Research ArticleDecember 2011, 2(2):6-10GERF Bulletin of Biosciences

Photolysis of alphacypermethrin as thin film on soil surface underUV and natural light

Mukesh Kumar Raikwar1* and Subir Kumar Nag2

1Reliable Analytical Laboratories Pvt. Ltd., Thane, Mumbai2Central Inland Fisheries Research Institute, Barrackpore, Kolkata

*Corresponding author: mraikwar@gmail.com

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Abstract

Photodegradation of Alphacypermethrin (1a) (IUPAC name (RS)-α cyano-3-phenoxy benzyl (1RS) (cis-3- 2, 2,dichlorovinyl)-2, 2-dimethyl cyclopropane carboxylate) was evaluated on soil (Black and Red soil) under UVlight and sunlight. In this study, two photoproducts were isolated from soil, separated on silica column andwere identified on Gas chromatography-Mass spectrometry (GC-MS) which gave two base peak at m/z 197(M+-H) with fragment peaks at m/z 169 (M+-CHO), 141 (M+-CHOCO), 115, 77 and m/z 208 (M+) with fragmentpeaks at 191 (M+-OH), 180 (M+-CO), 163, 140, 128, 123, 107 indicating the presence of 3-phenoxy-benzaldehydeand 3-(2, 2–dichlorovinyl)-2, 2-dimethylcyclopropane carboxylic acid, respectively. It showed thatphotodegradation occur in first order kinetics with correlation coefficient of 0.97 and 0.99 on black and redsoil(s) respectively. Rate constant of photodegradation of Alphacypermethrin (1a) was found to be greater onblack soil (0.03084/day) than red soil (0.2181/day).

Key Words: Photodegradation, black soil, red soil, UV, sunlight, alphacypermethrin (1a).

Introduction

Alphacypermethrin (1a) (1R cis, α S and 1S cis α Renantiomeric pair of α -cyano-3-phenoxy benzyl-3-(2,2-dichlorovinyl)-2,2-dimethyl cyclopropane carboxylate) isa synthetic pyrethroid insecticide having a broad spectrumof activity. It is a stereo selective compound consisting ofthe mixture of four stereo isomers owing to the presenceof two chiral carbon atoms. The stereo selectivity ofAlphacypermethrin (1a) is very high. It is effective againsta wide range of agricultural pests, particularly ofLepidoptera and Coleoptera order and used in differentcrops at a low dosage 5–30 g a.i./ha Worthing (1987).

Alphacypermethrin is also used for seed protectionduring storage, eradication of vectors of endemic diseaseslike malaria and to fight household insects mentioned byDemoute, (1989); Hirano, (1989); Gupta and Bhumi,(1988). Persistence and residual activities ofAlphacypermethrin have been evaluated on tea, cotton,cabbage and mustard crops in Indian conditions, Barooahand Borthakur, (1994); Tamilselva, (1995), Pandit et al.(1996); Kumar and Dixit, (2002).

Additionally, environmental safety aspect ofCypermethrin metabolism has been studied in mammalsby Hutson et al. (1981); Crawford et al. (1981); Cole etal. (1982), in soils by Roberts and Standen (1977,

1981); Sakata et al. (1986) and in plants by Kumar andDixit (2002), Wright et al. (1980), Furuzawa et al (1986).Cypermethrin photodegradation study by Takahashi etal (1985) in water and on soil surfaces revealed that bothcis and trans isomers were rapidly degraded on soil withinitial half-life of 0.6–1.9 days. Major reactions involvedin the degradation were, cleavage of ester or diphenylether linkage, oxidation of –CHO, hydration of –CN,oxidative cleavage and dehalogenation. Althoughliterature about photodegradation of Cypermethrin in soilare available but the detail elucidation ofAlphacypermethrin (1a) degradation and the rate ofphotodegradation are not understood. This study wasundertaken to analyze the photo degradation productsand the rate of photolysis on different soil conditionsunder ultra-violet light (UV) and sunlight using GCMSand TLC methods.

Materials and methods

Materials and Purification

Compound Alphacypermethrin (1a) (99%) wasobtained from M/s. Meghamani Organics Ltd.(Ahemdabad). It was further purified by repeatedcrystallization from (or using) hexane. The purity of thecompound was checked by TLC and Melting point (at800C). All Laboratory Grade (LR) solvents from Merckwere used for the experiments. Further purification ofHexane (or Alphacypermethrin) was done by distillation

Soil Characteristics

Chromatography

Gas Liquid Chromatography (GLC)

Rate of Photo degradation of Alphacypermethrin (1a)on Soil Surface

The rate of photo degradation of Alphacypermethrin(1a) was studied on black and red soil. Soil passed through

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GERF Bulletin of Biosciences 2011, 2(2): 6-10 7

over anhydrous sodium sulfate in the boiling range of60–800C. Acetone was refluxed over KMnO4 and thendistilled.

Two types of soils were used in the experiment, blacksoil and red soil. Their properties are given in Table 1.

Alphacypermethrin (1a) degradation rates weredetermined using a PC based gas liquid chromatograph(Varian CP–3800) equipped with an electron capturedetector (ECD, Ni63) and a capillary column (CP SIL 5CB,30 m × 0.53 mm i.d. × 0.25 mm film thickness). Theoperating conditions were as follows: Columntemperature: 2500C for 1 min, then 50C/min up to 2800C(5 min), Injection port temperature: 2800C and Detectortemperature: 3000C. Nitrogen was used as the carrier gaswith the flow rate of 1 ml/min through column and makeup to 30 ml/min.

Gas Chromatography–Mass Spectroscopy (GC-MS)

The GC-MS spectra were performed on Fison gaschromatograph (trace GC) connected with an electronimpact mass detector (MD–800) and fitted with acapillary Column (BD-17, Jand W Scientific, 30 m ×0.32 mm i.d.× 0.25 µm film). The conditions were asfollows. Injection port temperature: 2600C (Split 1:10);Oven temperature: 150–2500C, 20C/min and Helium wasused as the carrier gas.Photolysis on Soil Surface

Soil samples collected from the Central Research Farmof the IGFRI, Jhansi and were dried in air under shade.They were pulverized and passed through a 2 mm sieve.Furthermore, soil samples were also sterilized in anautoclave for two hours at a temperature of 1210C andpressure of 15 lb/sq inch. Slurry of soil (50 g) wasprepared with double distilled water, sprayed uniformlyon to petri plates (20 cm dia.) to generate a layer of 2mm thickness and then dried in air. Alphacypermethrin(1a) in hexane (10 ml, 1 mg/ml) was applied uniformlyto the surface of the soil using a pipette. The plates werefurther dried in air and then irradiated under UV lightfor four hours and (or) under sunlight for 18–20 hours.After irradiation the soil was removed from plates andextracted with hexane (5 × 5 ml). The extracts fromseveral plates were combined and concentrated at lowtemperature. Photoproducts thus formed were separatedby column chromatography, preparative TLC andcharacterized by GC-MS.

2 mm sieve was suspended in distilled water (1 g in 2ml) and the suspension was used to prepare a thin layeron the bottom of a petriplate (5 cm dia). Air drying of theplates resulted in a thin uniform layer of soil on the glasssurface. A solution of Alphacypermethrin (1a) (10 mg/l,1 ml) was applied uniformly on the petri plates and thesolvent was allowed to evaporate at room temperature.The plates were exposed to UV light and sunlight fordifferent durations. In case of UV irradiation, sampleswere withdrawn at random in triplicate at intervals of 0,30, 60, 90, 120, 150 and 180 min (black soil) and 0, 30,60, 90, 120, 180, 240 and 300 min (red soil). Samplesirradiated under sunlight were withdrawn at 0, 1, 2, 3, 4,5, 6 and 7 days (black soil) and 0, 1, 2, 3, 4, 5, 6, 7, 8, 9and 10 days (red soil). After irradiation, soil was scrapedfrom each plate and extracted with hexane (5 × 3 ml).The combined extracts were then centrifuged. Thesupernatant was then concentrated to 1 ml at a lowtemperature and analyzed by GLC.

Column Chromatography (CC)

Photoproducts were separated by columnchromatography (CC) using a glass column of 75 cm × 2cm id containing 500 g of 60–100 mesh silica gel forcolumn chromatography (pre activated at 1200C) inhexane. The column was successively eluted with hexane,mixture of hexane and toluene and finally with mixtureof toluene and acetone in different proportions. Differentfractions (25ml) were collected and evaporated on a rotaryvacuum evaporator. The fractions containing sameproduct were combined and purified further.

Preparative TLC

Single or mixtures of compounds of different polarity,obtained through column chromatography, were spottedin 20 × 20 cm plates coated with silica gel G and separatedin a suitable solvent system as mentioned above. Afterseparation of compounds and visualization by UVchamber at 254 nm, spots appearing at the same position(i.e. same Rf value) were marked, scratched along withthe coated silica and put in a small conical flask. Theywere dissolved in a small quantity of acetone, filteredand further concentrated to obtain purified compound. Anumber of plates were used for the experiment to generatea higher yield. Finally, the total concentrated mixturewas purified and isolated through columnchromatography.

Results and Discussion

The UV spectrum of Alphacypermethrin (1a) inmethanol and water exhibited a band at 245.6 nm (ε=24,560) for the allowed π-π* transition of the phenylrings and again a band at 276 nm (ε =27,600), which isessentially corresponding to n-π* in character resultingfrom the combined transition of the carbonyl group and

Soil properties

Black soil Red soil

pH(1:2.5) 7.62 6.72 EC (1:2.5) 0.87 dSm-1 0.76 dSm-1 Organic carbon (%) 0.60 0.47 Soil order Inceptisol Alfisol Particle size analysis Silt (%) Clay (%) Sand (%)

49.5 27.2 23.3

30.0 28.3 41.7

Soil Texture Loam Clay loam Available Nitrogen (Kg/ha) 288.24 185.02 Available P2O5(Kg/ha) 22.27 12.23 Available K2O (Kg/ha) 476 280

Fig. 1: Possible photodegradation products ofAlphacypermethrin (1a), (I) 3-phenoxy-benzaldehydeand (II) 3-(2, 2–dichlorovinyl)-2, 2-dimethylcyclopropanecarboxylic acid on thin film soil surface under UV light.

Fig. 2: Proposed photochemical pathways to accountsfor the observed photoproducts. Alphacypermethrin (1a),(I) 3-phenoxy-benzaldehyde and (II) 3-(2, 2–dichlorovinyl)-2, 2-dimethylcyclopropane carboxylicacid.

Table 1: Physicochemical properties of the soils usedin the experiment.

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8 GERF Bulletin of Biosciences 2011, 2(2): 6-10

the lower energy bond of the aromatic rings. This π-π*andn-π* transition could lead to the production of eithersinglet or triplet excited states. So no unique excited statecould be invoked to explain the variety of photochemicalreactions.

Elution of photoproducts

The column was successively eluted with singlesolvent hexane, mixture of hexane and toluene and withmixture of toluene and acetone in different proportions.Elution of column with hexane and toluene (7:3 v/v,fractions I–VI) and distillation of elute generated a paleyellow viscous oil, which was further purified by shortpath distillation under reduced pressure (1200C bathtemperature). Fractionating on TLC, it gave a single spot(solvent system hexane: acetone 9:1, Rf

= 0.51). Thecompound was eluted at 14.83 min with a molecular ionpeak at m/z 198, the base peak at m/z 197 (M+-1) and afragment peak m/z 169 (M+-CHO), 141 (M+-CHOCO),115, 77. Analysis of the results leads to identification ofthe compound as 3-phenoxy-benzaldehyde (I). Thisproduct was also identified by Michal Segal-Rosenheimerand Yael Dubowski (2008) in cypermethrin using in situFTIR.

Elution of column with toluene and acetone (99:1, v/v, fraction I–VII) yielded a solid which was furtherpurified by preparative TLC (solvent system hexane:acetone 8.5: 1.5, Rf. 0.56). The compound was eluted at51.74 min and generated a molecular ion peak at m/z208 (M+) and fragment peaks at m/z 191 (M+-OH), 180(M+-CO), 163, 140, 128, 123, 107 indicating the presenceof the chlorine atom. The compound was identified as 3-(2, 2–dichlorovinyl)-2, 2-dimethylcyclopropanecarboxylic acid (II).

Photodegradation of Alphacypermethrin (1a) in to 3-(2, 2–dichlorovinyl)-2, 2-dimethylcyclopropanecarboxylic acid (II) and 3-phenoxy-benzaldehyde (I) wasexpected to occur directly from (1a) compound. However,using GCMS it was determined that intermediateformation occur before final photodegradation product

(Fig. 2) because it generated possible fragments(Molecular Masses), suggesting the intermediateformation during photodegradation.

Rate of Photodegradation of Alphacypermethrin (1a)

The rate of photodegradation of Alphacypermethrin(1a) was studied on black and red soil surface under UVlight as well as under sunlight. Results showed that nodegradation of Alphacypermethrin (1a) occurred in thedark(control), since more than 90 per cent of the appliedAlphacypermethrin (1a) was recovered unchanged duringthe time frame of the study which indicated thatAlphacypermethrin (1a) was stable under theseconditions. Comparison of the control (degradation indark) and the experiments concluded that the degradationobserved in the study could be attributed to photolysisonly. The rate of photodegradation followed first-orderkinetics with good correlation coefficients (Table 2). Therate of first-order reaction of Deltamethrin andFenvelarate in soil was characterized by Pengyan et al.(2010). Fig. 3 and 4 show the first-order kinetic of

Rate of Photodegradation

Surface Source of irradiation

Rate constant (k)

Half-life R2 R2

Black soil UV 0.0122/ min 56.8 min 0.98 Black soil Sunlight 0.3084/day 2.24 day 0.96 Red soil UV 0.0073/ min 94.07 min 0.99 Red soil Sunlight 0.2181/day 3.17 day 0.99 Glass UV 0.0486 /min 14.26 min 0.95 Glass Sunlight 0.0191/ day 36.26 day 0.95

Fig. 3: Linear plot for first order kinetics ofAlphacypermethrin (1a) as thin film on black and redsoil under UV light.

Fig 4: Linear plot for first order kinetics ofAlphacypermethrin (1a) as thin film on black and redsoil under sunlight.

Acknowledgment

The authors are grateful to the Head, PAR Divisionand the Director, Indian Grassland and Fodder ResearchInstitute, Jhansi, Uttar Pradesh, India for providingnecessary facilities to carry out the experiment, constantencouragement and valuable suggestions.

References

1. Barooah AK and Borthakur MC (1994).Residues of Alphacypermethrin (1a) in tea andits potential daily intake. Pestic. Res. J. 6(2):161-166.

2. Cole LM, Ruzo LO, Wood EJ and Casida JE(1982). Pyrethroid metabolism: Comparativefate in rats of tralomethrin, tralocythrin,deltamethrin and (1RS)-cis cypermethrin. J.Agric. Food Chem. 30: 631-636.

Table 2: Rate constants and halfilife values forAlphacypermethrin (1a) on different soin surface un-der UV and sunlight

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GERF Bulletin of Biosciences 2011, 2(2):6-10 9

Alphacypermethrin (1a) in red and black soil under UVand sunlight respectively and Table 2 shows the rateconstant and half life of the photo degradation ofAlphacypermethrin (1a).

Photolysis on Glass and Soil Surface

The identification of photoproducts ofAlphacypermethrin (1a) formed as thin film on soilsurface can be rationalized as possible photodegradationpathways originating from one of the followingphotochemical processes, such as cleavage of esterlinkage, cleavage of diphenyl ether linkage, hydration ofCN group to CONH2 group and hydrolysis of CONH2group to COOH group Katagi (1991). Product I and IIwere also formed in both black and red soil under bothUV and sunlight. These photoproducts were alsoidentified as a thin film of Alphamethrin on glass plateexposed in to UV light by Raikwar and Nag (2008).

The rate of photodegradation was greater on glass thanon soil surface (Table 2). This may be attributed to the

fact that some pesticides get adsorbed on soil clays or onother colloidal substances and become less available tolight. The rate of degradation was higher on black soilthan on red soil under both UV and sunlight (Fig. 3-4).This may be due to characteristic difference in organic

matter content and pH of the soil (Table 1).

Conclusion

Irradiation of Alphacypermethrin (1a) as a thin filmon soil surface under UV and sunlight has revealedcharacteristic information about photodegradation rateon black soil (0.03084/day) and red soil (0.2181/day),photoproducts (3-phenoxy-benzaldehyde and 3-(2, 2–dichlorovinyl)-2,2-dimethylcyclopropane carboxylic acid)and further regarding possible chemical/photodegradation pathways. The study also indicated thatmany photo degradation products were formed on theglass surface during photodegradation process; however,only two major products were generated on soil surface.Major photodegradation process could include, hydrolysisof ester bond, cleavage of diphenyl ether bond,dehalogenation, decarboxylation and hydration of CNgroups. The study further concludes that in theenvironment, Alphacypermethrin (1a) undergoes rapidbreakdown on glass than on soil surface.

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Degradation of the pyrethroidcypermethrin,NRDC 149 and the respective cis-(NRDC-160) and trans-(NRDC-159) isomers insoils. Pestic. Sci. 8: 305-319.