Pestic. Sci. 1995, 43, 333-331

Photostabilization of Quinalphos by Crystal Violet on the Surface of Kaolinite and Pal y g o r s ki te Kaushik Banerjee & Prem Dureja* Division of Agricultural Chemicals, Indian Agricultural Research Institute, New Delhi-110012, India (Received 14 November 1994; accepted 19 December 1994)

Abstract: Photochemical stability of the insecticide, quinalphos (0,O-diethyl 0- quinoxalin-2-yl phosphorothioate), adsorbed on kaolinite and palygorskite was studied. In all cases considerable photostabilization was achieved in comparison with the sample of pesticide in its free form. Improvement of photostabilization was achieved when a cationic dye (crystal violet) was co-adsorbed with the quin- alphos. The interactions between quinalphos and clays were studied by Fourier- transform infra-red spectroscopy. It is suggested that photostabilization is due to steric hindrance imposed by the clay surface to the isomerization step of photo- chemical reaction and pacification of clay surface by crystal violet.

Key words : photostabilization, crystal violet, kaolinite, palygorskite.

1 INTRODUCTION

Sensitivity to sunlight limits the use of some potential pesticides in agriculture. Classical approaches to over- come this obstacle have involved chemical modification of the molecular structure of the pesticide or the use of UV-absorbing molecules in the formulations. However, the first method suffers from serious drawbacks, since any chemical modification may affect the pesticidal activity of the compound or its biodegradability and may introduce ecological problems related to soil and water pollution.

A new method for photostabilizing an insecticide, free from the above-mentioned disadvantages has been sug- ge~ted . ' -~ This method is based on the adsorption of the photolabile bioactive compound on the surface of clays to which a selected organic chromophore has pre- viously been attached. This chromophore, with specific spectroscopic and charge properties, is absorbed at a concentration similar to that of the pesticide. The surface properties of the clay determine the orientation and distance between the two adsorbed organic mol- ecules so that intramolecular interactions can take place. The chromophore can either absorb light and

* To whom correspondence should be addressed.

prevent photo-excitation of the pesticide or accept the excess energy from the already excited pesticide mol- ecules by different energy transfer or charge transfer rnechani~ms,~ ,~ resulting in substantial extension of the activity of the pesticide.

Quinalphos (0,O-diethyl 0-quinoxaline-2-yl phos- phorothioate) was selected in the present study. It is a powerful contact insecticide, effective against a wide range of insect pests. Despite these advantages, its use in agriculture is constrained by its rapid photo- decomposition.6 It undergoes degradation even on storage at ambient temperature' (38°C) and therefore, for effective use, it must be stabilized. In our investiga- tion, the intermolecular interaction on the surface of kaolinite and palygorskite of photolabile quinalphos and an organic dye, considered as a possible stabilizer, was studied.

2 MATERIALS AND METHODS

2.1 Materials

Technical quinalphos (Fig. 1, 1; 96%) was obtained from Sandoz Ltd, Bombay, India, which was further purified to analytical grade (m.p. 36°C) by column chro- matography. The purity was determined by gas liquid

333 Pestic. Sci. 0031-613X/95/$09.00 63 1995 SCI. Printed in Great Britain

334 Kaushik Banerjee, Prem Dureja

1 2

Fig. 1. Structures of (1) quinalphos and (2) crystal violet.

chromatography (GLC) and found to be greater than 99%. Laboratory grade reagents were locally procured and purified before use. The clays used were kaolinite and palygorskite purchased from MCA Industries, New Delhi, India. The cationic dye, crystal violet (2) was pur- chased from Aldrich Chemical Company. It was used without further purification.

2.2 Adsorption isotherm

To measure the adsorption isotherm of crystal violet (CV) to kaolinite and palygorskite, aliquots of an aqueous solution of crystal violet (1 x M) were added dropwise to a suspension of kaolinite or paly- gorskite in distilled water (5 g kg-'; 10 ml), under con- tinuous stirring. The final volume in both the clays was brought to 40 ml. After six days at room temperature, the suspension was centrifuged for 30 min at 60000g. The concentration of the dye in the supernatant liquid was determined by UV-VIS spectroscopy, measuring the absorption at 590 nm.

The adsorption of quinalphos to kaolinite, palygors- kite, kaolinite-CV and palygorskite-CV was determined by shaking 0.15 g of the clays or clay-dye complexes overnight with 25 ml of a solution of quinalphos in toluene in the appropriate concentration. After precipa- tion of the clay, the concentration of quinalphos in the supernatant liquid was measured by UV-VIS spectro- scopy.

2.3 Preparation of clay-organic complexes

The clay-dye complexes (kaolinite-CV and palygorskite-CV) were prepared by dropwise addition of aliquots of an aqueous solution of the crystal violet (1 x M) to an aqueous suspension of the clay (5 g kg-') under continuous stirring. After a 30-min centrifugation at 60 0009, the flocculated clay was washed three times with distilled water, freeze-dried and ground to < 50 pm. In no case could dye be detected in the supernatant by UV-VIS spectroscopy, indicating that all the added dye had been adsorbed by the clay.

To prepare clay-quinalphos (kaolinite-quinalphos and palygorskite-quinalphos) and clay, quinalphos and crystal violet complexes, the appropriate volume of a solution of quinalphos in hexane (1 x M) was added to the powdered clays, or to the powdered clay- crystal violet, and the solvent was removed under reduced pressure.

2.4 Irradiation

For UV irradiation treatments, powdered clay or clay- organic complex (2 g) was homogeneously spread on the bottom of Petri dishes (5 cm in diam.), quinalphos dissolved in hexane was applied to the Petri dishes and the solvent subsequently evaporated. For the quinalphos-crystal violet mixture, a methanolic solu- tion of the two organic compounds was used. The treated clay samples were exposed to UV light for various times using a Rayonet RPP photochemical reactor. Samples were taken at different time intervals. Dishes covered with aluminium foil served as controls.

2.5 Bioassay

Bioassay experiments were carried out using adults of the red flour beetle Tribolium castaneum Herbst (Scabb strains). Following irradiation, 22 one-week-old beetles were transferred from stock culture to Petri dishes. After 48 h exposure, the insects were removed to vials containing food, and mortality was assessed seven days later. All irradiation and bioassay experiments were repeated at least twice. In each experiment mean mor- tality values were calculated from three replicates. Dif- ferences between replicates were within 10%. LD,, values expressed as pg quinalphos were estimated from log-dose probit mortality regression lines.

2.6 Extraction

Irradiated samples of clay-quinalphos, and clay-CV- quinalphos were extracted as follows. The clays were scrapped from the Petri dishes and extracted thor- oughly with hexane (10 ml), followed by centrifugation at 3000 rev min-' for 20 min. The supernatant liquid was concentrated to 1 ml and analysed by GLC.

2.7 Spectroscopic measurements

UV-VIS absorption spectra of aqueous solutions of quinalphos and crystal violet were measured in a Hitachi Model V-2000 double-beam UV-VIS spectro- photometer. Gas liquid chromatography was carried out using a Hewlett-Packard Model 5890 series I1 gas liquid chromatograph, fitted with 530 mu megabore column (10 m x 0.53 mm i.d., 2.65 pm thickness) HP-1 (methyl silicone gum), in nitrogen phosphorus detector (NPD) mode. The following operating conditions were used : column temperature 180°C injector and detector temperature, 250°C. Carrier gas used was nitrogen at a flow rate of 20 ml min-'.

Fourier-transform infra-red (FTIR) spectra of the compounds and of the adsorption complexes were mea-

Photostabilization of quinalphos 335

sured in potassium bromide pellets using a Phillips PU 9706 infra-red spectrophotometer.

3 RESULTS AND DISCUSSION

The ultra-violet spectrum of the aqueous solution of quinalphos exhibited maximum absorption at 240 nm and 322 nm due to n-n* and n-n* transitions of the quinoxaline ring. The ultra-violet spectrum of the aqueous solution of the cationic dye crystal violet exhibited strongest absorbance at 590 nm. These differ- ences in ultra-violet spectra of quinalphos and dye were expected to affect the efficiency of possible energy pro- cesses between pesticide and organic chromophore.

3.1 Adsorption isotherm

The adsorption isotherms of crystal violet to kaolinite and crystal violet to palygorskite are shown in Figs 2 and 3. For amounts of added dye less than or equal to the cationic exchange capacity (CEC) of the kaolinite (14 mmol per 100 g of clay) and palygorskite (25 mmol per 100 g of clay) practically all the added dye was removed by the clays since no trace of dye could be detected in the supernatant.

In Figs 4 and 5, the results of adsorption of quin- alphos from toluene to kaolinite and palygorskite, and to kaolinite-crystal violet, and palygorskite-crystal violet complexes having different concentration of crystal violet are presented. The adsorption of pesticide increases as its concentration in equilibrium solution is increased. The adsorption decreases with increasing coverage of clay surface by organic cations.

3.2 Irradiation and recovery

Non-irradiated samples of quinalphos, quinalphos with crystal violet, quinalphos adsorbed on kaolinite or paly-

1 I I I I I I

0.2 0.6 1.0 1.4 CV equilibrium concentration (mM)

Fig. 2. Adsorption isotherm of crystal violet (CV) on paly- gorskite.

2 1 6 - 0

m -

U

> u 0 4

t 0' 1 1

I I 1 1 1

0.2 0 .6 1.Q 1.4 CV equilibrium concentration (mM)

Fig. 3. Adsorption isotherm of crystal violet (CV) on kaolin- ite.

gorskite, and quinalphos adsorbed on clay-dye com- plexes gave 98% recovery. Adsorption to a clay surface considerably affected the lability of quinalphos to UV radiation. The results obtained in a photochemical experiment aimed at evaluating different clays as poten- tial photoprotectors of quinalphos are shown in Fig. 6. When quinalphos was adsorbed to kaolinite and paly- gorskite, about 23% of the initial amount of quinalphos remained unaffected after 15 h of irradiation, whereas samples of free quinalphos and quinalphos with crystal violet were almost completely degraded (2% recovery) following 7 h exposure to UV light.

Considerable improvement in the photostabilization of quinalphos was obtained when crystal violet was co- adsorbed to kaolinite or palygorskite in three different

1.6 Quinalphos equilibrium concentration (mM) .

Fig. 4. Adsorption isotherm of quinalphos on complexes of Kaolinite-CV (0) Kaolinite-CV (0.4); ( +) Kaolinite-CV

(0.2); (*) Kaolinite-CV (0.1); (0) Kaolinite.

336 Kaushik Banerjee, Prem Dureja

I I t I I I

. 2 .4 .6' . 8 1 1.2 1.4 1.6 Quinalphos equilibrium concentration (mM)

Fig. 5. Adsorption isotherm of quinalphos on complexes of Palygorskite-CV (0) Palygorskite; ( +) Palygorskite-CV

(0.1); (*) Palygorskite-CV (0.2); (0) Palygorskite-CV (0.4).

concentrations (0.1, 0.2 and 0.4 mmol g-' clay), while the concentration of quinalphos (0.2 mmol g- ' clay) was kept constant (Figs 7 and 8).

The three complexes gave similar results, although better stabilization was observed with 0.2 mmol crystal violet g- ' palygorskite and 0.1 mmol crystal violet g- ' kaolinite. In both cases, the recovery of unreacted quin- alphos after 24 h irradiation was higher (40-50%) than for the sample without the cationic dye as protecting chromophore (23%).

3.3 Irradiation and bioassay experiments

Non-irradiated samples of all materials used in the experiments were tested for bioactivity against

'*O[

Time ( Hours

Fig. 6. Degradation of quinalphos on (0) glass, (+) kaolinite and (*) palygorskite surfaces.

Y

\., , , 1

1 5 9 13 17 21 25 Time (Hours)

Fig. 7. Degradation of quinalphos on glass, kaolinite (Ka) and Ka-CV complexes. (0) Glass; (+) Ka-CV (0.1); (*)

Ka-CV (0.2); (0) Ka-CV (0.4); ( x ) Ka.

T. castaneum. Crystal violet and the clays, either separately or as a mixture, were inactive, whereas quin- alphos, either alone or in mixture with the crystal violet or adsorbed on clays with or without the crystal violet gave 100% mortality. These bioassays were carried out using the same concentrations of crystal violet and quinalphos as those used in the irradiation experiment.

Results of typical bioassays aimed at evaluating crystal violet as a potential photoprotector of quin- alphos are shown in Figs 9 and 10 respectively. Despite some variation in percentage mortality, which was related to the method of bioassay used, the trends are evident. Free quinalphos lost its insecticidal activity after 3 h of irradiation. Mixtures of quinalphos and crystal violet were also inactive after 3 h of irradiation (not shown). Some insecticidal activity was observed in the samples in which quinalphos was adsorbed on kaolinite or palygorskite at a load of 0.2 mmol quin- alphos g- ' clay. Maximum insecticidal activity was retained by the samples containing crystal violet co-

120 r

1

1 5 9 13 17 21 25 Time (Hours)

Fig. 8. Degradation of quinalphos on glass, palygorskite (P) and P-CV complexes. (0) Glass; (+) P ; (*) P-CV (0.1); (0)

P-CV (0.2); ( x ) P-CV (0.4).

Photostuhilization of quinalphos 337

100

- 80. s

$ 60.

4 0 .

20.

0

'

- z

9

0 1 3 5 7 1 5 Exposure t o UV l ight (Hours)

Fig. 9. Evaluation of photostabilization of quinalphos using Triholium custaneum (m) glass, (a) kaolinite, (a) kaolinite-

cv (0.1).

adsorbed with the quinalphos on the surface of either kaolinite or palygorskite.

In the case of kaolinite, maximum insecticidal activity was shown by the sample containing crystal violet 0.1 mmol g- I kaolinite (kaolinite-crystal violet,.,- quinalphos,., Fig. 9), whereas, in the case of palygors- kite, maximum insecticidal activity was shown by the sample containing crystal violet 0.2 mmol g- ' paly- gorskite and 0.2 mmol quinalphos (Fig. 10). A decrease in insecticidal activity was observed in the samples con- taining crystal violet at 0.4 mmol g- ' kaolinite or paly- gorskite, apparently because the CEC of the clays had been exceeded by the dye. Lower degree of photo- stabilization were achieved on kaolinite clay than on palygorskite. This difference may be due to the different content of lattice iron.

Some details of these interactions at the molecular level can be obtained using Fourier-transform infra-red (FTIR) spectroscopy. A comparison of the spectra of free quinalphos and quinalphos adsorbed on clay reveals that stretching frequency of the P=S bond shifts

from 760 cm-' (in quinalphos) to 720 cm-' and 740 cm- ' (in quinalphos adsorbed on kaolinite and palygorskite respectively) due to adsorption. A similar shift was observed in the position of this band in the spectrum of quinalphos adsorbed on clays in the pres- ence of crystal violet. This clearly shows that quin- alphos molecules in the clay-crystal violet-quinalphos complex are adsorbed in a similar manner to that in quinalphos4ay.

The infra-red spectrum of photolysed samples of quinalphos and quinalphos with crystal violet showed bands at 1100 cm-' due to P-0, showing that P-S present in quinalphos had been changed to P-0 during photolysis, whereas quinalphos adsorbed on clays and on clays in the presence of crystal violet did not show any band at 1100 cm-' in the infra-red spec- trum due to P=O. This shows that steric hindrance from the clay lattice, restricts the rearrangement of quinalphos. The degradation of quinalphos on clay sur- faces may be due to catalytic decomposition by the surface. The stability of quinalphos on clay surface in presence of crystal violet arises from modification of the clay surface by crystal violet so that the insecticide is neither decomposed by surface catalysis nor by rearrangement. This also explains why crystal violet has no direct effect on quinalphos technical.

4 CONCLUSION

Several factors contribute to the efficiency of the photo- stabilization of quinalphos incorporated in clay-organic complexes. It may be that the biological activity of many other photolabile pesticides can be extended by an appropriate choice of clays and chromophores. The clay is not merely a carrier but rather an active factor in the stabilization process.

REFERENCES

/I m * - I a A - C V l O l ) 120

100

80 s -$ 60

40

20

n

- I

r I

0 1 3 5 7 1 5 Exposure t o UV l ight (Hours)

Fig. 10. Evaluation of photostabilization of quinalphos using Triholium castaneum (m) glass, (m) palygorskite, (0)

palygorskite-CV (0.2).

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