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Journal of Hazardous Materials 263 (2013) 584– 592

Contents lists available at ScienceDirect

Journal of Hazardous Materials

j o ur nal homep age: www.elsev ier .com/ locate / jhazmat

fficient removal of endosulfan from aqueous solution byV-C/peroxides: A comparative study

oor S. Shaha,b, Xuexiang Heb, Hasan M. Khana, Javed Ali Khana,b, Kevin E. O’Sheac,ominic L. Boccelli b, Dionysios D. Dionysioub,d,∗

Radiation Chemistry Laboratory, National Centre of Excellence in Physical Chemistry, University of Peshawar, Peshawar 25120, PakistanEnvironmental Engineering and Science Program, University of Cincinnati, 705 Engineering Research Center, Cincinnati, OH 45221-0012, USADepartment of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, USANireas-International Water Research Centre, University of Cyprus, 20537 Nicosia, Cyprus

i g h l i g h t s

Removal of endosulfan was studiedby UV-C and UV-based advanced oxi-dation processes (AOPs).Among UV/S2O8

2−, UV/HSO5−, and

UV/H2O2, endosulfan was removedthe most efficiently by UV/S2O8

2−.Hydroxyl and/or sulfate radicals wereinvolved in the destruction of endo-sulfan and its by-products.Presence of natural organic matteror alcohol inhibited the removal ofendosulfan and its by-products.Degradation pathways were pro-posed based on the nature of endo-sulfan degradation intermediates.

g r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 17 April 2013eceived in revised form 8 October 2013ccepted 9 October 2013vailable online 18 October 2013

eywords:ndosulfan

a b s t r a c t

This study explored the efficiency of UV-C-based advanced oxidation processes (AOPs), i.e., UV/S2O82−,

UV/HSO5−, and UV/H2O2 for the degradation of endosulfan, an organochlorine insecticide and an emerg-

ing water pollutant. A significant removal, 91%, 86%, and 64%, of endosulfan, at an initial concentrationof 2.45 �M and UV fluence of 480 mJ/cm2, was achieved by UV/S2O8

2−, UV/HSO5−, and UV/H2O2 pro-

cesses, respectively, at a [peroxide]0/[endosulfan]0 molar ratio of 20. The efficiency of these processeswas, however, inhibited in the presence of radical scavengers, such as alcohols (e.g., tertiary butyl alco-hol and isopropyl alcohol) and natural organic matter (NOM). The inhibition was also influenced by

V-Ceroxidesdvanced oxidation processes (AOPs)dvanced Oxidation Technologies (AOTs)ater treatment

common inorganic anions in the order of nitrite > bicarbonate > chloride > nitrate � sulfate. The observedpseudo-first-order rate constant decreased while the degradation rate increased with increasing initialconcentration of the target contaminant. The degradation mechanism of endosulfan by the AOPs wasevaluated revealing the main by-product as endosulfan ether. Results of this study suggest that UV-C-based AOPs are potential methods for the removal of pesticides, such as endosulfan and its by-products,from contaminated water.

∗ Corresponding author at: Environmental Engineering and Science Program, Uni-ersity of Cincinnati, 705 Engineering Research Center, Cincinnati, OH 45221-0012,SA. Tel.: +1 513 556 0724; fax: +1 513 556 2599.

E-mail addresses: dionysios.d.dionysiou@uc.edu, dionysdd@ucmail.uc.eduD.D. Dionysiou).

304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jhazmat.2013.10.019

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Pesticides are a diverse group of chemical substances, appliedintentionally to control pests and pest causing diseases. Some ofthem, such as organochlorine pesticides, are highly persistent andremain for a long time in various environmental matrices [1]. The

rdous

cagcerfw[wtTvsatr

ecborcactcS1tapspiicaahositmaprpu[

piso(d

2

2

f

N.S. Shah et al. / Journal of Haza

ontamination of water by pesticides has been reported worldwidend constitutes an issue of great concern at regional, national, andlobal levels [2,3]. One of the most important organochlorine pesti-ides is endosulfan, commonly found as a mixture of two isomers,ndosulfan I and endosulfan II in a ratio of 64–70% and 29–32%,espectively [4]. Endosulfan is used on crops, vegetables, and forestsor the control of tsetse fly, mites, home garden pests, as well as aood preservative [5]. According to the World Health Organization

6], the total worldwide production of endosulfan in the early 1980sas about 9,000 metric tons. It has been found to be highly persis-

ent with a long half life ranging from nine months to six years [7].he half life of endosulfan is pH dependent in water with a reportedalue of 5 weeks at pH 7.0 to 5 months at pH 5.5 [8]. Moreover, endo-ulfan is classified as a category 1b (highly hazardous) pesticidend has been associated with genotoxic effects, such as gene muta-ions, chromosomal alterations, and mitotic effects [9,10]. Thus theemoval of such a compound from contaminated water is vital.

Advanced oxidation processes (AOPs) have been introduced asfficient and environmentally friendly treatment technologies foronversion of organic pollutant to less complex and less harmfuly-products, and even mineralization to carbon dioxide, water, andther inorganics [11–14]. The AOPs rely on in situ generation ofeactive radicals, such as hydroxyl radical (HR, •OH) or sulfate radi-al anion (SR, SO4

•−). HR is a strong and non-selective oxidant with high redox potential of 2.72 V (depending on the experimentalonditions) [15] while SR (with a redox potential of 2.5–3.1 V), onhe other hand, reacts more selectively with most of the organicompounds. The reported second-order rate constants of HR andR with organic compounds range from 108 to 1010 M−1 s−1 and07 to 1010 M−1 s−1, respectively [16,17]. As a result, in this study,he removal of endosulfan from water environment by both HR-nd SR-AOPs was investigated through the activation of commoneroxides, i.e., H2O2, persulfate (PS, S2O8

2−), and peroxymono-ulfate (PMS, HSO5

−), under UV-254 nm irradiation. The UV/H2O2rocess is relatively conventional and the most studied AOP for

ts powerful oxidation ability, no sludge production and simplic-ty of operation [16,18,19]. The use of PS is, however, preferred inertain circumstance due to its easy storage and transport, moder-te stability in the subsurface environments, high water solubility,nd relatively low cost [20,21]. Consequently, SR-AOP (e.g., UV/PS)as recently emerged as an efficient alternative treatment technol-gy for the remediation of various classes of organic pollutants,uch as polyvinyl alcohol, trichloroethylene (TCE), pharmaceut-cals, and pesticides in contaminated soil and water [22,23]. PMS ishe active component of a triple salt, 2KHSO5·KHSO4·K2SO4, com-

only known as Oxone®, which has been applied significantly as non-chlorine shock-oxidizer in swimming pool disinfection thatrovides effective oxidation of organic contaminant and avoid chlo-ine shocking [24]. Due to environmental friendly and high activityroperties, PMS combined with UV to produce HR and SR was eval-ated for the decontamination of polluted water in the recent years25,26].

The main objectives of the present study were to investigate theerformance of different peroxides and their dosages under UV-C

rradiation for the degradation of endosulfan. Various parameters,uch as different initial concentrations of endosulfan, the presencef commonly found inorganic anions and natural organic matterNOM), were evaluated. The main reaction mechanism of the degra-ation of endosulfan by these AOPs was also examined.

. Materials and methods

.1. Materials

All the chemicals in this study were used as received withouturther purification. Solid endosulfan (99.6%) and endosulfan ether

Materials 263 (2013) 584– 592 585

(99.5%) were purchased from Supelco (PA, USA). Hydrogen perox-ide (50% v/v, Fisher Scientific), sodium persulfate (Sigma–Aldrich)and Oxone® (Sigma–Aldrich) were used as the oxidants. Otherchemicals, such as tertiary butyl alcohol (t-BuOH), isopropylalcohol (i-PrOH), methanol, sodium nitrite (NaNO2), sodiumnitrate (NaNO3), potassium chloride (KCl), sodium bicarbonate(NaHCO3), sodium sulfate (Na2SO4), sodium dihydrogen phosphate(NaH2PO4), and disodium hydrogen phosphate (Na2HPO4) werealso of high purity and obtained from Fisher Scientific. StandardSuwannee River humic acid (SRHA) and fulvic acid (SRFA) fromInternational Humic Substances Society were used as a represen-tative of NOM.

2.2. Analysis

The concentration of endosulfan and its by-products weredetermined by an Agilent 7890 series gas chromatography (GC)technique using solid phase microextraction (SPME, with the fibermade of polydimethylsiloxane, PDMS, and purchased from Supelco,USA). The extraction was carried out at room temperature and thesample was directly injected into GC–MS with the aid of a manualholder. In this manuscript, the concentration of endosulfan is repre-sented by the sum of its isomers, i.e., endosulfan I and endosulfan II.Analysis was conducted using an Agilent 5975 mass spectrometricdetector (MS) and an HP-5MS (5% phenyl methylsiloxane) capil-lary column (30 m, i.d. 0.25 �m). The conditions of the injector,MS source, and MS quad were set at 250 ◦C, 230 ◦C, and 150 ◦C,respectively. The temperature of the oven was programmed from80 ◦C (hold time 2 min) to 150 ◦C (hold for time 0 min) at a rate of20 ◦C/min followed by an increase to 220 ◦C (hold time 6 min) at arate of 10 ◦C/min. Helium gas (purity 99.9995%) was used as a car-rier gas at a flow rate of 1.5 mL/min. Analysis was carried out in anelectron impact ionization mode (EI+) with the ionization energy of70 eV. The MS was set at scan mode using a scan range from 50 to450 m/z for the analysis of endosulfan and its by-products. A massspectral search program (NIST, USA) was used for the analysis of theresults, by comparing the spectra of the samples and those from thelibrary [27].

Spectrophotometric measurement using a UV–vis spectropho-tometer (Hewlett Packard, 8452A) was carried out to quantify theconcentration of PS, PMS, and H2O2 [28,29].

2.3. Reactor and procedure

The photo-assisted irradiation experiments were conductedusing a bench-scale photochemical reactor equipped with two15 W low-pressure Hg lamps (Cole-Parmer) emitting light at awavelength of 253.7 nm. A fluence rate of 0.1 mW/cm2 was usedin the present study [30]. The fluence rate was checked regularlyusing a calibrated radiometer (Model IL 1700, XRD (XRL) 140T254low profile germicidal probe, International Light, Co., Newburyport,MA) to ensure the consistency in lamp emission. The initial pH ofthe solution was adjusted using phosphate buffer at a concentra-tion of 0.5 mM in the case of pH 5.7 and a concentration of 5 mMin the case of pH 7.0. The sample (0.15 mL) was taken at each UVfluence interval and mixed with the same volume methanol (usedfor quenching radicals) before GC–MS analysis. All the experimentswere carried out in triplicate; the error bars shown in the givenfigures represent the standard error of the mean.

3. Results and discussion

3.1. Effect of peroxide type

The effect of different peroxides was assessed using three dif-ferent methods, peroxides only, UV-C only, and UV-C-based AOPs.

586 N.S. Shah et al. / Journal of Hazardous

Time (min)

806040200

C/C

0

0.0

0.2

0.4

0.6

0.8

1.0

UV Fluence (mJ/cm2)

5004003002001000

Dark/H2O2Dark/PS Dark/PMS UV-C only UV-C/H2O2UV-C/P MS UV-C/P S

Fig. 1. Degradation of endosulfan with different processes: peroxideso[w

Farewe

[ε3oepAeUdt5b

k , Eq. (2) can be written as,

TR

nly, UV only and AOPs. Experimental conditions: [endosulfan]0 = 2.45 �M,peroxide]0/[endosulfan]0 = 10, initial pH = 5.7. (Note: In dark reactions, samplesere taken at a time interval corresponding to the same UV fluence equivalent.)

ig. 1 shows that no observed degradation of endosulfan waschieved by only H2O2, while direct oxidation by PMS and PSesulted in a 10% and 4% removal of endosulfan, respectively, at anquivalent time interval as UV fluence of 480 mJ/cm2 [21]. Thereas, however, a significant improvement in the degradation of

ndosulfan under the conditions of UV-C only and AOPs.Endosulfan has a maximum absorption at 264 nm

31] and a large molar extinction coefficient at 254 nm,254 = 40,877 M−1 cm−1 (compared to a literature value of7,893 M−1 cm−1 [31]). Consequently, direct photo-excitationf endosulfan could occur and lead to photolytic degradation ofndosulfan. Peroxides under UV-C irradiation dissociated androduced free radicals as shown by Eqs. (R1)–(R3) in Table 1.mong the three peroxides tested, the highest degradation ofndosulfan was achieved by UV/PS, followed by UV/PMS, and thenV/H2O2. One possible reason for the observed trends could be

educed from the structure of the peroxides. The bond energy ofhe O O bond was estimated to be 33.5 kcal/mol for PS [15] and1 kcal/mol for H2O2 [32], while that for PMS was proposed toe between PS and H2O2 [25]. The order of the O O bond energy

able 1ate constant for the potential elementary reactions in the AOPs under different conditio

Reaction

H2O2/HO2− + hv → 2•OH (R1)

S2O82− + h� → 2SO4

•− (R2)

HSO5− + h� → •OH + SO4

•− (R3)

SO4•− + NO2

− → NO2• + SO4

2− (R4)

•OH + NO2− → NO2

• + OH− (R5)

SO4•− + HCO3

− → CO3•− + SO4

2− + H+ (R6)

•OH + HCO3− → CO3

•− + H2O (R7)SO4

•− + NO3− → NO3

• + SO42− (R8)

•OH + Cl− → ClOH•− (R9)

Cl• + Cl− → Cl2•− (R10)

SO4•− + Cl− → Cl• + SO4

2− (R11)

Cl• + H2O → H+ + •OH + Cl− (R12)

SO4•− + i-PrOH → (CH3)2

•COH + SO42− + H+ (R13)

SO4•− + t-BuOH → •CH2C(CH3)2OH + SO4

2− + H+ (R14)

•OH + i-PrOH → (CH3)2•COH + H2O (R15)

•OH + t-BuOH → •CH2C(CH3)2OH + H2O (R16)

•OH + NOM → Products (R17)

SO4•− + NOM → Products (R18)

•OH + SO42− → SO4

•− + −OH (R19)

SO4•− + SO4

2− → S2O82− + eaq

− (R20)

Materials 263 (2013) 584– 592

corresponds well with the reported molar extinction coefficientorder of these peroxides [33], suggesting that PS followed by PMScan be cleaved more easily under UV-C irradiation than H2O2.

Second-order rate constants of endosulfan (ES) with •OH(k•OH/ES) and SO4

•− (kSO4•−/ES) was determined by competi-

tion kinetics as shown in Eq. (1) [34] using para-chlorobenzoicacid (pCBA, k•OH/pCBA = 5.0 × 109 M−1 s−1) [35] and m-toluic acid

(m-TA, kSO4•−/m-TA = 2.0 × 109 M−1 s−1) [36] as the competitor,

respectively [30].

k•OH(s) =(

ks-UV/H2O2− ks-UV

kref-UV/H2O2− kref-UV

)k•OH(ref) (1)

In Eq. (1), the k•OH, kUV/H2O2, and kUV refer to the second-order

rate constant with •OH, observed fluence-based pseudo-first-orderdegradation rate constants (kobs) in UV/H2O2 and UV only con-ditions, while “s” and “ref” stand for substrate and referencecompound, respectively. The same Eq. (1) was used for determi-nation of the second-order rate constant of SO4

•− with endosulfan,except by using SO4

•− and PS instead of •OH and H2O2, respectively.Comparable second-order rate constants of 1.83 × 109 M−1 s−1 fork•OH/ES and 1.50 × 109 M−1 s−1 for kso4

•−/ES were obtained. Thefaster degradation of endosulfan by UV/PS thus probably resultedfrom the higher radical quantum yield of 1.8 for UV/PS compared to1.04 for UV/PMS and 1.0 for UV/H2O2 as shown by Eqs. (R1)–(R3)in Table 1, which is again consistent with the above mentioneddissociation of the peroxides by UV-254 nm.

3.2. Effect of peroxide dosage

Peroxide dosage was varied to assess the optimum initialconcentration of peroxide for an efficient removal of endosul-fan. The destruction of endosulfan using different molar ratios of[peroxide]0/[endosulfan]0 under UV-C followed pseudo-first-orderkinetics, and the observed fluence-based pseudo-first-order degra-dation rate constants (kobs, cm2/mJ), as shown in Table 2, weredetermined using Eq. (3) [37].

C = C0 exp(−kt) (2)

By integrating Eq. (2), replacing time with UV fluence and k with

obs

−ln(

C

C0

)= kobsUV fluence (3)

ns.

Rate constant (M−1 s−1) Reference

= 1.0 [54] = 1.8 [55] = 1.04 (pH = 7) [56]

8.8 × 108 [57]8.0 × 109 [58]9.1 × 106 [59]8.5 × 106 [35]5.0 × 104 [60]4.3 × 109 [61]8.0 × 109 [62]6.6 × 108 [63]2.0 × 105 [35]8.2 × 107 [64]8.4 × 105 [64]1.9 × 109 [35]6.0 × 108 [35]2.23 × 108 L (mol C)−1 s−1 [65]

>6 × 106 L (mol C)−1 s−1 [66]3.5 × 105 [17]

[35]

N.S. Shah et al. / Journal of Hazardous Materials 263 (2013) 584– 592 587

Scheme 1.

Scheme 2.

ClCl

Cl

Cl

Cl

Cl

O ClCl

Cl

Cl

Cl

Cl

O ClCl

Cl

Cl

Cl

Cl

O

O

ClCl

Cl

ClCl

ClO

O

(3) Endo sul fan ether(4) Endosul fan lacton e

(5) 1,3-cyclopentadie ne,

Loss ofClCl

Cl

Cl

Cl

Cl

O

O

(4) Endosul fan lacton e

O2

eme

ilr[ostwi

TF[

1, 2, 3, 4, 5, 5-hexachlor o

Sch

The rate of •OH and SO4•− generation was expected to

ncrease with increasing initial concentrations of peroxideeading to a higher observed pseudo-first-order degradationate constants of endosulfan (Table 2). The kobs at eachperoxide]0/[endosulfan]0 molar ratio was found to be in an orderf UV/PS > UV/PMS > UV/H2O2, consistent with the previous discus-

ion on the activity of peroxides under UV irradiation [30]. Amonghe tested AOPs, UV/PS was found to be the most efficient. UV/PSas further studied to evaluate the effect of reaction conditions,

ncluding initial concentrations of endosulfan, and the presence of

able 2luence-based pseudo-first-order rate constants (kobs, cm2/mJ) of endosulfan as a functiendosulfan]0 = 2.45 �M, [peroxide]0/[endosulfan]0 = 5, 10, and 20.

Molar ratio of peroxide/endosulfan H2O2

kobs (×10−3) R2

5 1.40 0.986

10 1.74 0.980

20 2.27 0.965

3.

inorganic anions, NOM, and other radical scavengers, i.e., t-BuOHand i-PrOH.

3.3. Effect of initial concentration of endosulfan

This study shows the destruction of endosulfan by UV/PS under

different initial concentration of endosulfan while keeping thatof PS constant at 49 �M. When the initial concentration of endo-sulfan increased, the number of molecules exposed to reactiveradicals increased correspondingly and thus resulted in a higher

on of different initial peroxide/endosulfan molar ratios. Experimental conditions:

PMS PS

kobs (×10−3) R2 kobs (×10−3) R2

3.29 0.998 3.58 0.9893.84 0.994 4.14 0.9774.22 0.992 4.72 0.984

588 N.S. Shah et al. / Journal of Hazardous Materials 263 (2013) 584– 592

Concentr ation ( µM)

3.53.02.52.01.51.00.5

k obs

(cm

2 /mJ)

0.00 40

0.00 42

0.0044

0.0046

0.0048

0.00 50

0.00 52

Deg

rada

tion

rate

(µM

/min

)

0.01

0.02

0.03

0.04

0.05

Fig. 2. Variation of kobs (cm2/mJ), and degradation rate (�M/min) as a functionotp

itii(dsoftotcspat

3

rfd[caTeiSTbarpsci•

brrpy

UV Flu ence (mJ/cm

(a)

(b)2)

5004003002001000

C/C

0

0.0

0.2

0.4

0.6

0.8

1.0 UV-C/PS + HA UV-C/PS + FA UV-C/PS onl y

UV Flu ence (mJ/cm2)

5004003002001000

C/C

0

0.0

0.2

0.4

0.6

0.8

1.0 UV-C /PS + i-PrOH UV-C /PS + t -BuOHUV-C/PS only

Fig. 3. (a) Removal of endosulfan by UV/PS in the presence of NOM.Experimental conditions: [endosulfan]0 = 2.45 �M, [PS]0/[endosulfan]0 = 20,

f different initial concentrations of endosulfan by UV/PS. Experimental condi-ions: [endosulfan]0 = 3.70 �M, 2.45 �M, 1.72 �M, and 0.86 �M, [PS]0 = 49 �M, initialH = 5.7.

nitial degradation rate (calculated by the change in concentra-ion with time at initial 20 min reaction time), as suggested byts linear increase from 0.02 to 0.05 �M/min (R2 = 0.999) whenncreasing initial concentrations of endosulfan from 0.86 to 3.7 �Mas shown in Fig. 2) [20]. It was revealed however that kobsecreased with increasing initial concentrations of endosulfan. Pos-ible explanations for this observation include: (1) a large numberf by-products, as shown in Schemes 1–3 in Section 3.6, wereormed during the destruction of endosulfan. The increased ini-ial concentrations of endosulfan was expected to increase thosef the by-products, which competed for reactive radicals [38]; (2)he optical density of the solution increased with increasing initialoncentrations of endosulfan that decreased the permeability ofolution for light absorption [39], which could influence the directhotolysis of the target contaminant as well as reduce the dissoci-tion of the peroxide and produce fewer reactive radicals to attackhe target contaminant.

.4. Effects of inorganic anion

Inorganic anions, i.e., nitrite (NO2−), bicarbonate (HCO3

−), chlo-ide (Cl−), nitrate (NO3

−), and sulfate (SO42−), are commonly

ound in water, with varying concentrations from 10−5 to 10−3 M,epending on geographical location and anthropogenic activities40]. Some of these inorganic anions have high bimolecular rateonstants with SO4

•− and •OH, as shown by Eqs. (R4)–(R7), (R9)nd (R11) in Table 1, and thus can scavenge these radicals [41].herefore, to examine the influence of these anions on the removalfficiency of endosulfan by UV/PS was considered highly importantn our study for the assessment of potential practical applications ofO4

•− and •OH in the treatment of endosulfan in water. As shown inable 3, the removal efficiency of endosulfan was strongly inhibitedy NO2

−, followed by HCO3−, and then Cl− while the other two

nions, i.e., NO3− and SO4

2−, had very little influence. The loweremoval of endosulfan in the presence of NO2

− and HCO3− was

robably because of their faster reaction with SO4•− and •OH as

hown by Eqs. (R4)–(R7) in Table 1, suggesting their significantompeting effect with endosulfan for reactive radicals [42]. Thenfluence of Cl− was less despite its fast rate constants with bothOH and SO4

•− (Eqs. (R9) and (R11) in Table 1) that was proba-ly resulted from the formation of Cl• (Eq. (R11)), which has a high

• −

edox potential of 2.4 V (Cl /Cl ) and react selectively with electronich organic compounds [43,44]. Moreover, the resulting Cl• couldossibly induce the propagation reactions and thus attribute to theield of more SO4

•− [43]. Further reactions of Cl• in water can also

[HA]0 = [FA]0 = 5 mg/L, initial pH = 7. (b) Removal of endosulfan by UV/PS in thepresence of t-BuOH and i-PrOH. Experimental conditions: [endosulfan]0 = 2.45 �M,[PS]0/[endosulfan]0 = 20, [t-BuOH]0 = [i-PrOH]0 = 60 mM, initial pH = 7.0.

lead to the yield of •OH (Eq. (R12) in Table 1). According to Lianget al. [42], Cl− at concentration of less than 200 mM did not inter-fere with the removal of trichloroethylene using PS as an oxidant.Wang et al. [45] found that Cl− at a concentration up to 10 mMdid not significantly influence the decolorization of Acid Orange7 by UV/PS either, which was consistent with these results. Therelatively greater removal efficiency of endosulfan in the presenceof SO4

2− and NO3−, compared with the presence of other anions,

could be due to their much slower reactions with SO4•− and •OH

as shown by Eqs. (R8) and (R19) in Table 1. Besides, reactions ofSO4

2− with •OH and SO4•− yielded highly reactive SO4

•− and aque-ous electron (eaq

−), respectively, as shown by Eqs. (R19) and (R20)in Table 1. A well-known reducing agent with a reduction potentialof −2.9 V, aqueous electron react rapidly with halogenated organiccompounds [46].

3.5. Influence of common radical scavengers, NOM and alcohols

The degradation of endosulfan by UV/PS in the presence of NOM,

i.e., SRHA and SRFA, is shown in Fig. 3a. NOM absorbs the inci-dent light and thus competes with endosulfan and PS for light [47].According to Fu et al. [48] and Hua et al. [49], FA and HA acted asan efficient binder to pesticide, leading to a transfer of the energy

N.S. Shah et al. / Journal of Hazardous Materials 263 (2013) 584– 592 589

Table 3Removal efficiency (%) of endosulfan by UV/PS in the presence of different inorganic anions, i.e., NO2

− , HCO3− , Cl− , NO3

− , and SO42− . Experimental condition:

[endosulfan]0 = 2.45 �M, [PS]0/[endosulfan]0 = 20, [inorganic anions]0 = 1 mM, initial pH = 7.0.

UV fluence (mJ/cm2) Percent degradation = ((C0 − C)/C0) × 100%

UV-C/PS only UV-C/PS + SO42− UV-C/PS + Cl− UV-C/PS + HCO3

− UV-C/PS + NO3− UV-C/PS + NO2

−

0 0.00 0.00 0.00 0.00 0.00 0.0040 17.0 12.7 14.7 10.0 11.7 8.0080 32.6 30.6 28.3 19.0 23.6 14.9120 57.5 49.0 47.8 36.0 47.6 20.9

5 49.6 58.8 33.06 60.7 66.6 43.02 71.3 79.4 52.0

asoeai

N

eSssdf1pcdb

3

awrtttbsSfteTtIaemADrawfUarhd

Fig. 4. (a) Changes in the concentration of endosulfan ether (�M) as a functionof different processes, UV-C only and UV-C based AOPs. Experimental conditions:[endosulfan]0 = 2.45 �M, [peroxide]0/[endosulfan]0 = 10, initial pH = 5.7. (b) Changesin the concentration of endosulfan ether (�M) by UV/PS in the presence of t-BuOH or

240 70.3 61.2 57.360 80.0 72.0 67.480 90.0 80.6 76.

bsorbed by endosulfan (ES) to the surrounding NOM molecule ashown by Eq. (4), which subsequently reduced direct photolysisf endosulfan. In addition, NOM can also work as a radical scav-nger, though at a faster rate with •OH than with SO4

•− (Eqs. (R17)nd (R18) in Table 1, respectively) [50], suggesting the significantmportance of radical species.

OM + ES + hv → ES∗ − NOM → ES − NOM∗ (4)

As for alcoholic radical scavengers, according to Anipsitakist al., [33], alcohols with �-hydrogens react efficiently with bothO4

•− and •OH. Therefore in this study, i-PrOH was used as acavenger for both SO4

•− and •OH while t-BuOH was used as acavenger of •OH as shown by Eqs. (R13)–(R16) in Table 1. Fig. 3bepicts that the observed degradation rate constants of endosul-an in solutions spiked with t-BuOH and i-PrOH were lower i.e.,.88 × 10−3 cm2/mJ and 1.60 × 10−3 cm2/mJ, respectively, as com-ared to 4.53 × 10−3 cm2/mJ in the absence of either alcohol. Theomparable scavenging effect of t-BuOH and i-PrOH on the degra-ation of endosulfan by UV/PS further revealed the involvement ofoth •OH and SO4

•−.

.6. Identification of intermediates and degradation pathways

The degradation of endosulfan by various processes mentionedbove resulted in several by-products. A total of five by-productsere detected and identified, as shown in Table 4 with their

espective retention time (RT), molecular weight (MW), and struc-ural formula. Most of them were not assessed quantitatively, dueo their low concentrations except product 3, which was iden-ified as endosulfan ether. Under UV-C initiated photolysis, twoy-products, 1 and 3, were identified. Barcelo-Quintal et al. [31]uggested that endosulfan photolysis began with the excitation of

O bond. The oxidation of endosulfan produced endosulfan sul-ate (1) that subsequently transformed to endosulfan ether (3) viahe loss of SO3 group. A steady increase in the concentration ofndosulfan ether was observed under UV-C photolysis (Fig. 4a).he significant increase in the formation of endosulfan ether byhe AOPs implicates the important role of radical species (Fig. 4a).n fact, when radical scavengers, i.e., t-BuOH and i-PrOH, weredded to the UV/PS system, the formation and further removal ofndosulfan ether were inhibited (Fig. 4b). Under extended treat-ent by the AOPs, by-products were eliminated. The three studiedOPs reported herein yielded the same by-products (Table 4) [51].espite their differences in mechanism action, •OH and SO4

•− caneact with organic compounds also through some common mech-nisms, i.e., hydrogen abstraction, and electron transfer. However,e were not able to distinguish whether the products were resulted

rom hydroxyl radical or sulfate radical chemistry in UV/PS andV/PMS system [51]. The degradation pathways occur primarily

t the S O group of endosulfan through electron abstraction. Theesulting radical cation intermediate transformed quickly throughydroxylation into an oxygen-centered radical which was then oxi-ized into endosulfan sulfate (1, Scheme 1) [52]. On the other hand,

i-PrOH. Experimental conditions: [endosulfan]0 = 2.45 �M, [PS]0/[endosulfan]0 = 20,[t-BuOH]0 = [i-PrOH]0 = 60 mM, initial pH = 7.0.

a route of elimination of the oxygen-centered radical intermedi-ate led to the formation of endosulfan alcohol (2, Scheme 2) andendosulfan ether (3, Scheme 2) [52].

The ether by-product is attacked by •OH and SO4•− by an

H-abstraction forming carbon-centered radical [46]. This carbon-centered radical reacted with dissolved molecular oxygen in

aqueous solution, and produced a corresponding peroxyl radical[46] that underwent several intermediate steps to form an endo-sulfan lactone by-product (4, Scheme 3). The resulting endosulfan

590 N.S. Shah et al. / Journal of Hazardous Materials 263 (2013) 584– 592

Table 4List of by-products formed during the degradation of endosulfan by AOPs under different conditions.

Compound RT MW Structural formula Detected in

UV-C only AOPs

Endosulfan I 14.1 406.9

Cl

Cl

Cl

Cl

OOCl Cl S O

Endosulfan II 15.8 406.9

Cl

Cl

Cl

Cl

OO SClCl O

1) Endosulfan sulfate 17.4 423.0

Cl

ClCl

Cl

ClClOO S O

O√ √

2) Endosulfan alcohol 15.6 360.0

ClCl

ClCl

OHOH

ClCl√

3) Endosulfan ether 11.2 342.0 O

ClCl

ClCl

ClCl √ √

4) Endosulfan lactone 13.7 356.0

ClCl

ClCl

OClCl

O

√

5) 1,3-Cyclopentadiene,1,2,3,4,5,5-hexachloro 11.3 273.0

ClCl

Cl √

l(

4

rfb(pPHoap

actone finally decomposed to product 5 via the loss of furan-25H)-one as shown in Scheme 3 [53].

. Conclusions

The following conclusions were obtained from this study on theemoval of endosulfan by the AOPs: (a) the degradation of endosul-an was influenced by UV fluence and was significantly increasedy the addition of peroxides with UV/PS being the most effective;b) the removal of endosulfan by UV/PS AOP was inhibited in theresence of radical scavengers, such as alcohols (i.e., t-BuOH and i-rOH), NOMs, and certain common inorganic anions (e.g., NO2

− and

CO3

−), showing the involvement of both •OH and SO4•−; (c) based

n GC–MS/NIST analysis, following the destruction of endosulfannd the formation of by-products, degradation pathways were pro-osed showing that the destruction of endosulfan was initiated

ClClCl

at the S O group; and (d) compared to direct photolysis, radicalbased processes were found to be more efficient in the removalof endosulfan ether, a major by-product during the degradation ofendosulfan. Results of this study suggest that UV-C-based AOPs areeffective processes for the removal of pesticides, such as endosulfanand its by-products, from a water environment.

Acknowledgments

The authors would like to acknowledge the Higher EducationCommission (HEC), Islamabad, Pakistan for funding this projectpartially through an International Research Support Initiative Pro-

gram (IRSIP). D.D. Dionysiou and X. He are grateful to the CyprusResearch Promotion Foundation for providing partial financialsupport for this project on the evaluation of emerging AOTsthrough Desmi 2009–2010 which was co-funded by the European

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