Pestic Sci. 1992, 34, 163-166 55/91

Soil Sensitised Generation of Singlet Oxygen in the Photodegradation of Bioresmethrin Paul Clements & Clifford H. J. Wells School of Applied Chemistry, Kingston Polytechnic, Kingston upon Thames, Surrey KT1 2EE, UK

(Revised manuscript received 12 August 1991 ; accepted 15 September 1991)

Abstract: The ability of soil samples of pH 42 , 5.5 and 7.2 to generate gaseous singlet oxygen was investigated using a separated-surface-sensitised reactor. I t was found that the soil samples can act as sensitisers for the production of singlet oxygen in a similar fashion to the well known sensitisers chlorophyll and Rose Bengal. The reaction of singlet oxygen so produced with the pyrethroid insecticide, bioresmethrin, has been investigated.

1 INTRODUCTION

It is important to have a knowledge of the various modes of photodegradation of agrochemicals in the environ- ment. A study of the photodegradation of pyrimidine- based fungicides and related compounds has shown that one possible mode of degradation is via reaction with singlet oxygen.’-3 This highly reactive electrophilic form of oxygen can be readily produced by energy transfer to ‘normal ’ molecular oxygen from a sensitiser which has taken up energy by absorption of visible radiation. There are a number of sensitisers available in the environment viz. chlorophyll,4 porphyrins5 and humic acids,5 which absorb visible radiation, and hence have the ability to absorb sunlight and produce singlet oxygen. Indeed, singlet oxygen has been detected in natural surface waters, and it acts as an oxidising agent in this environment.‘.’

The role of singlet oxygen as an environmental oxidising agent has been postulated in the photo- degradation of pesticides and of other compounds on soil surface^.^-'^ Although these studies implicated singlet oxygen they did not preclude the possibility that other oxidising species, such as superoxide anion, hydrogen peroxide and hydroxyl radicals, could be involved in the photodegradation. This problem of deciding whether or not a particular oxidising species is in part or wholly responsible for the environmental photodegradation of an agrochemical exists when the sensitiser is present in solution along with the substrate, or is immobilised on a surface, or is in the solid state in a heterogeneous system.

The separated-surface-sensitised (s-s-s) reactor de-

veloped by Midden and Wang” overcomes the above mentioned difficulties and provides a conclusive means of demonstrating the intermediacy of singlet oxygen in chemical reactions. By immobilising an illuminated sensitiser on a glass plate placed a small distance above a substrate solution, a physical separation between sensitiser and substrate is maintained, which prevents non-singlet oxygen reactions from occurring whilst allowing singlet oxygen to diffuse across the gap to react with the substrate.

We have chosen the s-s-s reactor to study the ability of soil samples to act as sensitisers for the generation of gaseous singlet oxygen. If soil surfaces can generate singlet oxygen, then there is a high probability that reaction with singlet oxygen is a primary pathway for the photodegradation of electron-rich xenobiotics in contact with soil.

In the systems reported on herein, the pyrethroid insecticide, bioresmethrin, was chosen as the target substrate since it is known to react rapidly with singlet oxygen” (also Clements, P., Wells, C. H. J., unpublished results) and that attack by singlet oxygen on pyrethroids causes their 0xidati0n.l~ The well-known singlet oxygen sensitisers, Rose Bengal and chlorophyll, were also used in the s-s-s reactor to provide results for comparison with those of the soil samples.

2 EXPERIMENTAL METHODS

Bioresmethrin was supplied by the Rothamstead Ex- perimental Station as ‘Technical’ grade and was purified

I63 Pesric. Sci. 0031-613X/92/$05.00 @ 1992 SCI. Printed in Great Britain

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1 64 Paul elements, eliflord H . J . Wells

by column chromatography on silica gel prior to use. After purification, a single spot was observed when a solution of bioresmethrin in methanol was loaded onto a Merck silica gel F,,, plate (thickness 0.2 mm, size 10 cm x 10 cm) and developed with a solvent mixture of chloroform + acetonitrile + hexane (8 + 1 + 1 by volume). Rose Bengal. 8-carotene, N,N-dimethyl-4-nitrosoaniline (RNO). and histidine (Aldrich Chemical Co Inc) were used as supplied. Chlorophylls a and h were extracted from spinach leaves and purified as previously de- s~ r ibed . '~ Soil samples were donated by the School of Geography, Kingston Polytechnic. The samples were dried under vacuum at 353 K and then passed through a 2-pm mesh sieve before use. Three samples were used of pH 4.2. 5.5 and 7.2 with total carbon contents of 2.9, 1.8 and 3.1 YO respectively.

The separated-surface-sensitised (s-s-s) reactor was of the same design as that previously described." The radiation source, when the coloured materials Rose Bengal and chlorophyll were used as sensitisers, was a 100 W Wotan spotlight bulb with the radiation filtered through a Wratten gelatine filter to remove radiation of wavelengths less than 420 nm. The radiation source when a soil sample was used as sensitiser, was a 400 W medium pressure Hg discharge lamp. This type of lamp emits radiation in the near ultraviolet region which can be absorbed by potential sensitisers in the soil samples. The Rose Bengal or chlorophyll sensitisers were coated onto the silica gel surface of a TLC plate by immersion into a saturated solution of the sensitiser in methanol. The TLC plate was left immersed for 10min and then placed in a vacuum of 30 mbar for 30 min. Any loose particles on the surface of the plate were removed by a high pressure stream of nitrogen. The soil sensitisers were sprinkled onto double-sided tape adhering to a glass slide and blown with a high pressure stream of nitrogen to remove any loose particles. The glass slide or cut-sections of the TLC plates were placed over the wells of a Boerner slide in the s-s-s reactor prior to irradiation with a separation of less than 1 mm between the glass slide or TLC plate and the surface of the reactant solution. The glass slide acts as a filter and prevents radiation of wavelengths shorter than 320 nm impinging on the soil surfaces.

The ability of the s-s-s reactor to produce gaseous singlet oxygen was tested using the histidine/RNO reaction.15 Solutions of histidine ( M) and RNO (lo-") in phosphate buffer (002 M, pH = 7.2) were placed in the wells of a Boerner slide and covered with a chlorophyll-loaded sensitiser plate, which was then irradiated. Control runs were carried out with a chlorophyll-loaded sensitiser plate placed with the sen- sitiser side upwards from the reactant solutions. Samples of the reaction solution were taken at regular intervals and the absorbance of RNO at 440 nm was monitored with respect to irradiation time.

Solutions containing bioresmethrin (lo-* M) were

prepared in benzene, as were solutions containing bioresmethrin (lo-' M) and 8-carotene ( M). Equal volumes of these solutions were placed in twelve wells of a Boerner slide in groups of four as follows:

Group C-control-solutions of bioresmethrin Group Q-quenched-solutions of bioresmethrin and

/%carotene Group U-unquenched-solutions of bioresmethrin.

Solutions in groups Q and U were covered by a sensitiser plate with the sensitiser side downwards towards the solution, whilst solutions in group C were covered by the sensitiser plate with the sensitiser side upwards. The concentration of bioresmethrin in the solutions was monitored during irradiation by hplc. The analysis was carried out in triplicate on a 5 pm nitrile column (20 x4.6 mm) at ambient temperature with a mobile phase of hexane + propan-2-01 (993 + 0.5 by volume). Detection was by uv detector set at 240 nrn. The retention time of bioresmethrin was 5 min.

3 RESULTS AND DISCUSSION

It is well known that the presence of singlet oxygen in the histidine/RNO system results in the loss of RNO". Conversely, if RNO loss is observed within a his- tidine/RNO system then it can be deduced that singlet oxygen is present. The concentration of RNO in the histidine/RNO system used in the s-s-s reactor, with chlorophyll as sensitiser, was monitored by the ab- sorbance of RNO at 440 nm. The results are shown in Fig. I. The change in absorbance on irradiation shows that there is a loss of RNO and, furthermore, this loss is a linear function of irradiation time. There was no change in absorbance of RNO on irradiation in the control system. It can be concluded, therefore, that the design of the s-s-s reactor used in the present study was satisfactory in that gaseous singlet oxygen is produced by the illuminated chlorophyll and that the singlet oxygen

x

I

3

/ x O

4

Irradiation time ( h )

Fig. I . Change with irradiation time of absorbance at 440 nm of RNO in RNO/histidine system using chlorophyll as sensitiser

in s-s-s reactor.

Soil sensitised generation of singlet oxygen 165

C ._ 40- 5

P 2 0 - P m

:

traverses the air gap between sensitiser and substrate solution to react with the substrate.

Figures 2-6 show plots, from one of a set o f duplicate trials, of the percentage bioresmethrin remaining in solution with irradiation time when different sensitisers were used in the s-s-s reactor. The similarity of the plots for the chlorophyll-sensitised reaction with those for Rose Bengal and the soil samples is evidence that the Rose Bengal and the soil samples also produced singlet oxygen in the s-s-s reactor. This is to be expected in the case of Rose Bengal, as it is a well known sensitiser for singlet oxygen formation. Further evidence that singlet oxygen is produced by chlorophyll, Rose Bengal and the soil samples comes from a comparison between the rate of removal of bioresmethrin in the control, quenched and unquenched solutions in each system. p-carotene is a known singlet oxygen quencher and it can be seen from Figs 2-6 that in each case the presence of p-carotene in the quenched solutions slows down the rate of removal of bioresmethrin as compared to the unquenched solutions when b-carotene is absent.

The observation that the presence of p-carotene in the

k 40 L

5 E 2 0 - P m

-

1 0 1 2 3 4

Irradiation time (h)

Fig. 2. Change with irradiation time of bioresmethrin in solution using chlorophyll as sensitiser in s-s-s reactor. Solvent = benzene. Temperature = 280 K. (0) Control;

(A) quenched ; (B) unquenched.

quenched solutions does not result in the same rate of loss of bioresmethrin as in the control solutions shows that, a t a concentration of M, the &carotene is not capable of completely quenching the reaction of singlet oxygen with bioresmethrin.

g 201 m

I ___-A

0 1 2 3 4 Irradiation time (h)

Fig. 4. Change with irradiation time of bioresmethrin in solution using soil sample of pH 4.2 as sensitiser in s-s-s reactor. Solvent = benzene. Temperature = 280 K. Symbols as Fig. 2.

I 0 1 2 3 4

Irradiation time (h)

Fig. 5. Change with irradiation time of bioresmethrin in solution using soil sample of pH 5.5 as sensitiser. Solvent =

benzene. Temperature = 280 K. Symbols as Fig. 2.

I

3 4 I -~~ I. 1-

0 1 2 3 4 0 1 2 Irradiation time (h ) Irradiation time (h)

Fig. 3. Change with irradiation time of bioresmethrin in Fig. 6. Change with irradiation time of bioresmethrin in solution using Rose Bengal as sensitiser in s-s-s reactor. Solvent solution using soil sample of pH 7.5 as sensitiser. Solvent =

= benzene. Temperature = 280 K. Symbols as Fig. 2. benzene. Temperature = 280 K. Symbols as Fig. 2.

166 Puul Clements, Cliford H . J . Wells

The plots for the control solutions given in Figs 2-6 show that some loss of bioresmethrin occurs, even though singlet oxygen generated by the sensitiser could not reach the control solution, because the sensitiser coated surface of the sensitiser plate/slide was positioned upwards and away from the control solution. The loss of bioresmethrin observed in the control solutions must be due to stray light from the radiation sources causing direct photolysis of the bioresmethrin. The direct photodecomposition of pyrethroid insecticides is well documented.'" l6

The results presented herein provide unambiguous evidence that irradiation of the soil samples can generate singlet oxygen. This finding has important implications, in that any soil surface in the environment in the presence of sunlight and oxygen could be expected to produce singlet oxygen, and thus any agrochemical on the surface which has an electron-rich site within its molecular framework will be susceptible to attack and degradation by the electrophilic singlet oxygen. The studies reported relate to a limited number of soil samples, and further studies are planned to ascertain the effect on singlet oxygen production of the characteristics of soils in different regions of the UK.

4 CONCLUSIONS

Irradiation of soil surfaces in the presence of oxygen can result in the generation of gaseous singlet oxygen which being electrophilic has the ability to react with electron- rich xenobiotics. Reaction with singlet oxygen could

provide a mode for the environmental degradation of agrochemicals on soil surfaces.

REFERENCES

I .

2.

3.

4.

5.

6.

7.

8.

9.

10.

1 1

12.

13.

14.

15.

Harkness, G. J. & Wells, C. H. J., Pestir. Sci., 12 (1981)

Dixon, S. R. & Wells, C. H. J. , Pestic. Sci., 14 (1983) 444-8. Dixon, S. R. & Wells, C. H. J. , Pestic. Sci., 21 (1987)

Krasnovsky, A. A., Photochem. Photobio/., 29 (1979)

Slawinski, J., Puzyna, W. & Slawinski, D., Photochem. Photobiol., 28 1978) 75-8 I . Zepp, R. G., Wolfe, N. L., Baughman, G. L. & Hollis, R. C. , Nature (London), 267 (1977) 421-3. Wolff, C. J . M., Halmans, M. T. H. & van der Heijde, H . B., Chemosphere 10 (1981) 59-62. Gohre, K. & Miller, G. C., J. Agric. Food Chem., 31 (1983)

Gohre, K. & Miller, G . C. , J. Agric. Food. Chem., 34 (1986)

Gohre, K. , Scholl, R. & Miller, G. C., Environ. Sri. Techno/., 20 (1986) 934-8. Midden, W. R. & Wang, S. Y., J . Amer. Chem. SOC., 105

Ueda, K., Gaughan, L. C. & Casida, J . E., J . Agric. Food Chem., 22 (1974) 212-20. Ruzo, L . 0. In Pesticide Chemistry: Human Weware and the Enrironment, Proceedings of the 5th International Congress of Pesticide Chemistry, ed. J. Miyamoto & P. C . Kearney. Pergamon, New York. 1983, 2, pp. 201-6. Omata, T. & Murata, N., Photochem. Photobiol., 31 (1980)

Kraljic, 1. & Mohsni, S. El., Photochem. Photobiol., 28

215-18.

155-63.

29-36.

1 104-8.

709-13.

( 1 983) 41 29-35.

183-5.

(1978) 577-81. 16. Ruzo, L. O., Prog. Pestic. Biochem., 2 (1982) 1-31