Use of UV spectroscopy to characterize the reaction between NOM and free chlorine

Simple and reliable relationships exist between the change in UV absorbance (Delta A) of NOM when it is chlorinated and the formation of chlorinated byproducts. These relationships provide an approach for obtaining large amounts of data that can be used to interpret the kinetics, stoichiometry, and mechanism of the reactions. Analysis of these relationships suggests that the functional groups that are the major precursors for DBPs might be highly activated aromatic rings, as has been suggested previously, but that these groups have some fundamental differences from highly activated rings in pure compounds. The key evidence for this difference is that the UV absorbance of NOM decreases when dosed with even very low concentrations of chlorine, whereas the absorbance of pure compounds such as 3,5-DHBA and resorcinol increases. When hypochlorite species (HOCl and OCl-) are added to a solution containing NOM, between 1.6 end 4.1 CI atoms become incorporated into NOM for each activated aromatic ring that is destroyed. The rate of CI incorporation into organic molecules is very rapid initially and decreases steadily thereafter. Chlorine reduction on the other hand (or, equivalently, NOM oxidation) is negligible initially and then increases over time. The effect of these parallel processes is that the amount of CI that becomes incorporated into organic molecules as a fraction of the amount of HOCl and OCl- consumed decreases from near 100% initially to near 20% over the course of the reaction. Considering that many of the carbon atoms in NOM probably do not participate in redox reactions with CI, those that do must be oxidized quite substantially during the process, and it is likely that atoms other than C (particularly RI) also provide some of the electrons to reduce CI. Expanded use of Delta A to study DBP-forming reactions is likely to lead to more insights into key aspects of the reaction mechanisms.