Desalination 299 (2012) 1–15

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Trends in electro-Fenton process for water and wastewater treatment: An overview

P.V. Nidheesh, R. Gandhimathi ⁎Department of Civil Engineering, National Institute of Technology, Tiruchirappalli, Tamilnadu, India

⁎ Corresponding author. Tel.: +91 431 2503171; fax:E-mail address: rgmathii@nitt.edu (R. Gandhimathi)

0011-9164/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.desal.2012.05.011

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 February 2012Received in revised form 3 May 2012Accepted 5 May 2012Available online 14 June 2012

Keywords:Electro FentonWastewater treatmentDegradationOrganic pollutant

Organic compound, especially aromatic compound is the main pollutant in industrial effluent. Conventionalwastewater treatment processes are inefficient for the removal of these types of toxic and hazardous pollut-ants from wastewater. Electro Fenton is one of the powerful and environmentally friendly emerging technol-ogies for the remediation of wastewaters containing organic, especially aromatic compounds. This paperreviews the fundamentals and recent developments in electro Fenton process. Electro Fenton process utilizesdifferent electrolytic reactors such as bubble reactor, filter press reactor, divided double-electrode electro-chemical cell, divided three-electrode electrochemical cell and double compartment cell. Different cathodesas working electrode and anodes as counter electrode used in this process are analyzed. The effects of variousoperating parameters and their optimum ranges for maximum pollutant removal and mineralization arereviewed. Also various pollutants removed by this process are evaluated. Quick removal and mineralizationof pollutants and their intermediate reaction products were reported.

© 2012 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. E-Fenton reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1. Bubble reactor (BR) (A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2. Filter press reactor (FPR) (B) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.3. Divided double-electrode electrochemical cell (DDEC) (C) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.4. Divided three-electrode electrochemical cell (DTEC) (D) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.5. Double compartment cell (DCC) (E) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3. Electrode materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44. Kinetics of E-Fenton process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65. Affecting factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

5.1. pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75.2. Oxygen sparging rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75.3. Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75.4. Applied current density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85.5. Fe2+ concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85.6. Hydrogen peroxide concentration and feeding mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95.7. Distance between the electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95.8. Nature of the supporting electrolyte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

6. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106.1. Dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106.2. Pesticides and herbicides. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106.3. Phenolic compounds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116.4. Leachate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116.5. Drugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116.6. Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

7. Degradation pathway of organic pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

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2 PV. Nidheesh, R. Gandhimathi / Desalination 299 (2012) 1–15

8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Table 1Advantages and disadvantages of the Fenton process.

Advantages Disadvantages

No energy input is necessary toactivate hydrogen peroxide [11]

Ferrous ions are consumed more rapidlythan they are regenerated [12]

Fenton's reagent is relativelyinexpensive and the process iseasy to operate and maintain [13]

Treatment of the sludge-containing Feions at the end of the wastewatertreatment is expensive and needs largeamount of chemicals and manpower [14]

Short reaction time among alladvanced oxidation processes [15]

It is limited by a narrow pH range(pH 2–3) [16]

There is no mass transfer limitationdue to its homogeneous catalyticnature [17]

Iron ions may be deactivated due tocomplexion with some iron complexingreagents such as phosphate anions andintermediate oxidation products [16]

There is no form of energy involvedas catalyst [17]

Additional water pollution caused bythe homogeneous catalyst that addedas an iron salt, cannot be retained in theprocess [11]

1. Introduction

The focus on waste minimization and water conservation in therecent years resulted in the discovery of various treatment processes,one of them being Advanced Oxidation Processes (AOPs). These referto the chemical treatment processes which follow oxidation routeand are particularly employed to degrade biologically toxic and nondegradable chemicals [1]. AOPs have been broadly defined as nearambient temperature treatment processes based on highly reactiveradicals, especially the hydroxyl radical (OH•) as the primary oxidant[2]. Many processes such as chemical oxidation, Fenton and photo-Fenton processes, ultraviolet (UV)-based processes, photo-catalyticredox processes, supercritical water oxidation, sonolysis, and electronbeams and γ-ray irradiation come under advanced oxidation tech-niques [1]. The main function of AOPs is the generation of highly reac-tive free radicals. Hydroxyl radicals (HO•) are effective in destroyingorganic chemicals because they are reactive electrophiles (electronpreferring) that react rapidly and non-selectively with nearly allelectron-rich organic compounds [3].

Among AOPs, oxidation using Fenton's reagent is an attractive andeffective technology for the degradation of a large number of hazard-ous and organic pollutants because of the lack of toxicity of the re-agents, eventually leaving no residues and the simplicity of thetechnology [4]. Maleic acid oxidation was the first Fenton processreported by Fenton [5]. The main steps involved in the Fenton processare (i) Oxidation, (ii) Neutralization, (iii) Flocculation and (iv) Sedi-mentation. The Fenton process is most effective at pH near to 3 [6].Normally the organic substances are removed at two stages of oxida-tion and coagulation [7]. Oxidation of organic substances is due to•OH radicals and coagulation is ascribed to the formation of ferrichydroxo complexes [8]. The degradation mechanism of organic pol-lutants by Fenton reaction is given in Eqs. (1) to (4) [9,10]. Themain advantages and disadvantages of Fenton process are given inTable 1.

Fe2þ þH2O2→Fe3þ þ OH− þHO· ð1Þ

RHþHO·→R· þH2O ð2Þ

where, RH denoting organic pollutants

R· þ Fe3þ→Rþ þ Fe2þ ð3Þ

Fe2þ þHO·→Fe3þ þ OH− ð4Þ

Electrochemical advanced oxidation processes (EAOPs) based onFenton's reaction chemistry are eco-friendly methods that have re-cently received much attention for water remediation [18]. Themost popular EAOP is the electro Fenton (E-Fenton) process [19]. E-Fenton process has two different configurations. In the first one,Fenton reagents are added to the reactor from outside and inert elec-trodes with high catalytic activity are used as anode material while inthe second configuration, only hydrogen peroxide is added from out-side and Fe2+ is provided from sacrificial cast iron anodes [20]. Com-pared to the conventional Fenton process, the electro-Fenton processhas the advantage of allowing better control of the process andavoiding the storing and transport of the H2O2 [21]. In this approach,H2O2 is continuously supplied to the contaminated solution by a two-electron oxygen reduction in an acidic medium according to Eq. (5)

[22–24]. Moreover, electricity as a clean energy source is used in theprocess, so the overall process does not create secondary pollutants[21]. Since E-Fenton process is not using any harmful reagents, it isan environment friendly method for water and wastewatertreatment [25]. The typical mechanism of E-Fenton process is illus-trated in Fig. 1.

O2 þ 2Hþ þ 2e−→H2O2 ð5Þ

This review reports on the most recent experimental studies anddevelopments in the field of E-Fenton process. Fundamentals, exper-imental setups, main reactions, the parameters that affect these pro-cesses and various applications are discussed in detail. Differentcathodes and anodes used for E-Fenton process are also analyzed inthis work.

2. E-Fenton reactors

Electrolytic reactor is one of the essential parts of E-Fenton pro-cess. There are several types of electrolytic cells used by researchers.Some of the important types of such cells are explained below.

2.1. Bubble reactor (BR) (Fig. 2A)

The E-Fenton bubble reactor (a glass cylindrical reactor) hav-ing working volume of 0.675 L was operated in batch mode withtotal reflux or continuous mode by Rosales et al. [27]. The cathodeand anode bars were placed at a distance of 30 mm and 270 mmabove the bottom of the cell, respectively. Steel bars having a totalcontact surface area of 3.14 cm2 or graphite bars having a total con-tact surface area of 1.27 cm2 were used. A constant potential differ-ence (15 V) was applied with a power supply and the process wasmonitored with a multimeter.

2.2. Filter press reactor (FPR) (Fig. 2B)

Prabhakaran et al. [28] used filter press reactor of capacity 2 L forthe removal of resin effluents. The fluid flow circuit consists of a res-ervoir, a magnetically driven self priming centrifugal pump, a flowmeter and the electrolytic cell. The electrical circuit consists of a

Fig. 1. Reaction mechanism of E-Fenton process [26].

3PV. Nidheesh, R. Gandhimathi / Desalination 299 (2012) 1–15

regulated D.C. power supply; ammeter and the cell with the voltme-ter are connected in parallel to the reactor.

2.3. Divided double-electrode electrochemical cell (DDEC) (Fig. 2C)

Reactor consisting of a 0.5 L glass beaker equippedwith iron cathodeand anode, installed in parallel was used by Kurt et al. [29]. Electrodeshaving a total effective electrode area of 45.0 cm2 were plunged intothe beaker, containing tannery wastewater with a 0.4 L working

(9)

(1)

(11) (11)

(3)(4)

(13)

(A)

A V

(C) (D)

(1)

(2)(3)

(5)

(6)

(6)

(7)

(4)

Fig. 2. Schematic diagram of electrolytic reactors (A) BR [27] (B) FPR [28] (C) DDEC [29] (D) Dcontrol unit, (6) pumps, and (7) reflux (8) rotameter (9) reservoir (10) electro chemical re

volume. A digital DC power supply was connected to the electrodes.1 N H2SO4 solution was used to adjust the pH between 3 and 5.

2.4. Divided three-electrode electrochemical cell (DTEC) (Fig. 2D)

Zhou et al. [30] used DTEC reactor for the removal of methyl redfrom aqueous solution. This type of reactor contains a saturated calomelelectrode (SCE) as reference electrode in addition to DDEC. Potentialcontrolled electrolysis is the main purpose of SCE in DTEC.

(1)

(6)

(8)

(10)

(12)

Compartment 1 Compartment 2

(4)(3)

(B)

(E)

(1)(1)

(3)

TEC [30] (E) DCC [31] (1) power supply, (2) air compressor, (3) cathode, (4) anode, (5)actor (11) magnetic stirrer (12) oxygen tank (13) SEC.

Fig. 5. Comparison of recently used cathodes in E-Fenton process.Fig. 3. Recent citations of E-Fenton reactors.

4 PV. Nidheesh, R. Gandhimathi / Desalination 299 (2012) 1–15

2.5. Double compartment cell (DCC) (Fig. 2E)

DCC contains two compartments for electrolytic action. Yuan et al.[31] conducted experiments with DCC type of reactor for the degrada-tion of Rhodamine B. Anode (Pt flakes) and cathode (Pt flakes) wereplaced in compartment 1, whereas in compartment 2 only cathodewas placed. Direct current was provided by a power supply. Saltbridge filled with saturated K2SO4 and agar was used to connect thecompartments. Micro bubbles of H2 and O2 were generated in com-partment 1 by water electrolysis. Excess production of H+ comparedwith OH− results in lowering of pH in between 2 and 3. Cathode 2 incompartment 2 was used as a bypass to accumulate OH− and to neu-tralize the low pH effluent from compartment 1 [31].

The recent citation of above reactors is shown in Fig. 3. From Fig. 3,it is observed that DDEC and DTEC reactors were used in the E-Fentonprocess by many researchers.

3. Electrode materials

Selection of anode and cathode material is one of the significantsteps in E-Fenton process. For example, selection of unstable anodewill cause deterioration of electrode in electrolytic cells. High-oxygen

Fig. 4. Comparison of recently used anodes in E-Fenton process.

overvoltage anode can produce hydroxyl radicals in E-Fenton processas in Eq. (6) [32],

H2O→HO· þHþ þ e− ð6Þ

Pt has been used for a long time as an electrode material due to itsgood conductivity and chemical stability even at high potentials andin very corrosive media [33]. Pt anode was used in more numbersfor the degradation of pollutants in E-Fenton system (Fig. 4). The dif-ferent forms of Pt anodes such as Pt sheet [19,34–39], Pt gauze[25,40–42], Pt foil [43], Pt flakes [31], Pt grid [44,45], platinumplate [46], Pt mesh [39] etc. were used for the degradation of pollut-ants. But it is rarely used for a practical purpose because of its highcost. The various anodes other than Pt being recently used areshown in Fig. 4. Boron-doped diamond (BDD) electrode synthesizedby the hot filament chemical vapor deposition technique on single-crystal p-type Si (100) wafers was used by Isarain-Chavez et al. [47]for the mineralization of atenolol. Borra's et al. [48], Özcan et al.[44], Pozzo et al. [49] and Sire's et al. [39] used BDD as an anode forwastewater treatment. Feasibility of Titanium (Ti) rod coated withIrO2/RuO2 in the E-Fenton process has been studied by Huang et al.[50] and Prabhakaran et al. [28]. RuO2/Ti mesh [51] and Iron [52] aresome other types of anodes being used recently in E-Fenton process.

Efficiency of E-Fenton system depends more on efficiency of cath-ode. Hence it is also known as working electrode. The various cathodematerials used for laboratory-scale and pilot-scale are shown in Fig. 5.Commercial graphite felt [34], carbon felt [35,38,40,42,46,53], BDDplate [54],carbon-polytetrafluoroethylene (PTFE) [37,39,43], graphite-PTFE [30], graphite [55], reticulated vitreous carbon (RVC) [25,41],activated carbon fiber (ACF) [51], Pt flakes [31], carbon sponge [44],stainless steel [28,48,49], and titanium [52] are some of the recentlyused working electrodes in wastewater treatment. The details of anodeand cathode including cell configuration are summarized in Table 2.

Different configurations of cathode were also used by various re-searchers. The usage of composite electrodes is the emerging trendin E-Fenton system. Fan et al. [64] prepared Fe-CHI/Ni|ACF|Fe-CHI/Ni sandwich film cathode for the removal of Rhodamine B (Note:Fe-CHI — Fe2+-chitosan, Ni — Nickel). Fe@Fe2O3/ACF [19], Cu2O/CNTs/PTFE (Note: CNT — Carbon Nanotubes) [36] and Fe@Fe2O3/CNT [65] were also used as composite cathodes in E-Fenton process.Wang et al. [66] used platinum wire anode placed inside a hollow cy-lindrical cathode (2.9 cm diameter and 7 cm height), composed ofone layer of polyacrylonitrile (PAN) based activated carbon fibercloth held by two plastic screens. This design makes the primary

Fig. 6. Pathway of Acid Red 97 degradation products using carbon felt cathode and Pt anode [40].

5PV. Nidheesh, R. Gandhimathi / Desalination 299 (2012) 1–15

current or potential distribution more uniform. The oxygen gas froman oxygen cylinder was dispensed directly at the bottom of the hol-low cylindrical cathode [66].

Different configurations were used for the application of electro-chemical remediation techniques and the electrodes could be madeof different materials. A significant interest in the effectiveness of dif-ferent electrode materials has recently arisen, with the use of stain-less steel, graphite, platinum, PbO2, titanium compounds, borondoped diamond, and ceramics [27]. Oxidation capacity and removalefficiency of different electrodes were also compared by different re-searchers. Sire's et al. [39] evaluated and compared the oxidizingpower of the four E-Fenton systems to mineralize chlorophene solu-tions from the Total Organic Carbon (TOC) decay. Four different E-Fenton systems are: (i) a Pt/O2 diffusion cell (ii) a BDD/O2 diffusioncell (iii) a Pt/carbon felt cell and (iv) a BDD/carbon felt cell. A

continuous, but slow, TOC abatement with 52% of mineralizationhas been observed in the Pt/O2 diffusion cell after 660 min of electrol-ysis at 300 mA. But BDD/O2 diffusion cell has a removal efficiency of100% with rapid and total mineralization by applying high currents.This indicates that BDD has greater oxidation ability than that of Pt[39]. Isarain-Chavez et al. [56] also reported a same trend for degrada-tion of atenolol from aqueous solution. The study demonstrated thatBDD/carbon felt cell has higher oxidizing power than Pt/carbon feltcell to decontaminate the solution completely in a shorter time [56].Siminiceanu et al. [37] reported better performances by the replacementof Pt with BDD anode. The mineralization current efficiency becomes1.53 to 3.25 times higher; whereas specific energy consumption is 1.43to 2.81 times lower for BDD than that of Pt [37]. Özcan et al. [44] reportedthat the formation rate of hydroxyl radicals and the decay rate of organicsare faster on BDD anode than that of Pt. Hammami et al. [67] also

Table 2Specifications of anodes and cathodes used in E-Fenton process.

No Reactor configuration Anode specification Cathode specification Reference

1 Open and undivided tank reactor containing100 mL of solution

Pt sheet of 99.99% purity Carbon-PTFE, carbon felt of area 3 cm2 [56]

2 A small, open and undivided cylindrical glasscell of 6 cm diameter and 250 mL capacity

Cylindrical Pt mesh or a 25 cm2 thin-filmboron-doped diamond Ti/RuO2

Carbon-felt of size 14 cm×5 cm each sideand 0.5 cm in width

[57]

3 A glass reactor of capacity 600 mL Stainless steel [58]4 0.5 L glass beaker with a 0.4 L working volume. Iron plate of size 6.0 cm×7.5 cm Iron of size 2.0 mm×6.0 cm×7.5 cm [29]5 Borosil glass of capacity 0.5 L Iron plate of size 55 mm×40 mm Graphite of size 75 mm×30 mm×6 mm and

having an effective electrode area of 22 cm2.[59]

6 0.40 L open and undivided cylindrical glasscell of internal diameter 60 mm

Cylindrical Pt grid Carbon felt of dimensions 6 cm×8 cm×0.6 cmand 6 cm×17 cm×0.6 cm.

[60]

7 Open undivided cylindrical glass cell of 6 cm diameterand 500 mL capacity

4.5 cm2 Pt cylindrical mesh Carbon felt having an area of 60 cm2

(15 cm×4 cm)[61]

8 Reservoir of capacity 5 L Titanium rod coated with RuO2/IrO2 with anoutside diameter of 1.5 cm and a height of 16 cm.

Stainless steel cylinder with an insidediameter of 8 cm and a height of 17.5 cm

[32]

9 Glass reactor of capacity 300 mL Platinum gauze Carbon felt of size 3 cm×5 cm. [62]10 Glass cell of 6 cm diameter and 250 mL capacity 3 cm2 Pt sheet, 3 cm2 BDD thin-film deposited

on conductive single crystal p-type Si wafersand a 4.5 cm2 Pt cylindrical mesh

A 3 cm2 carbon-PTFE and 70 cm2

(17 cm×4.1 cm) carbon felt[39]

11 An undivided cylindrical glass cell of capacity0.175 L

BDD thin-film electrode which is deposited onboth sides of a niobium substrate (3.0 cm×4.0 cm)

Carbon sponge of size 1.0 cm×1.0 cm×4.0 cm [44]

12 Open undivided cylindrical glass cell of 6 cmdiameter and 250 mL capacity

4.5 cm2 Pt cylindrical mesh A 60 cm2 carbon felt [63]

13 Open undivided cylindrical glass cell of 500 mLcapacity

Pt sheet of 1 cm2 area Graphite felt of thickness=0.4 cm, havingan area of 9.5 cm2

[34]

14 Cylindrical cell of 250 mL capacity Pt gauze of area 6 cm2 Carbon felt of size 12.5 cm×4 cm [40]15 An open undivided cylindrical glass cell of

100 mL capacityPlatinum (geometric area, 4 cm2) BDD plate having geometric area of 2 cm2 [54]

16 A divided thermostatic cell of 150 mL in volume Pt sheet (purity: 99.99%) of area 2.0 cm2 Cu2O/CNTs/PTFE having an area of 3.0 cm2 [36]17 Undivided glass electrochemical cell of 600 mL

capacityPlatinum gauze of an area 3.8 cm2 Reticulated vitreous carbon (RVC) sheet of

an area of 35 cm2 and thickness of 0.9 cm.[25]

18 An open and undivided cell with a capacityof 0.55 L

RuO2/Ti mesh of area 20 cm2 20 cm2 area of ACF felt [51]

19 An undivided glass electrochemical cell ofcapacity 500 mL

Platinum gauze of an area 3.8 cm2 RVC sheet of dimensions 5 cm×7 cm×0.9 cm [41]

20 Glass beakers of capacity 250 mL Pt flakes of size 1×1 cm Pt flakes of size 1×1 cm [31]

6 PV. Nidheesh, R. Gandhimathi / Desalination 299 (2012) 1–15

reported similar results for BDD and Pt anodes in the E-Fenton process.Wang et al. [51] compared the efficiency of activated carbon fiber andgraphite cathodes for removing azo dye and Acid Red 14 from aqueoussolution. For ACF cathode, after 360 min of electrolysis under the operat-ing conditions of 0.36 A current, 1 mmol/L Fe2+ at pH 3, 70% TOC remov-al and 100% color removal were achieved. For the same operatingconditions, graphite cathode shows less removal efficiency than that ofACF [51]. Sudoh et al. [68] reported that the graphite was the best cath-ode material for electrogeneration of H2O2 while metal cathodes suchas copper, stainless steel, lead and nickel were likely to decomposeH2O2. This agrees with the results reported by Rosales et al. [27] for lis-samine green B dye (LGB) removal from aqueous solution using graphiteand stainless steel electrodes. The graphite electrodes have the highestLGB discoloration rate, with total discoloration after 10 h. Moreover,gradual corrosion was detected in stainless steel electrodes [27]. Özcanet al. [44] compared the propham degradation rate for BDD//Pt, Pt//CS(Note: CS — Carbon Sponge) and BDD//CS systems in acidic media. Thedecay rate of propham was higher in BDD//CS system than in BDD//Ptsystem. This result indicates that H2O2 production ability of CS washigher than that of Pt. A slight increase in removal rate was observed inthe case of BDD//CS compared to Pt//CS system [44].

4. Kinetics of E-Fenton process

The decay of organic pollutants via E-Fenton process can be repre-sented as:

RHþ OH·→Oxidation products ð7Þ

As •OH is a very reactive species it does not accumulate in the so-lution, and its concentration takes a steady-state value during

treatment [69]. The rate of decay of organic pollutants can be writtenas:

d RH½ �dt

¼ kabs RH½ � OH·½ � ð8Þ

Since [•OH] is constant at steady state, kabs [•OH] is equal to kapp.Where kabs and kapp are absolute and apparent rate constants, respec-tively. The value of kabs can be determined by kinetic competitionmethod using the benzoic acid as standard substrate having a kabsvalue of 4.3×109 L mol−1 s−1 [70,71]. Eq. (8) can be rewritten as:

d RH½ �dt

¼ kapp RH½ � ð9Þ

Integration of Eq. (9) gives the first order kinetic equation of E-Fenton process as:

InRH½ �oRH½ �t

¼ kappt ð10Þ

Where [RH]0 and [RH]t are the concentration of organic pollutantat beginning and time t, respectively. The kapp can be determined an-alytically from the slope of concentration vs. time plot in accordancewith the above equation. The second order kinetic equation of E-Fenton is given as [4]:

RH½ �tRH½ �o

¼ 11þ kt RH½ �o

ð11Þ

Table 3Optimum pH values of E-Fenton process in various studies.

No Pollutant Electrodes Optimum pH Efficiency (%) Time (min) Reference

1 Dyeing wastewater Activated carbon fiber-Pt wire 3 75.2 240 [66]2 LGB Steel-graphite 2 40 120 [27]3 Methyl red Graphite PTFE-Pt 3 80 20 [30]4 Sunset yellow FCF RVC-Pt 3 100 120 [25]5 Dyeing wastewater Graphite-Pt/Ti 3 70 150 [55]6 Rhodamine B Fe@Fe2O3/CNT-Pt 3 99.6 120 [65]7 Biologically treated coking wastewater ACF-Ti/RuO2 4 55 480 [79]

7PV. Nidheesh, R. Gandhimathi / Desalination 299 (2012) 1–15

5. Affecting factors

5.1. pH

The pH is one of the most important factors for the E-Fentonprocess. Generally Fenton process was conducted in acidic medi-um. Most of the studies reported that the optimum pH of Fentonprocess is around 3 [25,30,55,65]. In traditional Fenton process,iron species begin to precipitate as ferric hydroxides at higherpH values. On the other hand, iron species form stable complexeswith H2O2 at lower pH values, leading to deactivation of catalysts.Consequently, the oxidation efficiency dramatically decreases [66].Acidic medium is the favorable condition for the production ofH2O2 (Eq. (5)) [66]. However, a low pH also promotes hydrogenevolution, as given in Eq. (12), reducing the number of activesites for generating hydrogen peroxide [66]. In addition, at pHbelow 3, hydrogen peroxide would remain steady according tothe formation of oxonium ion (e.g., H3O2

+ as in Eq. (13)) [30].Due to the regeneration of Fe2+, through reaction between Fe3+

and H2O2, Fenton process becomes less effective at pHb3[72,73]. At higher pH, the efficiency of E-Fenton process decreasesrapidly, especially pH>5. This is due to the fact that H2O2 is un-stable in basic solution. H2O2 rapidly decomposes to oxygen andwater at neutral to high pH with rate constant of 2.3×10−2

and 7.4×10−2 min−1 at pH 7.0 and 10.5, respectively [74,75].

H2O2 þ 2Hþ þ 2e−→2H2O ð12Þ

H2O2 þHþ→H3Oþ2 ð13Þ

The increase in pH during E-Fenton process leads to electro-coagulation whereby pollutants are removed by electrostatic attrac-tion and/or complexation of reactions due to the conversion of Fe2+

and Fe3+ to Fe(OH)n type structures [76]. Ting et al. [77] reportedthat 2,6-dimethylaniline concentration decreased from 36% to 25%in 2 h when pH was increased from 1.5 to 2.0. The complete removalof 2,6-dimethylaniline was achieved after 140 min at pH 2. Also a fur-ther increase of pH from 2 to 4 increased 2,6-dimethylaniline concen-tration from 25% to 85%. Daneshvar et al. [78] suggested theperchloric acid instead of hydrochloric or sulfuric acid to adjust theoptimal pH value to obtain optimal removal of dyes. But the optimumpH value shows the disadvantage of E-Fenton process because the pHof most wastewater samples is not within the optimal range. The op-timum pH necessary for the different pollutant removal by E-Fenton

Table 4Optimum temperatures of E-Fenton process in various studies.

No Pollutant Electrodes Experimental conditions

1 Dyeingwastewater

Pt-PAN-based activatedcarbon fiber

Current density=3.2 mA/cm2; oxygepH=3; [Fe2+]=2 mM.

2 Dyeingwastewater

Graphite-Pt/Ti pH=3; oxygen sparging rate=0.3 L/m2; [Fe2+]=15 mM.

3 Nitrotoluenes Pt–Pt Electrode potential=6 V, O2=100 m

process is reported in Table 3. From Table 3, it was found that the op-timum pH for E-Fenton process varies from 2 to 4.

5.2. Oxygen sparging rate

Oxygen is one of the major factors that limit the performance of E-Fenton system, because increasing the oxygen sparging rate can in-crease the dissolved oxygen concentration and the mass transferrate of dissolved oxygen and finally increase the production of hydro-gen peroxide [66]. Chen and Lin [80] reported that the electrochemi-cal oxidation of TOC correlates well with the hydrogen peroxidegenerated at the cathode, wherein the saturated solubility of oxygenin wastewater has been almost achieved at the oxygen flow rate of100 mL/min. But the color removal efficiency remained almost con-stant at a current density of 68 A/m2 even when the oxygen spargingrate increased from 0.3 to 0.4 L/min [55]. The results indicate thatcolor removal began to be controlled by the kinetics of the productionof hydrogen peroxides when the oxygen sparging rate exceeded0.3 L/min [55]. Similar results are reported by Wang et al. [66] afterthe oxygen sparging rate was over 150 L/min for COD removal.

5.3. Temperature

Although temperature has a positive effect on the treatment effi-ciency in Fenton and related processes, the increase in organic com-pound removal due to temperature is relatively small compared tothe other factors. Too low and too high temperatures negatively im-pact the process efficiency. An optimal temperature of 30 °C hasbeen reported by Guedes et al. [81] for the degradation of corkcooking wastewater. Temperature between 20 and 30 °C can be con-sidered as an optimum range because of relatively higher treatmentefficiency in this temperature range [82]. Zhang et al. [83] reportedthat COD removal efficiency of Fenton process increased slightly asthe temperature increased from 15 to 36 °C. Wang [4] reported thatthe rate of dye degradation was lower at low temperature and the ex-tent of degradation was higher at 20–30 °C before 100 min. On theother hand Wu et al. [84] found an optimal temperature of 45 °C forthe degradation of humic acid. Also up to 100% degradation ofdiisopropanolamine at 60 °C has been reported by Khamaruddin etal. [85]. “Temperature has two effects on the accumulated H2O2 con-centration during the electrolysis. First one is the decrease of O2 solu-bility in water with increase of the temperature and second one is thelow stability of H2O2 at high temperatures” [86]. Dye degradation ratehas been decreased when the temperature was greater than 30 °C

Temperature(°C)

Removalefficiency (%)

Reference

n sparging rate=150 cm3/min; 20 75 [66]

min; applied current density=68 A/ 45 70.6 [55]

L/min, pH=0.2, [Fe2+]=15 mg/L. 40 ~100 [80]

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due to decomposition of H2O2 at a higher temperature [4]. The nega-tive effect of temperature on the pollutant removal percentage canalso be explained by the lower concentration of dissolved oxygenand the self-decomposition of hydrogen peroxide at higher tempera-tures [66]. Namely, the concentration of hydrogen peroxide decreasedas the temperature increased because increasing temperatures can de-crease oxygen solubility in the wastewater [66]. In addition, the rateof self decomposition of hydrogen peroxide to water and oxygen in-creased with the temperature [55,66]. But Hameed and Lee [87]reported that degradation of malachite green increased from 85.59 to98.14% as a consequence of increasing the temperature from 30 to50 °C within the first 10 min of Fenton process. Also Homem et al.[88] reported that the reaction rate of amoxicillin degradation was in-creased by increasing the temperature (in between 22 and 57 °C).Zazo et al. [89] reported that increasing the temperature clearly im-proves both the oxidation rate and the degree of mineralization of phe-nol by Fenton oxidation allowing working with reduced amounts ofH2O2 and Fe2+. This may be due to the fact that the increase in reactionrate between hydrogen peroxide and any form of ferrous/ferric iron(chelated or not) at higher temperature increases the rate of generationof oxidizing species such as •OH radical or high-valence iron species[90]. The optimum temperatures and corresponding removal efficien-cies reported by various researchers are given in Table 4.

5.4. Applied current density

The applied current is the driving force for the reduction of oxygenleading to the generation of hydrogen peroxide at the cathode. Higherapplied current increases the quantum of hydrogen peroxide pro-duced, thus increasing the number of hydroxyl radicals in the electro-lyte medium, which are highly reactive and responsible for thedegradation [91]. Higher applied current density means higher ap-plied voltage on the electrochemical system [66]. Also higherelectro-regeneration of ferrous ion from ferric ion (Eq. (14)) with in-creasing current increased the efficiency of Fenton chain reac-tions [32].

Fe3þ þ e−→Fe2þ ð14Þ

The efficiency of E-Fenton will be less at higher current density ef-ficiency. This is due to the competitive electrode reactions in the elec-trolytic cell. The discharge of oxygen at anode (Eq. (15)) and theevolution of hydrogen at cathode (Eq. (16)) occur at a higher current.These reactions inhibit main reactions such as reactions (6) and (14)[32], which lead to decrease in efficiency of E-Fenton.

2H2O→4Hþ þ O2 þ 4e− ð15Þ

Table 5Optimum current/current densities of E-Fenton process in various studies.

No Pollutant Electrodes Experimental conditions

1 o-Chlorophenol Stainless steel — Pt gauze [Pollutant]=80 mg/L, [Fe2+]=2 4-Nitrophenol Stainless steel — Ti/RuO2/IrO2 [Pollutant]=200 mg/L, [H2O2]=3 Dyeing

wastewaterPt-PAN based activated carbonfiber

Oxygen sparging rate=150 cmtemperature=20 °C

4 Acid Red 14 RuO2/Ti — ACF [Pollutant]=200 mg/L, [Na2SO5 Dyeing

wastewaterGraphite — Pt/Ti pH=3; oxygen sparging rate=

6 Picloram Carbon felt — Pt [Pollutant]=0.125 mM; [Fe3+]pH=3

7 2,4,6-Trinitrotoluene

Pt — carbon felt [TNT]0=0.2 mM, [Fe2+]=0.2 m

8 2,6-Dimethylaniline

Ti-RuO2/IrO2 — stainless steel [Pollutant]=1 mM; [Fe2+]=1

2Hþ þ 2e−→H2 ð16Þ

The degradation rate of organic pollutants is constant after300 mA. This is due to the formation of H2O as in Eq. (17) [42].

4Hþ þ O2 þ 4e−→2H2O ð17Þ

Some studies indicated that the current density in the E-Fentonprocess should be no larger than 10 A/m2, while others indicatedthat the upper limit value should be 6.4 A/m2 [92]. The optimum cur-rent or current densities and corresponding removal efficiencies of E-Fenton process are given in Table 5.

5.5. Fe2+ concentration

Suitable ferrous ion concentration is an important prerequisite inthe E-Fenton process [30]. Generally the efficiency of E-Fenton pro-cess increases with Fe2+ concentration because the concentration ofhydroxyl radical, which is the main oxidizing agent in the E-Fentonprocess increases with the increase in Fe2+ concentration (Eq. (1)).Also the oxidizing power of hydrogen peroxide was not enough to de-stroy large molecules, such as dyestuffs in real dyeing wastewater inthe absence of ferrous ions [66]. The color removal efficiency wasmarkedly increased from 9% to 46% by externally adding a smallamount of ferrous ions (5 mM) [55]. Wang et al. [66] reported thatthe presence of Fe2+ significantly improved the COD removal per-centage. The COD removal percentage markedly increased from19.8% to 43.1% by externally adding a Fe2+ concentration of0.33 mM [66]. Zhou et al. [30] reported that the removal of methylred increased from 45% to 75% in 10 min in the presence of Fe2+.Wang et al. [51] observed an increase in the rate of TOC decay by E-Fenton process, when initial Fe2+ concentrations were increasedfrom 0 to 1 mM. However, ferrous ions in the electrolyte solution,when present in excess, could consume the hydroxyl radicals and af-fect the extent of degradation [91]. The plausible interpretation isgiven by a competitive reaction between hydroxyl radicals and fer-rous ions, which could diminish the concentration of hydroxyl radi-cals as in Eq. (4) [93,94]. The effect of Fe2+ concentration on thekinetic rate constants, for 2,6-dimethylaniline degradation was stud-ied by Ting et al. [77]. The kinetic rate constant increased with in-creasing Fe2+ concentration from 1.0 to 1.5 mM and does notincrease significantly as the dosage of ferrous ions increased from1.5 to 2.0 mM [77]. The optimum Fe2+ concentrations andcorresponding E-Fenton process removal efficiencies are given inTable 6.

Applied current orcurrent density

Removalefficiency (%)

Reference

0.5 mg/L, electrolysis time=60 min 600 mA 59.12 [91]9.12 mmol/L, Fe(II)/H2O2=0.050 1 A 65.0 [32]

3/min; pH=3; [Fe2+]=2 mM; 3.2 mA/cm2 75.2 [66]

4]=0.05 M, pH=3, [Fe2+]=1 mM 0.50 A 73.3 [51]0.3 L/min; [Fe2+]=15 mM 68 A/m2 70 [55]

=0.1 mM; [Na2SO4]=50 mM; 300 mA 100 [42]

M 250 mA 99 [61]

mM; [H2O2]=20 mM; pH=2.0 10.6 A/m2 91 [77]

Table 6Optimum Fe2+ concentrations of E-Fenton process in various studies.

No Pollutant Electrodes Experimental conditions Fe2+ concentration Removal efficiency (%) Reference

1 o-Chlorophenol Stainless steel — Pt gauze [Pollutant]=80 mg/L, applied current=60 mA,electrolysis time=60 min

2 mg/L 80.4 [91]

2 Dyeing wastewater Pt-PAN-based activatedcarbon fiber

Current density=3.2 mA/cm2; oxygen spargingrate=150 cm3/min; pH=3; temperature=20 °C.

2 mM 75 [66]

3 Methyl red Graphite PTFE — Pt pH=3.0, [Na2SO4]=0.1 M, [pollutant]=100 mg/L,oxygen flow rate=0.4 L/min

0.25 mM 75 [30]

4 Sunset yellow FCF RVC — Pt [pollutant]=0.2 mM, [Na2SO4]=0.05 M, pH=3 0.1 mM 100 [25]5 Acid Red 14 RuO2/Ti — ACF [pollutant]=200 mg/L, [Na2SO4]=0.05 M, pH=3 1 mM 68.2 [51]6 Dyeing wastewater Graphite — Pt/Ti pH=3; oxygen sparging rate=0.3 L/min; applied

current density=68 A/m215 mM 70 [55]

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5.6. Hydrogen peroxide concentration and feeding mode

The initial concentration of H2O2 plays an important role in the E-Fenton process [77]. Removal of pollutants increases with increase inH2O2 concentration. The increase in the removal efficiency was due tothe increase in hydroxyl radical concentration as a result of the addi-tion of H2O2 (Eq. (1)) [77]. Zhang et al. [32] reported that efficiency ofhydrogen peroxide for removing organic materials in the leachate de-creased with the increase of Fenton's reagent dosage. At a high dosageof H2O2, the decrease in removal efficiency was due to the hydroxylradical scavenging effect of H2O2 (Eqs. (18) and (19)) and the recom-bination of the hydroxyl radical (Eq. (20)) [95].

HO· þH2O2→HO·2 þH2O ð18Þ

HO·2 þ HO·→H2Oþ O2 ð19Þ

2HO·→H2O2 ð20Þ

Improvement of E-Fenton process efficiency by feeding Fenton'sreagent in multiple steps or continuous mode was reported byZhang et al. [12]. Anotai et al. [96] compared the one-step and thetwo-step addition with H2O2 for aniline degradation. The results indi-cated that the aniline oxidation for the system of one-step additionwith H2O2 was similar to that of the two-step. In both cases anilineoxidation was about 95% after reacting for 60 min. The removal effi-ciency of COD in the one-step addition with H2O2 was similar tothat in the two-step. On the other hand, the TOC removal efficiencyand remaining Fe2+ concentrations were not affected by the H2O2

feeding mode [96].Effect of H2O2 addition in a single step and in continuous mode on

the degradation of 4-nitrophenol was reported by Zhang et al. [32]. Ithas been reported that decreasing feeding time increased the initialCOD removal rate. The optimum COD removal has been reachedwhen H2O2 was applied in a single step. But for continuous mode,the concentration of H2O2 increased with decrease in feeding timeduring the initial period. This will cause the production of more hy-droxyl radicals as in Eq. (1) and increases the efficiency of E-Fentonprocess. The final COD removal efficiency increased with the decreas-ing feeding time and reached highest when feeding time was 60 min.But due to side reactions, the efficiency of E-Fenton process decreaseswith further decrease of feeding time [32]. The concentration of hy-drogen peroxide during the initial period would be higher when hy-drogen peroxide was added in a single step or fed more quickly in acontinuous mode, the produced hydroxyl radicals would be scav-enged by hydrogen peroxide. This reaction leads to the productionof hydroperoxyl radical (Eq. (18)), a species with much weaker oxi-dizing power compared to hydroxyl radical [32].

5.7. Distance between the electrodes

In the E-Fenton process, distance between electrodes is anotherimportant factor that affects the removal of pollutants. The decreaseof the distance between the electrodes leads to a decrease of theohmic drop through the electrolyte and then an equivalent decreaseof the cell voltage and energy consumption [97]. Zhang et al. [12]reported that the COD removal efficiency from landfill leachateremained the same for electrode distance between 1.3 and 2.1 cm.The removal efficiency of E-Fenton system was less for the shorteror larger distance. This is because electro-regenerated Fe2+ could beeasily oxidized to ferric ion at the anode, when the electrodes wereplaced too short [12]. Longer distance causes the limiting mass trans-fer of ferric ion to the cathode surface that governs ferrous ion regen-eration [12,98]. Atmaca [20] reported that the changes in the distancebetween the electrodes have an insignificant effect on treatment effi-ciency. Use of long electrode distance in E-Fenton reactor causes a sig-nificant increase in energy consumption [20].

5.8. Nature of the supporting electrolyte

Electrolyte improves the solution conductivity, and accelerates theelectron transfer, thus benefiting the E-Fenton reaction. Therefore,supporting electrolyte is necessary, especially in the solution withoutenough conductivity [21]. In E-Fenton process, sodium sulfate is com-monly used as the supporting electrolyte. Zhou et al. [30] reportedthat higher Na2SO4 concentration led to higher current density,which resulted in faster and larger production of hydrogen peroxideand increases the efficiency of E-Fenton system. But Diagne et al.[99] observed a faster methyl parathion degradation rate in the pres-ence of NO3

− than that of SO42−. Daneshvar et al. [78] also reported

that the change in Na2SO4 concentration from 0.05 to 0.1 M did nothave any effect on H2O2 accumulation. Ghoneim et al. [25] reportedthat optimal Na2SO4 concentration for sunset yellow FCF is 0.05 M.But Pt-graphite PTFE electrolytic system has an optimal Na2SO4 con-centration of 0.1 M for the degradation of methyl red [30]. Also theauthors noticed a significant drop in efficiency of the system at0.2 M Na2SO4 concentration. This may be due to the consumption ofthe generated hydroxyl radical by high SO4

2− concentration as inEq. (21) [30].

HO· þ SO2−4 →HO− þ SO·−

4 ð21Þ

Efficiency of 0.05 M Na2SO4, 0.05 M NaCl and 0.05 M KCl assupporting electrolyte was compared by Ghoneim et al. [25] andreported that aqueous solutions with SO4

2− have the highest rate ofdecolorization of sun set yellow compared to Cl− electrolytes of thesame concentration. But approximately 100% removal of the azo-dye was achieved for all electrolytes. The time taken for the removalof dye is 120, 180 and 180 min respectively for 0.05 M Na2SO4,

10 PV. Nidheesh, R. Gandhimathi / Desalination 299 (2012) 1–15

0.05 M NaCl and 0.05 M KCl solution [25]. Pimentel et al. [60] com-pared the efficiency of iron, cobalt, manganese, and copper salts toprovide the metal cations as catalyst of Fenton reaction to producehydroxyl radicals and concluded that ferrous ions were the most ef-fective catalysts with optimum concentration of 0.1 mM. 0.1 mM ofsoluble FeSO4 supplied the optimum catalytic condition, allowing100% removal of TOC of aqueous phenol solutions [60]. But duringE-Fenton process, degradation of some other compounds using ironas catalyst may lead to formation of complexes and changing ironconcentration in the media [60]. Özcan et al. [86] reported that theproduction of H2O2 is lower in the case of NaCl than that of NaNO3

and NaSO4. Significantly a faster decrease of Orange II in the NaClO4

media than in Na2SO4 or NaCl media has been noticed by Daneshvaret al. [78]. Concentration of electrolyte also affects the efficiency ofE-Fenton process. But Daneshvar et al. [78] reported that the concen-tration of supporting electrolyte does not have any effect in orange IIdegradation, when the NaClO4 concentration has been increased from0.05 to 0.1 M.

6. Applications

In recent years there is a great interest in the development of prac-tical electrochemical methods for the destruction of toxic and bio-refractory organic pollutants for wastewater treatment [91]. The E-Fenton process is widely used to treat non-biodegradable or refractoryorganic compounds with moderate energy costs [100]. The applica-tions of E-Fenton process is explained below.

6.1. Dyes

The dyes used in the textile dyeing and printing industries notonly can impart color to water sources but also can cause environ-mental damage to living organisms by stopping the reoxygenation ca-pacity of water and also blocking sunlight, thereby disturbing thenatural growth activity of aquatic life [4]. Some of the dyes are alsotoxic and carcinogenic in nature [101]. Presence of very smallamounts of dyes in water (less than 1 ppm for some dyes) is highlyvisible and undesirable [102]. Synthetic dyes are used extensivelyby several industries including textile dyeing (60%), paper (10%)and plastic matter (10%) [35]. It is estimated that 10–15% of the dyeis lost in the effluent during dyeing processes [103]. Presently, morethan 10,000 types of different commercial dyes and pigments areavailable [101], and more than 7×105 t per year are produced world-wide [102].

E-Fenton process has been identified as a powerful tool for remov-al of dye fromwastewater very effectively. Guivarch et al. [35] studiedthe degradation of the three azo dyes azobenzene, methyl orange andp-methyl red by the E-Fenton and concluded that this process is effi-cient for azo dye degradation, achieving an efficient removal (over80%) of COD. The degradation mechanism begins with the azo bondcleavage and is followed by the hydroxylation of the aromatic ring[35]. A synthetic dye wastewater sample composed of yellowdrimaren, Congo red and methylene blue and having an initial CODof 3782 mg O2 L−1 was successfully mineralized using E-Fenton pro-cess by Lahkimi et al. [38] with the COD abatement ratio of 89% after10,000 C of electrical charge passed. Decolorization of acid yellow 36in acidic aqueous medium was studied by Cruz-González et al. [54]and reported 97.8% of acid yellow 36 removal at optimum conditionssuch as Fe2+ of 0.24 mmol/L, current density of 23 mA/cm2 and elec-trolysis time of 48 min. Complete mineralization of indigo carmine of220 mg/L concentration is feasible when E-Fenton is carried out witha BDD anode [43]. The degradation of different dyes by E-Fenton ox-idation was carried out successfully in a continuous reactor by Rosaleset al. [27]. The reactor was very efficient for dye removal and high dis-coloration percentage depends on the residence time. The operationalproblems of the reactor were also very less [27]. Degradation of

rhodamine B in aqueous solution was evaluated by Ai et al. [36].The study reported that degradation of rhodamine B reached 80.2%and 89.3% in 120 min at neutral pH and pH 3, respectively [36].Zhou et al. [30] reported that the degradation of methyl red was ac-complished at two different stages, and the consumption of ferrousion and formation of hard-to-treat intermediates led to the slowerdegradation in the second stage. Under the optimal conditions, theinitial methyl red concentration of 100 mg/L could be degraded 80%in 20 min [30]. Ghoneim et al. [25] reported that for a contact timeof 120 min, complete color removal and significant mineralization(approximately 97%) of sunset yellow FCF have been achieved. 70%TOC removal for 500 mL of a 200 mg/L Acid Red 14 after 360 min ofelectrolysis was reported by Wang et al. [51]. El-Desoky et al. [41] ap-plied optimized E-Fenton system successfully for complete degrada-tion and significant mineralization (approximately 85–90%) ofLevafix blue and red reactive azo-dyes in real industrial wastewatersamples of textile dyeing house. The removal efficiency of colorfrom real dyeing wastewater in the cathodic chamber reached 70.6%under specified operation conditions in 150 min [55]. This studyreported that the best oxygen contact mode for removing the colorwas the three-phase contact mode, resulting in the optimal transfer-ence of the dissolved oxygen to the electrode surface [55]. COD re-moval efficiency from real dyeing wastewater by using Fe2+ incombination with electrogenerated hydrogen peroxide at the polyac-rylonitrile based activated carbon fiber cloth cathode was studied byWang et al. [66]. In this study, the highest COD removal efficiency(75.2%) was achieved at an applied current density of 3.2 mA/cm2.

6.2. Pesticides and herbicides

Due to the extensive utilization in agricultural activities, pesticidesand herbicides are widely detected in many surface water, groundwa-ter and wastewater effluents and are among the most frequentlyfound organic pollutants in natural waters [104,105]. Many of thesepesticides are utilized in amounts over 50,000 kg/year [106]. Relative-ly high pesticide and herbicide contamination levels are found ingroundwater and surface water: 0.1–0.3 μg/L in US groundwaterand 0.03–0.5 μg/L in European groundwater [107,108]. Similar resultshave been observed in India also. The residue levels of persistentchlorinated pesticides such as HCH (hexachlorocyclo-hexane) iso-mers and DDT (dichlorodiphenyltrichloroethane) compound samplesfrom the river Kaveri, Tamil Nadu, South India have been reported byRajendran and Subramanian [109]. River Ganges in Kanpur containshigh concentrations of γ-HCH (0.259 μg/l) and malathion (2.618 μg/l)[110]. γ-HCH, malathion and dieldrin concentrations of 0.900,29.835 and 16.227 μg/l, respectively were detected in ground watersamples [110]. Sanghi et al. [111] reported that the endosulfan con-centrations in the human milk were the highest and exceeded theS-HCH, chlorpyrifos, and malathion concentrations by 3.5-, 1.5-,and 8.4-fold, respectively. Similarly Jani et al. [112] reported an aver-age concentration of alpha-HCH, gamma HCH, beta HCH, p,p′-DDE(Note: DDE-Dichlorodiphenyldichloroethylene), and p,p′-DDT as17.51, 1.62, 205.48, 244.71, and 53.43 μg/kg, respectively in humanmilk. Also dairymilk and buffalomilk from Jaipur citywere contaminatedwith DDT and its metabolites (DDE and p,p′-dichlorodiphenyl-dichloroethane DDD), isomers of hexachlorocyclohexane (HCH; alpha,beta, and gamma), heptachlor and its epoxide, and aldrin [113].

This situation is considered as a pervasive problem because thesecompounds usually have direct adverse effects on the living organ-isms [114]. The E-Fenton process has been proved to be powerfulenough to degrade hazardous pesticides and herbicides. Yatmaz andUzman [52] reported that E-Fenton reaction by using both Fe elec-trodes with supply of H2O2 is the most efficient method for the degra-dation of monocrotophos in acidic medium. Monocrotophos wasrapidly degraded within 5 min and the energy consumption of com-plete degradation was 0.834 kWh/kg [52]. Boye et al. [108] reported

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80% mineralization of herbicide 4-chlorophenoxyacetic acid. Özcan etal. [44] compared the propham removal efficiency of BDD, CS and Ptelectrodes as anode and/or cathode at four different configurationsin the E-Fenton process. The authors reported that the highest TOCremoval efficiency and the lowest mineralization current efficiencyvalues were observed at 500 mA in the presence of 0.2 mM Fe3+.The best mineralization current efficiency value of 81% was obtainedat 100 mA in the presence of 0.2 mM Fe3+ for 30 min treatment[44]. Edelahi et al. [46] reported 93% COD removal within 10 min bydegrading diuron using E-Fenton process. This process is also efficientfor imazapyr degradation and the COD removal was found to behigher than 95% [115]. The degradation of herbicide chlortoluron inaqueous medium by E-Fenton process using a carbon felt cathodeand a platinum anode was studied by Abdessalem et al. [69]. Eventhe degradation of 0.05 mM chlortoluron happened within 4 min,98% of TOC removal was recorded only after 8 h [69]. Similar resultswere also reported by Özcan et al. [42] for the removal of picloram.Kaichouh et al. [116] reported that the imazaquin is degraded morequickly than the imazapyr. But 97% mineralization of each herbicidewas reached after 3.5 h of treatment under optimal operating condi-tions of I=0.2 A and [Fe2+]0=0.1 mM [116]. E-Fenton with a Ptanode, stainless steel sheet cathode and 1 mmol/L Fe2+ as catalystyields the quickest and complete depollution of amitrole [49]. A com-parative study of a mixture of three pesticides (chlortoluron,carbofuran and bentazon) has been investigated by Abdessalem etal. [117]. It was reported that based on cost, E-Fenton process ismuch more interesting than photo-Fenton process [117].

6.3. Phenolic compounds

Among the various wastes, phenolic compounds constitute a fam-ily of pollutants particularly toxic to the aquatic fauna and flora. Thesecompounds are released in the surface water by a considerable num-ber of industries, mainly, by pharmaceutical plants, oil refineries, cokeplants, pulp, and food-processing industries and several other chem-ical plants [118,119]. Nitrophenols are anthropogenic, toxic, inhibito-ry and biorefractory organic compounds used extensively in chemicalindustries for the manufacture of pesticides, pharmaceuticals andsynthetic dyes [120]. Chlorophenols have been detected during themanufacture of pesticides, bleaching of industrial wastewater, andchlorination of drinking water [121]. Phenolic compounds are verytoxic to human health and aquatic life. Consequently, removal ofthem from wastewater is an environmental concern. These com-pounds are removed very effectively by E-Fenton process. Pimentelet al. [60] reported that total mineralization of phenol was obtainedat optimum experimental conditions. The degradation of variouschlorophenols by E-Fenton method was carried out by Song-hu andXiao-hua [122]. The degradation sequence of various chlorophenolswere in the following order: 2,4-dichlorophenol>2,4,6-trichlorophe-nol>pentachlorophenol>4-chlorophenol. This order is different fromother AOP studies. The degradation pathways of chlorophenolswere pro-posed as ortho- and para-reaction by hydroxyl attack and directdechlorination by cathode reduction [122]. Further oxidation waspreceded by hydroxyl radical. Most of the residual chlorine was left inthe ring opening low molecule compounds [122]. 280.7 C electricalcharge was consumed during 450 min of electrolysis to attaindegradation of 4-chloro-2-methylphenol and 14.9% TOC removal and89.3% dechlorination have been reported by Irmak et al. [62]. The kineticsof the oxidative degradation of several chlorophenols, such as mono-chlorophenols (2-chlorophenol and 4-chlorophenol), dichlorophenols(2,4-dichlorophenol and 2,6-dichlorophenol), trichlorophenols (2,3,5-trichlorophenol and 2,4,5-trichlorophenol), 2,3,5,6-tetrachlorophenol,and pentachlorophenol by the E-Fenton process have been investigatedusing a carbon felt cathode [123]. It was demonstrated that the num-ber and the position of the chlorine atoms in the aromatic ring signif-icantly influence the oxidation. The degradation followed a pseudo-

first-order kinetics and the apparent rate constant follows thesequence 4-chlorophenol>2-chlorophenol>2,4-dichlorophenol>and2,6-dichlorophenol>2,3,5-trichlorophenol>2,4,5-trichlorophenol>2,3,5,6-tetrachlorophenol>pentachlorophenol. It was also observed thatthe mineralization rate of chlorophenols decreased with increasing thenumber of chlorine atoms in the aromatic ring, confirming that themore chlorinated phenols are, the more difficult to mineralize [123]. E-Fenton process is highly efficient for the degradation of o-chlorophenol[91]. Sankara Narayanan et al. [91] concluded that “the non-availabilityof sufficient concentrations of ferrous ions with electrolysis time limitsthe efficiency of the process. Under optimized conditions it is possibleto achieve more than 70% degradation and addition of goethite as a cat-alyst did not show any significant increase in the degradation of o-chlorophenol”. Irmak et al. [62] reported that “in E-Fenton application,degradation of the refractory aromatic ring was quite straightforwardbut decomposition of all aliphatic products into carbon dioxide andwater, i.e. complete mineralization is time and energy consuming. Onthe other hand, release of chlorine atoms from organic structure intothe aqueous solution as chloride ions takes place almost simultaneouslywith the breaking-up of the aromatic ring. In other words, toxicity relat-ed to the organochlorine structures is diminished in the early stages ofthe E-Fenton treatment. Therefore, one can advise to continue to treatthe aqueous system by using cheaper conventional techniques followingthe breakdown of aromatic structures and in situ release of most of thechlorine as chloride ions” [62]. Degradation of 4-nitrophenol was carriedout in batch recirculation mode by Zhang et al. [32]. It was reported thateven under successful COD removal, E-Fenton process induces a syner-getic effect on COD removal. Treatment of high concentration of 4-nitrophenol in the undivided cell removed more than 98% of 4-nitrophenol and about 13% of TOC [124]. Negligible quantity of nitrateand nitrite ions detected indicates that there is no direct release of –NO2 and –NOgroups from4-nitrophenol and its degradation intermedi-ates [124].

6.4. Leachate

Landfills are ubiquitous in modern society, and the proper man-agement of their potential environmental impact is of the highest pri-ority [125]. Landfill leachate is defined as those aqueous streamsgenerated as a consequence of rainwater percolation through wastes,biochemical processes in the wastes' cells and the inherent watercontent of the wastes themselves [126]. Leachate may contain largeamounts of organic matter, of which humic-type constituents are animportant group, as well as ammonia-nitrogen, heavy metals andchlorinated organic and inorganic salts. The removal of organicmaterial based on COD, biological oxygen demand and ammoniumfrom leachate is the usual prerequisite before discharging the leach-ate into natural waters [126]. Only a few works on leachate treatmentby E-Fenton are reported. Altin [127] reported more than 90% colorremoval efficiency by E-Fenton process. 87% PO4–P and 26% NH4–Nremoval from landfill leachate under optimum conditions wasreported by Atmaca [20]. Also this study reported 72% COD and 90%color removal. Mohajeri et al. [128] achieved 94% COD removal and95.8%color removal from landfill leachate. Treatment of high strengthlandfill leachate by E-Fenton process was studied by Zhang et al. [12].Ti/RuO2 and IrO2 type electrodes were used as anode. They reportedthat the process was very fast in the first 30 min and then sloweddown till it was complete in 75 min. Atmaca [20] studied the charac-teristics of sludge produced from landfill leachate treatment by E-Fenton process. The author reported that the sedimentation charac-teristics of the waste are fairly good [20].

6.5. Drugs

A large number of pharmaceutical drugs have been recentlydetected in water sources. For example chlorophene, a widespread

Table 7Various pollutants removed by E-Fenton process.

No Pollutant/purpose Research highlights Reference

1 Aniline • After 6 h of electrolysis at 100 mA 68% of TOC removal achieved. [131]• A fast aniline mineralization• Ammonium ions (75–80% of initial nitrogen) were generated

2 Petrochemical wastewater • More than 50% of COD removal efficiency [132]• E-Fenton method is effective in treating this wastewater

3 Biological coking wastewater • Optimum parameters were: pH 4, 1.8 h reaction time, 0.6 mM of Fe2+

and 3.7 mA/cm2 of current density[79]

• 55% TOC removal4 Industrial wastewater containing morpholine and

diethylethanolamine, as well as sodium salts of naphthalenesulfonic acid and of ethylenediamine tetraacetic acid

• Up to 64.5% of running costs can be cut when E-Fenton used [133]• 100% current efficiency in solutions polluted by organic substances

5 Dinitrotoluenes and 2,4,6-trinitrotoluene • Nearly complete decomposition of nitrotoluenes under the optimal conditionsof electrode potential=6 V, T=303 K, O2=100 mL/min and iron(II)=15 mg/L.

[80]

6 2,4,6-Trinitrotoluene • After 5 min electrolysis, 70 and 99% of the initial TNT content were degradedfor 60 and 250 mA current intensity values

[61]

• Minimum 22% of aromatic rings were cleaved at TNT disappearance time• 35% of initial material reached the last step before mineralization,

7 2,6-Dimethylaniline • 60% TOC removal efficiency [77]• Oxalic acid was the major intermediate detected from 2,6-dimethylaniline degradation

8 Nonylphenol polyethoxylate • Around 50% COD reduction [134]• 95% nonylphenol polyethoxylate removal was achieved in 5 min for aqueoussolution and 10 min for wastewater treatment

9 COD reduction of rayon industry wastewater • 88% COD was reduced in 50 min [59]10 COD reduction of leather tanning industry wastewaters • COD was reduced by 60–70% within 10 min [29]

• Over 70% COD removal for pH 3.• At a neutral pH, greater than 60% COD removal• Sulfide concentration in the tannery industry wastewater was almost 100%removed in 10 min

11 Benzene sulfonic acid • 64% of TOC removal [135]

12 PV. Nidheesh, R. Gandhimathi / Desalination 299 (2012) 1–15

broad-spectrum antimicrobial pharmaceutical has been detected atconcentrations up to 50 mg/L in activated sludge sewage systemsand up to 10 mg/L in sewage treatment plant effluents and rivers[39]. This pollution is originated from emission from productionsites, direct disposal of over plus drugs in households, excretionafter drug administration to humans and animals and treatmentsthroughout the water in fish and other animal farms [129]. Some ofthe drugs are carcinogenic and mutagenic [130]. To avoid the danger-ous health effects of such pollutants, potent oxidation methods areneeded to remove drugs and their metabolites from wastewaters[56]. E-Fenton is one of such powerful tool for removing drugs fromwastewater. The catalytic behavior of the Fe3+/Fe2+ system in theE-Fenton degradation of the antimicrobial drug chlorophene mainlydepends on the cathode [39]. The authors reported that that E-Fenton is a viable environmentally friendly technology for the reme-diation of wastewaters containing chlorophene. The removal efficien-cy of chlorophene by Pt/O2 diffusion, BDD/O2 diffusion, BDD/carbonfelt and Pt/carbon felt E-Fenton systems was compared. Maximum re-moval of chlorophene is in the order of Pt/carbon felt system followedby BDD/carbon felt, BDD/O2 diffusion and Pt/O2 diffusion systems[39]. Isarain-Chávez et al. [47] used two-electrode cells with a Pt orboron-doped diamond anode and an air-diffusion cathode for H2O2

electrogeneration, and four-electrode combined cells containing theabove pair of electrodes coupled in parallel to a Pt anode and acarbon-felt cathode, to degrade the pharmaceutical β blocker ateno-lol by E-Fenton. “Compared with the single cells, the correspondingnovel four-electrode combined systems enhance strongly the miner-alization rate of atenolol in E-Fenton. Because of the fast Fe2+ regen-eration at the carbon-felt cathode favoring: (i) the production ofmore amounts of •OH from Fenton's reaction that destroy more rap-idly aromatic pollutants and (ii) the formation of Fe(II) complexeswith final carboxylic acids such as oxalic and oxamic, which aremore quickly oxidized with •OH” [47]. Solutions of about 0.25 mMof the β-blocker metoprolol tartrate (100 mg/L total organic carbon)with 0.5 mM Fe2+ in the presence and absence of 0.1 mM Cu2+ withpH 3.0 have been degraded under single and combined E-Fenton

conditions [56]. The study reported that “the combined cell wasmuch more potent than the single one by the larger •OH generationfrom the continuous Fe2+ regeneration at the carbon felt cathode,accelerating the oxidation of organics. Total mineralization was fea-sible using the combined cell in the presence of 0.1 mM Cu2+, be-cause of the parallel quick oxidation of Cu(II) carboxylatecomplexes by •OH” [56].

6.6. Others

Many other pollutant degradation was carried out successfully byE-Fenton process. The details are given in Table 7. From Table 7, it isvery clear that E-Fenton is a very powerful and environmentallyfriendly tool for wastewater treatment.

7. Degradation pathway of organic pollutants

One of the main advantages of E-Fenton process is the completemineralization and degradation of organic compounds. High perfor-mance liquid chromatography (HPLC) analysis and Gas chromatogra-phy–mass spectrometry (GC–MS) of electrolyzed wastewaterrevealed the formation of different oxidation products. The decay ki-netics for initial pollutants can be monitored by reversed-phaseHPLC. The proposed reaction sequence for the degradation of dyeAcid Red 97 using carbon felt cathode and Pt anode [40] is shownin Fig. 6. The oxidation of Acid Red 97 under the action of •OH gives1,2-naphthalenediol, 1,1′-biphenyl-4-amino-4-ol, 2-naphthalenol di-azonium and 2-naphthalenol by the reduction of azo bonds. Theseproducts undergo further oxidation and release 2,3-dihydroxy-1,4-naphthalenedion, phthalic anhydride, 1,2-benzenedicarboxylic acid,phthaldehyde, 3-hydroxy-1,2-benzenedicarboxylic acid, 4-amino-benzoic acid and 2-formyl-benzoic acid. During this reaction mostof the nitrogen element went away from the dye structure asNO3

− and NH4+ ions or nitrogen. After that gradual cleavage of aro-

matic ring occurs, and this leads to the formation of CO2 as finalproduct [40].

13PV. Nidheesh, R. Gandhimathi / Desalination 299 (2012) 1–15

8. Conclusions

The application of E-Fenton on organic pollutant removal fromwastewater has received increased attention in the last decade. Dif-ferent types of electrolytic reactors were used for E-Fenton study. Ef-ficiencies of various anode and cathode were evaluated andcompared. This process is very much dependent on pH, oxygen sparg-ing rate, temperature of solution, applied current density, Fe2+ con-centration, hydrogen peroxide concentration and feeding mode,distance between the electrodes and nature of the supporting electro-lyte. Removal of various organic pollutants including dyes, drugs, her-bicides and pesticides, leachate, phenolic compounds etc. by thisprocess was evaluated. Overall, E-Fenton process is a promising tech-nology for applications in wastewater treatment.

Acknowledgments

The authors are thankful to K. Sharath, Ranjith Kumar and T.G.Parameswaran for the great support.

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