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ORGANOPHOSPHATE PESTICIDE DEGRADATION UNDER DRINKING WATER TREATMENT CONDITIONS
Citation:
DUIRK, S. E. AND T. W. COLLETTE. ORGANOPHOSPHATE PESTICIDE DEGRADATION UNDER DRINKING WATER TREATMENT CONDITIONS. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-05/103 (NTIS PB2006-101091), 2005.
Impact/Purpose:
Conditions for treatment of DW vary widely. However, most all processes involve some form of conventional treatment (filtration, etc.), and some form of disinfection. Also, systems sometimes use various other treatments, including softening by the addition of a base. Treatment processes can have profound effects on the pesticides and toxics that occur in DW sources. For example, hydrophobic chemicals may be partially removed by conventional treatment, however, percent removal can vary significantly depending on conditions. On the other hand, conventional treatment generally has little or no effect on hydrophilic chemicals.
If pollutants are not removed by conventional treatment, they may be altered by other treatment processes. For example, disinfection can transform some chemicals via oxidation; however, little is known about the identity of products formed by this process. Limited information shows that disinfection can yield products that are more toxic than the parent. Also, some chemicals are transformed via base-catalyzed hydrolysis during the softening process. The nature and extent of transformations vary greatly depending on treatment conditions.
EPA Program Offices recognize that treatment often has a large effect on pesticides and toxics that occur in DW sources; and they have articulated a need to incorporate these effects into risk assessments. This task will provide regulators with methods, tools, and databases to forecast the fate of pesticides and toxics during DW treatment. The early task outputs will be chemical-specific information from bench-scale studies that simulate disinfection and softening. However, all task efforts will be focused on the long-range goal of providing predictive models for chemical removal and transformation that cross chemical class and treatment conditions. Early experiments will provide information to elucidate transformation mechanisms. Next, we will investigate effects of varying treatment conditions and chemical speciation. This strategy will lead to broadly applicable tools for forecasting fate for a wide range of chemicals. Finally, we envision that the output of our predictive fate tools will be used as input into models developed under the ORD Computational Toxicology Initiative. In this fashion, the final contaminants and concentrations predicted by our models to occur in finished DW can then be considered for toxic potential. This will provide Program Offices with an integrated system for risk assessment and management for the pesticides and toxics in drinking water.
Description:
Chlorpyrifos (CP) was used as a model compound to develop experimental methods and prototype modeling tools to forecast the fate of organophosphate (OP) pesticides under drinking water treatment conditions. CP was found to rapidly oxidize to chlorpyrifos oxon (CPO) in the presence of free chlorine. The primary oxidant is hypochlorous acid (HOCl); thus, oxidation is more rapid at lower pH (i.e., below the pKa of HOCl at 7.5). At elevated pH, both CP and CPO are susceptible to alkaline hydrolysis and degrade to 3,5,6-trichloro-2-pyridinol (TCP), a stable end product. Furthermore, the hydrolysis of both CP and CPO to TCP was shown to be accelerated in the presence of free chlorine by OCl-. These observations regarding oxidation and hydrolysis are relevant to common drinking water treatment processes: disinfection and water softening, respectively. In this work, intrinsic rate constants for these processes were determined, and simple computer models have been developed that accurately predict the concentration of CP, CPO, and TCP as a function of pH, chlorine dose, CP concentration, and time after chlorine dosing. These models serve as a first step toward the development of tools to assess the assessment of risk associated with consuming treated drinking water whose source is contaminated with OP pesticides.