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THE FATE OF THE HALOACETATES IN DRINKING WATER - CHEMICAL KINETICS IN AQUEOUS SOLUTION
Citation:
Urbansky*, E T. THE FATE OF THE HALOACETATES IN DRINKING WATER - CHEMICAL KINETICS IN AQUEOUS SOLUTION. Michl, J. (ed.), CHEMICAL REVIEWS. American Chemical Society, Washington, DC, 101(11):3233-3244, (2001).
Description:
Haloacetates comprise about 13% of the measurable halogenated organic matter in potable water supplies after chlorination. Some of these species have been linked with animal carcinogenesis and are regulated under the Stage 1 Disinfection Byproduct (DBP) Rule. However, it is known that post-disinfection pre-distribution concentrations do not necessarily reflect concentrations at the tap. The inability to accurately predict the speciation and concentrations of DBPs in a general way has led the EPA's Office of Water to focus on minimizing DBP formation via remobal of precursor material, i.e., natural organic matter (NOM). The process of predicting DBP concentrations is further complicated by the continuous formation and interconversion of DBPs in the distribution system. The behavior of haloacetates remains an issue for the EPA. Of particular interest is the variation in concentrations of haloacetates and trihalomethanes as a function of time. One concern has been the possibility for conversion of trihaloacetates to trihalomethanes. Whether the concentrations of these two species reach their maxima at the same time and location has been a continued source of debate in drinking water chemistry. This report highlights the need for fundamental understanding of mechanisms and kinetics for reactions whose rates can be readily measured in the lab. Rate constants and activation energies thus determined can then be applied directly in modeling the fate of DBPs. Important reaction types include nucleophilic substitution, hydrolysis, and decarboxylation. By discerning rate constants for the formation and/or loss of identifiable and quantitatable species, reliance on purely empirical models (i.e., those without chemical rationales) can be reduced. Although it is not possible to do this for all species, efforts should be made to take advantage of known rate and equilibrium constants whenever possible. Calculations based on kinetic models in the literature suggest that rates of strictly chemical decomposition are too small to have detectable impacts and offer further support for the hypothesis that biodegradation is perhaps the most important factor affecting haloacetate loss in the distribution system. Gaps in knowledge are presented as questions to be answered about haloacetate behavior. Answering these questions will improve predidctive modeling and strengthen the scientific foundation on which future drinking water regulations will be built.