Absorbance spectroscopy-based examination of effects of coagulation on the reactivity of fractions of natural organic matter with varying apparent molecular weights

Absorbance spectra of fractions of natural organic matter (NOM) with varying apparent molecular weights (AMWs) were examined in this study. Size exclusion chromatography (SEC) was employed to obtain AMW distributions for three Australian water sources which represented low- and high-dissolved organic carbon (DOC) surface waters and a source with highly degraded NOM. These waters were coagulated with alum and other coagulants. Effects of coagulation on AMW distributions were quantified based on an absorbance slope index (ASI) calculated using NOM absorbance measured at 220, 230, 254 and 272 nm. This index can be calculated for any AMW fraction of NOM. Similarly to SUVA254, ASI values decrease consistently in coagulated waters and are correlated with trihalomethane yields. Comparison of ASI indexes in different water sources indicates the presence of both common trends and differences indicative of NOM site-specificity.