UV spectra of natural organic matter (NOM) can be represented by three bands, each a Gaussian function of energy. The bands, referred to as the local-excitation (LE), benzenoid (Bz) and electron-transfer (ET) bands, can be ascribed to three types of electronic transitions typical in aromatic compounds. At wavelengths from 200 to 400 nm, an adequate fit to the spectrum can be obtained by consideration of only the Bz and ET bands. The parameters of these two bands are sensitive to the composition of NOM and to alterations in NOM caused by coagulation and chlorination. Removal of NOM fractions with higher-than-average concentrations of carboxyl-, hydroxyl- and ester-substituted aromatic rings or oxidation of those fractions by chlorine decreases the intensity and width of the ET and Bz bands. The ratio of absorbances at the band maxima is an indicator of the average degree of activation of the aromatic rings and permits prediction of the reactivity of the aromatic moiety in chlorination reactions. In addition, measurements of absorbance changes induced by chlorination permit one to monitor certain aspects of chlorination reactions in situ and to evaluate the formation of disinfection by-products.
