Multi-site proton interactions with natural organic matter

Water samples from various locations in Norway were used to isolate natural organic matter (NOM). The NOM was isolated using both reverse osmosis and low-pressure low-temperature evaporation for each sample site. These samples were titrated from -log[H+] 3 to 11 at 0.1 unit intervals. The data were analyzed using the Discrete Site Analysis (DISI) technique for pK(a) intervals of 0.2. The acidity constants are grouped into four classes: strong (pK(a) < 5), intermediate strong (5.1 < pK(a) < 7.5), intermediate weak (7.6 < pK(a) < 9.2), and weak (pK(a) > 9.3). All samples, regardless of isolation method, were found to contain strong and weak ligands, along with some intermediate ligands. For the same sample site, the concentration of these ligands were found to be dependent on isolation method and titration direction (acid then base vs. base then acid). In addition, the concentration and classes of ligands present were found to vary between sample sites. Suwannee River fulvic acid was analyzed as a reference sample, and was qualitatively similar to the NOM samples but quantitatively different. Overall, the differences in pKa spectra due to isolation method and/or titration direction are almost as significant as differences between sample location, but there are no consistent trends in effects of isolation method or titration direction on characterization of NOM. (C) 1999 Elsevier Science Ltd.