| Abstract |
The fate of organophosphorus (OP) pesticides in the presence of chlorinated oxidants was investigated under drinking water treatment conditions. In the presence of aqueous chlorine, intrinsic rate coefficients were found for the reaction of hypochlorous acid (kHOCl,OP) and hypochlorite ion (kOCl,OP) with eight OP pesticides. The reaction of hypochlorous acid (HOCl) with each OP pesticide was relatively rapid at near neutral pH, kHOCl,OP = 0.86 - 3.56 x 106 M-1h-1. The reaction of HOCl with OP pesticides occurs at the thiophosphate (P=S) moiety resulting in the formation of the corresponding oxon (P=O), which is more toxic than the parent OP pesticide. Hypochlorite ion (OCl-) was found not to oxidize the pesticide but act like a nucleophile accelerating hydrolysis, kOCl,OP = 37.3 - 15,908.9 M-1h-1. Both the kHOCl,OP and the kOCl,OP were found to correlate well with molecular descriptors within each subgroup of OP pesticide class. The most commonly detected OP pesticides in drinking water sources were then investigated in the presence of monochloramine (NH2Cl). Monochloramine was found not to be very reactive with OP pesticides, kNH2Cl,OP = 10.6 - 21.4 M-1h-1. Dichloramine (NHCl2) was found to be two orders of magnitude more reactive with the OP pesticides investigated than monochloramine, kNHCl2,OP = 1995.0 - 2931.9 M-1h-1. The reactivity of the three chlorinated oxidants was then found to correlate with half-wave potentials (E1/2) for each OP pesticide respectively. A model was developed to predict the transformation of OP pesticides in the presence of chlorinated oxidants. With hydrolysis rate coefficients, the transformation of OP pesticides under drinking water treatment condition was found to be adequately predicted over the pH range of 6.5-9. The structure-activity relationships and mechanistic models developed here could be used by regulators to determine if drinking potable water contaminated with OP pesticides represents significant risk to a receiving population serviced by a community water system. |
