Thermal degradation of haloacetic acids in water

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Haloacetic acids are commonly found in most natural waters. These are known as degradation products of some halogenated compounds such as C2- chlorocarbons and CFC replacement compounds: hydroflurocarbons (HFCs) and hydrochloroflurocarbons (HCFCs). While knowledge clarifying the particular sources of these compounds and precursor degradation mechanisms are progressing, there is less understanding of mechanisms for the environmental degradation resulting from haloacetic acids. In particular, increasing concentrations of trifluoroacetic acid (TFAA) and its stability to degradation have prompted concerns that it will accumulate in the environment. Here we present the results of experiments on the non-biological decomposition of aqueous haloacetic acids. The decarboxylation of trichloroacetic acid (TCAA) and tribromoacetic acid (TBAA) was investigated in the 1930’s, so this process seemed a potentially important pathway for degradation of trihaloacetic acids (THAAs) in the environment. We have measured the rate of decarboxylation of TFAA, TCAA, and TBAA and also the hydrolysis rate constants for some mono-, di-, and mixed halogen haloacetic acids in water at temperatures above ambient. The results suggest long lifetimes in natural waters. Tri-substituted acids degrade through decarboxylation with half-lives (extrapolated) at 15°C for 103 days, 46 years and 40,000 years for TBAA, TCAA and TFAA respectively. The mono and di-substituted haloacetic acids degrade via hydrolysis with half-lives (extrapolated) of 2, 12, 15, and 68 years at 15°C for monobromo- (MBAA), dibromo- (DBAA), monochloro- (MCAA) and dichloro- (DCAA) acetic acids respectively. The mixed haloacetic acids, bromochloro- (BCAA) and chlorodifluoro- (CDFAA) degrade by hydrolysis with half-lives (extrapolated) of 6 and 83 years respectively at 15°C. The overall stability of nine haloacetic acids investigated in this study of thermal degradation regardless the process, is in the order: TFAA >> CDFAA > DCAA > DBAA > MCAA> BCAA > MBAA > TCAA > TBAA. We found no catalytic effect of iron, copper and manganese on the rate of decarboxylation in water. Key words: Trifluoroacetic acid, trichloroacetic acid, decarboxylation, HFC, HCFC, hydrolysis, residence time.