Transformation of Organophosphorus Pesticides in the Presence of Aqueous Chlorine: Kinetics, Pathways, and Structure-Activity Relationships

The fate of organophosphorus (OP) pesticides in the presence of aqueous chlorine was investigated under simulated drinking water treatment conditions. Intrinsic rate coefficients were found for the reaction of hypochlorous acid (k(HOCl,OP)) and hypochlorite ion (k(OCl,OP)) for several OP pesticides. The reaction of hypochlorous acid (HOCl) with each OP pesticide was relatively rapid near neutral pH, k(HOCl,OP) = 0.86 - 3.56 x 10(6) M(-1)h(-1). HOCl reacts at the thiophosphate (P = S) moiety of the OP pesticide resuffing in the formation of the corresponding oxon (P = 0), which is more toxic than the parent pesticide. Hypochlorite ion (OCl(-)) was found not to oxidize OP pesticides but act like a nucleophile accelerating hydrolysis, k(OCl,OP) = 37.3-15 910 M(-1)h(-1). Both the k(HOCl,OP) and the k(OCl,OP) were found to correlate well with molecular descriptors within each subgroup of the OP pesticide class. A model was developed to predict the transformation of OP pesticides in the presence of aqueous chlorine. With hydrolysis rate coefficients, the transformation of OP pesticides under drinking water treatment conditions was found to be adequately predicted. The structure-activity relationships and model developed here could be used by risk assessors to determine exposure to OP pesticides and their transformation products in potable water.