An Innovative In Situ Spectroscopic Approach to Characterize Functional Groups in Natural Organic Matters (NOMs) and Their Interactions with Protons and Metals

Abstract

Differential absorbance spectroscopy (DAS) was used in this study as an in situ approach to characterize the functional groups in natural organic matters (NOMs) with respect to their conformational change at varying pHs and interactions with metal ions. Suwannee River fulvic acid (SRFA) was profiled by DAS as the feature bands of phenolic groups evolved at wavelengths of 244 nm and 330–380 nm and a peak signifying carboxylic groups at 280 nm. Water treatment process (e.g., chlorination) altered the properties of functional groups, as could be reflected by DAS profiles. Furthermore, NOM-calcium interactions were investigated using nonideal competitive adsorption (NICA) model to describe the specific binding of calcium ions to NOMs. Correlations between the observed changes in the intensity of the representative DAS features as well as spectral slopes of NOM showed that these parameters were strongly correlated with the amount of NOM-bound calcium. The result suggested that the NICA-Donnan model could be expanded to reflect and predict the true diversity of NOM groups engaged in interactions with metal cations in biogeochemical and engineering systems.