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DETERMINING ACTIVE OXIDANT SPECIES REACTING WITH ORGANOPHOSPHATE PESTICIDES IN CHLORINATED DRINKING WATER
DUIRK, S. E., D. CHERNEY, C. TARR, AND T. W. COLLETTE. DETERMINING ACTIVE OXIDANT SPECIES REACTING WITH ORGANOPHOSPHATE PESTICIDES IN CHLORINATED DRINKING WATER. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-06/103 (NTIS PB2007-100144), 2006.
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.
Chlorpyrifos (CP) is an organophosphate (OP) pesticide that was used as a model compound to investigate the transformation of OP pesticides at low pH and in the presence of bromide and natural organic matter (NOM) under drinking water treatment conditions. Raman spectroscopy was used to determine which active chlorine species was responsible for the rapid oxidation of CP below neutral pH. Over the pH range of 2-11, three active chlorine species were identified with Raman bands of 725, 711, and 538 cm-1. The first two bands were easily assigned to HOCl and OCl- respectively. The 538 cm-1 band was identified as Cl2(aq) after bubbling chlorine gas through phosphate buffered water at pH 2. Either at low pH or in water treatment plants that use direct injection of Cl2(g), molecular chlorine rapidly reacts with CP transforming it to chlorpyrifos oxon (CPO). In the presence of bromide, the loss of CP was found to be accelerated when aqueous chlorine was added. It was found that bromide acts as a catalyst in the oxidation of CP to CPO via the formation of hypobromous acid (HOBr) at concentrations relevant to drinking water treatment. Also, NOM was found to not inhibit the oxidation of CP to CPO in the presence of free chlorine under the experimental conditions in this study. A previously developed screening-level model used to predict OP pesticide degradation pathways was modified to model the effects of both bromide and NOM. The model adequately described the oxidation of CP to CPO by free chlorine as well as the stability of CPO over the course of the experiment. This work demonstrated the applicability of screening-level models to predict the fate of OP pesticides under drinking water treatment conditions.