Decomposition of trihaloacetic acids and formation of the corresponding trihalomethanes in drinking water

The decomposition of trihaloacetic acids [bromodichloroacetic acid (BDCAA), dibromochloroacetic acid (DBCAA), tribromoacetic acid (TBAA)], and the formation of the corresponding trihalomethanes [bromodichloromethane (BDCM), dibromochloromethane (DBCM), tribromomethane (TBM)] were studied. Like TBAA, the two mixed chlorobromo-species, BDCAA and DBCAA, were found to decompose to form BDCM and DBCM, respectively, via a decarboxylation pathway. The decomposition of BDCAA, DBCAA and TBAA in water at neutral pH follows a first-order reaction, with rate constants of 0.0011, 0.0062 and 0.040 day−1 at 23°C, respectively; and 0.000028, 0.00014 and 0.0016 day−1 at 4°C, respectively. The activation energies for the decomposition reaction of BDCAA, DBCAA and TBAA in water at neutral pH were found to be 35.0, 34.5 and 29.2 kcal/mol, respectively. The effect of pH in the range of 6–9 and the effect of a drinking water matrix on the decomposition of BDCAA, DBCAA, and TBAA in water were found to be insignificant. Measurement and health implications due to decomposition of trihaloacetic acids and formation of the corresponding trihalomethanes were discussed. By applying the technique of quantitative structure–activity relationships (QSAR), the decomposition rate constants of six iodinated trihaloacetic acids were estimated.