Degradation and byproduct formation of parathion in aqueous solutions by UV and UV/H2O2 treatment

The photodegradation of parathion in aqueous solutions by UV and UV/H2O2 processes was evaluated. Direct photolysis of parathion both by LP (low pressure) and MP (medium pressure) lamps at pH 7 was very slow with quantum yields of 6.67 ± 0.33 × 10−4 and 6.00 ± 1.06 × 10−4 mol E−1, respectively. Hydrogen peroxide enhanced the photodegradation of parathion through the reaction between UV generated hydroxyl radical and parathion with a second-order reaction rate constant of 9.70 ± 0.45 × 109 M−1 s−1. An optimum molar ratio between hydrogen peroxide and parathion was determined to be between 300 and 400. Photodegradation of parathion yielded several organic byproducts, of which the paraoxon, 4-nitrophenol, O,O,O-triethyl thiophosphate and O,O-diethyl-methyl thiophosphate were quantified and their occurrence during UV/H2O2 processes were discussed. NO2−, PO43−, NO3− and SO42− were the major anionic byproducts of parathion photodegradation and their recover ratios were also discussed. A photodegradation scheme suggesting three simultaneous pathways was proposed in the study.