophenols by means of advanced oxidation processes: A general review

Chlorophenols

Ozone based process, UV-based process, Electrochemical process, Catalytic process. Ozone-based process, UV light-based process, Fenton and photo/electro-Fenton processes.

Organic pollutants

UV LED-induced oxidation processes.

Persistent organic pollutants (Synthetic and real wastewaters)

Anodic oxidation process, Anodic oxidation with electrogenerated H2O2, Electro-Fenton process, Photoelectro-Fenton process, Solar photoelectro-Fenton process. Ozone based process, Fenton process, Photo-Fenton process UV/O3 process, Photocatalytic process, Ultrasonic process, γ –Radiolysis process. Ozonation Fenton-process Photo assessed Fenton process Sonolysis Hydrogen peroxide (H2O2 + UV, Fenton, photo-Fenton and Fenton-like processes), Photolysis, photocatalysis, and processes based on ozone (O3, O3 + UV and O3 + catalyst) Fenton's peroxidation process

8. A review of classic Fenton's peroxidation as an advanced oxidation technique 9. Advanced Oxidation Processes in Water/Wastewater Treatment: Principles and Applications. A Review 10. Sono-photocatalysis in advanced oxidation process: A short review 11. Treatment of textile wastewater by advanced oxidation processes – a review

Organic and inorganic pollutants Water/Wastewater

12. Coupling of membrane filtration and advanced oxidation processes for removal of pharmaceutical residues: A critical review 13. Use of selected advanced oxidation processes (AOPs) for wastewater treatment - a mini review 14. Application of advanced oxidation processes for TNT removal: A review

Pharmaceutical wastewater

15. Application of advanced oxidation methods for landfill leachate treatment – A review

Landfill leachate

16. Removal mechanisms for endocrine disrupting compounds (EDCs) in wastewater treatment — physical means, biodegradation, and chemical advanced oxidation: A review 17. Aqueous Pesticide Degradation by Ozonation and Ozone-Based Advanced Oxidation Processes: A Review (Part I)

Endocrine disrupting compounds (EDC)

18. Degradation of Recalcitrant Surfactants in Wastewater by Ozonation and Advanced Oxidation Processes: A Review 19. Aqueous pesticide degradation by hydrogen peroxide/ultraviolet

Surfactants

Reference Others Miklos et al. (2018)

Coupled with membrane filtration, Adsorption,

Matafonova and Batoev (2018)

Combined with other Biological treatment and Electrocoagulation process.

Biological treatment (pre-treatment and post-treatment)

Oller et al. (2011)

–

Pera-Titus et al. (2004)

–

Neyens and Baeyens (2003) Oturan and Aaron (2014)

–

Organic wastewater

Sonication, Photocatalysis

–

Textile wastewaters

Combination of O3, H2O2, TiO2, UV radiation, electron-beam irradiation and ultrasound Ozonation process, Peroxone (O3/H2O2), UV/H2O2, Photo-Fenton, Photocatalysis and Electrochemical advanced oxidation processes (EAOPs) Titanium dioxide/UV light process, hydrogen peroxide/UV light process and Fenton's process Hydrogen peroxide, photocatalysis, processes based on ozone and electrochemical processes Ozone, ozone with hydrogen peroxide, ozone with ultraviolet light, hydrogen peroxide with ultraviolet light, Fenton process, and photo-Fenton process Chemical advanced oxidation

–

Pesticides

Pesticides

Ozone/hydrogen peroxide, Ozone/ultraviolet irradiation, and Ozone/hydrogen peroxide/ultraviolet irradiation Ozone, hydrogen peroxide, ultraviolet light irradiation

Hydrogen peroxide/ultraviolet irradiation, Fenton, photo-Fenton, and Electro-Fenton processes

Moreira et al. (2017)

Antonopoulou et al. (2014)

Chemical, photochemical, sonochemical, and electrochemical process

Recalcitrant organic constituents from industrial and municipal wastewater 2,4,6-trinitrotoluene

Sillanpää et al. (2018)

Joseph et al. (2009) Al-Kdasi et al. (2004)

Coupled with membrane filtration

Ganiyu et al. (2015)

–

Stasinakis (2008)

–

Ayoub et al. (2010)

–

Wang et al. (2003)

–

Liu et al. (2009)

Ikehata and Gamal El-Din (2005) –

Ikehata and El-Din (2004)

–

Ikehata and El-Din (2006) (continued on next page)

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Table 2 (continued) Title of the review article

Contaminants

Process used AOPs

irradiation and Fenton-type advanced oxidation processes: a review 20. Destruction of microcystins by conventional and advanced oxidation processes: A review 21. Advanced oxidation processes and their application in the petroleum industry: a review 22. Review of photochemical reaction constants of organic micro-pollutants required for UV advanced oxidation processes in water 23.

24.

25.

26.

27. 28.

29.

Microcystins

Petroleum industry wastewater

Pharmaceutical compounds, Personal care products, Pesticides, Hormones, Surfactants, Fire retardants, Fuel additives A comprehensive review on removal Arsenite As(III) and Arsenate As(V) of arsenic using activated carbon prepared from easily available waste materials Technology alternatives for deconArsenic-rich groundwater tamination of arsenic-rich groundwater—A critical review Arsenic contaminated Arsenic removal by synthetic and real field electrocoagulation process: Recent industrial wastewater trends and removal mechanism Industrial wastewater Microbial biotechnology as an contaminated with arsenic emerging industrial wastewater treatment process for arsenic mitigation: A critical review Application of low-cost adsorbents Arsenic for arsenic removal: A review Arsenic Arsenic removal from water/wastewater using adsorbents—A critical review Arsenic Arsenic — a Review. Part II: Oxidation of Arsenic and its Removal in Water Treatment

30. Arsenic Removal from Natural Water Using Low Cost Granulated Adsorbents: A Review 31. Arsenic Removal from Water by Adsorption Using Iron Oxide Minerals as Adsorbents: A Review 32. Removing arsenic from groundwater for the developing world - a review 33. Removal of arsenic from water using nano adsorbents and challenges: A review 34. Application of titanium dioxide in arsenic removal from water: A review 35. Electrocoagulation treatment of arsenic in wastewaters: A comprehensive review

Reference Others

Photolysis, UV/H2O2 process, Fenton reagent, Sulfate radical-based AOPs, Radiolysis, Ultrasonic degradation, TiO2 photocatalysis Ozonation, Photolysis, Photo-Fenton, UV/H2O2, O3 + UV.

Sharma et al. (2012)

–

Mota et al. (2008)

Photolysis

–

Wols and Hofman-Caris (2012)

–

Adsorption (Activated carbon from various waste materials)

Mondal and Garg (2017)

–

Electrocoagulation

Ghosh et al. (2019)

–

Electrocoagulation

Nidheesh and Singh (2017)) Hayat et al. (2017)

–

Bioremediation process Bacterial, Fungal, algae Adsorption

–

Adsorption

Mohan and Pittman (2007)

Ozone, H2O2 and Fenton's process, Photochemical oxidation,

Bissen and Frimmel (2003)

Chiban (2012)

Arsenic

–

Adsorption, anion exchange, electrocoagulation, and membrane filtration by ultrafiltratio, nanofiltration or reverse osmosis. Adsorption

Arsenic

–

Adsorption

Gallegos-Garcia et al. (2012)

Arsenic

–

Coagulation/precipitation and Adsorption

Jiang (2001)

Arsenic

–

Nano-Adsorption

Arsenic

Photocatalytic oxidation

Adsorption

Lata and Samadder (2016) Guan et al. (2012)

Arsenic

–

Electrocoagulation

organisms employed include bacteria (Vibrio fischeri, Photobacterium phosphoreum (Standards catalogue, 2008), Bacillus subtilis, Staphylococcus epidermidis (Kusvuran et al., 2011). Invertebrates such as Sea urchin (Paracentrotus lividius) (Pagano et al., 1982), Daphnia magna (Rizzo et al., 2009), Artemia salina (Campos et al., 2002), algal species such as Pseudokirchneriella subcapitata (De Schepper et al., 2009), Selenastrum capricornutum (Ferrari et al., 1999), Scenedesmus subspicatus (Cotman et al., 2004) or Dunaliella tertiolecta (Meriç et al., 2005), Scenedesmus obliques (Oturan et al., 2008), Nitellopsis obtusa (Vengris et al., 2007) Chlorella vulgaris (Moradas et al., 2008), and Synechococcus leopoliensis (Andreozzi et al., 2006) and plants such as Avena sativa, Brassica campestris (Ferrari et al., 1999), and finally some fish species Poecilia vivipara, i.e. the guppy (Corrêa et al., 2010). Oncorhynchus mykiss, i.e. the rainbow trout (Ferraris et al., 2005; Stalter et al., 2010), Danio

Baig et al. (2015)

Song et al. (2017)

rerio, i.e. the zebrafish (Cotman et al., 2004; He et al., 2009), These bioassays evaluate the effects of the toxic compound on these living organisms in a more cost effective, rapid manner and are useful in predicting the potent environmental hazards on the aquatic ecosystem. Table 3 summarizes these organisms of different group's viz., bacteria, algae, invertebrates, plants, and fishes, etc., utilized in bioassays for the applications in a wide variety of wastewater. 2.1.1. Bacterial bioassays Different bacterial toxicity test methods utilize metabolic activity of the microorganisms as the basis of the tests like growth inhibition tests or bioluminescence tests for screening toxicity of samples. In bioluminescence test, the use of bacterial species for toxicity testing employs the inhibition of the bioluminescence as a result of the

D.S. Babu et al. / Science of the Total Environment 696 (2019) 133961

alteration of the metabolic activity of the bacterium cell on the exposure of the chemical (Farré and Barceló, 2003). The property of the marine bacterium to naturally emit light, as well as their sensitivity towards pollutants, is exploited in these bioluminescence tests. The toxicity is expressed as effective concentration EC50 which represents the 50% reduction in the light on the exposure of the effective concentration of the toxic substance. The inhibition of the enzyme bacterial luciferase on exposure to the toxic substance is primarily responsible for the decrease in the luminescence. Bacterial species, whose property of bioluminescence has been exploited, include Vibrio fischeri, Photobacterium phosphoreum, Pseudomonas fluorescens P-17. Biodegradability tests employing V. fischeri has also been utilized for toxicity testing of wastewater from wood pulp mill plant (Yeber et al., 1999), resins acids based solutions (Ledakowicz et al., 2006), etc. Other microorganisms include Escherichia coli, Pseudomonas putida where growth inhibition tests were carried out for toxicity testing of the wastewater from nitrocellulose production (Ribeiro et al., 2013). In these tests, growth in terms of Optical Density of the test concentration to that of the seed control was expressed. However, the limitation of this methodology effluents in generating error makes it unsuitable for the strongly coloured effluents (Ribeiro et al., 2013). Kusvuran et al. (Kusvuran et al., 2011)employed antibacterial activity tests by studying zones of inhibition of B. subtilis and S. epidermidis against Malachite Green dye and its oxidation products post ozonation treatment. Activated sludge respiration inhibition test is one of the emerging approaches of acute toxicity measurement where the effect of pollutants on the bacterial community of the aquatic environment is elucidated by means of inhibition of microbial respiration (ISO, 2007; OECD/OCDE, 2010). This ISO 8192 (ISO, 2007) protocol utilizes the rate of decrease of oxygen consumption of the biodegradable matter with different concentrations of the substrate. 2.1.2. Invertebrate bioassays Some of the invertebrate species such as Daphnia magna owing to its high sensitivity and short reproductive cycle (Tothill and Turner, 1996) and Ceriodaphnia have also been used for the toxicity tests. The acute toxicity of Daphnia magna is an established USEPA (United States Environmental Protection Agency, 2002); ISO (ISO, 1996a) test. Live mobile daphnias on exposure to the toxic contaminant under controlled conditions are tested for their mortality as the toxicity test end point. Tests with D. magna for testing the toxicity of methomyl, a WHO classified hazardous pesticide is reported (Fernández-Alba et al., 2002). Besides this, D. magna has also been used for the bioassays of effluent of chemical and mechanical industrial wastewater treatment (Schrank and Jose, 2005), raw textile wastewater (Selçuk et al., 2006) and D. magna and V. fischeri were used for coagulated/settled tannery wastewater (Schrank and Jose, 2005). 2.1.3. Algae Many algal species have been also used for toxicity analysis of a wide range of industrial effluents, pesticides, antibiotics as discussed in this section. A 72 h growth inhibition tests using fluorospectrometric growth rate determination of algal species such as Pseudokirchneriella subcapitata have been employed for toxicity testing of the concentrate of tank cleaning wastewater (De Schepper et al., 2009), P subcapitata along with Artemia salina has also been used for the toxicity testing of non-steroidal anti-inflammatory drug and Diclofenac (Rizzo et al., 2009). Other algal species used for the toxicity testing post AOP processes include Selenastrum capricornutum against toxicity testing of a developing agent, metol-N-methyl-p-aminophenol (Andreozzi et al., 2000), Chlorella vulgaris for organochlorine pesticide trans- chloradane (Moradas et al., 2008), Scenedesmus obliquus for a herbicide of a phenylurea family Diuron ((N′-[3,4-dichlorophenyl]-N,Ndimethylurea) (Oturan et al., 2008), Synechococcus leopoliensis for an antibiotic molecule, lincomycin (Andreozzi et al., 2000). In all these toxicity tests utilizing different algal species, growth inhibition test is

7

generally used as the indicator of toxicity against the industrial effluents. The algal species are grown in industrial effluents to be tested for 72 to 96 h, subsequent to which the algal number is estimated. The major limitations of the algal toxicity test lie in culture as well as differences in reproducibility of the results in the consecutive assays (Farré and Barceló, 2003). 2.1.4. Plants Plant based bioassays in toxicity testing include the study of the seed germination rate, leaf number, measurement of the length of stems and roots, enzyme activity after exposing the plants to different concentrations of the effluent (Karci, 2014). Plant species such as Lepidium sativum along with D. magna and P. subcapitata for toxicity testing of pharmaceuticals (Naddeo et al., 2009; Rizzo et al., 2009) and L. sativum with Chlorella vulgaris for toxicity testing of organoclorine pesticide (Moradas et al., 2008) have been exploited. Similarly, Avena sativa, Brassica campestris and Lactuca sativa L have also been used as toxicity tests species for the toxicity of municipal solid waste incinerator bottom ash leachate toxicity (Ferrari et al., 1999). The activity of oxidant stress enzymes such as catalase, peroxidase, glutathione reductase and superoxide dismutase, direct phytoxicity tests such as fresh weight of biomass and germination rate test were used in these tests. Long exposure times for studying root measurements and complications due to different soil and sediment characteristics are some of the shortcomings of using plant bioassays (Wang and Freemark, 1995). 2.1.5. Fish bioassay Fish bioassay offer advantage over other bioassays for fish being virtually present everywhere in the aquatic ecosystem as well as poses a direct impact on the food chain. These bioassays vary from larval growth and survival tests on exposure of the fish larvae to the oxidized industrial effluents for 1–7 d, measurements of Adenosine Triphosphate (ATP) as energy stress bioindicator to direct fish lethality assays. In fish lethality assays, on exposure with the contaminant for 96 h, the percentage volume responsible for lethality of 50% of the organisms is expressed in terms of Lethal Concentration 50 (LC 50). Acute lethality tests have been used for fish species like rainbow trout (Oncorhynchus mykiss), bluegill sunfish (Lepomis macrochirus), fathead minnow (Pimephales promelas) for which standardized methods of EPA and ISO are available (ISO, 1996b; United States Environmental Protection Agency, 2002). Other fish species include Poecilia vivipara for toxicity testing of wastewater from petroleum refineries (Corrêa et al., 2010), Oncorhynchus mykiss for fish early life stage toxicity testing of ozonated wastewater (Stalter et al., 2010) and Danio rerio form-Nitrotoluene in aqueous solution (He et al., 2009). In yet another example, fish embryo tests offer a greater advantage of being more sensitive as it detects even minor toxicity as against other fish tests. The embryo test using Zebrafish Danio rerio is a recommended substitute test over acute fish test (Nagel, 2002). This 48-h short duration test utilizes examination of different toxicological endpoints such as effects on pigmentation disturbance and heartbeat frequency. Therefore, this test can serve as a model in toxicology for testing of chemicals. The requirement of adequate skill and specialised equipment, standardisation problems and long testing times are some of the restrictions of these assays (Tothill and Turner, 1996). However, good reproducibility and comprehensive information of toxicity of intermediates make it a bioassay of choice. 2.2. Detoxification by AOPs Advanced oxidation processes, grounded on the production of hydroxyl radicals, are proving to be successful viable options in detoxifying a variety of industrial effluents, pesticides, textile industry effluents, pharmaceuticals, pesticides, herbicides, explosives, and the range is manifold. These processes when employed not only aid in decreasing the toxicity in some cases but also benefits in improving the biodegradability of the treated effluent. However, in some cases, the production of

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Table 3 Toxicity tests used in different wastewater post treatment for assessing risk to human health. Group

Organism

Applications

Comments

Reference

Bacteria

Vibrio fischeri

Wood pulp mill plant

5 min of O3/UV advanced oxidation process was efficient in increasing the biodegradability of the organic matter with 50% COD reduction of the paper mill effluent AOPs: H2O2/UV at pH 3 and Fenton at pH 3.5 reduced TOC content of the organic compoundsby mineralization Ozonation, UV irradiation, and H2O2/UV AOP treatments increased the biodegradability by changinf kinetic parameters such as increasing maximum specific rate of substrate and decreasing the Monod's constant O3 and O3/UV oxidation methods along with Membrane Bioreactor Treatment (MBR) and Powdered Activated Carbon (PAC) were efficient for removing the persistent pharmaceuticals Ozonation treatment of 10 min resulted in 86% decolorisation of Malachite Green dye; antibacterial activity of remaining intermediates removed by 26 min of ozonation

Yeber et al. (1999)

Tannery wastewater Resins acids-based solutions

Photobacteriumphosphoreum Wastewater containing pharmaceutical compounds Bacillus subtilis Staphylococcus epidermidis Escherichia coli Pseudomonas putida Algae

Selenastrum capricornutum

Scenedesmus subspicatus Dunaliella tertiolecta Pseudokirchneriella subcapitata Chlorella vulgaris Scenedesmus obliquus Nitellopsis obtusa Synechococcus leopoliensis Artemia salina

Invertebrates Daphnia magna

Plants

Paracentrotuslividius (Sea urchin) Avena sativa Brassica campestris Lepidium sativum

Fish

Danio rerio

Oncorhynchus mykiss

Poecilia vivipara

Schrank and Jose (2005) Ledakowicz et al. (2006) Baumgarten et al. (2007)

Kusvuran et al. (2011) Kusvuran et al. (2011) Wastewater from Ribeiro et al. nitrocellulose production (2013) Ribeiro et al. (2013) Tannery wastewater Coagulation/flocculation process followed by biological treatment was resposible Oral et al. for decreasing the toxic content of the wastewater (2007) Sewage treatment plant UV/H2O2 process in the pH range 3.0–9.0 was capable in reducing the toxicity of Andreozzi effluents metol aqueous solutions et al. (2000) Tannery wastewater treatment Removal of the toxic agent responsible for the toxicity helped in declining the Cotman toxicity level of the wastewater. et al. (2004) Leather tanning wastewater Fenton (FeSO4/H2O2, pH = 3.0) and Ozonation for 20 min were effective in Meriç et al. effluents removing the toxicity of wastewater (2005) De Schepper Tank truck cleaning generated 79% toxicity reduction achieved by direct ozonation (pH = 7.5) while 53% by et al. (2009) concentrate advanced ozonation process (pH = 11.5) by 500 mg ozone L−1 for both processes Organochlorine pesticide Reduction of 95–100% toxicity by UVC, UVC/H2O2 or UVC/TiO2 treatment with Moradas trans- chloradane 250 mg TritonX-114 L−1 et al. (2008) Herbicide Diuron Electro- Fenton method for 10–30 min at 200 mA constant current intensity Oturan et al. effective for decreasing the toxicity (2008) −1 −1 2+ Treatment of spent 1.0 mol L H2O2 + 0.1 mol L Fe Fenton's reagent concentration effective for Vengris offset-printing developer 99% COD reduction et al. (2007) Antibiotic lincomycin 1 h of ozonation treatment reduced the toxicity of the ozonated solution of Andreozzi sewage treatment plant conating Lincomycin et al. (2000) Oilfield wastewater treatment Microfilration by Mixed Cellulose Ester effective in reducing 65% COD Campos et al. (2002) TiO2 photocatalysis effective for detoxification of DCF.A salina found not sensitive Rizzo et al. Non-steroidal anti-inflammatory drug in the investigation (2009) (NSAID) and Diclofenac (DCF) Advanced treatment of urban D. magna sensitive for the treated samples and in line with P. subcapitata for 20 Rizzo et al. wastewater min treatment (2009) Toxicity found to be decreased by using D. magna with no observed Schrank Effluent of mechanical and biodegradability post UV, TiO2/UV (120 min), O3 (60 min) and O3/UV(30 min) et al. (2005) chemical industrial wastewater treatment AOP treatments −1 Raw textile wastewater Ozonation (18.5–24 mg L ozone) effective in reducing acute toxicity to D. Selçuk et al. magna (2006) Industrial wastewater Pagano et al. treatment (1982) Ferrari et al. Solid wastes Mass fresh weight, germination rate for A. sativa and oxidant stress enzyme (1999) activities for Brassica campestris are used as the endpoints for toxicity Ferrari et al. assessment. Municipal solid waste incinerator bottom ash (MSWIBA) toxicity (1999) was revealed however MSWIBA leachate did not exhibit any toxicity Pharmaceuticals Sonication (20 kHz ultrasound generator) with pH 3–11 decreased the toxicity of Naddeo et al. (2009) 3 pharmaceuticals amoxicillin, carbamazepine and diclofenac at varied initial concentrations Testing of Nitrotoluene in Sonolytic ozonation (US/O3 oxidation) of 2.4 g h−1 Ozone dose at pH 10.0 Cotman aqueous solution et al. (2004) achieved 98% removal of 400 mg L−1 initial concentration of m-nitrotoluene He et al. (MNT) after 120 min treatment and also effective in reduction of MNT toxicity (2009) −1 Ozonated wastewater Ozonation (1 mg O3 mg ) resulted in developmental retardation of Ferraris Oncorhynchus mykiss et al. (2005) Stalter et al. (2010) Petroleum refineries 60 min of Batch Ozone-Photocatalytic oxidation (O3/UV/TiO2) resulted in 89.2% Corrêa et al. wastewater of toxicity reduction for bacteria (2010) Degradation of Malachite green dyes and intermediates

transformation products or toxic intermediates is one of the critical aspects of this research which needs to be looked upon. The role of AOPs in detoxifying the contaminants belonging to different classes has been discussed with the major interesting outcomes of the study.

Application of AOP processes like H2O2 + UV, photocatalysis, processes based on ozone (O3, O3 + UV), Fenton, photo-Fenton and Fenton-like processes and electrochemical processes are investigated for the removal of 2,4,6-trinitrotoluene (TNT). TNT is one of the

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compounds having explosive properties and known to contaminate groundwater owing to his low biodegradability and high toxicity (Ayoub et al., 2010). The toxicology aspects of the oil sands process-affected water (OSPW) towards Vibrio fischeri had shown decreased toxicity subsequent to its treatment by AOPs (Meshref et al., 2017). It was suggested that the residual toxicity still generated was due to the oxidized species of Naphthenic Acids. Different AOP treatments, however, resulted in varied toxicity reductions. 51% of the toxicity inhibition by V. fischeri by the raw OSPW, however highest reduction of 25.4% of toxicity was observed with 1:2 H2O2: O3 treatment. The 1:1 H2O2: O3 treatment showed similar toxicity reduction as only ozone only treatment but was lesser than 1:2 peroxone treatment as seen in Fig. 1. In at another example, ozonation and photocatalytic ozonation were also shown to be successful for reducing the toxicity of the pollutants from the hospital laundry wastewater by Kern et al. (Kern et al., 2013). Reduction in the toxicity of D. magna viz. EC50 = 47.3% with Ozone, EC50 = 50.6% with UV and EC50 = 45.4% with UV/O3/Fe2+ were observed coupled with significant reductions in some parameters like COD, BOD, and TKN. The photochemical VUV/UVC/O3 treatment system was also effective in removing the toxicity of hospital laundry wastewater against D. magna from EC50 6.7% to EC50 100% (De Oliveira Schwaickhardt et al., 2017). The application of VUV/H2O2 advanced oxidation process was tested for the treatment of an endrocrine disrupting compound, Bisphenol A. The treatment was effective in reducing the cytotoxicity of bisphenol A (Moussavi et al., 2018). Application of AOPs is also demonstrated in the degradation of formaline wastewater where combinations of UV/H2O2, Fenton and photo Fenton processes where investigated in removing the toxicity of the byproducts of the effluent for its safe disposal (Kajitvichyanukul et al., 2006). However, it was observed that while applying UV/H2O2 process, the complete reduction of formaldehyde and methanol was not accompanied by any reduction in the toxicity. But comparatively low toxicity of the effluents was achieved in the effluents after Fenton and photo Fenton processes after treatment of 120 min with complete mineralization of the formaline wastewater (Kajitvichyanukul et al., 2006). Influence of AOP (O3, H2O2, and UV light) on Polish textile industry was also evaluated by Ledakowicz et al. (Ledakowicz et al., 2006) where a decrease in the inhibitory action of microbial growth up to 30% was observed when using UV irradiation and further 26% inhibitory effect on addition of H2O2. Coking wastewater is an example of a potent toxic contaminant source which contains many toxic chemicals hazardous. Na et al. (Na et al., 2017) studied zebra fish embryo toxicity test with wastewater samples obtained from coking wastewater treatment plants A and B. The treatment plant A involves anaerobic unit, anoxic unit and oxic unit with ozone oxidation (A2O-ozonation) process and plant B contains anaerobic unit, anoxic unit and oxic unit with Fenton oxidation (A2OFenton oxidation) process. Toxic units (TU), survival index (SI), total survival rate (TSR), and hatching rate (HR) were measured to observe the effects of coking wastewater on the zebrafish embryos. It was observed that when the embryos were exposed to the coking wastewater there was a significant toxic health effect on the embryos as the coking wastewater does DNA damage and have a long - term effect like carcinogenesis and mutagenesis on the organisms. The effects were observed on the body length of the zebrafish and the phenotype is shown in Fig. 2. The EC 50 (Effective Concentration) values in plant A of raw influent on zebra fish are 4.24% and 2.44% of hatching rate (HR) and survival rate after hatching (SRAH), respectively. And total units for HR 23.7, total survival rate (TSR) 41.0, SRAH 41.0, and survival index (SI) 41.0, for raw influent water. It is observed that larva exposed to raw influent found decreased body length with reference to negative control at dilution 2.1%. In the raw influent (exposure at 1.5% dilution), ot (Opaque Tissue), usb (Uninflated Swim bladder) and yse (Yolk sac edeme) were observed. Significant malfunction of ot, usb, yse, cm

9

(Craniofacial malformation), sc (spinal curvature) and pe (pericardial edema) was observed after 2.1% and 3.0% dilutions exposure. These results indicate that the raw influent in plant A had inhibitory effects on zebrafish embryo hatching, development (body length) and survival; and teratogenic effects on larvae after exposure of the embryos to a certain level. While after passing the coking wastewater through the anaerobic unit, anoxic unit, and oxic unit (A2O) process unit, the TUs for HR, TSR, SRAH and SI were negligible and no remarkable toxic effects occurred after exposure to the A2O effluent, rather than a little bending of tails which was observed at 80% dilution. No embryo toxicity was observed at 80% dilution of after ozonation. Whereas in plant B, the EC50 values of raw influent on zebrafish is HR, 14.1% and SRAH, 2.93%. The toxic units for SI, HR, SRAH and TSR for the raw influent were 35.2, 7.67, 33.8 and 34.3 respectively. Exposure to the raw influent causes a notably decreased body length as compared to the negative control at dilution 3.0%. usb was observed in the raw influent and after exposure at dilutions of 1.5% and 2.1%. usb, ot, and cm were seen after exposure at 3.0% dilution. The raw influent in plant B showed notable zebrafish embryo toxicity. After passing through the A2O process unit, the TUs for SI, SRAH, TSR, and HR for the A2O effluent were found insignificant. This indicates the insignificant toxic effect of A2O effluent on embryo hatching and survival. However, exposure of A2O effluent at 80% and 40% dilution resulted in a slight bending of tails and a significant decrease of body length. Even at 80% dilution of final effluent after Fenton oxidation showed no significant embryonic toxicity. By taking all above results in to account both A2O-ozonation and A2O Fenton oxidation treatment processes are able to eliminate complete embryo toxicity of zebrafish. The combination with biological treatment helps in treating the more recalcitrant compounds to increase their biodegradability using AOPs. Combination of such Fenton and biological aerated filters (BAFs) was also found to be suitable for detoxification of 1000 mg L−1 Phenol and formaldehyde (1:1) concentration (Méndez et al., 2015). Besides being used as a detoxification method, the utility of the hydrogen peroxide (H2O2) and ferrous sulfate (FeSO4) was also examined to ferment the otherwise difficult to ferment spruce hydrolysates. An increased ethanol production as high as 8.3 g L−1 was achieved with treatment of the hydrolysates with H2O2 and FeSO4 as against b0.4 g L−1 ethanol production without the treatment (Soudham et al., 2014). This shows the efficacy of both the compounds in enhancing the ethanol production

Fig. 1. Toxicity towards Vibrio fischeri after various ozone treatment techniques (OSPW = oil sands process-affected water, O30 = treatment denoted as 30 mg L−1 ozone, O50 = treatment denoted as 50 mg L−1 ozone, peroxone treatment at different molar rations P (1:1) = 20 mg L−1 H2O2: 30 mg L−1 O3, P(1:2) = 20 mg L−1 H2O2: 50 mg L−1 O3). Reprinted with permission from Meshref et al. (Meshref et al., 2017). Copy right ©2017 Elsevier Ltd.

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Fig. 2. Effects of exposure to coking wastewaters on body length (A) and embryonic phenotypes (B) of larvae at the end of the zebrafish embryo toxicity test. (RIa = raw influent from plant A, A2OEa = anaerobic, anoxic and oxic effluent from plant A, O3Ea = ozonation unit effluent from plant A, RIb = raw influent from plant A, A2OEb = anaerobic, anoxic and oxic effluent from plant B, FEb = Fenton oxidation unit effluent from plant B, NC = negative control, pe = pericardial edema, yse = yolk sac edema, ot = opaque tissues, sc = spinal curvature, cm = craniofacial malformation, usb = uninflated swim bladder, RIa 1.5% = raw influent from plant A with 1.5% dilution, RIa 2.1% = raw influent from plant A with 2.1% dilution, RIa 3.0% = raw influent from plant A with 3.0% dilution, A2OEa 80% = anaerobic, anoxic and oxic effluent from plant A with 80% dilution, O3Ea 80% = ozonation unit effluent from plant A with 80% dilution, RIb 1.5% = raw influent from plant B with 1.5% dilution, RIb 2.1% = raw influent from plant B with 2.1% dilution, RIb 3.0% = raw influent from plant B with 3.0% dilution, RIb 4.2% = raw influent from plant B with 4.2% dilution, A2OEb 80% = anaerobic, anoxic and oxic effluent from plant B with 80% dilution, EFb 80% = Fenton oxidation unit effluent from plant B with 80% dilution). Reprinted with permission from Na et al. (Na et al., 2017). Copy right ©2017 Elsevier B.V.

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and substrate fermentability by significantly removing the inhibitory compounds. Atrazine, one of the herbicides was also tested for its cytotoxicity in Human Hepatic Cells (HepG2) cell viability before and after photocatalytic treatment using In, S˗TiO2@rGO aerogel catalyst (Khavar et al., 2018). The tests and the control did not show a significant difference in the toxicity suggesting a reduction in cytotoxicity post photocatalytic oxidation treatment. In another example of propanil, one of the herbicides, the mineralization, and detoxification of the herbicide was studied utilizing photocatalysis through TiO2 in combination with two catalysts and Fenton reaction (Sánchez et al., 2014). The combination of Fenton reaction with TiO2-photocatalysis and one of the tested catalysts ECT1023t showed increased mineralization coupled with a fourfold increase in the rate of detoxification of propanil compared to the use of either Fenton process or TiO2-photocatalysis. The reason could be due to the competing scavenger compounds in case of using Fenton process and interaction of the catalyst surface with propanil in case of catalyst which could be inhibiting the detoxification process or mineralization while using these processes. Application of Fenton processes post biological treatment is also suggested as a cost-effective strategy for the removal of toxic azo dyes (Brindha et al., 2018). The aromatic amines formed as a result of anaerobic degradation of monoazodye Mordant Yellow (MY) by Pseudomonas aeruginosa was then subsequently treated with Fe catalyzed Fenton reaction. Significant improvement on the mineralization as well as detoxification of MY dye was achieved when toxicity studies were done on human small cell lung cancer cells A549, shown in Fig. 3. Yeber et al. (Yeber et al., 1999) demonstrated about 50% removal of the acute toxicity, reduction in absorbable organic compounds (AOX) values of the cellulose bleaching effluent when photocatalysis using TiO2 and ZnO on glass Raschig rings were used. This treatment offers an effective approach in oxidation and subsequent removal of toxicity of the organic compounds present in paper and pulp bleaching effluents. A fabricated AgBr nano particle decorated TiO2 nanotube arrays (AgBr/TiO2 NTAs) photo electrode has been employed in reducing the toxicological effects of 4-Chlorphenol (4CP), one of the phenolic compounds used in industrial processes (Cui et al., 2018). Around

11

0–140 min of the photocatalytic treatment on the luminescent bacterial tests resulted in inhibition from 39.2% to b1% indicating that the toxicity of the intermediates is b4 CP. Treatments combining the different technologies of Ozone, Hydrogen Peroxide and UV radiation for the removal of tamoxifen, a recalcitrant chemotherapy drug was investigated by (Ferrando-Climent et al., 2016). The complete removal of the drug was observed in all the treatments using Ozone. However, an increase in the toxicity was observed for ozone-based oxidation processes due to the formation of the transformation products during the process Fig. 4. This study is a representative example that although AOPs can be utilized in removing the pollutants however in some cases it may be effect the generation of more toxic compounds than the parental compounds itself as in the case of tamoxifen. Similarly, in the case of antibiotic Sulfachloropyridazine (SCP), it was treated with the hydroxyl radicals generated with BDD anode. Although the rate of mineralization was quite effective with the increasing current, the assessment of toxicity during this treatment of SCP revealed the formation of some more toxic compounds than the parent compound. The overall total detoxification at the end of the treatment nevertheless justified the efficacy of this treatment (Haidar et al., 2013). The use of AOPs has similarly been investigated for the treatment of pesticides. In the treatment of diuron, the toxicity tests on D. magna and Selenastrum capricornotum showed the formation of the toxic intermediates that led to initially the increase in the toxicity to the original value and later subsequent decrease in toxicity (Malato et al., 2003). This inconsistent behavior of the treatment process calls for the need for control over the treatment processes to guarantee the quality of treated wastewater. In another study done on diuron treatment by electrochemical advanced oxidation process by Oturan and co-workers (Oturan et al., 2008), the formation of more toxic metabolites than the parent herbicide was observed using microtox and alga toxicity test methods. Application of high electrolysis current 250 mA for long hours was suggested to achieve complete mineralization and circumvent the limitation of the production of toxic metabolites. Contrary to this, in case of metobrumon, the reduction in toxicity was accompanied by no virtual change in the biodegradability and recalcitrance while for isoproturon, the herbicide solution was almost

Fig. 3. Cytotoxicity of biological/photo-Fenton Mordant Yellow 10 samples in human small lung carcinoma cells, A549, (MY10 = Mordant Yellow 10, BRPO3 treated = Pseudomonas aeroginosa BRPO3 treated, PF treated = Photo Fenton treated). Reprinted with permission from Brindha et al. (Brindha et al., 2018). Copy right © 2018 Elsevier Ltd.

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Fig. 4. Removal of tamoxifen by advanced oxidation treatments (O3, O3/H2O2, O3/UV, UV and UV/H2O2) and the toxicity generated during the processes (EC50). Reprinted with permission from Ferrando-Climent et al. (Ferrando-Climent et al., 2016), copy right © 2016 Elsevier Ltd.

completely detoxified as well as mineralization up to 95% (Parra et al., 2000). Some pesticides like octachlorodibenzo-p-dioxin presence of some toxic intermediates was encountered during their treatment via AOPs (Ikehata and El-Din, 2006). Comparison of the treatments involving Fenton processes, TiO2 catalysis and lab scale constructed wetlands was tried by Herrera-Melián et al. (Herrera-Melián et al., 2012) for the detoxification and degradation of 4 -Nitrophenol (4NP). The combination of the constructed wetlands at 16 h with TiO2-photocatalysis of 6 h was found to be efficient in treating 200 mg L−1 of the organic of the 4NP effluent. But for higher concentrations up to 500 mg L−1, Fenton reaction alone was efficient in degrading completely in lesser time span. In this, the toxicity test for Lemna minor showed b20% toxicity for concentrations below 300 mg L−1 while using Fenton process. Advanced oxidation processes namely photolysis by hydrogen peroxide, Fenton reagent and combination of ozonation and hydrogen peroxide and UV radiation also showed N90% degradation of 4NP and removal of toxicity of its byproducts when tested for acute toxicity test for D. magna (Trapido and Kallas, 2010). The cytotoxicity of petroleum refinery wastewater (PRW) was evaluated for Allium cepa, brine shrimp using TiO2/UV/H2O2 and gamma ray/ H2O2 which was found to be effective in detoxifying the mutagenic and cytogenic agents present in PRW (Iqbal et al., 2017). There was an increase in root count (70%), root length (64%) and mitotic index (89%) of A. cepa as well as reduction in mutagenicity up to 86% for the microbial strains were observed post AOP treatment.

3. Arsenic toxicity reduction by AOP treatment 3.1. Arsenic toxicity Globally N150 million individuals are affected due to the consumption of arsenic contaminated water (Sarkar and Paul, 2016). Arsenic is commonly observed in the alluviums of the Indian states of West Bengal, Bihar, Bangladesh, Jharkhand, Assam, and Uttar Pradesh. Parts of other countries like Afghanistan, Argentina, Bolivia, Cambodia, China, Ghana, Hungary, Japan, Mexico, Mongolia, Sri Lanka, Nepal, Pakistan, Taiwan, Myanmar, Thailand, and Vietnam, are fairly affected by arsenic toxicity (Mandal and Suzuki, 2002; Sarkar and Paul, 2016). The presence of arsenic in water or soil beyond the safe limit have also come from parts of Australia, Brazil, Canada, parts of European Union, Iceland, Iran, New Zealand, and U.S.A. (Garelick et al., 2009; L. J. and A. P., 2013; Mukherjee et al., 2008; Sarkar and Paul, 2016). Arsenic toxicity is mainly determined by arsenic metabolism. It has been identified that

metabolic capacity and the toxic effects of arsenic changing from one community to other, even with in family (Guha Mazumder, 2008; Sarkar and Paul, 2016). Arsenic is widely distributed in the environment by many environmental sources like anthropogenic activities, burning of fossil fuels and weathering of rocks (Martin et al., 2000), mining (Wang and Mulligan, 2006), hydrothermal (Wang and Mulligan, 2006) and geothermal activities (Sarkar and Paul, 2016), use of arsenical pesticides (Martin et al., 2000), herbicides (Grund et al., 2008) and arsenic as an additive in poultry feed (Matschullat, 2000). Groundwater in Assam, Bihar, Chattisgarh, Jharkhand, Manipur, Uttar Pradesh, and West Bengal, are among the most contaminated states, with arsenic levels above 50 μg L−1 (Hossain, 2006). The mobility of arsenic in water mainly depends on proliferation of microorganisms and oxygen concentration (Sirés et al., 2014). The maximum drinking water permissible limit of arsenic is 10 μg L−1 (Graham, 1999). In terms of severity, arsenic poisoning is third in position. Generally, arsenic poisoning happens due to contaminated water and the food made by the irrigation of that contaminated water. It is estimated that 40% of human body with arsenic toxicity is due to food contamination (Huq et al., 2006). For different countries, soil contamination as well irrigation with contaminated water, the arsenic intake concentration through food varies from 17 to 291 μg d−1 (Delgado-Andrade et al., 2003). Rice is one of the major staple food in India and accounts for 42% of total food grain production (Jain and Chandramani, 2018). West Bengal is one of the major rice cultivators in India and relies on groundwater which is greatly polluted with arsenic and which is used for the irrigation of the crops (Abedin et al., 2002; Das, 2007). The WHO acceptable limit of arsenic in various food items including rice is 0.5–2 mg L−1 (Abedin et al., 2002; FAO/WHO, 2011; Misbahuddin, 2007). Vegetables that contain arsenic in various concentrations include potato (448 μg kg−1), amaranth leaf (458 μg kg−1), arum root and eggplant (893 μg kg−1), and arum lati (1143 μg kg−1) (Misbahuddin, 2007). 3.1.1. Acute toxicity Acute toxicity of arsenic occurs generally when it is either consumed or inhaled accidentally. The dose of b5 mg of arsenic compounds causes diarrhoea and vomiting which can be resolved within 12 h and does not require any treatment (Ratnaike, 2003). Inorganic arsenite and arsenate are highly toxic than their methylated forms such as monomethylarsonic acid and dimethylarsinic acid (Benramdane et al., 1999). 3 mg kg−1 is the estimated lethal dosage for adult humans (Hughes, 2002). The mostly encountered manifestations of acute arsenic toxicity include excessive salivation, severe abdominal pain, nausea, profuse watery diarrhoea (often bloody), vomiting, etc. (Choong et al., 2007; Guha Mazumder, 2008; Hu et al., 1998; Ratnaike, 2003; Sarkar and Paul, 2016). 3.1.2. Chronic toxicity Arsenicosis is a chronic level of arsenic toxicity (Guha Mazumder, 2008). Different body systems or organs like renal system, nervous system, skin, liver and respiratory system, are vulnerable to chronic arsenic poisoning (Guha Mazumder, 2008; Kim and Kim, 2015; Mandal and Suzuki, 2002; Rahman et al., 2010; Sarkar and Paul, 2016; Smith and Steinmaus, 2009). One of the main important symptoms of chronic arsenic poisoning is skin pigmentation and keratosis (Guha Mazumder et al., 1988; Hughes, 2002; Rahman et al., 2009; Sarkar and Paul, 2016). Tseng et al. (Tseng et al., 2005) identified Black foot disease, which is a cardiovascular disease observed in the 20th century is also caused due to consumption of arsenic for a long time. 3.1.3. Effects of arsenic on immune cells 3.1.3.1. Macrophages and dendritic cells. Macrophages and dendritic cells are proficient phagocytes that control innate immunity and manage

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immune surveillance against diseases and malignancies. The prime function of these cells is to engulf the foreign bodies' cellular debris etc. Macrophages release a broad range of cytokines to the infection or tissue damage with help of monocytes as precursors. Banerjee and coworkers (Banerjee et al., 2009) reported that arsenic affected individual with skin lesions observed reduced cell adhesion capacity, nitric oxide production, F-actin expression and phagocytic activity due to release of macrophages. Recent studies show that arsenic affects the central nervous system by altering the functions of microglia (Chen et al., 2016; Singh et al., 2016). Singh et al. (Singh et al., 2016) observed exvivo secretion of IL-6 and TNF- α from microglia isolated from arsenic (0.38 mg kg−1 arsenite) exposed mice for a week. These all effects may lead to enhanced incidence of developmental neurotoxicity in children, chronically exposed to arsenic (Tsuji et al., 2015). 3.1.3.2. Lymphocytes. The cellular and humoral responses of adaptive immunity are mediated by T and B lymphocytes. The dendritic cells activate the T lymphocytes and produce different cells, which are CD4+ T helper cell subtypes and CD8+ T cytotoxic cells. The T lymphocytes number and their functions are influenced by exposure of arsenic (Biswas et al., 2008; Liao et al., 2009; Soto-Peña et al., 2006). The decreased amount of CD4+ T cells and CD4/CD8 ratios in peripheral blood of children and adults which are exposed to chronic exposure is because of reduced ex-vivo proliferation of T lymphocytes (Biswas et al., 2008; Liao et al., 2009; Soto-Peña et al., 2006). Wu et al. (Wu et al., 2003) observed expression of growth factors and chemokine's, and upregulation of inflammatory cytokines from high arsenic exposed individuals blood lymphocytes. 3.1.3.3. Epigenetic effects. It is suggested with the help of molecular epidemiological studies, that arsenic can modify the white blood cell epigenome and regulate the gene expression (Salnikow and Zhitkovich, 2008; Terry et al., 2011). Adults from Bangladesh are identified with DNA methylation in the peripheral blood mononuclear cells which is due to drinking water highly contaminated with arsenic (Kile et al., 2012; Niedzwiecki et al., 2013). Particularly the highly methylated DNA levels are observed in individuals with highest arsenic exposure (Niedzwiecki et al., 2013). DNA methylation modifications can be detected in gene level. The chronic exposed individuals show either reduced or enhanced cell functions like pH regulation in intercellular activate, nuclear factor κ B signalling pathway, lipid metabolism (Argos, 2006). Similarly, results were observed in Argentinean women with alterations of the CD4+ T cell methylome (Engström et al., 2017). Both hypo- and hypermethylated regions were detected in genes regulating T cell activation and differentiation. 3.2. Advanced oxidation methods for treatment of arsenite Compared to arsenate, toxicity of arsenite is quite high and arsenite conversion to arsenate is essential for the effective removal of arsenic from aqueous medium. AOPs are effective methods to oxidize arsenite from water medium and the various AOPs used for arsenite oxidation are described below. Ozonation is the oxidizing technique, which converts more toxic arsenite to less toxic arsenate by utilizing ozone molecules. Almost complete oxidation is possible through this method (Yoon et al., 2011). More the initial concentration of pollutants, more is the time consumption (Otgon et al., 2017). pH is also an important parameter (Khuntia et al., 2014). The highly acidic condition favors effective arsenite oxidation (Otgon et al., 2017). Khuntia et al. (Khuntia et al., 2014) studied arsenic removal with ozone dosage 0.56 mg/s by varying initial pollutant concentration from 50 to 200 μg L−1 and found N90% of oxidation in 25 min. Oxidation processes involving hydroxyl radicals are promising techniques for the effective removal of pollutants. The reaction of arsenite with •OH radicals forms the As(IV) intermediate, which further oxidizes

13

to arsenate. The proposed reaction mechanism is as follows (Bissen and Frimmel, 2003; Hug and Leupin, 2003; von Gunten, 2003). AsðIIIÞ þ OH˙→As ðIVÞ

ð1Þ

AsðIVÞ þ O2 →As ðVÞ þ O:2

ð2Þ

The process in which the oxidation of arsenite occurs in presence of light and hydrogen peroxide, whereas H2O2 itself acts as an oxidant but the combination of both will increase the oxidation rate (Daniels, 1962; Kim et al., 2015). The rate of reaction for conversion depends on the light dosage (Sorlini et al., 2014). Study on arsenite oxidation reported that in presence of 5 mg L−1 H2O2 and the light dosage of 600 mJ cm−2 resulted only 21% removal, while at 2000 mJ cm−2 it reached up to 85% (Sorlini et al., 2014). Combined UV/H2O2 process is giving high conversion values within short response times by utilizing low H2O2 consumption. The observed pH of 7.9 in the system remains constant and reaction attains the steady state within 3 min in both ultra and groundwater samples with a rate of reaction of 1.23 × 10−2 S−1 and 4.9 × 10−3 S−1 respectively (Lescano et al., 2011). Photocatalytic oxidation technique for converting arsenite to arsenate has been widely accepted due to its environmental acceptance. TiO2 is the extensively used material for the photocatalytic oxidation of pollutants (Ameta et al., 2003; Kumar and Devi, 2011; Singh et al., 2018). Process of UV/TiO2 is pH independent (Dutta et al., 2005), while the arsenite oxidation, as well as its uptake, is influenced by the pH of the aqueous medium (López-Muñoz et al., 2017). Lopez Munoz et al. (López-Muñoz et al., 2017) observed the complete oxidation of arsenite in 10 min at pH 9 and 35 min at pH 9. ZrO2 (Sun et al., 2017), ZnO (Rivera-Reyna et al., 2013) CuO/ZnO (Samad et al., 2016)V2O5/TiO2 (Xie et al., 2016) are also showing efficient photocatalytic ability in oxidizing arsenite to arsenate under UV light irradiation. Fenton process is a very efficient process for transformation of arsenite (Hug and Leupin, 2003). Complete oxidation of 6.6 μM arsenite by the external addition of 20 μM hydrogen peroxide and ions of ferrous in aerated solutions was observed by Hug and Leupin (Hug and Leupin, 2003). Highly reactive hydroxyl radicals oxidizes the arsenite (Wang et al., 2013). In the bio-EF process, the required electricity for the EF reactions is in-situ generated by biochemical reactions. Wang et al. (Wang et al., 2014) studied Bio-EF process for arsenite oxidation, where they used Shewanella putrefaciens SP200 pure culture as microbial source in the process. Lepidocrocite (γ-FeOOH) coated on the surface of electrode in the cathode region as iron source, while lactate in the anodic chamber acts as electron donor. The generated protons in anodic chamber passes through the membrane while an electron goes outwardly through connected wire. Thus, the generated electricity in the system, in turn, generates hydrogen peroxide and subsequently hydroxyl radicals in cathodic chamber. As per their findings, γ-FeOOH dosage is an important factor in determining the system performance at the neutral pH (Wang et al., 2014). Zhang et al. (Zhang et al., 2014) removed arsenic from the aqueous medium by employing anodic oxidation. They used iron as cathode and SnO2 coated TiO2 nanotube as anode for the electrolysis process at 50 mA for 60 min and achieved complete arsenite oxidation. The oxidized pollutant removed from the system by electrocoagulation process for 10 min, simply by reversing the polarity of the electrodes. Thus within 70 min, they found complete oxidative removal of arsenite from aqueous medium. The hydroxyl radicals generation in the sono-chemical system is due to the thermal decomposition of water (Henglein and Kormann, 1985). As arsenite is unstable, it is oxidized by the oxygen and hydroxyl radicals (Neppolian et al., 2009). Arsenite oxidation rate depends on the dissolved oxygen present in the aqueous medium. Improved oxidation of arsenite up to 99.1% was observed on sonication with external addition of ferrous ions (Cui et al., 2011). Application of zero valent aluminium

14

D.S. Babu et al. / Science of the Total Environment 696 (2019) 133961

Table 4 Comparison of arsenite detoxification potential of various AOPs. Process

Initial concentration of arsenite

Optimal operating conditions

Removal efficiency

Reference

Ozone oxidation

42–62 μg L−1

Complete conversion

Kim and Nriagu (2000)

Ozone micro-bubbles H2O2 + UV irradiation Photocatalytic oxidation (TiO2) Photocatalytic oxidation

200 mg L−1 0.1 mg L−1 0.1 mg L−1 10 mg L−1

≥90% of arsenate conversion 85% of conversion Complete conversion Complete oxidation

Khuntia et al. (2014) Sorlini et al. (2014) Zhang and Itoh (2006) López-Muñoz et al. (2017)

Photocatalytic oxidation (TiO2) Anodic oxidation process Ultrasound/persulfate process

50 mg L−1 6.67 μg L−1 0.067 mM

Flow rate 2–2.5 L/Min; Time 20 min pH 4–9, ozone con 0.56 mg/s, Time 25 min Dose 2000 mJ cm−2, Time 315 min pH 3, 2–5 g L−1 Adsorbent, time 3 h, pH 3 within 10 min and at pH 9, 35 min pH 7, time 3 h pH 7, current density of 50 mA, 60 min pH 3–8, ultrasound frequency of 211 kH, 10 mg L−1 dissolved oxygen; time 5 min

Complete oxidation Complete oxidation 80% of conversion

Yu et al. (2013) Zhang et al. (2014) Neppolian et al. (2010).

(ZVAl) for oxidative removal mechanism of pollutant is an emerging technology (Khatri et al., 2018; Nidheesh et al., 2018a, 2018b). It is observed to be effective for transforming arsenite to less toxic arsenate species. The enhanced arsenite oxidation is because of amplified dissolution of aluminium layer leading to increased H2O2 generation (Wu et al., 2013). Presence of ions like ferrous and polyoxometalate can improve the performance of ZVAl/O2 process considerably (Zhang et al., 2017). The addition of nZVI and hydrogen peroxide in the aerated wastewater at neutral condition also shows enhanced arsenite oxidation (Katsoyiannis et al., 2015). Sulphate radicals are more powerful oxidizing agents than hydroxyl radicals (Deng and Ezyske, 2011; Nidheesh and Rajan, 2016). The sulphate radicals are generated from parent compounds such as persulfate or peroxy-monosulfate by UV irradiation, ultrasound, heat, etc. (Shukla et al., 2010; Su et al., 2012; Xie et al., 2012). UV/persulfate mediated process was observed to be efficient for arsenite oxidation. The pH of the system is not having any significant influence on the process (Neppolian et al., 2008). Ultrasound mediated persulfate process also resulted in complete oxidation of arsenite (Neppolian et al., 2010). Zhou et al. (Zhou et al., 2013) reported the enhanced activity of ferrous activated persulfate for arsenic oxidation. In arsenite oxidation dissolved oxygen of the water medium plays a major role in sulfate radical mediated AOPs (Neppolian et al., 2010, 2008). Through ultrasound/persulfate process in the absence of enough dissolved oxygen, only 40% arsenite oxidation was observed, while it increased up to 80% in 5 min with 10 mg L−1 dissolved oxygen presence (Neppolian et al., 2010). The arsenite oxidation mechanism by sulphate radical based AOP has explained below [Eqs. (3)–(7)] (Neppolian et al., 2008).

tests with a particular organism may be effective for a specified purpose while it may not be accurate for other intended use. It is also imperative to understand that acute toxicity tests might delimit the ecological testing of the low concentrations of micro pollutants. In these cases, studies are warranted for chronic toxicity testing of such pollutants. Wide range of AOPs such as Fenton, Anodic oxidation, electroFenton, photo-catalysis, photo-Fenton, ozonation and catalytic ozonation, sonochemical methods, sono-Fenton, and zero valent metal/H+/ O2, as well as persulfate/peroxymonosulfate oxidation as were found as an effective tool for reducing toxicity from water and wastewater. Even, the toxicity of arsenic could be reduced considerably by employing these AOPs. Toxicity tests employed for various oxidized chemicals post AOP treatment serves only as a screening test for the toxicity levels and not necessarily determine the effect of AOP treatment on the biodegradability of the industrial effluents. Hence the application of specific biodegradability tests is necessary for effective characterization of the AOP treated effluent (Rizzo, 2011). Although AOPs have been proved to be effective in detoxifying a wide range of industrial effluents, pesticides, pharmaceutical products, etc., often the intermediate/transformations products are more toxic than the parent compound. Understanding this, toxicity tests should be employed at various stages of the AOP treatment in order to check the toxicity effects of these compounds as well prior to their final discharge in the aquatic environment.

2− AsðIIIÞ þ SO− 4 ˙⟶AsðIV Þ þ SO4

ð3Þ

2− AsðIV Þ þ SO− 4 ˙⟶AsðV Þ þ SO4

ð4Þ

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

AsðIV Þ þ O2 ⟶AsðV  Þ þ O− 2 ˙

ð5Þ

Acknowledgments

2− þ SO− 4 ˙ þ H 2 O⟶HO˙ þ H þ SO4

ð6Þ

AsðIIIÞ þ HO˙⟶AsðIV Þ þ HO−

ð7Þ

Authors are thankful to the Director, CSIR-NEERI, Nagpur, India for providing encouragement, and kind permission for publishing the article. This work was partially supported by Science and Engineering Research Board, India (File Number: ECR/2017/000005) and the authors are grateful to Science and Engineering Research Board for supporting.

Comparison of arsenite oxidation ability of various AOPs are given in Table 4. From the table, it can be seen that most of the AOPs are effective for the oxidation of arsenite at a wide range of initial concentrations. 4. Conclusions and future perspectives The acute toxicity tests or bioassays are sensitive analytical tools for detecting the toxicity of various industrial effluents on target organisms. These toxicity test methods, on one hand, are rapid, sensitive and cost effective; on the other hand, each test comes with their own demerits and limitations. Choice of the different acute toxicity tests should be wisely made in terms of the final use of the treated wastewater. Some

Declaration of Competing Interest

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