Formation of disinfection by-products and applicability of differential absorbance spectroscopy to monitor halogenation in chlorinated coastal and deep ocean seawater

Formation of disinfection by-products (DBPs) in chlorinated coastal (CS) and deep ocean (sampled at a 1500-m depth) seawater (DO) was explored using conventional chromatographic methods and differential absorbance spectroscopy. It was determined that trihalomethanes (THMs) predominated by bromoform, haloacetonitriles and haloacetic acids (HAA) form in chlorinated seawater. The yield of identified DBPs, especially THMs, was higher in CS, while in DO seawater the relative contribution of HAAs was almost four times higher than that of THMs. The fraction of consumed chlorine incorporated into the identified DBPs in DO natural organic matter (NOM) was much lower than that for coastal marine NOM, which was also more active in forming brominated DBPs. The kinetics of seawater chlorination was relatively rapid, and high levels of DBPs were released within the first 20–30 min of reaction. The kinetics of THMs and HAAs release could be approximated using a logarithmic function of reaction time. Absorbance measurements showed that chlorination caused the absorbance of DO and CS to decrease at wavelengths >250 nm. The features of the differential absorbance spectra of chlorinated seawater were close to those reported for drinking water, but their intensity was lower. The concentrations of CHBr3, CHBr2Cl and CHBrCl2 formed in CS and DO chlorinated at varying chlorine doses and reaction times were well correlated with the corresponding !)A272 and/or !)A405 values. (The differential absorbance at 405 nm can be measured in the presence of residual chlorine). These results show that differential absorbance provides a number of sensitive surrogate parameters that can be used to monitor and quantify the formation of halogen-containing DBPs in seawater.

The results of this study indicate that both volatile (THMs, haloacetonitriles) and hydrophilic DBPs (HAAs) form in chlorinated DO and coastal seawater. Although in both cases bromoform is the predominant THM species, the overall yield and speciation of the identified DBPs exhibit notable site-specificity. That is, the concentrations of brominated THMs and the yield of all identified DBPs were considerably higher in coastal seawater, while in DO seawater the relative contribution of HAAs was almost four times higher than that of THMs. HAAs formed in DO and CS water were predominated by TCAA and TBAA, respectively. In general, NOM found in DO seawater was less active in the formation of bromine-containing DBPs. The fraction of active chlorine incorporated into identified DBP species in chlorinated DO NOM was several times lower than that for coastal marine NOM.