Performance of various catalysts on treatment of refractory pollutants in industrial wastewater by catalytic wet air oxidation: A review

Journal of Environmental Management 228 (2018) 169–188

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Review

Performance of various catalysts on treatment of refractory pollutants in industrial wastewater by catalytic wet air oxidation: A review

T

Sushma∗, Manjari Kumari, Anil K. Saroha Department of Chemical Engineering, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi, 110016, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Catalytic wet air oxidation Catalysts Catalyst deactivation Removal efficiency Biodegradability

The tremendous increase of industrialization and urbanization worldwide causes the depletion of natural resources such as water and air which urges the necessity to follow the environmental sustainability across the globe. This requires eco-friendly and economical technologies for depollution of wastewater and gases or zero emission approach. Therefore, in this context the treatment and reuse of wastewater is an environmental friendly approach due to shortage of fresh water. Catalytic wet air oxidation (CWAO) is a promising technology for the treatment of toxic and non-biodegradable organic pollutants in the wastewater generated from various industries. Various heterogeneous catalysts have been extensively used for treatment of various model pollutants such as phenols, carboxylic acids, nitrogenous compounds and different types of industrial effluents. The present review focuses on the literature published on the performances of various noble and non-noble metal catalysts for the treatment of various pollutants by CWAO. Reports on biodegradability enhancement of industrial wastewater containing toxic contaminants by CWAO are reviewed. Detailed discussion is made on catalyst deactivation and their mitigation study and also on the various factors which affects the CWAO reaction.

1. Introduction In the last few decades, the rapid industrialization and urbanization across the world is one of the major causes of depletion of natural resources such as water. The industrial effluent generated from various industries such as pharmaceutical, textile, chemical and petrochemical contains huge amounts of toxic and refractory organic compounds. The improper discharge of industrial as well as domestic effluent can cause a severe threat to the aqueous ecosystem. Various methods are available for the treatment of wastewater depending on its composition. Therefore, economical and efficient treatment technology is required specific to a particular type of wastewater. Conventional treatment technologies like biological treatment (activated sludge process, trickling filter, rotating biological contactor, etc.) are used for domestic as well as non-toxic industrial wastewater while wastewater containing toxic and refractory compounds requires advanced treatment technologies. The technologies commonly used in industries are physicochemical (adsorption, reverse osmosis, coagulation, precipitation, membrane separation etc.), incineration, advanced oxidation processes (wet air oxidation, ozonation etc.). Sometimes a combination of these techniques is employed for the wastewater treatment. In coagulation technique, chemical (coagulant) is added to the wastewater to destabilize the charge of pollutant particles. The ∗

coagulant has charge opposite to the dispersed non-settable solids present in wastewater. The coagulant neutralizes the charge of these particles which later stick together and make micro-flocs which settle down easily. However, this technique results in generation of large amount of sludge and is not effective for dissolved solids. Membrane separation is a method in which a membrane is used to remove particles, colloids and macromolecules from the wastewater. It is an effective technology for water purification due to less space requirement and no sludge generation. However, fouling of membrane takes place after a period of time and disposal of pollutant concentrated effluent is another problem. Adsorption is an efficient technique for the removal of wide range of contaminants which get adsorbed on the surface of adsorbent. Many researchers have developed a large number of cost effective adsorbents with high surface area and capability to adsorb significant quantity of pollutants. However, this process does not ultimately destroy the pollutants and requires an additional step for regeneration of the spent adsorbent. The disposal of concentrated pollutants on the adsorbent is another issue which ultimately increases the operational cost. Incineration is the controlled combustion of concentrated and toxic organic pollutants at high temperature (between 1000 °C and 1700 °C) to degrade the organic compounds into carbon dioxide and water. Although incineration is a complete pollutant destruction method, it is

Corresponding author. Present Address: Department of Industrial Waste Management, Central University of Haryana, Mahendergarh, Haryana, 123031, India. E-mail address: [email protected] (Sushma).

https://doi.org/10.1016/j.jenvman.2018.09.003 Received 11 June 2018; Received in revised form 29 August 2018; Accepted 1 September 2018 0301-4797/ © 2018 Elsevier Ltd. All rights reserved.

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hazardous and costly oxidizing agents. However, certain limitations are associated with it, such as requirement of high temperature and pressure and corrosive environment (low pH due to carboxylic acids intermediates formation during reaction). Therefore, to reduce the severe operating conditions of temperature and pressure in WAO, a catalyst is used and the use of a catalyst in conjunction with WAO is known as catalytic wet air oxidation (CWAO). In CWAO, higher oxidation of organic compounds is achieved at mild operating conditions (low temperature and pressure) in presence of a catalyst, thereby reducing the capital and operating cost compared to WAO. Since most of the review papers have reported the reaction mechanisms and kinetics for CWAO of organics and performances of various catalyst used in CWAO. However, there are very less studies reported on the biodegradability enhancements and toxicity study of the effluent obtained after CWAO treatment. Moreover, CWAO reaction condition depends not only on the catalyst but also on various other parameters such as reaction temperature, operating pressure, pollutant concentration, solution pH, catalyst dosage etc. which also a lack of study in recent review papers on CWAO. Therefore, in the present review article performances of various heterogeneous catalysts with their deactivation and mitigation studies for CWAO of various pollutants are discussed. Additionally, a detailed discussion on role of CWAO for biodegradability enhancement of various industrial effluents has been made. Finally various other parameters like reaction temperature, operating pressure, pollutants concentration, solution pH, catalyst dosage etc. have been discussed for CWAO reaction performance.

a costly process and suitable for effluent with organic load more than 25% in order to achieve auto thermal oxidation. The major drawback is the generation of toxic compounds like dioxins and furans during the incomplete combustion of the organics which require additional air pollution control equipment. Additionally, the handling of the toxic residual ash containing concentrated amount of the pollutants is another environmental concern (Kim and Ihm, 2011). The biological treatment methods are economical and widely used for domestic wastewater treatment. Although, conventional biological processes are commonly employed due to environment concerns and low cost, they are not suitable for the treatment of effluent containing toxic organics due to biomass poisoning. Advanced oxidation processes (AOPs) which include Fenton, photoFenton, wet air oxidation (WAO), ozonation (O3), hydrogen peroxide oxidation, photocatalysis etc., are the emerging technologies for the treatment of toxic and refractory industrial wastewater. The generation of hydroxyl radicals which are highly reactive and have high oxidation potential in AOPs makes them capable to oxidize various organic compounds. The Fenton's oxidation is very effective for treatment of toxic industrial wastewater. However, the separation of iron from the treated solution is required and generated sludge has to be disposed off carefully. Ozone has high oxidation potential of 2.07 compared to hydrogen peroxide (1.78) and chlorine (1.36). The combination of ozone with other oxidizing agents such as hydrogen peroxide, UV light is used for the treatment of wide range of toxic organic pollutants (Ghuge and Saroha, 2018). But at high effluent flow rate and organic load, these processes are less useful. Additionally, as ozone is a harmful gas, the unreacted ozone requires an ozone destruction unit which increases the cost of the process. Photocatalysis is a good technology in which organic compounds breakdown take place with the help of UV radiation energy. However these photocatalysts are active only under UV-irradiation which contributes only 5% intensity in solar energy while other absorbing visible light are not stable during this process, therefore, unsatisfactory photocatalytic efficiency due to the insufficient solar light absorption, low surface area of photocatalysts make this process less useful (Jiang et al., 2017, 2018a; 2018b; Zhang et al., 2018a, 2018b). Therefore, the effective removal of toxic organic pollutants requires a new, compact and more efficient technique. WAO (Wet air Oxidation) is a promising technology for the treatment of effluent containing toxic as well as biologically refractory compounds. Reaction temperature, dissolved oxygen and mixing intensity are the main parameters for the degradation of organic pollutants in WAO process. WAO is an eco-friendly process compared to other AOPs like ozonation and hydrogen peroxide oxidation which use harmful and expensive oxidizing agents. It is an efficient technique for treating wastewater with high organic loading (10–100 g/L of COD) which is too dilute for incineration and toxic for biological treatment (Kim and Ihm, 2011). Further WAO does not generate NOx, SO2, HCl, dioxins, furans, fly ash, etc. The WAO process is carried out at high reaction temperature (200–325 °C) and pressure (5–15 MPa) in presence of air/oxygen and hydroxyl radicals are generated which have a great potential for the treatment of wastewater containing toxic organic pollutants. The organic matter present in wastewater is oxidized, via free radical reaction mechanism either to partially oxidized intermediates or completely mineralized. The WAO technique does not require any

2. Catalytic wet air oxidation (CWAO) The CWAO is a promising technique for the degradation of toxic organic compounds present in the industrial effluent. In CWAO process, large molecular weight organic compounds are oxidized using air/ oxygen over surface of catalyst either partially into lower molecular weight organic compounds or completely into carbon dioxide and water as shown in Fig. 1. These lower molecular weight organic compounds are further mineralized into carbon dioxide and water but need more energy due to their refractory nature. The CWAO is mainly used for achieving two objectives (Fig. 2): (i) for complete mineralization of organic compounds into carbon dioxide and water (ii) for enhancing the biodegradability and decreasing the toxicity of the effluent by conversion of toxic compounds to biodegradable intermediates thereby allowing the use of biological methods for its further treatment. The conversion of complex organic compounds to biodegradable intermediates is much cheaper compared to complete mineralization as complete oxidation requires more energy. In CWAO process, the organic nitrogen may be decomposed into ammonia, nitrate and nitrogen depending on the reaction conditions and other toxic elements like phosphorus is converted to phosphates, halogens to halides and sulphur to sulphate. The high molecular weight organic compounds such as pyridine, phenol, chlorophenols etc. are oxidized into lower carboxylic acids which are more biodegradable in nature. Therefore, CWAO can be employed at mild reaction conditions for treatment of toxic effluent when biological techniques are ineffective and the main emphasis would be to convert toxic organic pollutants in the effluent into

Fig. 1. Degradation of organic compounds by CWAO. 170

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the same reaction mechanism but at milder temperature. Moreover, homogenous Cu2+, Fe2+ and Mn2+ catalysts result in desired intermediates by selective rupture of aromatic ring following slow kinetic regime (initial step). In this process, oxidation of phenol results in catechol and hydroquinone and further oxidation results in many intermediates which follows free radical autocatalytic pathway. The addition of radical promoter improves the activity of catalyst resulting in higher oxidation of organics. Vaidya and Mahajani (2002) reported the enhanced oxidation of phenol using copper catalyst by addition of hydroquinone, which is a free radical generator. Moreover, the co-oxidation of organic compounds such as nitrobenzene and phenol showed higher oxidation of phenol using copper catalyst (Fu et al., 2015). The performance of homogenous catalysts increased by combination of copper and iron salts compared to their individual salts for the treatment of sludge by CWAO at 200–250 °C under 4–15 bar O2 partial pressure (Bernardi et al., 2010). Copper sulphate had high activity but prevented dissolution of suspended organic material into aqueous medium while FeSO4 had moderate activity but showed activity for transferring suspended organic matter into liquid phase. The combination of both these catalysts showed synergistic effect and catalytic activity was found to increase. Wang et al. (2012) studied copper as co-catalyst (Cu2+[PxWmOy]q−) for the pre-treatment of fosfomycin pharmaceutical wastewater. The pharmaceutical wastewater contained WO3− and PO43− and berberine wastewater contained Cu2+ and the mixture of these two form polyoxometalates (POMs) co-catalyst and COD removal of 40% was obtained at 250 °C and 1.4 MPa pressure. Although, the BOD/COD ratio was not significantly improved and the results revealed that toxicity remained in the CWAO effluent and was not suitable for biological treatment. It was due to the precipitation of solid crystals and the dissolution of copper did not occur easily. However, the homogenous catalyst system is not economically viable despite higher oxidation rates of organics than heterogeneous catalysts as the requirement of additional step for separation of dissolved catalysts after CWAO treatment which makes the process costly.

Complete CWAO of wastewater Biodegradability enhancement and toxicity removal

Post-treatment by biological processes

Fig. 2. CWAO objectives scheme for treatment of wastewater.

intermediates more amenable to biological treatment. The biodegradable effluent can be easily treated using biological process for complete removal of organic compounds. Therefore, the integration of CWAO at mild operating conditions and biological treatment could be effective step for treatment of toxic industrial effluent. CWAO is one of the most commonly used processes for treatment of toxic organic effluent at less severe operating conditions. In this process, the dissolved organic carbon is oxidized in liquid phase in presence of air or oxygen over a catalyst at lower temperature and pressure compared to WAO process. It is more efficient than WAO for oxidation of refractory compounds due to lower energy demand. The catalyst should be cheap and should have sufficient active sites on the catalyst surface for the treatment of industrial wastewater (Levec and Pintar, 2007). The catalyst should be stable in corrosive environment, mechanically strong and stable after several runs and should result in higher oxidation or complete mineralization of organic compounds (Matatov-Meytal and Sheintuch, 1998). Since, the catalyst is the heart of CWAO process, the selection of the catalyst is important for the feasibility of CWAO. Various homogenous and heterogeneous catalysts used for the CWAO of synthetic and real industrial effluents have been described below: 3. Homogenous catalysts Salts of iron, copper and manganese are commonly used as homogenous catalysts in CWAO process. Homogenous catalysts result in better performance and the process control and reactor design is less complex compared to the heterogeneous CWAO process. Homogeneous catalysts are widely used for treatment of toxic organic pollutants and industrial wastewater (Garg et al., 2004; Fu et al., 2015; Yadav and Garg, 2014; Arena et al., 2010; Bernardi et al., 2010; Velegraki et al., 2011). Garg and Mishra (2013) reported the degradation of phenol by CWAO process at 120 °C and 0.5 MPa pressure using CuSO4 as homogenous catalyst. The phenol conversion and TOC removal of 90% and 67% respectively were observed in 4 h of reaction and oxalic acid and acetic acid were found to be the main intermediate compounds in the reaction mixture. Yang et al. (2010b) studied CWAO of industrial wastewater containing epoxy acrylate monomer using copper, manganese and iron (homogenous) catalysts. The copper catalyst showed the highest activity for COD removal (77%) and an increase in biodegradability of the CWAO effluent was observed. Collado et al. (2010) studied CWAO of thiocynate with homogenous copper sulphate at 180 °C and 8.1 MPa oxygen pressure. The thiocynate degradation of 95% was obtained within 1 h and sulphates, carbonates and ammonium ions were found in the CWAO effluent. The higher activity of Cu2+ was attributed due to reduction of active Cu2+ to Cu+ during oxidation of thiocynate and re-oxidation of Cu+ to Cu2+ by dissolved oxygen. The homogenous reaction mechanism follows free radical autocatalytic pathway of 3 steps: initiation, propagation and termination (Arena et al., 2015). In WAO process, for aromatic compounds like phenol, the initiation step is very slow which follows faster kinetic reaction resulting in low TOC removal and accompanied with various intermediate compounds. The formation of undesired intermediates due to non-selective rupture of aromatic ring (propagation step) results in slow kinetics of termination step. The homogenous catalysts follow

4. Heterogeneous catalysts The heterogeneous CWAO process has been reported to be one of the most promising methods for waste minimization on large scale applications. It is an effective and cheap technology with the capability to treat various types of industrial wastewater. Various heterogeneous catalysts such as noble metals, non-noble metals have been used in recent years.

4.1. Noble metals The noble metals platinum (Pt), palladium (Pd), ruthenium (Ru), and rhodium (Rh) are the most promising metals used in CWAO process for abatement of pollutants. In most of the studies reported in literature, the activity of metal depends on the characteristics of wastewater such as pH, pollutant type and concentration of pollutant. The noble metals are generally used because of their high activity towards oxidation of organic pollutants and high catalyst stability at severe reaction conditions. The noble metals are supported on the support and the active metal sites are embedded on the surface as well as on pores of the support. The most commonly used supports are CeO2, SiO2, Al2O3, ZrO2, TiO2, activated carbon (AC), carbon nano tubes (CNT), carbon nano fibers (CNF) and a combination of them. The noble metal catalysts have been used by various researchers (as shown in Table 1) for the treatment of various pollutants such as phenol (Espinosa de los Monteros et al., 2015), substituted phenols (Suarez-Ojeda et al., 2005), carboxylic acids (Gaálová et al., 2010), nitrogenous organics (Song and Lu, 2015) and real industrial effluents (Minh et al., 2008) by CWAO process. 171

172 180–230 150–200 180–220 140 150 160–200 95 20–53

SR SR SR SR SR FBR SR SR SR TBR SR SR SR FBR SR, FBR

Ru/TiO2 Pt,Pd, Ru/CNF Pt/Al2O3

Ru/ZrO2 graphite

Ru/CeO2-TiO2 Ru/CeO2, ZrO2 Ru, Pt/TiO2, ZrO2

Phenol (1 g/L) Resin effluent (COD 19500 mg/L) Phenol (0.44 mol/L, COD 50–1200 mg/L) Formic acid (5 g/L, TOC 1305 mg/L) Diuron (pesticide) (TOC 19.5 mg/L) Phenol (1 g/L) Butyric acid, maleic acid (3 g/L)

Aniline, phenol (20–23 mmol/L)

p-hydroxy benzoic acid (10 mmol/L) Acetic acid (4.7 g/L) Olive oil mill wastewater (OMW)- (TOC 1 g/L)

N-N- dimethyl formamide (5 g/L) N-ethyl-ethanol (500 mg/L) Methyl methacrylate (500 mg/L) Phenol (2.5 g/L)

140 140 190

350

140–180 180–240 60

5 MPa 0.7 MPa (O2) 4 MPa 2 MPa

190 200 220 200

5 MPa (O2) 4 MPa 7 MPa

2 MPa

5 MPa 1 MPa (O2) Atm. Pr.

1.4 MPa (O2) 1.5 MPa (O2) 0.1 MPa Atm. Pr.

5–7 MPa 0.34–1.38 MPa (O2) 2 MPa (O2) 2 MPa (O2)

1 MPa (O2) 5 MPa

0.1–1.1 MPa 5.7 MPa 0.9 MPa (O2) 3.6 MPa

230 230–200

Estrogen 17β-estradiol (E2) (0.272 mg/L) Wastewater (COD 178183 mg/L) Tert-amyl-methyl ether (227 g/L) Succinic acid (2032 mg/L) Aniline (2 g/L) Model compound (COD 5–20 g/L) Acetic acid (COD 5 g/L)

180 260 150 200

2 MPa (O2) 2.5 MPa 5 MPa Atm. Pr. 2 MPa (O2) 0.69 MPa

1 MPa (O2) 5 MPa 2 MPa (O2)

1 MPa (O2) O2 flow rate- 150 mL/min 5 MPa

0.69–2.07 MPa Atm. Pr. 7 MPa

Ru/TiO2 Ru and Ir/Ti monolith Sn/CeO2, Rh/Al2O3 Au/CeO2 Pt/AC Rh and Ru/Ce-γ-Al2O3 Ru and Pt/CeO2 Zr.1(Ce.75Pr.25).9O2 Pt, Pd, Ru/TiO2, ZrO2 Ru/TiO2 Pt/Al2O3 Pt/MnO2-CeO2 MnOx-CeOx Pt/Al2O3, CeO2 Ru/AC, ceramic spheres Pt/Al2O3 Pt/CexZr1-xO2

Ru/ZrSiO2 and ZrO2 Ru/TiZrO4 Pt, MnCeOx/CeO2 Pt/TiO2 and ZrO2

200 200 160 40 160 200

180–220 245 190 190 160 200 160

150–325 70 210

5h 120 min ST-2 h gRu/gTOC

320 min

19 h 50 h

3h 4h 5h 6h

6h 2h 3h

8h 3.5 h 1h 5h 3h 2.5 h

1.5 h 18–160 min 60 min 6h

3h LHSV 2/h 6h 8h 3h 120 min

3h LHSV 1.5/h 6h LHSV 0.6/h 2h 6h 3h

9h 2h

11, 7.9 78 100 99

100

96 97 100

90 65

99

100 85

100

79–99 92 100 N2 Sel. 97.5

100 100 100 98 100

96 98 100

100

95

42

Pollutant

Time

Temp (oC) Pressure

Removal (%)

Operating conditions

SR SR SR SR FBR TBR TBR SR SR SR SR SR SR

SR FBR SR SR SR SR

Ru/CeO2 Ru/xTiZrO4 Ru/N-CNFs Pt/CeO2 Pt and Ru/Ce Pd/AC, PdCl2-60%

SR PBR SR PBR SR SR SR

Ru–Cu/carbon Ru-Ce/γ-Al2O3 Pt and Ru/CexZr1−xO2 Pt-Ru, Pt and Ru/ZrO2 Pt/TiO2-Ce Pt/TiO2 Pt/TiO2–CeO2

Aniline (20 mmol/L) Isothiazolone (TOC 467 mg/L) Phenol (0.02 M) Formic acid (0.1–1.2 g/L) Phenol (2.098 g/L) Tannery NF reject COD 165 mg/L, TOC 34 mg/ L Pentachlorophenol (2 g/L) Isophorone (5 g/L) Phenol (1 g/L) Ammonium acetate (58 mmol/L, TOC1392 mg/L)

SR SR SR

Ru/C Pt/Al2O3 Ru and Pt/TiO2-ZrO2

Quinolone (0.1 g/L) Industrial Effluent (COD 15 g/L) Glucose-glycine melanoidins solution (TOC 2.2 g/L) Ammonia (1 g/L) Wastewater (20 g/L) Succinic acid (5 g/L) Methylamine (2.4 g/L) Phenol (1 g/L) Ammonia (60 mmol/L) Phenol (2.098 g/L)

Reactor type

Catalyst

Pollutant

Table 1 Studies on CWAO of various organic compounds using noble metal catalysts.

COD-96 TOC-97

Mineral. 17, 70

TOC-88 TOC-91, 88

TOC- 95 COD- 92 COD-95.2

TOC-90 TOC-90

COD-96.3

TOC-68

COD 95

TOC 95 TOC 25.7

TOC 28-46

TOC 85

TOC 82.6 TOC 93

COD 99.5 TOC 93 TOC 100

COD 45 TOC 60

TOC/COD

(continued on next page)

Triki et al. (2009) Wang et al. (2008) Minh et al. (2008)

Lopez et al. (2009)

Carrier et al. (2009) Taboada et al. (2009) Dukkanci and Gündüz (2009)

Lee et al. (2010) Liu et al. (2010) Sulman et al. (2010) Yang et al. (2010a)

Grosjean et al. (2010) Gunale and Mahajani (2010) Ji et al. (2010) Kouraichi et al. (2010)

Bistan et al. (2012) Hosseini et al. (2012) Cuauhtémoc et al. (2011) Tran et al. (2011) Morales-Torres et al. (2011) Yu et al. (2011) Gaálová et al. (2010)

Wei et al. (2013a) Wei et al. (2013b) Arena et al. (2012) Bernardi et al. (2012)

Fu et al. (2016) Yu et al. (2016a) Yang et al. (2015) Song and Lu (2015) Rocha et al. (2015b) Lousteau et al. (2015) Espinosa de los Monteros et al. (2015) Lafaye et al. (2015) Wei et al. (2015) Ayusheev et al. (2014) Cau et al. (2014) Keav et al. (2014) Tripathi et al. (2013)

Pachupate and Vaidya (2018) Sushma and Saroha (2017) Thu and Michèle (2016)

Reference

Sushma et al.

Journal of Environmental Management 228 (2018) 169–188

Ru/TiO2 Pt/CeO2, CexZr1-xO2 Ru/CexZr1−xO2, Ce-Zr-PrOx, Ce-Zr-NdOx Pt, Ru/TiO2, ZrO2 Ru/TiO2, ZrO2 Pt, Ru/CeO2, Zr0.1Ce0.9O2, Zr0.1(Ce0.75Pr0.25)0.9O2 Pt/CeO2, Zr0.1(Ce0.75Pr0.25)0.9O2 Ru/CeO2/Al2O3 Ru, Pd, Pt/CeO2 Pt/AC RuO2 - CeO2/γ-Al2O3 Ru/TiO2 Pt/Graphite, TiO2, Al2O3, AC Pt, Ru/AC, γ-Al2O3 Pt/Graphite Pt, Pd, Ru/CBC Pd, Pt, Ru/AC, Al2O3, CeO2 Ru, Ru-CeO2/AC Pt/γ-Al2O3 Pt/γ-Al2O3 Pt/TiO2 Pt, Pd, Ru/CeO2

p-hydroxybenzoic acid (10 mmol/L) Phenol (22.3 mmol/L) 2-chlorophenol (2 g/L, TOC-1120 mg/L) p-hydroxybenzoic acid (10 mmol/L) p-hydroxybenzoic acid (10 mmol/L) Acetic acid (78 mmol/L)

140 160 140 140 140 200 200 140 230 200 150 100–140 120–170 200 200 120–160 180 160 80–180 175 150–200 170

SR TBR SR SR SR SR SR SR SR TBR SR SR SR SR SR SR

Temp ( C)

o

2 MPa (O2) 0.7 MPa (O2) 2 MPa (O2) 0.69 MPa (O2) 3 MPa (O2) 0.1 MPa (O2) 1.7 MPa 0.42 MPa (O2) 0.01–0.08 MPa (O2) 0.96–1.5 MPa (O2) 2.6 MPa 2 MPa of (O2) Atm. Pr. 0.5–1.5 MPa (O2) 34-82 Atm. Pr. 2 MPa (O2)

5 MPa 0.5–1.5 MPa (O2) 3.0–5.0 MPa 5 MPa 5 MPa 2 MPa (O2)

Pressure

Operating conditions

SR SR SR SR SR, TBR SR

Reactor type

40–180 min 4h 2h 3h

3h 1h 3h 2h 2.5 h 24 h 1h 105 min 70 min LHSV 0.5–6.0/h 1h

6.6 h 3h 6h 7h 7h 3h

Time

100 62 99 99 98 99 100 95 100 100

100 60 98

94 80–100 70 100

Pollutant

Removal (%)

TOC 85

Sel. of CO2 100

Min. 45 Sel. of CO2 90

83 Min. 90

TOC 65-93

TOC-64

TOC/COD

Mikulová et al. (2007a) Massa et al. (2007) Barbier et al. (2005) Gomes et al. (2005) Yang et al. (2005) Kojima et al. (2005) Masende et al. (2005) Cao et al. (2003) Masende et al. (2003) Trawczyński (2003) Qin et al. (2001) Oliviero et al. (2000) Lee and Kim (2000) Hamoudi et al. (1998) Maugans and Akgerman (1998) Duprez et al. (1996)

Triki et al. (2008) Nousir et al. (2008) Li et al. (2007a) Minh et al. (2007a) Minh et al. (2007b) Mikulová et al. (2007b)

Reference

*LHSV - Liquid hourly space velocity; SR - Batch or semi-batch operated stirred-tank reactor with slurry; FBR - Continuously operated fixed-bed reactor; PBR -Continuously operated packed bed reactor; TBR- Trickle bed reactor; ST - Space time, NF –nano filtration, Sel.-Selectivity, Min.-Mineralization.

Acetic acid (78 mmol/L) Phenol (5 g/L) Aniline (20 mmol/L) Acetic, propionic, butyric acids (5 g/L) Phenol (4.2 g/L) o-Chlorophenol (100 mg/L) Phenol (1 g/L) Ammonium sulphate (1000 mg/L N) Phenol (0.02 mol/L) Phenol (5 g/L) p-chlorophenol (1.5 g/L) Phenol and acrylic acid (1 g/L) Maleic, oxalic, and formic acids (3 g/L) Phenol (25 mg/L) Phenol (H2O:Phenol::5:0.3) Phenol and Acetic acid (COD 5 g/L)

Catalyst

Pollutant

Table 1 (continued)

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promising compared to large platinum crystallites or higher OSC for oxidation of acetic acid. Perkas et al. (2005) studied CWAO of model compounds such as succinic, acetic and p-coumaric acids to demonstrate the activity of platinum and ruthenium metals supported on TiO2 and ZrO2 supports and reported that the Pt/TiO2 catalyst showed higher activity and stability towards mineralization of succinic and p-coumaric acids. However, Tran et al. (2011) observed that 4 wt. % Au/CeO2 showed better performance for succinic acid oxidation than platinum or ruthenium supported on TiO2 and ZrO2. The efficiency of oxidation of succinic acid was found to increase with an increase in gold loading. The support (CeO2) alone exhibited the oxidation of succinic acid and acetic acid was found to be an intermediate refractory compound. The introduction of gold into CeO2 resulted in complete mineralization of succinic acid due to synergistic effect between gold and CeO2. Yang et al. (2015) studied the CWAO of succinic acid at 190 °C and 50 bar pressure using platinum and ruthenium metals on CexZr1−xO2 mixed oxide support. The degradation of succinic acid was found to increase with an increase in the cerium content of the support. Both catalysts were found to be stable for leaching and carbonaceous deposition. Dükkanci and Gündüz (2009) studied CWAO of butyric acid and maleic acid over platinum, palladium and ruthenium metals supported on TiO2 and commercial Pt/γ-Al2O3 at 60 °C and atmospheric pressure. The maleic and butyric acids were oxidized into refractory intermediates such as oxalic acid, formic acid and acetic acid. The commercial Pt/γ-Al2O3 catalyst showed higher activity towards oxidation of maleic and butyric acids. However, acetic acid produced during the reaction caused the deactivation of catalyst by blocking the active sites of the catalyst. Yang et al. (2010a) used Pt/CexZr1−xO2 catalyst at 21–53 °C and atmospheric pressure for degradation of formic acid and complete mineralization of formic acid was obtained with Pt/ Ce0.9Zr0.1O2 (0.08 wt. %) catalyst (0.5 g/L). The initial reaction rate was proportional to dissolved oxygen concentration at 40 °C while the initial reaction rate at 53 °C was influenced by both formic acid and oxygen concentrations. These changes in initial reaction rate showed the role of ceria-zirconia mixed oxides support in activation of oxygen, which overcame the oxygen barrier onto the support surface and therefore higher conversion of formic acid was obtained at 53 °C. Similarly, Cau et al. (2014) studied the formic acid oxidation at 40 °C and atmospheric pressure using Pt/(Ce0.5Zr0.5)O2 and Pt/CeO2 catalysts. The Pt/(Ce0.5Zr0.5)O2 catalyst showed higher activity than Pt/CeO2 catalyst due to preparation of mixed oxides support by sonolysis method which increases the surface area with mesoporous and microporous combination of support morphology. Renard et al. (2005) studied stearic acid oxidation over Pt/CeO2 and Ru/CeO2 catalysts and found that the reaction mechanism of stearic acid oxidation occurs mainly via (1) carboxy-decarboxylation yielding CO2 and (2) C-C bond rupture which produces acetic acid and CO2. The Pt/CeO2 catalyst followed (1) pathway while (2) pathway was more selective for Ru/ CeO2 catalyst. In conclusion, the platinum or ruthenium supported on CeO2 or mixed oxide support (Ce, Zr) were found to be efficient for the oxidation of carboxylic acids. The p-hydroxybenzoic acid is mainly found in the olive oil mill effluent as a pollutant. It is toxic and refractory in nature and not suitable for treatment by biological processes. It is also found as an intermediate during phenol oxidation (Eftaxias et al., 2006). The treatment of phydroxybenzoic acid was studied by Minh et al. (2007b) using ruthenium supported on TiO2 or ZrO2 at 140 °C and 5 MPa pressure. The ruthenium catalyst was found to be stable in both batch and continuous reactors. A small loss in efficiency was observed due to over oxidation of ruthenium catalyst while deactivation by coke was overcome by reversible nature of reduction and oxidation of ruthenium metal. Minh et al. (2007a) studied CWAO of p-hydroxyphenylacetic acid and p-hydroxybenzoic acid at 140 °C and 5 MPa pressure using ruthenium and platinum supported on TiO2 or ZrO2. The conversion and mineralization of pollutants was influenced by nature of ruthenium precursor. The

4.1.1. Performance of noble metals in treatment of various model compounds The application of noble metals for the treatment of phenols is widely reported in literature as phenol is extensively used in various industries such as petrochemicals, oil refineries, pharmaceutical etc. (Liu et al., 2015). The commonly used noble metals for the treatment of phenol in wastewater by CWAO process are platinum, palladium, ruthenium and rhodium. Taboada et al. (2009) reported comparison in terms of activity of platinum, palladium and ruthenium metals supported on carbon nano fibers (CNF) for degradation of phenol. The catalysts were prepared by incipient wetness impregnation method with 2 wt. % metal content and found the following sequence of TOC removal and catalyst stability for phenol oxidation by CWAO process: Pd/CNF ˃ Pt/CNF ˃ Ru/CNF. Barbier et al. (2005) reported the activity order of platinum, palladium and ruthenium catalysts supported on CeO2 for conversion of phenol as follows: Ru/CeO2 ˃ Pd/CeO2 ˃ Pt/ CeO2. Espinosa de los Monteros et al. (2015) studied the oxidation of phenol using platinum and ruthenium metals supported on TiO2-CeO2 and platinum was found to be more efficient than rhodium catalyst. The comparison between the data of these studies suggests that the activity of noble metals vary with the nature of support as well as metal dispersion on the support. The selectivity of support towards phenols and byproducts formed during CWAO reaction also vary with varying operating conditions of temperature and pressure. Therefore, various supports have been used for the treatment of phenol by CWAO process using platinum, palladium and ruthenium catalysts. Lopez et al. (2009) studied the effect of ZrO2, graphite and activated carbon (AC) supports with ruthenium for the treatment of phenol by CWAO process at 140 °C and 2 MPa pressure. The phenol conversion of 100% and mineralization of 70% was observed with Ru/ AC. It was due to high surface area and high pore volume of AC support which provides higher adsorption capacity to the catalyst. However, despite the higher oxidation of phenol with ruthenium catalyst, formation of carbonaceous deposits on the catalyst surface was found which lowers the catalyst activity. The catalytic activity also depends on the preparation method of the catalyst. The Ru/CeO2-Al2O3 catalysts prepared by impregnation method were found to be more active than co-impregnation, co-precipitation and surfactant method (Massa et al., 2007). The ruthenium particles were found to be in direct contact with the reactants in the Ru/CeO2-Al2O3 catalyst prepared by impregnation method and showed the highest CO2 selectivity (> 90%). The carboxylic acids such as formic, acetic, oxalic, succinic and pcoumaric acids are the refractory intermediates formed during the oxidation of various higher molecular weight organic compounds. Acetic acid is one of the most refractory compounds and found as an end product compound in CWAO effluent. Therefore, it is commonly used as a model compound in the studies reported for treatment of wastewater by CWAO process. The noble metals platinum and ruthenium have been widely used for the treatment of acetic acid on various supports. The catalytic activity was found to increase by doping of zirconium and praseodymium (Pr) metal cations into CeO2 support by increasing oxygen transfer and the oxygen storage capacity (OSC) of the catalyst. Various combinations of supports such as CeO2, Zr0.1Ce0.9O2 and Zr0.1(Ce0.75Pr0.25)O2 mixed oxides have been used for the CWAO of acetic acid using platinum and ruthenium metals (Mikulova et al., 2007b; Gaálová et al., 2010). Mikulova et al. (2007) reported that Pt/ CeO2 catalysts exhibited higher conversion compared to other supports at 200 °C and 2 MPa oxygen pressure and acetic acid mineralization of 90% was achieved in 3 h. The uniform dispersion of small sized platinum sites of Pt/CeO2 catalyst resulted in higher conversion compared to other mixed oxides supports. The catalytic activity of mixed oxides based platinum catalyst was found to decrease due to poisoning of the catalysts by the formation of carbonates due to high OSC of mixed oxides support. The higher OSC favors both oxygen and CO2 migration resulting in carbonation of catalyst by capturing CO2. Therefore, moderate OSC and good dispersion of platinum particles are more 174

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metal species increased with introduction of Pt metal in Pt-Ru/Al2O3CeO2 catalyst which resulted in higher catalytic activity. The complete conversion of methylamine was observed with high selectivity of N2 at 210 °C using Pt-Ru/Al2O3-CeO2 catalyst. Similarly, Song and Lu (2015) observed the complete oxidation of methylamine at 190 °C using Pt-Ru/ ZrO2 catalyst due to ZrO2 support effect. Isothiazolone is another nitrogenous organic which is used in cosmetics, shampoos and to control bacteria, fungi and algae in various systems and therefore found in different types of wastewater. The treatment of isothiazolone was reported by Wei et al. (2015) using Ru/ xTiZrO4 catalyst by CWAO process. It was observed that introduction of TiO2 hindered the crystallization of ZrO2 which improved the crushing strength of the catalyst and increased the bond strength of Ru with support. The TiO2 addition favors the TOC removal due to enhancement in catalytic activity and highest TOC removal (82.6%) and TN removal (44.5%) was achieved using Ru/0.2TiZrO4 catalyst with good mechanical strength. The demand for ammonia has been increasing due to its high usability in various industries (fertilizers, urea, petroleum refineries etc.). Thus, it is found in various industrial effluents and also formed as an intermediate during oxidation of various nitrogenous organic pollutants. It is a refractory compound and therefore, various studies have been reported in the literature for CWAO of ammonia. Cao et al. (2003) studied the ammonia conversion using platinum and ruthenium supported on AC and found Pt/AC to be more efficient catalyst in terms of ammonia removal, pH sensitivity and stability. An ammonia removal of 88% was obtained at 200 °C and initial pH of 12 but small fraction of nitrate and nitrite were also found. Lee et al. (2005) studied ammonia conversion over Ru/TiO2 catalyst for the preferential formation of nitrogen and observed that the catalyst oxidized ammonia into nitrous acid and has no direct role in the selectivity of nitrogen. However, nitrogen is highly favored by homogenous aqueous phase reaction of + NO− 2 (nitrous acid dissociated) with NH4 ions. Bernardi et al. (2012) studied CWAO of ammonia at 200 °C and 3.6 MPa pressure over Pt/ TiO2 catalyst using ammonium acetate as a model compound. Ammonia was found to be completely removed with N2 selectivity of 97.5%. Kim et al. (2012) studied CWAO of ammonia using Pt/Fe/ZSM at 175 °C and an ammonia conversion of 81% and N2 selectivity of 93% were achieved. The dispersion of platinum with iron oxide was found to be better on ZSM5 support, compared to other supports like Al2O3 and SiO2. The ammonia conversion was enhanced by adjusting iron loading to achieve good dispersion of platinum on the support. Lousteau et al. (2015) studied the ammonia conversion using (platinum, palladium, ruthenium, iridium and rhodium) noble metals supported on TiO2 and ZrO2 at 200 °C and 50 bar pressure and found Pt/TiO2 catalyst to be most active and selective towards N2 formation. The metal-oxygen bond energy plays a very important role in the formation of N2 and an increase in metal-oxygen bond energy decreased the activity of the catalyst and lowered the selectivity towards N2 (increased oxygen leads to increase in nitrates and nitrites formation).

non-chlorine Ru(NO)(NO3)3 precursor was found better than RuCl3 precursor but the activity of platinum catalyst was not affected by nature of platinum precursor and same conversion was achieved using both chlorinated and non-chlorinated platinum precursors. Similarly, Triki et al. (2008) studied oxidation of p-hydroxybenzoic acid at 140 °C and 5 MPa pressure using Ru/TiO2 catalyst and Ru(NO)(NO3)3 precursor resulted in higher TOC removal efficiency compared to RuCl3. In another study, Triki et al. (2009) reported high activity of Ru/(Ce-Ti) catalyst in the oxidation of p-hydroxybenzoic acid. The synergistic effect of CeO2 and TiO2 affects the surface structural properties of Ce–Ti support which resulted in higher dispersion of ruthenium on the catalyst surface and enhanced the catalytic activity. 4.1.2. Performance of noble metals in degradation of nitrogenous compounds Aniline is the key compound used in the manufacture of chemicals, dyes, medicines, rubber and hence found in various industrial effluents. It is more refractory compared to phenol and is unsuitable for biological treatment. Morales-Torres et al. (2011) studied aniline conversion by CWAO over Pt/AC catalyst and found that the aniline conversion and mineralization were dependent on the porosity of AC and platinum dispersion. The catalytic activity decreased after 1st cycle run due to saturation of adsorption capacity and was found constant after 2nd and 3rd cycle runs. Similarly, Garcia et al. (2005) studied the aniline oxidation using Pt/MWNT catalyst at 200 °C and 6.9 bar of oxygen partial pressure. The catalysts showed highest activity with H2PtCl6.6H2O metal precursor. N,N-dimethyl formamide (DMF) is a polar solvent and used widely in various industries. It is carcinogenic and non-biodegradable in nature. The CWAO of DMF was studied by Sun et al. (2008) using Ru/ ZrO2 catalyst and DMF conversion of 98.3% and N2 selectivity of 88% were obtained at 240 °C and 2 MPa of oxygen pressure. However, Grosjean et al. (2010) studied the CWAO of DMF using Pt, Pd and Ru metal supported on TiO2 and ZrO2 support and observed that various major N-containing products such as dimethylamine (DMA), methylamine (MA), and ammonia were formed during DMF oxidation. Moreover, leaching of noble metals was also observed due to complexation products which were formed due to free lone pair electrons on nitrogen atom of these amines. Therefore, it was recommended for not using noble metals for the CWAO of DMF. Another nitrogenous organic such as diuron [N-(3,4-dichlorophenyl)-N,N-dimethylurea], a herbicide which is widely used in agriculture and its widespread use causes ground and surface water contamination. Therefore, to avoid the accumulation of this herbicide in the water bodies, the treatment of diuron is very necessary. The CWAO of diuron was studied using Ru/ TiO2 catalyst at 140–180 °C and 5 MPa of pressure and intermediates such as 3,4-dichloroaniline (DCA) and dimethylamine (DMA) were found in CWAO reaction mixture (Carrier et al., 2009). The intermediate DMA (refractory) was not further degraded by CWAO while DCA reacted with other carboxylic acids in CWAO reaction mixture and formed condensation products. The leaching of Ru catalyst was also observed by amines formed during CWAO reaction. Therefore, the CWAO of diuron was not a viable process and not recommended for further use. Similarly, N-ethylethanolamine (EEA) is another nitrogenous refractory organic compound found in various industrial effluents. The CWAO of EEA was carried out at 150–200 °C and 0.34–1.38 MPa of O2 partial pressure over Ru/TiO2 catalyst (Gunale and Mahajani, 2010). The Ru/TiO2 catalyst was found to be effective for the complete degradation of EEA and 70% conversion of –NH- group of EEA to N2 was obtained. Ethylamine (EA) and methyl amine were the refractory intermediates found in CWAO reaction mixture and reaction was found under mass transfer controlled regime. As, methylamine and ethylamine are the main intermediates formed during oxidation of nitrogenous organic compounds. Therefore, the CWAO of methylamine was reported by Song and Lu (2014) using Pt, Ru, Pt-Ru metal supported on Al2O3-CeO2. It was observed that the dispersion of active

4.1.3. Application of noble metals in treatment of industrial wastewater and their biodegradability enhancement Industrial wastewater contains various toxic and refractory organic compounds which are not suitable for biological treatment. Very few studies have been reported in the literature on the treatment of industrial effluent by CWAO process. Pintar et al. (2004a) studied the treatment of Kraft-bleach plant effluent by CWAO process using ruthenium (3 wt. %) Pt/TiO2 catalyst at 190 °C and 5.5 MPa pressure. The complex organic compounds were oxidized to CO2, H2O, acetic acid and innocuous salts and TOC removal of 99.6% was obtained. The bioassay test of CWAO effluent for acute toxicity was carried out using Daphnia magna and Vibrio fischeri for 48 h and 30 min, respectively (Pintar et al., 2004b). Despite the significant decrease in TOC, the CWAO effluent was found to be more toxic to Daphnia magna by factor of 3–33 compared to raw effluent due to synergistic effect of acetic acid and salts present in 175

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the solution. However, the toxicity of CWAO effluent for Vibrio fischeri was reduced, suggesting low sensitivity of Vibrio fischeri towards synergistic effect of acetic acid and salts. Liu et al. (2010) carried out CWAO of highly acidic (pH-0.2) resin effluent containing various compounds such as phenols, formaldehyde, methanol etc. over Ru/AC catalyst and the COD removal of 92% and phenol removal of 96% were achieved at 200 °C and 1.5 MPa of oxygen partial pressure. Olive oil mill effluent contains various organic pollutants which are phyto-toxic and antibacterial in nature and is not suitable for biological treatment. Minh et al. (2008) studied CWAO of olive oil mill effluent at 190 °C and 7 MPa pressure using platinum and ruthenium supported on TiO2 and ZrO2. The CWAO effluent obtained using Ru/TiO2 catalyst was found to be biodegradable. The bioassay test of CWAO effluent was performed using Vibrio fischeri and the toxicity of olive oil mill effluent was reduced after treatment. The coupling of CWAO and anaerobic biological treatment was carried out and an increment in bio-methane production was observed for CWAO effluent (effluent obtained after CWAO treatment) compared to raw olive oil mill effluent. Similarly, Gomes et al. (2007) studied CWAO of olive oil mill effluent using platinum and iridium supported on carbon at 200 °C and 6.9 bar oxygen pressure and complete TOC and color removal were obtained using Pt/ C catalyst in 8 h. Hosseini et al. (2012) studied CWAO of industrial process effluent using Ru-Ir/monolith Ti catalysts at 200–230 °C and 5 MPa pressure. The bimetallic Ru-Ir catalyst showed higher activity compared to ruthenium, iridium, and palladium catalyst. However, the catalytic activity decreased after CWAO experiments due to leaching of active metal species. The X-ray photoelectron spectroscopy (XPS) analysis showed the ratio 0.5: 0.7: 1 of Ru-Ir-Ti oxide catalyst optimum for catalytic activity. Benitez et al. (2011) studied CWAO of effluent containing pharmaceutical compounds naproxen, phenacetin and amoxicillin at 120 °C and 2 MPa pressure using Pt/CNT catalyst. The removal efficiency with following sequence was achieved: amoxicillin (98.3%) > naproxen (96.7%) > phenacetin (24%). The CWAO of pharmaceutical compounds dissolved in ultra-pure water resulted in higher removal efficiency compared to other sources of water (ground water, reservoir water etc.) due to additional organics present in the water systems. The increasing use of membrane technology leads to generation of concentrated effluent, termed as ‘reject’. In tannery industry, the nanofiltration (NF) reject contains high amount of organics which are refractory and recalcitrant in nature. Tripathi et al. (2013) studied the CWAO of NF reject using Pd/AC catalyst at 200 °C and 0.69 MPa pressure and obtained TOC removal of 85%. The palladium metal was found to be stable after CWAO experiments but AC support underwent some morphological changes (transformation of amorphous carbon to graphite). The toxic industrial effluent of a pyridine manufacturing industry containing pyridine and its derivatives was not suitable for biological treatment (Sushma and Saroha, 2017). Therefore, it was pretreated with CWAO using Pt/Al2O3 catalyst for its biodegradability enhancement and toxicity removal. The effluent obtained after CWAO treatment was found to be biodegradable and was further post treated with aerobic biological treatment. The COD removal of 98.4% was achieved by integration of CWAO and biological treatment confirms the biodegradability enhancement of industrial effluent by CWAO treatment.

4.2.1. Performance of metal oxide catalysts in treatment of various model compounds The metal oxide catalysts have been widely used for CWAO of model compounds like phenol. Santos et al. (2005) studied CWAO of phenol using copper oxide catalyst at 127–160 °C and 8–16 bar pressure and phenol mineralization of 77% was obtained. The leaching of copper was minimized using sodium bicarbonate and the intermediates formed in CWAO effluent were found to be less toxic than phenol. Kim and Ihm (2007) reported optimum copper loading of 20 wt. % for CuOx/TiO2 catalyst for CWAO of phenol. However, Kim et al. (2007) reported optimum copper loading of 7 wt. % for CuOx/Al2O3 catalyst for CWAO of phenol due to highly dispersed Cu2+ clusters found in 5–7 wt. % copper loading while bulk CuO dominates for 10–25 wt. % copper loading in CuOx/Al2O3 catalyst. The main drawback observed in copper oxide catalyst was leaching of active metal into the reaction solution and hence limiting its use in industrial applications. Khanikar and Bhattacharyya (2013) reported leaching of copper metal for CWAO of chlorophenols using copper supported on kaolinite and montmorillonite support. It was due to low pH of the reaction solution which favored high conversion of chlorophenols but increased the leaching of copper metal and resulting in loss of catalytic activity. Various studies have been reported on iron (Fe) catalyst supported on various supports for the CWAO of organics. Quintanilla et al. (2006) studied CWAO of phenol using Fe/AC catalyst at 127 °C and 8 atmospheric pressures. The acetic acid was found as main refractory intermediate and the toxicity of the CWAO effluent was found to be reduced. Similarly, Quintanilla et al. (2007) studied oxidation of phenol at 127 °C and 8 atmospheric pressures using Fe/AC and phenol conversion of 95% was obtained. However, the catalytic activity of spent catalyst was found to decrease significantly after 1st cycle run due to leaching of iron from the support. Soira-Sanchez et al. (2009) studied oxidation of phenol using iron acetylacetonate supported on CNFs at 140 °C and 2 MPa of oxygen pressure and complete phenol conversion was achieved. Xu et al. (2009) studied oxidation of phenol at 150 °C and 1 MPa of oxygen pressure using iron catalyst. The hematite (impure iron) acted as active sites and showed good catalytic activity. The leaching of iron was found to be higher but was recovered through precipitation route by adjusting the pH of the solution. Therefore, the higher leaching of iron in the CWAO effluent caused secondary pollution problem and hence was not applicable for real industrial applications. The ceria based catalysts have lots of application in CWAO of phenolic effluents due to its high redox and morphological properties (Chen et al., 2004). Chang et al. (2005) observed the optimum dose of ceria (20 wt. %) for CeO2/γ-Al2O3 catalyst and phenol conversion of 100% was achieved within 2 h at 180 °C and 1.5 MPa of O2 partial pressure. Chen et al. (2007) reported that the introduction of copper in the CeO2/γ-Al2O3 catalyst increased the catalytic activity and 100% phenol conversion was obtained within 1 h. However, the leaching of copper metal caused secondary pollution problem and was not recommended for further use. Yang et al. (2008) studied CWAO of phenol using CeO2-TiO2 catalyst and the COD removal of 100% was achieved at 150 °C and 3.5 MPa of pressure in 2 h. The leaching of cerium and titanium metal ions were found to be very low and the catalyst showed higher activity and stability. Similarly, Delgado et al. (2012) reported the higher catalytic activity and enhanced redox properties of Ce-Zr mixed oxides catalyst. Yang et al. (2006) studied CWAO of acetic acid using CeO2-TiO2 catalyst with an optimum ratio of 1:1 of the metals. The COD removal of 64% was achieved at 230 °C and 5 MPa pressure and the catalyst was found to be quite stable. Erjavec et al. (2013) studied CWAO of bisphenol A at 200 °C and 1 MPa pressure using titanate nanotube catalyst and TOC removal of 70% and complete conversion of bisphenol A was achieved. The catalyst was found to be stable towards leaching and very less poisoning was observed due to carbonaceous deposition on the catalyst when continuous experiment was performed for 4 days. However, to reduce the operating conditions

4.2. Non-noble metal based catalyst Although, noble metals have very good activity towards oxidation of organic pollutants in effluent, they are very expensive and easily poisoned by halogens and sulphur group containing compounds. Therefore, to reduce the cost of the process, metal oxides catalysts such as copper, iron, manganese, nickel oxides have been widely used for the treatment of various organic compounds. Some of the recent studies reported in the literature are shown in Table 2. 176

177

Phenol (1 g/L) Landfill leachate (4920 mg/L) Salicylic acid (5 mM) Ammonia (400 mg/L) Vitamin B6 (COD 120000 mg/L)

Ammonia (1 g/L) Β-naphthol (COD-1400 mg/L) Paracetamol (1 g/L) Non azo dye (Basic Yellow 11) (1–3 g/L) Phenol (4.29 g/L) Fulvic acid (0.4 g/L)

Azo dye (10–50 mg/L)

2-chlorophenol, 4-chlorophenol and 2,4dichlorophenol (10−3 M) Salicylic acid (100–200 mg/L) Lignin (10 g/L) Phenol (1 g/L) Phenol (1 g/L), Acetic acid, Oxalic acid Azo dye (methyl Orange) 1000 mg/L Basic yellow 11 (200 mg/L) Basic yellow 11 (200 mg/L) Bisphenol A (10 mg/L) Methyl Orange (500 mg/L)

Cu0.1Zn0.9Al1.9Fe0.1O4 AC K2S2O4 (promoter) MWCNT Cu2+-250-750 mg/L and H2O2 (0–1500 mg/L) FeSO4 Cu, La, Ce composite CuO/Al2O3

CuO/CeO2 MnOx/Al2O3-TiO2 AC Ni/hydrotalcite Mg-AlO

CuO/γ-Al2O3 Ni/MgAlO Ni and Fe/hydrotalcites (MgAlO) Titanate nano tube Cu2(OH)3NO3/ZnO Cu:Zn- 2:1, 4:1, 6:1 Ni/MgAlO

Cu(II)-kaolinite, Cu(II)-montmorillonite and H2O2 800 ppm of Fe (II) La0.9Sr0.1MnO3 LaMnO3 CuSO4, FeSO4 (Homo) CuO/CeO2

MnO2/TiO2, ZrO2 Ce0.7Zr0.3O2, CeO2 Nitrogen- doped carbon xerogel Ce0.4Fe0.6O2 MnCeOx Wcat/Wsus = 1 Fe/AC Carbon nano tubes

140 120 140 90–150

SR SR SR SR

SR SR SR SR SR

SR SR

TBR FBR FBR SR SR TBR

SR SR TBR TBR SR

155 160–200 150 150 250

2–5 MPa 2.5 MPa (O2) 1 MPa 4 MPa (O2)

1 MPa (O2) 0.5 MPa (O2)

10 MPa (O2) 2 MPa 3.2 bar (O2) 5 MPa

250 100 150 120–180 170 150

2.5 MPa

Atm. Pr. 5 MPa (O2) 2.5–6 MPa 1 MPa (O2) Atm. Pr.

1 MPa 0.2 MPa (O2) 0.4 MPa (O2) 0.5 MPa

Atm. Pr.

2.9–3 MPa 7 bar (O2)

0.54 MPa (O2) 0.7 MPa (O2) Atm. Pr. 0.9 MPa (O2)

Atm. Pr.

Atm. Pr. 1 bar Atm. Pr. Atm. Pr. 1 MPa (O2) 0.9 MPa (O2) 1.5 MPa (O2) Atm. Pr. 2 MPa (O2) Atm. Pr. 0.66 MPa (O2) 25 MPa

150

80 180 150 210 25

135–142 140 −160 60

SR SR SR

190 140 90 110–150

50

70 30 20 90 160 160 160 Room temp. 200 40 160 400–600

1h

2h 15–30 min 3–8 h

2h 4h

30 min 4h

–

2h 15 min 2h – 20 min

1h 1h 2h 4h

8h

2h 3h

4h 0.75 h 2h 6h

5h

9h 150 min 3h 2h 3.5 h LST 0.14 h 2h 10 h 2h 0.5 h 1.5 h 2.5 h

90–100 88

100

100

98 95

100

99 98 100 100 99

87.6, 74.7 and 52.4 85 80 95 90

80 100

100 100

63.4

100 100 85

85

100 97

100

Pollutant

Time

Temp (oC) Pressure

Removal (%)

Operating conditions

SR SR SR SR

SR

SR SR SR SR SR PBR SR SR SR SR FBR PBR

CeO2 promoted MnOx/Al2O3 catalyst MnMoO4/α-MoO3 and NiMoO4/α-MoO3 Co/carbon xerogel CuO-CeO2 nanocatalysts Fe/CNF AC CuO/SiO2 Cu-Ni/organic framework Cu-Fe-La/γ-Al2O3 M2Mo4O13/α-MoO3 (M = Li, Na or K) Sewage sludge based catalyst Ni/Al2O3 30% H2O2 Mn(II), Co(II) and Ni(II)/fly ash

Industrial Effluent (COD 15 g/L) Cationic dye (200 mg/L) Phenol (100 mg/L) Paper industry effluent (865 mg/L) Phenol (1.5 g/L) Phenol (5 mg/L) Phenol (1.3 g/L) Bismarck brown dye (30 mg/L) Methyl orange (COD 3 g/L) Cationic red dye (200 mg/L) m-Cresol (5 g/L) Landfill leachate (TOC 2535 mg/L)

4-chlorophenol (CP) (640 mg/L) (H2O2:CP1:1) Methylamine (30 mmol/L) Oxalic acid (1 g/L) Paper industry wastewater (COD 865 mg/L) Phenol (102 mol/L) and acetic, oxalic, formic acid Phenol (1 g/L) Phenol (75 mg/L)

Reactor type

Catalyst

Pollutant

Table 2 Studies on CWAO of various organic compounds using non-noble metal catalysts.

COD-50

TOC-80 COD-78

COD-95 COD 77

COD 93.7 COD 62 TOC 85

TOC 70 TOC 90, 67, 81 TOC 70 TOC 49 TOC-64 TOC-70 COD-98 TOC-94 TOC 82

76.5, 57.1 and 33.3

TOC 56

TOC 100 TOC 100 COD 72 CO2 selectivity 80

COD 91 TOC 93 TOC 85 TOC 98.2

COD 93

TOC 72 COD 67

COD 42

TOC/COD

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Yang et al. (2012) Anglada et al. (2011) Collado et al. (2011) Hung (2011) Jianxiong et al. (2011)

Xu and Sun (2012) Xu et al. (2012)

Wang et al. (2013) Liu et al. (2012) Penate et al. (2012) Vallet et al. (2012)

Vallet et al. (2013)

Hua et al. (2013) Ovejero et al. (2013a) Ovejero et al. (2013b) Erjavec et al. (2013) Srikhaow and Smith (2013)

Khanikar and Bhattacharyya (2013) Collado et al. (2013) Gao et al. (2013a) Gao et al. (2013b) Garg and Mishra (2013)

Dastgheib et al. (2014) Rocha et al. (2014)

Deka and Bhattacharyya (2015) Schmit et al. (2015) Rocha et al. (2015a) Anushree et al. (2015) Arena et al. (2014)

Sushma and Saroha (2018) Zhang et al. (2018) Chicinaş et al. (2018) Anushree et al. (2017) Yadav et al. (2016) Abid et al. (2016) Xie et al. (2016) Abbasi et al. (2016) Zhang et al. (2016a) Zhang et al. (2016b) Yu et al. (2016b) Civan et al. (2015)

Reference

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Journal of Environmental Management 228 (2018) 169–188

178 SR SR SR SR FBR SR TBR

TBR SR SR FBR SR TBR SR TBR

Mn, Fe, Co, Ni, Cu/CuOx/Ce0.65Zr0.35O2 Fe/Na-Y zeolite MoO3:Ce

Fe2O3/SBA-15

Iron acetyl acetone/CNF Cu/aeromated carbon fiber (ACF)

Mn-Ce-O/zeolite CeO2/TiO2 La1−xA'xBO3 (A’ = Sr, Ce; B = Co, Mn) Cu, Mn/CeO2 -γ-Al2O3 MnCeOx and K-promoted MnCeOx Cu, Ni, Co, Fe, and Mn/γ-Al2O3 LaFeO3 Fe/AC

Fe/AC MnCeOx and K-promoted MnCeOx Cu/CeO2 AC

CeO2/TiO2 AC

CeO2/γ-Al2O3, SiO2, TiO2 CuO-ZnO/γ-Al2O3

Phenol (10 g/L) Azo dye (Congo Red) (500 mg/L) Organic dye-ST (0.3 g/L)

Pharmaceutical effluent (COD 4901 mg/L, TOC 860 mg/L) Phenol (20 mmol/L) Ammonia (400 mg/L)

Phenol (1 g/L) Phenol (1 g/L) Stearic acid (not given) Phenol (1 g/L) Phenol (1 g/L) Phenol (10 g/L) Salicylic acid (2 g/L) Phenol (1 g/L)

Phenol (1 g/L) Phenol (0.5 g/L) o-chlorophenol, p-nitrophenol (0.5 g/L) o-nitrophenol, p-nitrophenol and o-cresol (1 g/L) Acetic acid (0.5 g/L) p-nitrophenol o-chlorophenol and o-cresol, (5 g/L) Phenol (1 g/L) Phenol (2 g/L)

SR SR SR SR SR SR SR TBR

FBR

SR SR SR SR SR

SR SR

180 100–130

230 140

100–127 130 160 160

110 150 200 160 100 150 100–180 127

140 190

80

150 90 25

250

150 200 240 157–227 25

40 Room temp.

20–35 150

150 160 220

SR SR SR SR SR

25

Cumene Isopropyl benzene (210 mg/L)

AC AC

ZnO/MoO3/SiO2 Homo-Cu, Mn, Fe Hetro- CuCeOx, MnCeOx/Ce Poly (DVB-Co-VBC) macroporous(P) ZnO/MoO3 nanotubes

Polyoxometalate [G6H33N(CH3)3]7 [PW10TiO2O38(O2)2 Co3O4/CeO2 Cuo Mn-Ce

160 200 140 160

Temp ( C)

o

1.5 MPa (O2) 0.46–1.3 MPa (O2)

5.0 MPa 1.31 MPa

0.8 MPa 2.07 MPa 1.0 MPa 1.6 MPa (O2)

2 MPa (O2) 2–4 MPa (O2) 0.5 MPa (O2) 3.0 MPa 2.0 MPa 1.5 MPa 0.9 MPa 5.05 MPa 0.5 MPa (O2) 0.8 MPa

2.02 MPa (O2) Atm. Pr. Atm. Pr.

2h

3h ST- 0.12 h

4h 2h 3h 4h 1h 4h Not given ST 20–320 gcat h/ gPh. ST 320 gcat h/gPh. 100 min 200 min TOS 300 h

3.3 h 10 h 5h –

3h 4h 45 min

3h

7h 2h 2h 2h 1.5 h

3.2 bar (O2) 0.7 MPa (O2) 0.4 MPa (O2) 2–3.5 MPa (O2) Atm. Pr. Atm. Pr.

4h 18 min

25 min 6h

Atm. Pr. Atm. Pr.

Atm. Pr. 0.9 Mpa (O2)

4 Atm. Pr. 0.3 MPa (O2) 1 MPa (O2)

120 min 120 min 80 min

240 min 180 min 15 min 4h 72 h 150 min

4.2 bar (O2) 1 MPa 0.7 MPa (O2) 4.2 bar (O2) Atm. Pr.

Time

Pressure

Operating conditions

SR SR SR SR TBR SR

Reactor type

CeO2/SiO2 Micellar molybdatevandophosphoric polyoxometalate Zeolite-NaX

Phenol (1 g/L) Safranin T (ST) (0.3 g/L) Phenol (0.05 mol/L) Trinitrophenol (125 mg/L) Polyether (2 g/L, COD 3933 mg/L) Wastewater (COD 4500 mg/L) Phenol (0.72 mM)

Aniline (1 g/L) Benzoic acid (50–150 mg/L) Pharmaceutical wastewater (COD 7–12 mg/ L) Organic dye-ST (Saffranin 10 mg/L) Phenol (1 g/L)

Thiocynate (456 mg/L)

Sewage sludge based AC

Phenol (5 mg/L) Fosfomycin (8.23 g/L, COD 72.7 g/L) Oxalic acid (1 g/L) Phenol, p-nitro phenol (5 g/L)

Multiwalled CNT Sewage sludge based AC

Catalyst

Pollutant

Table 2 (continued)

100 56

64 55

95 100, 63 82

95

64 65 100 100 100

90 70–95

94 99

100

95.3

98 99 94

93 98

40 100

99 95

Pollutant

Removal (%)

COD 80

COD 45

(continued on next page)

Chen et al. (2004) Singh et al. (2004)

Yang et al. (2006) Suarez-Ojeda et al. (2005)

Quintanilla et al. (2006) Santiago et al. (2006) Posada et al. (2006) Santos et al. (2006)

TOC 80 TOC 90

TOC-80

Hussain et al. (2009) Yang et al. (2008) Royer et al. (2008) Chen et al. (2007) Arena et al. (2007) Kim et al. (2007) Yang et al. (2007) Quintanilla et al. (2007)

Soira-Sanchez et al. (2009) Hung (2009b)

Melero et al. (2009)

Kim et al. (2009) Kondru et al. (2009) Li et al. (2009)

Beauchet et al. (2009)

Lebigue et al. (2010) Morales-Torres et al. (2010) Tang et al. (2010) Goi et al. (2010) Zhao et al. (2010)

Castro et al. (2010) Huang et al. (2010)

Yuan et al. (2011) Arena et al. (2010)

Ersöz and Atalay (2010) Velegraki et al. (2011) Zheng (2011)

Sun et al. (2011)

Marques et al. (2011) Qiu et al. (2011) Rocha et al. (2011) Stüber et al. (2011)

TOC-100

COD 94.47 COD-50 COD 98, TOC 93 Selectivity of CO2 95 TOC-80 COD-69 TOC-96.5 COD-98 TOC-50

COD 71

TOC 43 COD 95

COD 95.3 TOC- 95

COD 95

COD 96

TOC 80, 70

COD 58

TOC/COD

Reference

Sushma et al.

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Journal of Environmental Management 228 (2018) 169–188

Alejandre et al. (1998) Deiber et al. (1997)

of the CWAO, doping of active metals on mixed oxide support such as Ce-Zr and Ce-Ti was recommended. Various studies have been reported on CWAO of synthetic wastewater using manganese based catalyst. Chaliha and Bhattacharyya (2008) used Mn(II)/MCM-41 catalyst for CWAO of 2-chlorophenol, 2,4dichlorophenol and 2,4,6-trichlorophenol and removal efficiency of 91.1%, 85% and 79.7% respectively were obtained. The oxidation of chlorophenols increased with a decrease in pH but acidic conditions increased the leaching of the active species. In another study, Liu et al. (2012) compared the catalytic activity of CuOx and MnOx supported on nano TiO2 by carrying out the CWAO of β-naphthol at 100 °C and 2 MPa pressure. The MnOx/nano TiO2 showed higher conversion and COD removal of 93.3% was achieved due to large amount of highly dispersed MnO2 and strong electron transfer between MnO2 and Mn2O3 species which increased the catalytic activity of MnOx/nano TiO2 catalyst. The introduction of ceria as promoter into the manganese catalyst seems to be promising in CWAO of pollutants. Kumar et al. (2006) studied the CWAO of acrylic acid at 140–180 °C and 4 bar using homogenous FeSO4, CuSO4, and heterogeneous Co:Bi, Mn:Ce catalysts. The Mn: Ce (1:1) catalyst showed better activity compared to other catalysts. The COD removal of 86% was achieved in 6 h and leaching of manganese was found to be negligible in CWAO effluent. Similarly, Lopes et al. (2007) compared various commercially available catalysts CuO-MnOx/ Al2O3, CuO-ZnO/Al2O3, Fe2O3-MnOx and laboratory prepared Ag-Ce-O, Mn-Ce-O, Mn-O, Ce-O catalysts for the oxidation of six phenolic acids: syringic, vanillic, 3,4,5-trimethoxybenzoic, veratric, protocatechuic and transcinnamic acids (pollutants of olive oil mill effluent). The laboratory prepared Mn-Ce-O catalyst showed highest catalytic activity with complete oxidation of organics at 200 °C and 30 bar pressure and negligible leaching of manganese was observed. Similar results were obtained by Silva et al. (2004) who reported highest activity of Mn-Ce-O catalyst among commercially available Pt/Al2O3, CuO–ZnO/Al2O3, CuO–MnOx/Al2O3, Fe2O3–MnOx, CuO–MnOx and laboratory prepared Ag–Ce–O, Mn–Ce–O and Ce–O catalysts for the oxidation of ethylene glycol. The leaching of manganese was found to be within the prescribed limit in CWAO effluent and TOC reduction of 99.3% was obtained. Arena et al. (2014) studied the CWAO of carboxylic acids (acetic, oxalic and formic acids) at 110–150 °C and 0.9 MPa partial pressure of O2 over MnCeOx catalyst. The catalyst resulted in complete mineralization of carboxylic acids and negligible leaching of manganese was observed. Similarly, Li et al. (2014) studied the formaldehyde oxidation using Mn0.5Ce0.5O2 catalyst at 270 °C and 100% removal was achieved. Therefore, it can be concluded that the ceria promoted manganese catalyst could be a promising catalyst for the oxidation of various organic compounds.

COD 95

Fortuny et al. (1998)

80 – 1h

4.2.2. Performance of metal oxide catalyst for treatment of nitrogenous compounds The treatment of various nitrogenous organic compounds such as nitrophenols, nitrobenzene, aniline, pyridine, ammonia etc. by CWAO has been widely reported in literature. Nitrophenols are the byproducts formed during nitrobenzene synthesis and found in the effluent of nitrobenzene process industry. Apolinario et al. (2008) studied CWAO of dinitrobenzene and trinitrobenzene using carbon xerogel and nano sized CeO2 catalyst. The carbon xerogel was found to be an effective catalyst and 83% mineralization of dinitrobenzene and trinitrobenzene was achieved in 120 min. The nitrates formed during the reaction were removed by coupling of CWAO to biological processes. Aniline is a nitrogenous organic compound and is not amenable to biological treatment. Gomes et al. (2005) studied CWAO of aniline using transition metals (copper, chromium and vanadium) supported on MCM-41. The Cu/MCM-41 exhibited highest activity with 96% conversion and 76% CO2 selectivity at 200 °C and 6.9 bar oxygen partial pressure within 2 h. However, the high leaching of copper resulted in loss of catalytic activity. Ersöz and Atalay (2012) studied aniline oxidation using Cu/CeO2 and NiO/Al2O3 catalysts. The Cu/CeO2 catalyst showed

*LST - Liquid space time; ST-Space time; TOS- Time on stream.

140 200–325

Phenol (5 g/L, TOC 3835 mg/L) Phenol (5 g/L) aniline, nitrophenol, β-alanine, ammonia (COD 8.84 g/L)

CuO/γ-Al2O3 Mn/CeO2

TBR SR

0.9 MPa (O2) 15 MPa (O2)

100 140 TBR

0.9 MPa (O2)

–

COD 90 100 97 100

TOC 80 93

5h 10 min 40 min ST 1 h 1.5 h 150 110 150 140–160 110–145 SR SR SR TBR SR

CuCex MnCeOx Ce1−xCuxO2−δ CuO/γ-Al2O3 Mn-Ce-O composites CuO, CuO-ZnO, CuChromate/γ-Al2O3 CuO/γ-Al2O3 (70 g/L) (1 g/L) (1 g/L) (5 g/L) (5 g/L) Phenol Phenol Phenol Phenol Phenol

0.7 MPa (O2) 0.5 MPa (O2) 0.73 MPa (O2) 0.9–1.2 MPa (O2) 0.1–0.65 MPa (O2)

Pollutant Pressure Temp ( C)

o

Reactor type Catalyst Pollutant

Table 2 (continued)

Operating conditions

Time

Removal (%)

TOC/COD

Reference

Arena et al. (2003) Chen et al. (2001) Hocevar et al. (2000) Fortuny et al. (1999) Akyurtlu et al. (1998)

Sushma et al.

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decreasing the toxicity of the textile industrial effluent. Jianxiong et al. (2011) investigated the feasibility of coupling CWAO and biological methods using Cu/Al2O3 catalyst for treatment of effluent generated by vitamin B6 production plant. The BOD/COD ratio was improved from 0.1 to 0.8 and the COD removal of 99.3% was obtained. However, the leaching of copper metal was not reported in the CWAO effluent. The industrial organic raffinate generated from a chemical industry contains refractory and toxic organic compounds pyridine, β-picoline and 3-cyanopyridine (Sushma and Saroha, 2018). The treatment has been carried out by coupling of catalytic wet air oxidation (CWAO) over ceria promoted MnOx/Al2O3 catalyst and biological processes. The biodegradability enhancement of the effluent obtained after CWAO treatment has been investigated and the BOD/COD ratio was found to increase significantly after the CWAO treatment. The CWAO effluent was subjected to aerobic biological and a COD removal of 98.4% was found.

higher activity for aniline removal (45%) compared to NiO/Al2O3 catalyst despite NiO/Al2O3 catalyst having higher surface area than Cu/ CeO2 catalyst. The high activity of Cu/CeO2 was due to small crystallites of active copper particles while NiO/Al2O3 had an amorphous structure. The leaching of metal in both catalysts resulted in loss of catalytic activity. Pyridine is a toxic nitrogenous organic compound and unsuitable for biological treatment. Chaudhary et al. (2006) studied CWAO of pyridine using Cu-Co/C catalyst at 160 °C and 9 bar and pyridine conversion of 71.8% was obtained at pH 4. Subbaramaiah et al. (2013) studied pyridine oxidation at 85 °C and atmospheric pressure in presence of peroxide using Ce/SBA15 catalyst. The COD and TOC removal of 77% and 75% respectively were obtained and the catalyst was found to be stable after 6 cycles run. Ammonia is one of the most refractory compounds among nitrogenous compounds. Kaewpuang-Ngam et al. (2004) investigated CWAO of ammonia at 230 °C and 2 MPa pressure using Ni/Al2O3 catalyst. The catalyst was found to be stable towards leaching and N2 selectivity of 90% was achieved but the ammonia removal was found significantly lower. Similarly, Hung (2011) studied ammonia removal using copper-lanthanum-cerium composite catalyst. The ammonia removal of 88% was achieved by CWAO at 150 °C and 4 MPa of oxygen partial pressure. In another study, Wang et al. (2013) studied ammonia oxidation using CuO-CeO2 catalyst. The copper loading of 10 wt. % was found to be optimum and the catalyst showed higher activity due to finely dispersed CuO species. The ammonia oxidation occurred through adsorption on the CuO species and further activation and transformation into NHx species by strong electron state interaction of CuO-CeO2 catalyst. The synergistic effect between two components (CuO-CeO2) played the crucial role in activation of ammonia. The NHx intermediates reacted with lattice oxygen which was provided by Cu-O-Ce solid solution to form N2, N2O and H2O. The lapsed oxygen was refilled by gaseous oxygen and resulted in rapid redox reaction cycle. However, the catalyst stability in terms of leaching of copper was not reported. Nakamura et al. (2014) reported the CWAO of ammonia from (Cu (NH3)4)SO4 and (NH4)2SO4 compounds at pH 8–11 using persulfate (Na2S2O8) catalyst at 70 °C and atmospheric pressure. The ammonia oxidation of 95% was achieved with higher selectivity of N2 (75%) in 120 min at pH 13.5 for (NH4)2SO4 compound and at pH 8–11 for (Cu (NH3)4)SO4 compound. The initial rate of ammonia oxidation was higher for copper based compound than (NH4)2SO4, although the N2 selectivity (85%) was higher for (NH4)2SO4 compared to (Cu(NH3)4) SO4 (75%). The catalyst showed high activity, stability and selectivity for N2 formation.

4.2.4. Application of CWAO in treatment of dyes The effluent generated from textile industry contains residual dyes which impart color and toxicity to the effluent. The CWAO of effluent containing dye (Safranin-T) was studied at room conditions using new polyoxometalate Zn1.5PMo12O40 nano tube catalyst prepared with native cellulose fiber templates (Zhang et al., 2009). The color removal of 98% and COD removal of 95% was achieved within 40 min. The catalyst was found to be stable and negligible leaching was observed after 6 cycle runs. Li et al. (2009) studied Safranin-T (ST) degradation by CWAO over ceria doped MoO3 nanofibers. The degradation of 98% dye at room conditions was obtained by CeO2 (11.86 wt. %) doped MoO3 nanofibers within 20 min. The catalyst showed excellent stability towards leaching and exhibited high activity due to synergistic effect between MoO3 and ceria. The ceria increased the mobility of oxygen atoms while the MoO3 nanofibers provided high surface area in the reaction mixture. Similarly, Huang et al. (2010) reported the ST degradation by CWAO using ZnO/MoO3 mixed oxide nanotube catalyst. The COD and TOC removal of 95% and 99.3% were achieved respectively at room conditions within 18 min. The higher activity of the catalyst was due to introduction of zinc ion in the MoO3 which provides the Lewis acid sites to the catalyst. The catalyst was found to be stable after 10 cycles run with negligible metal leaching. Similarly, Yuan et al. (2011) observed CWAO of ST dye degradation using ZnO/MoO3/SiO2 hybrid catalyst and COD and TOC removal of 93.2% and 95.4% were achieved respectively at ambient conditions in 25 min. Hua et al. (2013) studied the degradation of three azo dyes (Methyl Orange, Direct Brown and Direct Green) by CWAO over CuO/γ-Al2O3 catalyst and the maximum COD removal of 70% was achieved in 2 h at 80 °C and atmospheric pressure. The free radicals produced during CWAO caused the strong oxidizing effects in solution and led the breakdown of the chromophoric groups of azo-benzene conjugates in the structure of dye. However, leaching of copper metal was not reported in the study. Peng et al. (2008) reported CWAO of azo dye over FeCl3/NaNO2 catalyst at 150 °C and 0.5 MPa pressure in acidic medium (pH 2.6). The TOC removal of 56% was achieved in 4 h and the carboxylic acids were the main intermediates found in the CWAO effluent. In another study, Srikhaow and Smith (2013) investigated the CWAO of methyl orange azo dye using Cu2(OH)3NO3/ZnO catalyst under ambient conditions. The COD and TOC removal of 98% and 94% respectively was achieved after 20 min. Vallet et al. (2013) studied the oxidation of Crystal Violet dye by CWAO in trickle bed reactor at 100–180 °C and 25 bar pressure using Ni/MgAlO catalyst. The catalyst showed high activity with same rate upto 57 h and TOC removal of 80% was observed at 180 °C. Similarly, Ovejero et al. (2012) reported the CWAO of Crystal Violet dye over Ni/Mg-Al mixed oxide catalyst at 180 °C and the TOC removal of 64% was obtained and only 6% leaching of nickel metal was found after 350 h in continuous experiment. Ovejero et al. (2013a) reported CWAO of Basic Yellow 11 (non azo) dye at 150 °C and 5 MPa oxygen pressure over Ni/MgAlO and 49%

4.2.3. Application of metal oxide catalysts in industrial wastewater treatment and biodegradability enhancement The pulp and paper mill effluent contains non-biodegradable compounds and is not suitable for biological treatment. The CWAO of pulp and paper mill effluent was carried out using CuO-ZnO supported on Al2O3, CeO2 and LaCoO3 supports (Garg et al., 2004). The COD removal of 83% was achieved at 95 °C and atmospheric pressure using CuOZnO/CeO2 catalyst at acidic conditions (pH 3). However, the leaching of active metal was not reported in the study. Similarly, Garg et al. (2007) reported the CWAO of pulp and paper mill effluent at 170 °C and 0.85 MPa pressure using CuO/CeO2 and CuO/AC catalyst and COD removal of 89% was achieved at initial pH of 8 in 4 h. However, leaching of the active metal species was not incorporated in the study. Domínguez et al. (2014) studied CWAO of winery wastewater using Fe/ graphite catalyst and the COD removal of 30% was obtained at 160 °C and 1 MPa pressure. The textile industrial effluent contains toxic compounds with high COD value. The CWAO of textile effluent was studied over Cu/CNFs catalyst at 120–160 °C and 6.3–8.7 bar pressure (Rodrıguez et al., 2008). The TOC reduction of 80.2% was observed at 160 °C and 8.7 bar pressure in 180 min. The CWAO process was found to be effective for 180

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of the catalytic system and prevented coke formation. Similarly, CWAO of ethyl tert butyl ether (ETBE) and tert amyl methyl ether (TAME) gasoline oxygenates was carried out using Rh/γ-Al2O3 at 100–150 °C at 10 bar oxygen pressure (Cuauhtemoc et al., 2008). The incorporation of CeO2 into Rh/γ-Al2O3 catalyst increased the CO2 selectivity by metalsupport interaction with formation of Ce4+-O2--M+ bond as shown by temperature programmed reduction (TPR) analysis, preventing the formation of refractory intermediates such as acetic acid (main coke forming compound). Moreover, Cuauhtémoc et al. (2011) reported the CWAO of TAME using Rh/CeO2-γ-Al2O3 catalyst and the activity of the catalyst was found to increase by addition of tin (Sn) in Rh/CeO2-γAl2O3 catalyst. The increased activity was found to be due to the formation of Snδ+ which acted as Lewis acid sites. These sites trap the TAME molecules for further oxidation on rhodium active sites while CeO2 improved the redox cycle resulting in high TAME mineralization and coke formation was not found on catalyst. Nousir et al. (2008) reported that the adsorption of carbonaceous deposits can be controlled by introduction of zirconium into the support lattice. The ZrO2 increases the mobility of oxygen atom in the lattice structure of ceria in the presence of platinum metal which resulted in oxidation of adsorbed carbon on the catalyst surface. Moreover, Wang et al. (2008) observed that introduction of ZrO2 into lattice structure of ceria in Ru/ZrO2-CeO2 catalyst increased the adsorption capacity by increasing the specific surface area and led to 100% phenol conversion. The carbonaceous deposits observed on the catalysts were fully oxidized at 300 °C.

degradation of dye with CO2 selectivity of 64.5% was obtained. The catalyst was found to be stable after 6 cycle runs and negligible leaching was observed. Vallet et al. (2012) reported higher activity of Ni/hydrotalcite (HT) catalyst compared to Ni/MgAlO catalyst for degradation of Basic Yellow 11 and TOC removal of 85% was obtained using Ni/HT at 180 °C and 5 MPa pressure. The catalyst was found to be stable towards leaching after 65 days of experiment. Ovejero et al. (2013b) observed higher activity of Ni/HT catalyst for the degradation of Basic Yellow 11 at 150 °C. The complete oxidation of the dye was observed within 2 h. The degradation of dye was increased through series of bond cleavage of aromatic ring via free radical reaction mechanism. Thus the Ni/HT seemed to be a promising catalyst for the degradation of azo and non-azo dyes. 5. Catalysts deactivation and mitigation The deactivation of catalyst during oxidation reaction due to incomplete oxidation and consequent deposition of carbon on catalyst surface and leaching of active metals in the reaction solution are severe problems for continuous operation in the CWAO process. 5.1. Noble metals based catalysts: deactivation and mitigation Despite the higher oxidation of organics with platinum and ruthenium based catalysts, formation of carbonaceous deposits on the catalysts surface was observed which lowered the catalyst activity (Garg et al., 2003; Keav et al., 2010, 2014; Massa et al., 2007; Kouraichi et al., 2010; Lee et al., 2010). The carbonaceous deposits were found mainly due to the formation of carbonates and polymeric carbon species during conversion of higher molecular weight organic compounds into intermediary by-products. Masende et al. (2003) observed deactivation of Pt/graphite catalyst for CWAO of phenol at 120–180 °C and 0.8 MPa of oxygen partial pressure. The catalyst activity was found to decrease mainly by coke formation and over oxidation of platinum active surfaces. Rocha et al. (2014) reported that the introduction of CeO2 into the Pt/TiO2 increased the oxygen storage capacity of the catalyst and provides good stability towards the deposits of carbonaceous species (coke) on the catalysts. The TOC removal of 96% and CO2 selectivity of 100% displayed the higher activity of Pt/TiO2 catalyst due to CeO2 addition. In another study reported by Roy and Saroha (2014), the ceria promoted Pt/Al2O3 catalyst exhibited higher stability compared to Pt/ Al2O3 catalyst for CWAO of oxalic acid at 90 °C and atmospheric pressure. Similarly, Pt/CeO2 catalyst exhibited higher conversion of phenol than Pt/Al2O3 catalyst mainly due to the catalyst deactivation by coke formation on the surface of Pt/Al2O3 catalyst (Lee et al., 2010). Similarly, Keav et al. (2014) studied the phenol conversion using Pt/ CeO2 and Ru/CeO2 catalysts. The CeO2 was found to promote the oxidation of deposited carbonaceous species on Pt/CeO2 catalyst surface and phenol conversion of 100% was achieved. The highly dispersed active metal in Pt/CeO2 catalysts showed higher activity for enhancing oxidation of heavy molecular weight compounds which caused carbonaceous deposition on the catalyst surface. In another study, the dispersion of ruthenium metal was found to be increased by CeO2 addition in Ru/γ-Al2O3 catalyst by dispersing the effective ruthenium active sites from the clusters at the catalyst surface (Yang et al., 2005; Yu et al., 2011). The CeO2 promoted Ru/γ-Al2O3 catalyst resulted in higher phenol degradation compared to Ru/γ-Al2O3 catalyst. It was due to the chemical state of ruthenium and oxygen elements which was affected by introduction of CeO2, resulting in an increase of chemisorbed oxygen contents at the catalyst surface which decreased the coke formation and increased the oxidation of phenol (Yang et al., 2005). Cervantes et al. (2013) observed CWAO of methyl tert-butyl ether (MTBE) using Rh/ TiO2 and Rh/TiO2-CeO2. The Rh/TiO2-CeO2 catalysts showed better results compared to Rh/TiO2 due to the severe deactivation of Rh/TiO2 by coke formation. The higher activity of Rh/TiO2-CeO2 was due to additional oxygen supply by CeO2 which improved the redox potential

5.2. Non-noble metals based catalysts: deactivation and mitigation The deactivation of metal oxide catalysts (non-noble metals) during oxidation reaction occurs by carbonaceous deposition on the active surface and leaching of active metals in the reaction solution resulting in loss of active sites. The leaching of active metals during CWAO occurs due to degradation of organic pollutants into acidic intermediates (short-chain carboxylic acids) which decreases pH value of reaction solution resulting in corrosive environment. Additionally, the leached metal ions in the CWAO effluent exhibit toxicity and causes secondary pollution. Various studies have been reported on the stability and/or deactivation of catalysts under varying reaction conditions. Ksontini et al. (2008) reported the oxidation of gallic acid at 90 °C and 5 bar and COD removal of 94% was achieved using Fe/(Al-Fe) pillared clay catalyst. However, leaching of iron from the catalyst in the CWAO effluent reduced the catalytic activity. In another study, Yang et al. (2006) studied the degradation of salicylic acid at 140 °C and 2.5 MPa pressure using LaFeO3 catalyst and the COD removal of 84% was achieved but the leaching of La2O3 and Fe2O3 was high. Royer et al. (2008) studied the CWAO of stearic acid using LaCoO3 perovskite catalyst. The stearic acid oxidation takes place via decarboxylation process which resulted in formation of carbonate species at the catalyst surface. The optimization of catalyst stability, while maintaining a high level of activity, is very important for CWAO of various organic pollutants, because of the high reaction temperature and acidic environment during oxidation of organics. Various studies have been focused on the stability of catalyst. Aihua et al. (2007) studied ZnFe0.25Al1.75O4 spinel catalyst for the oxidation of phenol. The catalyst was found to be highly stable and phenol conversion of 100% and COD removal of 88.7% was obtained at 160 °C and 1 MPa of O2 pressure and the leaching of iron and zinc were found to be low in the CWAO effluent. Similarly, Gao et al. (2013a) studied CWAO of lignin compound at 120 °C and 0.2 MPa of O2 pressure using LaFexMn1-xO3 hollow nanosphere catalyst and lignin conversion of 80% was achieved in 1 h. The catalyst was found to be stable towards leaching due to its perovskite phase structure. In another study, Beauchet et al. (2009) reported the CWAO of cumene (isopropyl benzene) at 250 °C using NaX zeolite catalyst and 100% conversion with 95% CO2 yield was achieved. The high activity of catalyst was due to fine dispersion of active species which were localized in the mouth of cavities on the zeolite surface. Similarly, Zhao 181

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of catalyst by decreasing the formation of bulk polymeric species on the catalyst surface. The geometry of the catalyst changed electronically by introduction of potassium and the zeolite support prohibited the transformation of ceria, manganese and potassium to higher oxidation states. Similarly, Santiago et al. (2006) studied the role of potassium in MnO2-CeO2 catalyst for CWAO of phenol. The potassium loading was varied from 0 to 10 wt. % in MnO2-CeO2 catalyst and TOC removal of 90% was obtained with 1 wt. % potassium in MnO2-CeO2 catalyst. The role of potassium in the catalyst was like a modifier or chemical promoter which improved the catalyst performance and increased the catalyst stability.

et al. (2010) reported high activity and stability of micellar polyoxometalate (POM) catalysts. The high activity (COD removal of phenol- 98.5%) of the catalyst was due to micellar structure which provided the surface to phenol and oxygen for reaction. The catalyst was found to be quite stable towards leaching after six runs. In another study, thiocynate oxidation was studied using POM catalyst under room conditions (Sun et al., 2011). The thiocynate removal of 97% was obtained after 150 min and SCN− was degraded into SO42−, HCO3−, NH4+ and NO3− compounds. The catalyst was found to be very stable towards leaching after five cycles runs. Similarly, Wei et al. (2013c) reported the oxidation of thiocynate at ambient conditions using polyoxoperoxo-metalates (POPMs) catalyst which showed higher activity and was found to be stable towards leaching. The micellar POM catalyst was found to be very effective for the oxidation of toxic thiocynate compound. The copper catalysts, in spite of their good catalytic activity, are prone to leaching in the CWAO conditions. Various studies have been reported in the literature on copper leaching. Yadav and Garg (2012) studied the oxidation of ferulic acid (lignin compound, found in cellulosic raw materials) at 90–160 °C and 0.55–0.8 MPa total pressure using CuO/CeO2 and COD removal of 70% was obtained. However, the deactivation of catalyst was observed due to carbonaceous deposition on active sites and leaching of active metal species. In another study, Yadav and Garg (2014) reported the minimization of leaching by addition of AC support to Cu/CeO2 catalyst and the TOC removal increased to 88% using Cu/Ce/AC catalyst due to high surface area and more binding sites of AC. However, the TOC removal decreased by 6% with spent catalyst after 1st run. Castro et al. (2010) used polymer supported copper complexes (iminodiacetic acid with copper acetylacetonate) to minimize the leaching of copper for catalytic wet peroxide oxidation of phenol at 30 °C and atmospheric pressure and phenol conversion of 93% was obtained. Similarly, Xu and Sun (2012) studied CWAO of phenol using Cu0.10Zn0.90Al1.90Fe0.10O4 spinel catalyst and 100% phenol conversion and 95% COD removal was obtained at 170 °C. It was found that the tetra-coordinated copper ions were stable towards leaching of copper after 20 runs while the highly dispersed Cu2+ ions were completely leached from the catalyst surface into the reaction solution.

6. Biodegradability enhancement of effluent by CWAO The CWAO is a very effective technique for biodegradability enhancement of effluent by converting toxic pollutants into biodegradable intermediates. Various studies have reported the toxicity as well as biodegradability of the effluent after CWAO treatment. Anushree et al. (2015) investigated the CWAO of paper industry wastewater at atmospheric pressure and 80 °C using Ce0.4Fe0.6O2 catalyst and the biodegradability of CWAO effluent was found to increase from 0.27 to 0.47. Wang et al. (2014) reported an enhancement in biodegradability of the landfill leachate from 0.1 to 0.39 after CWAO at 150 °C and 0.5 MPa of oxygen pressure using cobalt catalyzed NaNO2 catalyst. Tripathi et al. (2013) observed an enhanced biodegradability (0.11–0.46) of NF-reject after CWAO treatment using Pd/activated carbon catalyst at 200 °C and 0.69 MPa of oxygen pressure. Li et al. (2013) studied the enrofloxacin (antibiotic compound) degradation using FeCl3 and NaNO2 catalyst. The CWAO experiment was carried out at 150 °C and 0.5 MPa oxygen pressure and enrofloxacin degradation of 99% and TOC removal of 51% was obtained after 120 min. The BOD/COD ratio was increased from 0.01 to 0.12 indicating the improvement in biodegradability. Bistan et al. (2012) performed the toxicity test using yeast to detect toxicity level of estrogen in CWAO effluent and complete toxicity removal was observed at 230 °C using Ru/TiO2 catalyst. Chen et al. (2012) reported an increment in biodegradability (BOD/COD ratio) from 0.23 to 0.84 of low biodegradable coking wastewater using CWAO treatment process at 140–160 °C and 0.2–1 MPa partial pressure of oxygen. MartínHernández et al. (2012) performed the CWAO of p-nitrophenol at 180 °C and 7.6 bar partial pressure of oxygen using Ru/TiO2 catalyst and increment in biodegradability of 50% was observed after treatment. Penate et al. (2012) observed an enhanced biodegradability of effluent containing paracetamol after CWAO treatment at 150 °C and 3.2 bar partial pressure of O2. Xu et al. (2012) studied CWAO of fulvic acid at 150 °C and 0.5 MPa of O2 using potassium persulfate (K2S2O8) promoted AC catalyst and biodegradability of fulvic acid was found to increase from 0.13 to 0.95 after CWAO. Katsoni et al. (2011) studied CWAO of trinitrophenol (TNP) using AC catalyst at 174 °C and 0.7 MPa of oxygen pressure and TNP degradation of 90% was obtained in 2 h and BOD/COD ratio was enhanced from 0 to 0.25. Goi et al. (2010) reported enhanced biodegradability of wastewater containing landfill leachate and organic halogens by CWAO at 157–227 °C using CeO2TiO2 catalyst. Rubalcaba et al. (2007) observed enhanced biodegradability of effluent containing phenolic compounds (phenol, o-cresol, pnitrophenol) after peroxide promoted CWAO treatment at 140 °C and 2 bar partial pressure of oxygen using AC catalyst and suggested the integration of CWAO and biological process for complete removal of organic compounds. Similarly, Suarez-Ojeda et al. (2007b) reported CWAO of phenolic compounds (phenol, o-cresol and 2-chlorophenol) at 160 °C and 2 bar partial pressure of oxygen and improvement in biodegradability of CWAO effluent. Zhang et al. (2006) studied CWAO of effluent containing H-acid (1-amino-8-naphthol-3, 6-disulfonic acid) at atmospheric pressure in presence of microwave and the biodegradability was found to increase from 0.008 to 0.467. Posada et al. (2006) reported the CWAO of phenol, 2-chlorophenols and 4-nitrophenol at 160 °C and 1 MPa pressure using Cu/CeO2 catalyst and the

5.2.1. Synergistic effects of Mn-Ce catalyst in mitigation of catalyst deactivation The catalysts synthesis process based on redox-precipitation reaction of Mn-Ce yields MnCeOx catalyst which improved physicochemical properties of catalyst and higher oxidation of phenol was achieved compared to conventional co-precipitated synthesis process (Arena et al., 2007). Arena et al. (2008) studied the CWAO of phenol at 100 °C and 1 MPa pressure using MnCeOx catalyst and higher activity of catalyst was observed with negligible leaching of manganese. The carbon deposition (coke formation) was minimized by selecting optimum average pore diameter of the catalyst which enhances the adsorption rate and redox properties of MnCeOx catalyst. Arena et al. (2010) studied the phenol conversion using different metal oxide catalysts including copper, manganese, iron and nickel supported on ceria. The MnCeOx catalyst exhibited the higher TOC conversion and stability against fouling. Similarly, Kim et al. (2009) reported phenol oxidation using transition metals (manganese, iron, cobalt, nickel and copper) supported on Ce0.65Zr0.35O2 mixed oxides (high surface area with excellent redox properties). The CuOx/Ce0.65Zr0.35O2 catalyst showed highest conversion but exhibited higher leaching while the MnOx/ Ce0.65Zr0.35O2 catalyst showed high conversion but deactivated due to carbon deposition. The co-impregnation of manganese and copper on the support decreased the deactivation of the catalyst (leaching and carbon deposition) and catalytic activity was improved. Hussain et al. (2009) studied the oxidation of phenol over Ce-Mn/zeolite catalyst at 110 °C and 0.5 MPa of O2 pressure. The introduction of potassium (K) in the catalyst increased the catalyst stability and reduced the deactivation 182

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Fig. 3. Approach for complete mineralization of industrial effluent by integration of CWAO and biological process.

leads to higher conversion and mineralization of organics (Li et al., 2007c). Moreover, at an optimum catalyst loading, free radicals concentration is increased with an increase in oxygen pressure (Lin et al., 2003). The free radicals such as °OH induced at high pressure of O2 (0.8 MPa) have very strong ability to oxidize the organics (Sun et al., 2008). However, high pressure leads to more oxidative environment resulting in an over oxidation of active metal species. Moreover, the oxidation of nitrogenous organics at high oxygen pressure results in nitrate formation instead of molecular nitrogen and leads to deactivation of catalyst (Kim et al., 2012). Therefore, for cost effective system, the oxygen partial pressure should vary according to pollutant concentration in the effluent.

biodegradability of the CWAO effluent was found to increase compared to raw effluent. Santos et al. (2006) reported the CWAO of phenol at 160 °C and 16 bar partial pressure of oxygen using AC catalyst and the biodegradability of CWAO effluent was found to improve due to conversion of phenol into biodegradable intermediates such as acetic acid, maleic acid and formic acid. Suarez-Ojeda et al. (2007a) used integrated CWAO and aerobic biological treatment to obtain COD removal of 98% of a high-strength wastewater containing o-cresol. Therefore, the approach for complete mineralization of toxic industrial effluents by integration of CWAO (at low operating conditions) and biological process is a promising step as shown in Fig. 3. 7. Factors affecting CWAO reaction

7.3. Effect of initial pH of effluent The CWAO of organics depends on various parameters such as reaction temperature, operating pressure, pollutant concentration, solution pH, catalyst dosage etc. These parameters influence the rate of oxidation of organics and therefore, it is necessary to optimize the parameters for CWAO of organics.

The initial pH of effluent is an important parameter affecting the oxidation of pollutants in various ways. The adsorption of pollutant on the surface of catalyst was affected by pH through surface charge and functional groups on the catalyst surface (Huang et al., 2010). Further, pH also affects the speciation of reactants and products during CWAO reaction (Li et al., 2007b). The oxidation of non-azo dye increases in alkaline pH (7–12) while azo dye oxidation increases in acidic pH (7–0). For non azo dye, at acidic pH the catalyst surface is positively charged and the reaction intermediates are in the same charge resulting in slow oxidation of non azo dye. While for azo dye, the catalyst surface and dye intermediates are oppositely charged and better adsorption of pollutants at the catalyst surface occurs. It has been reported that oxidation of various organic compounds is higher at acidic or basic pH and decreases when approaches to neutral pH (Kim and Ihm, 2011). The catalyst support, reactants and the solution pH has direct effect on oxidation of reactant. Sometimes, the solution pH affects the catalyst support and decreases or increases the oxidation of pollutants. The support exhibit positive charge when the solution pH is less than pHpzc and negative when solution pH is higher than pHpzc (Zhang et al., 2014). Moreover, the formation of total free radicals varies with change in the pH of the solution (Yadav and Garg, 2012). The pH affects the formation of intermediate compounds and reaction pathway. The toxic reaction intermediates concentration in the solution at low pH was found to be higher and also, the low pH caused corrosion of equipment as well as leaching of metal catalyst (Martín-Hernández et al., 2012).

7.1. Effect of reaction temperature Reaction temperature plays a very important role in the CWAO of organic compounds in the effluent. The increase in temperature influences the combination of beneficial effects which may affect the CWAO reaction (Li et al., 2007c): 1. The higher the reaction temperature, the faster the reaction (Arrhenius law). 2. Above 100 °C, the oxygen solubility in water increases with temperature. 3. The activation energies are high for the carboxylic acids which are refractory to oxidation. However, from a practical point of view, the high reaction temperature increases the cost of the process. The catalyst with high activity can lower down the activation energy required for the reaction. Li et al. (2007b) studied the activity of Ru/ZrO2 to lower down the activation energy and obtained higher TOC reduction at lower temperature. The temperature increased the rate of reaction due to which the reaction intermediates formed during the reaction are affected. The formation of more carboxylic acids was observed at higher temperature than correspondingly lower temperature (Martin-Hernandez et al., 2012). Moreover, the temperature of 160 °C was found to be suitable for increasing the biodegradability of the effluent by converting the toxic phenolic compounds to more biodegradable compounds (Suarez-Ojeda et al., 2007b). Hung (2009a) observed the conversion of ammonia to molecular nitrogen at 230 °C and further increase in temperature increased the oxidation driving force which resulted in formation of more nitrates.

7.4. Effect of catalyst dosage Catalyst loading is an important parameter in CWAO of organic pollutants in the effluent. The oxidation of pollutants increases with an increase in catalyst dosage as the increase in catalyst dosage provides more active sites and surface area to the organic compounds to react with oxygen (Santiago et al., 2006). Moreover, the catalyst addition reduces the energy barrier and also helps in generation of free radicals resulting in higher oxidation of organics (Yadav and Garg, 2012). At higher catalyst loading, the temperature effect would be minimized and the conversion of nitrogenous compounds to nitrogen gas was found to be favorable (Ersöz and Atalay, 2012). However, much higher dosages decrease the catalyst performance by decreasing the overall oxidation through radical scavenging effect (Garg and Mishra, 2013). Also, the high catalyst dosage result in aggregation of active metal particles

7.2. Effect of operating pressure The availability of dissolved oxygen at the catalyst surface is necessary for the oxidation of organic compounds. According to Henry's law, the oxygen concentration dissolved in liquid phase is proportional to partial pressure of oxygen in gas phase. The higher oxygen pressure 183

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References

which decreases the available active sites and hence overall oxidation decreases (Fathima et al., 2008). Therefore, to reduce the cost of the process, the catalyst dosage needs to be optimized depending on the concentration of pollutants in effluent.

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7.5. Effect of initial pollutant concentration The initial pollutant concentration is an important parameter as the CWAO efficiency mainly depends on organic load in wastewater. When the pollutant concentration is increased, the degradation efficiency is increased due to more adsorption of pollutant to available active surface and interaction to oxygen molecules (Subbaramaiah et al., 2013). Although, the increase in degradation rate is up to a value and after that value, it decreases with an increase in initial concentration. The higher initial concentration causes concentration gradient which increases the driving force by overcoming the mass transfer resistance and result in overall increase in adsorption of pollutant at catalyst surface. The higher surface accommodating substrates inhibits the oxygen interaction and results in decrease in removal efficiency of pollutants (Vallet et al., 2012). 8. Conclusions CWAO is a promising technology for the treatment of toxic and nonbiodegradable organic pollutants in the wastewater of various industries. The CWAO as a pretreatment technique for biodegradability enhancement of toxic effluent at mild operating conditions is an environmental friendly approach which does not involve any harmful chemicals. The biodegradability and toxicity of the CWAO effluent should be checked to evaluate the suitability for the subsequent biological process. The integration of CWAO with biological process can be more attractive for the treatment of industrial wastewater containing toxic pollutants because the CWAO effluent which contains refractory short-chain carboxylic acids, especially acetic acid, formic acid are readily biodegradable due to their low eco-toxicity. Further studies are necessary to develop more active and stable catalysts which can be effectively utilized on industrial scale. The performance of various noble and non-noble metals has been studied for the treatment of various model compounds and real industrial wastewaters. The results showed that the catalytic activity and stability depends on metal support combinations, dispersion of active metals, preparation of catalysts, nature of pollutants and reaction conditions. Noble metals showed very good activity for degradation of various model compounds and real industrial wastewater. Although they are highly resistant to dissolution, they are vulnerable to poisoning by carbonaceous deposition and economically not suitable. The metal oxide catalysts showed high activity towards oxidation of various model compounds. Among them, Cu, Mn, and Ce are the most promising ones to compete with noble metals. However, the leaching of active metal and carbonaceous deposition are the major drawbacks for the non-noble metals catalysts. Insertion of active metal into the lattice structure of support, e.g. mixed metal oxides, and the addition of promoter can significantly reduce the metal leaching and coke deposition. The MnCeOx mixed oxide catalyst is found to more promising in terms of activity and resistance to metal leaching. Ceria based materials have large applications either as a support or as a promoter due to its redox and morphological properties. The performance of CWAO of organics depends on various other parameters such as reaction temperature, operating pressure, pollutant concentration, solution pH, catalyst dosage etc. These parameters influence the rate of oxidation of organics and therefore, should be optimized to get the better removal efficiency of pollutants in wastewater. Declaration of interest None. 184

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