2016Electrochemical and photoassisted electrochemical

Electrochemical and photo-assisted electrochemical

treatment of the pesticide imidacloprid in aqueous

solution by the Fenton process:effect of operational

parameters

Marjan Sedaghat 1?Behrouz Vahid 2?

Soheil Aber 3?Mohammad H.Rasoulifard 4?

Alireza Khataee 1?Nezamalddin Daneshvar 5

Received:29November 2014/Accepted:11April 2015/Published online:6May 2015

óSpringer Science+Business Media Dordrecht 2015

Abstract The aim of this study was to compare the degradation ef?ciency (DE%)of imidacloprid as a model pesticide by electro-Fenton (EF)and photoelectro-Fenton processes (PEF)using undivided three-electrode electrochemical cell and UV irradiation in a batch mode.The potential of the working electrode (graphite)was ?xed at -1.0V versus the saturated calomel electrode.The selected operating conditions for treatment of imidacloprid (20mg/L)were:pH 2.8,Fe 2?concentra-tion of 0.36mM and Na 2SO 4concentration of 0.15M as the background elec-trolyte,which produced a DE%of 59.23and 80.49%for EF and PEF after 180min,respectively.Considerable synergistic effect between EF and UV pro-cesses was observed due to the regeneration of Fe 2?ions and more production of hydroxyl radicals (áOH).Besides,accumulation of the electro-generated H 2O 2in the electrochemical system as the source of áOH radicals was con?rmed.Moreover,total organic carbon measurements under the optimized condition demonstrated that 50.73and 67.15%of the organic substrates were mineralized after 300min of the treatment by the EF and PEF,respectively.Eventually,the experimental results revealed that the degradation and mineralization rates of the pesticide followed pseudo-?rst-order kinetics;however,the rate constants of the mineralization were &Marjan Sedaghat marjansedaghat60@https://www.wendangku.net/doc/b71637928.html,

1Research Laboratory of Advanced Water and Wastewater Treatment Processes,Department of

Applied Chemistry,Faculty of Chemistry,University of Tabriz,Tabriz,Iran

2Department of Chemical Engineering,Tabriz Branch,Islamic Azad University,Tabriz,Iran 3Research Laboratory of Environmental Protection Technology,Department of Applied

Chemistry,Faculty of Chemistry,University of Tabriz,Tabriz,Iran

4Water and Wastewater Treatment Research Laboratory,Department of Chemistry,Faculty of Science,University of Zanjan,Zanjan,Iran

5

Department of Applied Chemistry,Faculty of Chemistry,University of Tabriz,Tabriz,

Iran Res Chem Intermed (2016)42:855–868

DOI 10.1007/s11164-015-2059-5

856M.Sedaghat et al. lower than the degradation ones owing to the generated intermediates,which re-quired more treatment during the processes.

Keywords Electrochemical treatmentáImidaclopridáDegradationáMineralizationáEFáPEF

Introduction

Extensive kinds of pesticides are discharged into the aquatic environments from various sources such as agricultural runoffs,industrial ef?uents and chemical spills. These compounds are stable,carcinogenic and toxic in the environment even at their low concentrations and they can affect the aesthetic value of environment; moreover,they often have unfavourable effects on living organisms.For these reasons,strict environmental regulations are used in order to remove them from industrial wastewaters[1,2].There are several processes for treatment of organic contaminants in aqueous solutions such as adsorption,chemical coagulation, membrane processes and bioremediation.However,the mentioned methods generate secondary wastes by solely transferring the contaminants from a liquid to solid phase,which requires extensive treatment or partial elimination of the wastes,depending on their chemical nature and the treatment process itself[3–6].

In recent years,electrochemical advanced oxidation processes(EAOPs), including electro-Fenton(EF),photoelectro-Fenton(PEF)and anodic oxidation, have received great interest for degradation and mineralization of organic pollutants in aqueous medium owing to the production of noticeable amounts of reactive species,especially an hydroxyl radical(áOH).This radical is a powerful oxidant (E o=2.8V/SHE)and can destroy and mineralize water contaminants,carbon dioxide and inorganic compounds effectively and non-selectively[7,8].In these methods,hydrogen peroxide,which is considered as a‘‘green’’(environmentally friendly)reagent,is generated by the two-electron reduction of oxygen on the cathode surface in acidic conditions(Eq.1)[9].Properties of cathodes with carbon material-like graphite exhibit electrical conductivity,wide utilizable potential and low activity for hydrogen peroxide decomposition(Eq.2),making them proper for in situ generation of H2O2;therefore,the EF process facilitates H2O2usage by preventing its risky storage and shipment[10,11].

O2t2Htt2eà!H2O2e1T

H2O2t2Htt2eà!2H2Oe2TAddition of Fe2?or Fe3?in a small amount to the acidic solution enhances the oxidation capability of the electro-generated H2O2considerably by producingáOH radicals via a Fenton reaction(Eq.3).The EF process proceeds by the catalytic performance of the Fe3?/Fe2?system,from the regeneration of Fe2?by the reduction of Fe3?on the cathode surface(Eq.4),which minimizes the iron species concentrations in the solution[12,13].

Electrochemical and photo-assisted electrochemical treatment (857)

Fe2ttH2O2!Fe3ttáOHtOHàe3T

Fe3tteà!Fe2te4TThe EF process can be promoted by applying UV irradiation simultaneously,which can be explained mainly by photoreduction of Fe(OH)2?(Eq.5),which is the predominant form of Fe3?in an acidic medium,and photodecomposition of stable Fe3? complexes with generated organic ligand intermediates like carboxylic acids(Eq.6); these two reactions in the PEF process regenerate Fe2?and enhance the formation of active species,particularlyáOH radicals and,consequently,the destruction of organic pollutants.Furthermore,the oxidative capacity of the PEF increases due to the photolysis of H2O2under UV irradiation to form more hydroxyl radicals(Eq.7)[14–16].

? 2tth m!Fe2ttáOHe5T

Fe OH

eT

Fe OOCàR

eT

? 2tth m!Fe2ttCO2tRáe6T

H2O2th m!2áOHe7TSeveral researchers have reported the degradation of various organic pollutants such as pesticides or dyes by the EF or PEF processes and the destruction mechanism of the pollutants was explained properly by pseudo-?rst order kinetics[17–21].Imidacloprid is a systemic pesticide used extensively in the world;moreover,it is mobile in soil,has high water solubility,is toxic and persistent in nature.It is classi?ed by the US Environmental Protection Agency(USEPA)to be a potential water contaminant and categorized by the World Health Organization(WHO)as moderately hazardous(Class II).Hence,its treatment is essential from an environmental point of view[22].To the best of our knowledge,there is no report for the degradation and mineralization of imidacloprid by EF and/or PEF processes in a comparative approach.

The primary aim of this research was to compare the degradation ef?ciency of imidacloprid as a model pesticide pollutant by the EF and PEF processes,utilizing a graphite cathode in a batch mode.Then,the effect of major operational parameters on the degradation of the contaminant,including initial concentration of imidaclo-prid,pH,concentration of Fe2?ions and concentration of background electrolyte, was investigated to obtain the desired conditions.Next,the obtained data was used to study the kinetics of imidacloprid removal.Eventually,imidacloprid mineral-ization was monitored during both treatment processes under the optimal conditions by total organic carbon(TOC)decay.

Experimental

Chemicals

Imidacloprid(1-((6-chloro-3-pyridinyl)methyl)-N-nitro-2-imidazolidinimine)with a purity of95%was obtained from Chem-Service(USA)and its chemical structure

and properties are presented in Table 1.Other chemicals were provided by Merck,Germany.

Experimental set-up and procedures

Experiments were carried out in an undivided three-electrode electrochemical cell controlled by a DC power supply (ADAK PS808,Iran).The working,counter and reference electrodes (Azar electrode Co.,Iran)were a graphite rod (9cm 2),a platinum piece (1cm 2)and a saturated calomel electrode (SCE),respectively.A low pressure mercury lamp (15W,UV-C,manufactured by Osram,Germany)was placed at the top of the cell with the distance of 15cm from the solution surface and was switched on in the PEF process.First,the cell was ?lled with aqueous solution of imidacloprid (150mL)containing Na 2SO 4to maintain conductivity with certain concentrations.Then,H 2SO 4was added to adjust the pH,and a catalytic quantity of ferrous ion (FeSO 4)was added into the solution just before the beginning of the electrolysis.Prior to the each run,oxygen was bubbled for 20min to saturate the solution with it,and during the electrolysis,oxygen was continuously sparged on the cathode surface with a ?ow rate of 20mL/min.For controlled potential electrolysis,the potential of the working electrode was ?xed at -1.0V versus SCE [23].Imidacloprid degradation was followed by a decrease in absorbance at the maximum wavelength of the pesticide (270nm),as per a UV–Vis spectrophotometer (Elmer-Perkin,SE550),and degra-dation ef?ciency was calculated using the following equation;where A 0and A were the absorbance at the initial and distinct time of each processes,respectively.

DE %?A 0àA eT=A 0e8T

Hydrogen peroxide concentration was determined spectrophotometrically by the standard iodide method [24].

Results and discussion

Comparison of EF and PEF processes in degradation of imidacloprid,and kinetics study

The degradation ef?ciency of imidacloprid (20mg/L)at the same operational conditions was 9.14,59.23,and 80.49%for photolysis,EF and PEF processes after Table 1Chemical structure and characteristics of imidacloprid

Pesticide Structure Molecular

weight (g/mol)

k max (nm)Solubility in water (mg/L)WHO class Imidacloprid N

N NH N

Cl NO 2

255.7

270610II 858

M.Sedaghat et al.

180min of treatment,respectively (Fig.1).UV irradiation had no considerable effect on imidacloprid removal.However,when the EF process was carried out in the exposure of UV light at k max =254nm (PEF),DE%was enhanced noticeably,owing to the increased production of áOH radicals (Eqs.5,7),and the regeneration of Fe 2?(Eqs.5,6);hence,high oxidative capability was observed in the PEF process compared to the EF at the same operational conditions [14–16,25].Accumulation of the electro-generated H 2O 2in the electrochemical system during the initial 3h of electrolysis was investigated in the presence and absence of UV irradiation or Fe 2?.Figure 2,curve (a)shows a gradual rise in H 2O 2concentration in solution during the electrolysis without Fe 2?and UV.When Fe 2?was added to the solution without UV irradiation,less H 2O 2was accumulated

[Fig.2,curve (b)]in comparison with Fig.2,curve (a);this slightly lower concentration can be related to the decomposition of H 2O 2by the Fenton reaction (Eq.3).In the presence of Fe 2?and UV light [Fig.2,curve (c)]the lowest H 2O 2concentration was observed compared to the others;this is owing to not only the Fenton reaction (Eq.3),but also the greater production of áOH radicals in UV exposure (Eqs.5,7)and their reaction with H 2O 2to yield oxygen and water (Eqs.9,

10).As a consequence,the generation of hydroxyl radicals as the main oxidant was proven by monitoring of the H 2O 2concentration [9,16,26].

H 2O 2táOH !HO á2tH 2O

e9THO á2táOH !H 2O tO 2e10

T

Fig.1Comparison of direct photolysis,EF and PEF processes in degradation of imidacloprid ([Fe 2?]=0.36mM,[Na 2SO 4]=0.15M and pH =2.8);the inset demonstrates the mentioned processes follow pseudo-?rst order kinetics

Electrochemical and photo-assisted electrochemical treatment (859)

Inset plot of Fig.1was depicted based on the pseudo-?rst order kinetics assumption (Eq.11),and the apparent reaction rate constant (k app )for each process was determined by the slope of the plot of ln(A 0/A )versus time (t )(Table 2)with a high correlation coef?cient (R 2),which con?rmed the proposed mechanism [14,27,28].

ln A 0=A eT?k app ?t e11T

The synergistic effect of UV irradiation and EF for degradation of imidacloprid was remarkable and expressed in the terms of the obtained apparent pseudo ?rst-order rate constants by Eq.(12)as 42%(Eq.12)[9,28].

Synergy %?100?k PEF àk UV tk EF eT

k PEF e12T

Effect of operational parameters on EF and PEF processes

The effects of the major operating conditions,including initial imidacloprid,Fe 2?and electrolyte concentrations and pH on the degradation of the pesticide were studied.Degradation of imidacloprid declined by increasing its

concentration Fig.2Hydrogen peroxide production during electrolysis,a without Fe 2?and UV,b in the presence of Fe 2?without UV,and c in the presence of Fe 2?and UV ([Fe 2?]=0.36mM,[Na 2SO 4]=0.15M and pH =2.8)

Table 2Pseudo-?rst order degradation and mineralization rate constants of 4-CNB in various processes Treatment

process

Degradation rate constant (min -1)R 2mineralization rate constant (min -1)R 2Direct photolysis

0.00050.957––EF

0.00480.9720.00240.995PEF 0.00910.9910.00350.996860M.Sedaghat et al.

(Fig.3a,b)because the same amounts of active oxidizing species generated in the identical conditions of electrolysis had to degrade more pesticide and degradation intermediates [14,25].The desired pH for EF and PEF processes is found to be acidic,around 3,where the maximum production of áOH radicals by the Fenton reaction is expected (Eq.3),owing to the adequate amount of H ?for the generation of H 2O 2.As can be seen from Fig.4a and b,the optimum pH was selected as

2.8,

Fig.3Effect of initial imidacloprid concentration on its degradation by the a EF and b PEF processes ([Fe 2?]=0.36mM,[Na 2SO 4]=0.15M and pH =2.8);the inset plot was depicted according to the pseudo-?rst order kinetics

Electrochemical and photo-assisted electrochemical treatment (861)

which is consistent with other research;however,at lower pHs,the reduction of H 2O 2and H ?(Eqs.13,14)as side reactions decreased hydrogen peroxide production [29,30].

H 2O 2t2H tt2e à!2H 2O e13

T

Fig.4Effect of pH on the degradation of imidacloprid by the a EF and b PEF processes ([Imidacloprid]=20mg/L,[Fe 2?]=0.36mM and [Na 2SO 4]=0.15M);the inset plot was depicted according to the pseudo-?rst order kinetics

862M.Sedaghat et al.

2H tt2e à!H 2e14T

A higher Fe 2?concentration catalyzed the formation of hydroxyl radicals (Eq.3).However,if its concentration was more than an optimal amount,which

was

Fig.5Effect of Fe 2?concentration on the degradation of imidacloprid by the a EF and b PEF processes ([Imidacloprid]=20mg/L,[Na 2SO 4]=0.15M and pH =2.8);the inset plot was depicted according to pseudo-?rst order kinetics

Electrochemical and photo-assisted electrochemical treatment (863)

found to be 0.36mmol/L (mM)in this study (Fig.5a,b),DE%was decreased owing to the scavenging effect of extra Fe 2?ions on hydroxyl radicals (Eq.15);furthermore,more generated Fe 3?ions also reacted with H 2O 2to form hydroper-oxyl radicals (HO 2á)(Eqs.16,17),which was a weaker oxidant than were the áOH radicals [25,31

].

Fig.6Effect of Na 2SO 4concentration on the degradation of imidacloprid by the a EF and b PEF processes ([Imidacloprid]=20mg/L,[Fe 2?]=0.36mM and pH =2.8);the inset plot was depicted according to pseudo-?rst order kinetics

864M.Sedaghat et al.

Electrochemical and photo-assisted electrochemical treatment (865)

Fe2ttáOH!Fe3ttOHàe15T

Fe3ttH2O2!FeàOOH2ttHte16T

FeàOOH2t!Fe2ttHOá2e17TAs can be seen in Fig.6a,b,DE%was increased with enhancing of the electrolyte(Na2SO4)concentration due to greater H2O2production resulting from the generation of S2O82-(Eqs.18,19).However,it was decreased when the electrolyte concentration was greater than the desired value(0.15M),owing to the SO42-performance as an hydroxyl radical scavenger(Eq.20)[32–34].

2HSOà4!S2O2à8tH2e18T

S2O2à8t2H2O!2HSOà4tH2O2e19TTable3Effect of experimental conditions on the k app of imidacloprid degradation for the EF and PEF processes

Operational parameters and amounts k app(min-1),EF R2k app(min-1),PEF(R2)

Imidacloprid concentration(mg/L)

200.00480.9720.00910.991 250.00450.9980.00810.997 300.00340.9950.00500.983 350.00240.9960.00370.970 pH

20.00460.9830.00830.988

2.80.00480.9720.00910.991

3.50.00310.9560.00700.997 40.00160.9800.00480.988 50.00070.9530.00190.964 Fe2?concentration(mM)

0.090.00320.9760.00550.987 0.360.00480.9720.00910.991

0.720.00440.9710.00900.995

1.070.00410.9790.00810.998 1.430.00300.9890.00540.984 Na2SO4(M)

0.030.00300.9940.00550.996 0.060.00390.9820.00690.996 0.150.00480.9720.00910.991 0.270.00340.9770.00680.993

Amount of experimental parameters,except for the above-mentioned ones:[Imidacloprid]=20mg/L, pH=2.8,[Fe2?]=0.36mM and[Na2SO4]=0.15M

SO 2à4táOH !SO á2à4tOH àe20T

All the apparent rate constants for degradation of imidacloprid in various operational conditions by the EF and PEF processes were determined from the slope of ln(A 0/A )against process time (t )(inset plots of Figs.3,4,5,6),and are presented in Table 3.

Mineralization of imidacloprid by EF and PEF processes

The oxidizing power of the EF and PEF processes to mineralize 20mg/L of imidacloprid in aqueous solution was monitored by TOC decay during 3h of treatment.Figure 7demonstrates that approximately 51and 67%of TOC were decreased by EF and PEF,respectively.Therefore,the applied electrochemical system can mineralize imidacloprid to inorganic compounds such as H 2O and CO 2.The mineralization kinetics of the pesticide also obeyed pseudo-?rst order kinetics (inset plot of Fig.7)and the apparent mineralization rate constants were presented in Table 2.However,the mineralization rate was less than the degradation rate under the same experimental conditions,owing to the generated intermediates,which needed to be oxidized more to water,carbon dioxide,and inorganic salts [35,36].

Conclusion

In this study,all of the utilized methods obeyed pseudo-?rst order kinetics.The synergistic effect of UV irradiation on EF was remarkable,owing to the regeneration of Fe 2?by photoreduction of Fe(OH)2?and production of extra áOH radicals by photodecomposition of stable Fe 3?complexes and H 2O 2.The optimal operational conditions for treatment of imidacloprid (20mg/L)by EF and

PEF Fig.7TOC removal during the EF and PEF processes ([Imidacloprid]=20mg/L,[Fe 2?]=0.36mM and pH =2.8);the inset plot was depicted according to the pseudo-?rst order kinetics

866M.Sedaghat et al.

Electrochemical and photo-assisted electrochemical treatment (867)

processes were a pH of2.8,an Fe2?concentration of0.36mM and a Na2SO4 concentration of0.15M as the electrolyte.Furthermore,the ability of the electrochemical system to produce H2O2was proven.TOC removal indicated that the EF and PEF processes were also able to mineralize the pesticide;however,the rate of mineralization was lower than the degradation in the same operational conditions,which can be attributed to the generated degradation intermediates. Acknowledgments The authors thank the University of Tabriz,Iran for?nancial and other supports.

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