Correlations between differential absorbance and the formation of individual DBPs

This study examined correlations between the differential absorbance at 272 nm (ΔA272) and the formation of disinfection by-products (DBPs) in chlorinated water from the Tolt River, a water source for Seattle, WA. The DBPs investigated included chloroform (CHCl3), dichlorobromomethane (CHCl2Br), mono-, di- and trichloroacetic acids (MCAA, DCAA, and TCAA, respectively), chloral hydrate (CH), dichloroacetonitrile (DCAN) and 1,1,1-trichloropropanone (TCP). Whereas the kinetics of DBP formation are complex and are non-linear, the same DBP data represented as a function of ΔA272 are quite simple. Absorbance decreases when the water is chlorinated, i.e., ΔA272 is always negative. The DBP vs. −ΔA272 correlations can almost always be quantified by linear equations, at least above some threshold value of −ΔA272, with R2 values >0.95. The only DBP that did not follow this trend was CH, for which an exponential relationship better described the data. TCP and DCAN were unstable at pH 7 and 8, but at pH 6 linear correlations between their concentrations and −ΔA272 were as strong as those for the more stable DBPs. The threshold −ΔA272 value is approximately the same for many of the DBPs studied, supporting the hypothesis that individual DBPs are released following the formation of a common intermediate, or at least a small group of similar intermediates. The DBP vs. −ΔA272 correlations may have practical value since they provide an alternative approach for monitoring the formation of individual DBP species on-line, but the generality of the relationships needs to be further examined.