Environmental Photochemistry in Heterogeneous Media

Investigation of photodegradation and hydrolysis of selected substituted urea and organophosphate pesticides in water

INTRODUCTION

Photodegradation and hydrolysis of two substituted urea herbicides, monolinuron [3-(4-chlorophenyl)-1-methoxy-1-methylurea] and linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea], and one organophosphorous insecticide, phoxim [2-(diethoxyphosphinothioyloxyimino)-2-phenylacetonitrile], were studied using buffered sterilized distilled water (pH 4, 7 and 9).

METHODS

Experiments were performed in the absence and presence of light (320-740 nm), while the effect of nitrates and humic acids on photodegradation was investigated for all pH values. An analytical method was developed and validated for the determination of target compounds in water samples using liquid chromatography positive ion electrospray-mass spectrometry.

RESULTS

According to the results, substituted ureas neither hydrolyzed, at all tested pH values, nor photodegraded at pH 7 and 9. Slow photodegradation of the compounds was observed at pH 4. During 70 days of light exposure, initial concentrations of linuron and monolinuron were decreased by 54% and 31%, respectively, while the presence of nitrates slightly enhanced photodegradation of these compounds. On the other hand, phoxim was found to be very unstable for all the tested conditions and an increase of pH resulted to higher degradation. During hydrolysis experiments, the degradation of the compound ranged from 41% (pH 4) to 85% (pH 9) and the half-lives varied from 10 h (pH 9) to 204 h (pH 4). The presence of light enhanced phoxim degradation and as a result half-lives of 37, 22 and 9h were calculated for pH 4, 7 and 9, respectively. The addition of nitrates and humic acids did not significantly affect the photodegradation of phoxim.

CONCLUSIONS

The results indicated that among the three tested pesticides, phoxim found to be the most sensitive in both photodegradation and hydrolysis.