Effects of Dissolved Water Constituents on the Photodegradation of Fenitrothion and Diazinon

The photochemical degradation of two widely used organophosphorothioate insecticides, fenitrothion and diazinon, was investigated in aqueous solutions containing three separate dissolved constituents commonly found in natural waters (NO 3 − , CO 3 2− and dissolved organic matter (DOC)). The effect of these constituents on pesticide photodegradation was compared to degradation in “constituent-free” pure water. Solutions were irradiated in an Atlas solar simulator fitted with a UV-filtered Xenon arc lamp with light irradiances (500 W m−2) measured using a spectral radiometer to allow derivation of quantum yields of degradation. Fenitrothion absorbs light within the solar UV range (λ, 295–400 nm) and underwent direct photolysis in pure water whereas diazinon (λ max ∼250 nm) showed no observable loss over the experimental period. However, photodegradation conforming to pseudo-first-order kinetics was observed for both chemicals in the presence of the dissolved constituents (at concentrations typically observed in natural waters), with the rates of photodecay observed in the order of NO 3 − > CO 3 2− ≅ DOC, with the highest rates observed in the 3 mM NO 3 − solutions (k Fen = 0.155 ± 0.041 h−1; k Dia = 0.084 ± 0.0007 h−1). For diazinon this rate was comparable to fenitrothion photolysis in pure water (k fen 0.072 ± 0.0078 h−1), highlighting the importance of NO 3 − on a non-photolabile pesticide, with indirect photodegradation probably attributable to the light-induced release of aqueous hydroxyl radicals (·OH) from NO 3 − . Suwannee river fulvic acid (serving as DOC) did not statistically affect the rate of photodecay for fenitrothion relative to its photolysis in MilliQ water, although measured rates in DOC solutions were slightly lower. However, measurable rates of photodecay were apparent for diazinon in the DOC solutions, indicating that fulvic acid, possibly in the form of “excited” triplet-state-DOC plays a role in diazinon transformation. Hydrolysis was not apparent for fenitrothion (in buffered solutions of pH 5–9) but was notable for diazinon at the lower pHs of 5 and 3 (k Dia-hyd 0.3414 h−1 at pH 3 and 0.228 h−1 at pH 5), resulting in the formation of the degradate, 2-isopropyl–6-methyl–4-pyrimidinol. This work highlights the importance of dissolved constituents on abiotic photodegradation of pesticides and it is recommended that these constituents be incorporated into laboratory-based fate-testing regimes.