Photodegradation of α-cypermethrin in soil in the presence of trace metals (Cu2+, Cd2+, Fe2+ and Zn2+)

EnvironmentalScienceProcesses & Impacts

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Photodegradatio

Department of Chemistry, Lahore College

E-mail: [email protected]; saa

2873869; Tel: +992-42-9208201-9

Cite this: Environ. Sci.: ProcessesImpacts, 2015, 17, 166

Received 9th August 2014Accepted 2nd November 2014

DOI: 10.1039/c4em00439f

rsc.li/process-impacts

166 | Environ. Sci.: Processes Impacts,

n of a-cypermethrin in soil in thepresence of trace metals (Cu2+, Cd2+, Fe2+ and Zn2+)

Nazia Rafique* and Saadia R. Tariq

The influence of trace metals (Cu2+, Zn2+, Cd2+ and Fe2+) on the photodegradation of a-cypermethrin (a-

CYM) in agricultural soil was studied. The soil samples were spiked with a-cypermethrin with/without the

presence of metal ions, irradiated under a UV irradiation chamber for a regular period of time and

analyzed by using HPLC. The dark control sterile and unsterile soil samples spiked with a-cypermethrin

and selected trace metals were incubated for the same interval of time at 25 �C. The results obtained

indicated that a-cypermethrin photodegradation followed biphasic kinetics. a-cypermethrin

photodegradation half-lives (t1/2) were increased to 0.71 and. 4.5 hours from 0.64 hours respectively in

the presence of elevated Zn2+ and Cu2+ concentrations. Fe2+ and Cd2+ increased the photodegradation

reaction kinetics from �1.078 h�1 to �1.175 h�1 and �1.397 h�1 and varied the t1/2 from 0.64 � 1.41 to

0.59 � 2.07 and 0.49 � 2.01 in the soil. Microbes also affected the degradation of a-cypermethrin in

metal contaminated soil. The degradation rate was inhibited in unsterile soil and was found to be in the

following order: Cd2+< Zn2+< Cu2+< Fe2+. The degradation/persistence of a-cypermethrin was affected

linearly with the increasing soil metal concentrations.

Environmental impact

The study focuses on trace metals induced photodegradation studies of a-cypermethrin – a very persistent pesticide, but still used for numerous crops, in soil.The results of the study were quite remarkable, as Fe2+ and Cd2+ were evidenced to signicantly enhance the rate of photodegradation of a-cypermethrin anddecrease its half-life while Cu2+ and Zn2+retard the photodegradation rate. Thus balanced trace metal levels in the soils may prove helpful in getting rid of theproblems that arise due to very persistent nature of a-cypermethrin.

1. Introduction

The use of pesticides to increase the crop production is acommon practice all over the world. This practice, however,generates residues that may be noxious to the environment. Theaccumulation and degradation of these pesticides and theirdispersion in the environment depends on the characteristicsand overall functions of the ecosystem.1 a-Cypermethrin (a-CYM) is widely used to control the Helicoverpa spp., the majorpests of cotton. It is highly hydrophobic as reected by its lowwater solubility and high octanol–water partition coefficient(Table 1). Low solubility and high lipoaffinity make it a highlytoxic agent to sh and aquatic invertebrates even at very lowlevels (<0.5 mg L�1, LD50 values).2 Moreover, it is metabolizedand eliminated signicantly more slowly by sh than bymammals or birds which explains its higher toxicity in sh thanin other organisms.3 Generally, the lethality of pyrethroids tosh increases with increasing octanol–water partition

for Women University, Lahore, Pakistan.

[email protected]; Fax: +992-42-

2015, 17, 166–176

coefficients.4 US Environmental protection agency (EPA) hasalso classied it as a possible human carcinogen.

A large proportion of cotton grown is irrigated by drainagewater, thus the risk of environmental damage may also besignicant.5,6 Moreover, pesticides when applied to soil asinsecticides, are not selective and may also kill benecial soilmicroorganisms.7,8 a-Cypermethrin is moderately persistent inthe soil environment with eld half-lives ranging from 4 to 12weeks.9,10 Due to its high hydrophobic property it causes strongsorption to soil particles, which may cause a build-up of boundresidues.11–13

Organic wastes and sludge are commonly applied to theagricultural soils as a source of organic materials and toimprove the soil properties.14 However, some studies haveshown that the addition of organic manure and N and P fertil-izers can affect the pesticide degradation in soils.15–18 Moreover,the use of these materials can lead to the problems associatedwith their heavy metal contents, especially their successiveapplications may result in heavy metal accumulation in the soil.

Pyrethroid can undergo photolysis in the soil with half-livesranging from 5 to 170 days.9 Enhanced concentrations of heavymetals and their strong binding with soil organic matter and

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Tab

le1

Elementary

propertiesofthepesticidealongwithitsdegradationprofile

inso

ila

Pesticide

Mol

wt(g)

Solubility

inwater

(mgL�

1 )Hen

ryconstan

tVap

orpressu

reApp

lication

rate

(gha�

1)

Chem

ical

stab

ility

intheda

rk

Aerob

ic/anaerobic

tran

sformation

insoil

Phototransformation

soilsu

rface

Stab

lede

grad

ation

prod

ucts

insoil

a-Cyp

ermethrin

C22H

19C

l2NO3

416.3

3.97

(pH

¼7),

25� C

1.25

(DDW)mgL�

1

at20

a� C

2.5b

�10

�7

atm

m�3mol

�1

5.1�

10�7nPa

at70

c� C

4�

10�8mm

Hg

at70

b� C

10–15gha�

1Verystab

lein

neu

tral

and

acidic

med

ia,h

ydrolyzed

instronglyalka

linemed

ia,

DT50(pH

4,50

� C)stab

leover

10da

ys,(pH

7,20

� C),

101da

ys(pH

9,20

� C)

7.3a

days

2–8weeks/63a

,d

days

UV;5

6.08

min

onclay

loam

,94.07

min

onloam

,sun

ligh

t2.24

days

onclay

loam

,3.14

onloam

e

3-PB

Aa

aTom

lin20

04(ref.7

3).b

Hayes

andLa

ws,

1990

(ref.7

4).c

Kiddan

dJames,1

991(ref.7

5).d

USDep

artm

entof

Agriculture,199

0(ref.7

6).e

Raikw

aran

dNag

,201

1(ref.4

1).


Paper Environmental Science: Processes & Impacts

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. View Article Online

clay minerals may lead to their persistence in the soil. Thisresults in a slow dispersion of synthetic pyrethroids and theirpotential for long-term effects on benecial soil microorgan-isms and aquatic species.1,19 Liu et al. (2007) have reported thatthe presence of Cu2+ (10 mg kg�1) in the soil may inhibit thedegradation of cypermethrin (increases t1/2 from 8.1 to 10.9days) that may be explained as the reduction in activity ofbacterial biomass due to Cu2+.20 Some of the metals like iron areknown to enhance the degradation of pesticides and reducetheir half-lives.21,22 The dissipation/persistence of pesticides inthe presence of trace metals was due to their effect on growthrate of the pesticide degrading bacterial population.23,24 Forexample the carbendazim degrading Variovorax and the diurondegrading Rhodococcus strains were extremely sensitive tocadmium as it decreased their degrading activity even at lowconcentrations. Cu2+ ions strongly inhibited the degradationprocess of ethylenethiourea (ETU) which is an importantdegradation product of ethylenebisdithiocarbamate fungicideswhile 2,4-D-degradation by Variovorax was highly accelerated byCu2+ ions. Zinc, copper (Cu2+) and manganese (20–50 mg L�1)accelerated the carbendazim and diuron degradation.23,24

Therefore the goal of the present study was to determine theinuence of Cu2+, Cd2+, Fe2+and Zn2+ ions on the dissipation/persistence of a-cypermethrin in soil. The study is importantbecause the trace metal levels in agricultural soil canenhance the catalytic photodegradation of pesticides.So major hazards related to the excessive and repeated useof pesticides in the agricultural soils may be abated inthis way.

2. Materials and methods2.1 Test materials and reference standards

Reference standard of a-cypermethrin (99% purity) wasobtained from Sigma-Aldrich, Ltd. (USA). The physical proper-ties of a-cypermethrin according to “OECD25 guidelines forphotodegradation of pesticides on soil surface” are listed inTable 1. HPLC grade methanol, acetonitrile, ferrous sulphate,zinc chloride, cadmium chloride, copper sulphate (CuSO4$5H2O)and anhydrous Na2SO4 (analytical Grade) were purchased fromMerck (Darmstadt, Germany). Highly pure double distilledwater for use during the experiment was prepared with a Milli-Qsystem from Millipore-Waters Co. (Bedford, MA). Na2SO4 wasbaked at 500 �C for 4 h before the beginning of the experimentand then stored in an airtight glass bottle until use.

2.2 Soil collection and characterization

Soil (0–20 cm top soil) used in the study was collected frombotanical garden of Lahore College for Women University,Lahore. Prior to use, the soil was passed through 2 mm sieve,and maintained at a 75% water holding capacity (WHC) inaccordance with the method described elsewhere.26 It was thenstored in the dark at 20 �C until analysis. Soil texture wasdetermined by using the hydrometer.27 The physical andchemical properties of the soil sample were measured by usingthe methods of Saltanpour and Schwab (1977)28 and are

Environ. Sci.: Processes Impacts, 2015, 17, 166–176 | 167


Table 2 Physico-chemical properties of the studied soil

Soil type

Soil texture

M. C % O.M % pH (1 : 2) CEC (mmol/kg)

Trace metals (mg kg�1)

% Clay % Sand % Silt Fe Zn Cd Cu

Sandy 4.5 87 8.5 2.34 4.62 7.4 8.3 863 26.9 0.7 15.9

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summarized in Table 2. Soil was cleaned from pesticides bystirring it with acetone for 24 h (three times) and aer decantingthe acetone, dried it rst at room temperature and then in ovenat 105 �C. Soil samples were sterilized by autoclaving for 2 h in acapped 100 mL Erlenmeyer ask at 121 �C.29

2.3 Photochemical experimental set up

Irradiation of the soil samples was performed in a self-designedphotoreactor, equipped with a 6 W UV tube (Atlas, Linsenger-icht, Germany), surrounded with a thermopore jacket and waterbath that circulated water through the oor of the photolysischamber for temperature control. An electric fan (3 volt) ttedinside the radiation chamber allowed constant purging of thesample headspace. The spiked soil samples contained in PyrexPetri plates were continuously irradiated with the UV tubeplaced 23 cm above. A reference plate containing the unspikedsoil sample was also irradiated for same time interval. Soilmoisture values were recorded rst aer every hour andsubsequently aer every 6 h. If necessary at each sampling, theweight of each soil tray was manually adjusted with distilledwater to ensure that the soil was being maintained at its initialweight and moisture content.

2.4 Control sterilized and unsterilized soil dark samples

In the laboratory, control soil samples were subdivided into twogroups to investigate the dissipation rates under sterilized andunsterilized dark conditions. The unsterilized samples wereused as bioactive controls and were not given any acetone wash.Each portion (10 g, dry weight) of the sample used for sterili-zation was autoclaved three times (at 24 h apart) for 30 min in acapped 100 mL Erlenmeyer ask at 121 �C. Double de-ionizedwater was added to the germ-free (autoclaved) original (un-autoclaved) soils, to obtain water content of 75% by WHC.These moistened sub-samples were spiked with the pesticideand then incubated at 25 �C in the dark for 0, 24, 48, 96, 144,192, 384 and 762 h respectively.

2.5 Standard solution preparations and spiking procedure

The spiking solutions (0.5 mg g�1) of a-cypermethrin wereprepared by appropriate dilution of stock solution (5 mg g�1)with acetonitrile. For metal assisted degradation tests, stocksolutions of CuSO4$5H2O, FeSO4$7H2O, CdCl2 and ZnCl2 wereprepared at concentrations of 1000mg L�1 in water. These stocksolutions were then diluted to 100 mg L�1 for use as a source ofexternal Cu2+, Fe2+, Cd2+ and Zn2+ ions. Soil samples werespiked with a-cypermethrin at the maximum eld concentra-tion of 0.5 mg kg�1. The nal concentrations of Cu2+ in the soil

168 | Environ. Sci.: Processes Impacts, 2015, 17, 166–176

were set at 15.9 (control treatment), 25.9, 35.9 and 45.9 mg kg�1,for Zn2+

nal concentrations were 26.9 (control treatment), 36.9,46.9 and 56.9 mg kg�1, for Cd+ 0.7 (control), 10.7, 17.7 and 27.7mg kg�1and for Fe the nal concentrations were 863 (control),873, 883 and 893 mg kg�1 (triplicate sample of each concen-tration were measured). Aer soil treatments, the samples wereincubated at 25 �C in the dark at a moisture content of 75%. Theresidual contents in the sterilized and unsterilized sampleswere monitored at regular intervals as described above.

Soil slurries were prepared by mixing 10.0 g of soil (dryweight) with 7.5 mL of water in Petri plates. The soil was evenlyspread across the plate to a depth of 2 mm and then spiked withan appropriate concentration of the pesticide. Subsequently,these soil samples were spiked separately with Cu2+, Cd2+, Fe2+

and Zn2+. To this effect, different volumes of diluted metalsolutions were dispensed evenly across the soil surface viamicro-syringe, while maintaining the similar moisture level forall the samples. Soil samples were manually shaken tohomogenize the samples. The Petri plates were then placedinside the photoreactor and irradiated for 0, 4, 24, 48, 96, 144,192, 384 and 762 h respectively. The control experiments withno addition of trace metals were carried out simultaneously.Aer irradiation, the triplicate samples and control wereremoved from the photoreactor and processed further.

2.6 Pesticide extraction and analysis

The USE method which is an extension of the EPA method3550C was used for extraction of a-cypermethrin from soil.30

Briey, the irradiated soil samples were placed in 50 mLErlenmeyer asks and extracted with 10 mL of ethyl acetate.These samples were rst manually agitated and then exposedthrice to USE in a 100H (80/160 W) ultrasonic bath (Sonorex,Germany) for 15 min. Aer each extraction, extracts werecollected by pouring the extractant through a funnel pluggedwith a small piece of cotton wool overlaid by a portion ofanhydrous sodium sulfate, which had been previously washedwith the same solvent. In order to achieve the adequateconcentration factor, 10 g aliquot of sample was submitted forextraction and the nal extract (ca. 30 mL) was evaporated todryness using a rotary evaporator and gentle steam of nitrogenwithout the need for any clean-up procedure and reconstitutedin 1 mL acetonitrile. The extraction method showed good effi-ciency and reproducibility with mean recoveries of 73–92% withstandard deviations of lower than 2.4% for the whole procedure.

a-Cypermethrin was analyzed by using the method of Met-wally et al., 1997 (ref. 31) and Martnez et al. 1996.32 The HPLCsystem consisting of an Agilent model 1100 pump, equippedwith a DAD detector, an autosampler (model G1313A) and a C8



Fig. 1 Photodegradation of soil incorporated a-cypermethrin.


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chromatographic column (Bondsil, 15 � 0.46 cm, 5 urn particlesize; Analytichem International) was used for analysis. Aceto-nitrile/water 75/25 at a ow rate of 1 mL min�1 was used as themobile phase. The areas of eluted peaks detected at 225 nmwere recorded by using a multi-wavelength UV detector Model G1315B. The retention time of a-cypermethrin under the aboveconditions was 8.3 min. Calibration was performed each timethe samples were analyzed by using external standards and thelinear regression analysis was used for quantication. TheHPLC procedure was linear in the range 0.01–100 mg mL�1 at225 nm with the regression coefficient of 0.994 (�0.02) (n¼ 12);the detection limit was 0.02 mg mL�1 and the limit of quanti-cation was 0.18 mg mL�1.

2.7 Data analysis

In the soil, the photolytic decline of a pesticide slows down withtime, either due to the adsorption of the pesticide to soil or itsmovement out of a photic zone. When the lag phase wasinvolved, the hockey-stick model was used for kinetics evalua-tion. This model consists of two sequential rst-order curves.The pesticide concentration initially declines according to rst-order kinetics with a rate constant k1. At a certain point in time(referred to as the breakpoint), the rate constant changes to adifferent value of k2.33

dM/dt ¼ �k1M for t # tb

dM/dt ¼ �k2M for t > tb

whereM ¼ total amount of the chemical present at time t,M0 ¼total amount of the chemical applied at time t ¼ 0, k1 ¼ rateconstant until t ¼ tb, k2 ¼ rate constant from t ¼ tb, tb ¼breakpoint (time at which rate constant changes)

DTx ¼ ln 100=100� x

k1if t# tb

DTx ¼ tb þ ½ln 100=100� x� t1b�k2

if t# tb

The tests were carried out in triplicate and the data wereexpressed as an average effect of the test points.

3. Results and discussion

a-Cypermethrin was chemically stable under neutral soilcondition with a half-life of 101 days. It was microbialydegraded with t1/2 of 13 weeks, but its photodegradation wasonly reported on the soil surface as a thin lm (Table 1). No soilincorporated photodegradation study has been reported untilnow.

3.1 Photodegradation of soil incorporated a-cypermethrin

The presence of unstable groups such as isobutyl and doublebonds in the structure of pyrethroids renders them to degradeusually through photolysis, photooxidation and


photoisomerization34 in the natural environment. The photo-degradation data of a-cypermethrin obtained aer irradiationof soil samples under the UV system versus irradiation time aredepicted in Fig. 1 as natural logarithmic declines. The data forcontrol samples are also elaborated in the same gure forcomparison. The photodegradation and photocatalysis rates ofpesticides on soil surfaces under UV light depend on differentparameters such as temperature, soil particle sizes, soil depthresponsible for photodegradation, and catalyst loads.35 The datarevealed that irradiation of moist soil follows biphasic dissipa-tion reaction kinetics in accordance with data provided by theFocus report.33 The present study revealed that a-cypermethrinphotodegradation in soil involves two distant phases, rst,active phase and second, lag phase. Swarcewicz and Gregorczyk(2012) also reported this type of biphasic degradation patternfor herbicide pendimethalin in soils. The dissipation curveshad the initial rapid rate of loss and were slower aer 8 days inthe dark unsterile control.36

UV light increased the reaction rate (k1) from �0.151 h�1 to�1.078 by reducing the active phase from 8 days in unsteriledark to only 1 day thereby enhancing the degradation DT50 from33.6 days to 0.64 hours (Table 3). Previous studies have reportedthe half-lives of 8–16 days for the photodegradation of cyper-methrin on soil surfaces.37 In sandy soils, its half life wasreported to be 2–4 weeks.38 It has been found that cypermethrindegrades more rapidly on sandy loam and sandy clay soils thanon clay soils and more rapidly in soils with low organicmatter.39,40 Raikwar and Nag (2011) reported the half-life of a-cypermethrin under the UV system to be 0.93 h in clay loam and1.57 h on loam soil.41 In fact only 8% of radiant solar energycomprises the UV radiation and on reaching the earth's surface,its intensity is further decreased. In the case of lab experiments,the source emits 100% only UV radiation with most of theintensity directed on the samples, that is why lower half liveswere observed in the present study and in all other studiescarried out under laboratory UV irradiated systems as comparedto the studies carried out on sunlit soil surfaces (Fig. 2).

Pesticides photodegradation was slow in dry soil as light wasunable to penetrate deep into the underneath soil and therewere no chances of interaction of light with the pesticide, thus



Table 3 Dissipation statistics of degradation of a-cypermethrinb

Pesticide Environment Trace metal Model k1 (h�1) r2 tb (days) k2 (h

�1) r2 t1/2 (days)

a-Cypermethrin UV HS �1.078 0.686 1 �0.236 0.916 0.64 � 1.41a

Dark unsterile HS �0.151 0.99 8 �0.046 0.747 33.63 � 1.92Dark sterile** HS �0.024 0.994 32 — — —UV Fe2+ HS �1.175 0.94 2 �0.024 0.663 0.59 � 2.07a

Dark unsterile HS �0.925 0.989 2 �0.035 0.855 54.03 � 1.4Dark sterile** HS �0.032 0.936 32 — — —UV Zn2+ HS �0.978 0.945 2 �0.017 0.935 0.71 � 1.2a

Dark unsterile HS �0.701 0.955 1.2 �0.021 0.697 41.6 � 2.9Dark sterile** HS �0.022 0.96 32 — — —UV Cd2+ HS �1.397 0.813 1 �0.088 0.734 0.49 � 2.01a

Dark unsterile HS �0.152 0.84 4 �0.042 0.918 17.7 � 0.429Dark sterile** HS �0.029 0.991 32 — — —UV Cu2+ HS �0.147 0.937 8 �0.067 0.944 4.7 � 1.04a

Dark unsterile HS �0.282 0.908 4 �0.024 0.847 49.7 � 1.12Dark sterile** HS �0.023 0.996 32 — — —

a DT50 is in hours. b tb not reached till 32 days of study period. So, DT50 was not possible to be calculated accurately by the HS model.

Fig. 2 Effect of concentrations of a-cypermethrin on itsphotodegradation.


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moist soil was used in the present study in accordance withndings of Graebing et al. (2004). a-Cypermethrin is likely tovolatilize as indicated by its low Henrys Law constant thereforeit can move into the photolytic zone of soil through evapo-condensation cycles where it degraded efficiently on irradiation.Furthermore, indirect photolysis by the hydroxyl radical, singletoxygen, and other radical species was believed to enhance therate of photodegradation under moist conditions.42

3.2 Microbial degradation of a-cypermethrin

Microbes also play signicant roles in degrading and detoxi-fying the a-cypermethrin residues in the environment.43,44 a-Cypermethrin was very slowly degraded during the whole studyperiod (t0.5 not reached till 32 days) under dark sterile condi-tions. The dissipation rate (k1) increased from�0.024 to�0.151which indicated the role of microbial degradation (Fig. 1 andTable 3). Tallur et al. (2007) studied thatMicrococcus sp. presentin soil utilized cypermethrin as a sole source of carbon leadingto hydrolysis of the ester linkage to yield 3-phenoxy benzoate.45


Sterilization eliminates the microbial population of the soil andthus increases the persistence of the pesticide. The a-cyper-methrin dissipation in the sterile soil in the dark may beattributed to the chemical dissipation because the possibility ofphotodegradation was ruled out by incubating the samples inthe dark.43 In the soil, the chemical dissipation of cypermethrintakes place through hydrolysis whereby the ester linkage is rsthydrolysed leading to the formation of 3-phenoxybenzoic acid(PBA) and cyclopropanecarboxylic acid derivatives,46 principally,3-(2,2-dichlorovinyl)-2,2-dimethyl cyclopropanecarboxylic acid(DCVA).43 This dissipation route was very lethargic and had DT50longer than 60 days (Table 1). Although it is a biodegradablepesticide, the microbial release of bound residues occurs ratherslowly.47

3.3 Effect of trace metals on photodegradation

The photodegradation rates of certain pesticides may beenhanced in the presence of certain metals in the soil byaltering the enzymatic activity of soil microorganisms.48–52

Similarly, trace metals are also known to inhibit the enzymaticreactions of microorganisms by complexation with thesubstrates, combination with the protein-active sites of theenzymes, or reaction with the enzyme–substrate complex. Thusbacterial biomass activity may also be inhibited in metalpolluted soils. Kools et al. (2005) have reported a positivecorrelation between glyphosate degradation rates and soil metalpollution (Kools et al., 2005).

3.3.1 Effect of Cu2+on a-cypermethrin. Photodegradationrates (k1 and k2) of a-cypermethrin were decreased from �1.078h�1 and �0.236 h�1 to �0.147 h�1 and �0.067 h�1 when 10 mgkg�1 of Cu2+ was added to the soil as evidenced by an increase int1/2 from 0.64 to 4.5 hours at p < 0.05 (Table 3). The percentphotodegradation of a-cypermethrin in the presence of 25.9 mgkg�1 (Co + 10mg kg�1) decreased from 95.7 to 61.7% aer 8 daysof continuous UV irradiation. This retarding effect becamepronounced when Cu2+ concentration was increased to 45.9 mgkg�1 (Co + 30 mg kg�1), the % photodegradation was observed




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to be reduced to only 50.5%. Copper is known to enhance thephotodegradation of pyrethroids in the presence of UV light.1,52

According to Sykora (1997) copper compounds may act ascatalysts for photodegradation of various pollutants in irradi-ated systems. The pollutants like a-cypermethrin may act asligands in the coordination sphere of the copper and a Cu2+–Cu1+ photocatalytic redox cycle was believed to occur in copperamended solutions. This catalytic effect might also arise due tosecondary thermal reactions of the active species producedphotochemically from the copper complex.53 The degradationrate of pesticides in the soil was closely related to its availabilityto the enzymatic systems of microorganisms.54,55

The dissipation rates of a-cypermethrin decreased signi-cantly at p > 0.05 in unsterilized soil during 32 days of incuba-tion when compared with the dark control sterile treatments(Fig. 5a). The half-life of a-cypermethrin was observed to beincreased from 33.6 to 49.7 days in unsterile control treatmentthat indicated that copper affects the activity of soil microbesinvolved in degradation of the pesticide (Table 3). The results ofthe present study were compatible with the ndings of Liu et al.(2007), who reported that the persistence of a-cypermethrin wasincreased from 8.1 to 10.9 days with the presence of 10 mg kg�1

copper ion in unsterile soil. The observed degradation might bedue to the fact that metals react with the sulydral group ofenzymes thereby leading to inhibition of their enzyme activity.1

Fig. 3 Effect of metal concentrations on photodegradation of a-cyperm


Ellis et al. (2001) and Fernandes et al. (2005) found that copper-tolerant communities may have replaced the soil microorgan-isms that were able to co-metabolize the pyrethroids.56,57

Moreover, soil microbial community was adversely impacted bythe presence of elevated concentrations of Cu2+. Copper ionshave been reported to strongly inhibit the degradation processof ethylthiourea (ETU).24 Different initial concentrations wereobserved in Fig. 3. The observation evidenced that the additionof Cu2+ caused the persistence of a-cypermethrin in the soil justaer its addition. Fig. 4 also depicted an initially high rate ofdegradation that later on was reduced aer some days and thenstalled completely. This fact was interpreted on the basis thatdesorption controls the biodegradation process.58 The sorptionof the substance determines its availability for microbialdegradation. The sorbed chemicals are less accessible tomicroorganisms that utilize exclusively or preferentially chem-icals in solution. Thus with passage of time, the sorbed quantityof pesticide is increased and thus rate of degradation isreduced. It is generally accepted that sorption limits thedegradation of pesticides by reducing their partitioning into thesoil.

The Cu2+ metal also affects the abiotic degradation of a-cypermethrin and thus exhibits an inhibitory effect. Theinhibitory effect was more pronounced when the Cu2+ concen-tration was increased up to 45.9 mg kg�1. The percent

ethrin.



Fig. 4 Effect of metal concentrations on microbial degradation of a-cypermethrin.


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degradation of a-cypermethrin decreased from 26.5% to 20.5%(Fig. 5c). These ndings pointed toward the fact that the a-cypermethrin dissipation in soils containing low concentra-tions of added Cu2+ was more dependent on biological dissi-pation than chemical dissipation, but when highconcentrations of added Cu2+ were present in soils it dependedon chemical dissipation.

3.3.2 Effects of Zn2+ on a-cypermethrin. Zn2+ additiondecreased the degradation of a-cypermethrin but the inhibitoryeffect was less severe than for Cu2+ (Fig. 3c and Table 3). Thephotodegradation of a-cypermethrin was decreased from 95.69to 79.5% aer 8 days of continuous UV irradiation. UV photo-degradation rate constants k1 and k2 decreased from�0.701 h�1

and �0.021 h�1 to �1.078 h�1 and �0.236 h�1 respectively. Thedegree of inhibition was increased with an increase in the soilZn concentration from 16.9 mg kg�1 to 36.5 mg kg�1 (i.e. fromCo + 10 mg kg�1 to Co + 30 mg kg�1). Different initial concen-trations were observed in Fig. 3, evidencing that the addition ofZn2+ in the soil caused the persistence of a-cypermethrin justaer its addition. The % dissipation was decreased from 64.55to 57.26% aer 32 days of continuous incubation (Fig. 4b). Thismight be due to a change in the functional diversity of themicrobial community. Under the Zn stress i.e., high Zn2+

concentrations, some soil microbial populations were shiedfrom sensitive to less sensitive areas and hence affected the soilmicrobial population and weakened the microbial activities (Guet al., 2010). Kamitani et al. (2006) reported that there was apositive correlation between available Zn2+ content of soil andsoil metabolic quotient, and a negative correlation between


available Zn2+ content andmicrobial biomass, carbonmicrobialbiomass, nitrogen and the microbia1 quotient.59 No signicantdifference was observed in dissipation of a-cypermethrin underdark sterile conditions at p > 0.05 in the presence of soil Zn2+

load. It was therefore suggested that microorganisms are themajor agents that are involved in the dissipation of pyrethroidsin the soil environment.60

3.3.3 Effects of Cd2+ on a-cypermethrin. The photo-degradation reaction rate k1 of a-cypermethrin in soil wasobserved to increase from �1.078 h�1 to �1.397 h�1 at p < 0.05.This resulted in decrease in persistence of a-cypermethrin from0.64 � 1.41 to 0.49 � 2.01 hours under the UV-irradiationsystem. The results were consistent with the ndings of Barakatet al. (2013), who reported the successful elimination of theharmful pesticide (methomyl) by using Cd based photocatalystsunder the sunlight radiation within a very short time with aremoval capacity being 1000 mg pesticide per gram of thephotocatalyst.61

Cd2+ also decreased the half-life of a-cypermethrin underdark unsterile conditions from 33.63 � 1.92 to 17.7 � 0.43 days(Table 3). It is explained on the basis of the fact that certainpesticide degrading strains of bacteria are extremely sensitive tocadmium. Cadmium decreases their degrading activity even atlow concentrations thus increasing the half life of the pesti-cide.24 In fact both the biotic and abiotic dissipation of pyre-throids might occur in soil simultaneously.62 Under sterileconditions in soils, chemical dissipation becomes moreimportant, in the presence of high concentrations of addedCd2+.



Fig. 5 Effect of metal concentrations on abiotic degradation of a -cypermethrin.


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3.3.4 Effects of Fe2+ on a-cypermethrin. Iron is one of themajor elements present in the soil mostly in the forms ofhydroxides/oxides/chlorides. Generally the most dominantoxidation state is Fe3+ and when reducing conditions (likesubsurface environment) prevail, iron exists as Fe2+ (ref. 63).Iron is known to accelerate the photolysis of pesticides throughthe photosensitizing effect.64–68 Raque et al. (2014) evidencedthat a 3-fold increase in percent degradation of imidaclopridwas observed in moist soils by the catalytic addition of iron tosoil.52 The present study also evidenced an accelerated photo-degradation of a-cypermethrin under the UV chamber by theaddition of iron. The reaction rate was observed to increasefrom �1.078 h�1 to �1.175 h�1. Hence, a decrease in the half-life was observed from 0.61 � 1.41 to 0.59 � 2.07 hours. Theseresults are in agreement with the ndings of several otherauthors who reported that Fe2+ catalyzed the photodegradationof several pesticides.21,52,69 The degradation of a-cypermethrin insoil was more efficient when soil iron levels were enhanced. Itwas degraded up to approximately 94 to 96% of initialconcentration aer 4 days of continuous UV irradiation in thepresence of Fe2+ (Co + 10 mg kg�1 and Co + 30 mg kg�1) ascompared with control of 74%. This enhanced effect was theresult of direct Fe2+ catalyzed photodegradation or indirectphotolysis due to reaction of Fe2+ with the OH� radical frommoist soil.21 Different initial concentrations of pesticides areobserved in Fig. 3, before and aer the addition of Fe2+ in thesoil samples that evidenced the fact that the addition of Fe2+ inthe soil caused the instant degradation of a-cypermethrin in thesoil.

Soil microbes are more efficient in degrading a-cypermeth-rin in the presence of higher Fe levels (CFe + 30 mg kg�1) asevidenced by % degradation which increased from 71.2% to96% in the presence of 30 mg kg�1 Fe2+ aer 32 days of


incubation. The zero valent iron (Fe0) has already been used as aremedial tool to enhance the degradation of HCHs and DDX insoil.70 The soil Fe levels also affected the abiotic dissipation of a-cypermethrin in soil with increase in the reaction rate to�0.032from �0.024 at p < 0.05 (Table 3). The % degradation enhancedfrom 26.6 to 46.1% aer 32 days of incubation in the dark at25 �C. Singhal et al. (2012) reported the degradation of mala-thion by zero-valent Fe nano-particles.71 When it was added tosoil under anaerobic conditions, corrosion (oxidation) of theiron might be effectively coupled to reductive dechlorinationand nitro group reduction.72

4. Conclusions

It is concluded that photodegradation half-life of a-cyper-methrin was retarded in the presence of elevated concentra-tions of Cu2+ and Zn2+. Cu2+ was evidenced to possess a slightlygreater inhibition effect than Zn2+ and increased the t1/2 from0.64 hours to 4.5 and 0.71 hours, respectively. These metals alsoretarded the microbial degradation of a-cypermethrin. Theproliferated soil Fe2+ and Cd2+ levels, however, enhanced thephoto and microbial degradation of a-cypermethrin.

Acknowledgements

The authors are highly thankful to the nancial supportprovided by the Higher Education commission of Pakistanunder the Indigenous 5000 Ph.D. Fellowship scheme. Theauthors are also thankful to DrMatten Abbas and Abdul Muqeetkhan for providing the support for HPLC analysis.




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Photodegradation of α-cypermethrin in soil in the presence of trace metals (Cu2+, Cd2+, Fe2+ and Zn2+)