| Abstract
| - Humic substances typically constitute 40−60% of thedissolved organic matter (DOM) in surface waters.However, little information is available regarding themetal binding properties of the nonhumic hydrophilic portionof the DOM. In this study, humic and nonhumic DOMsamples were isolated from the South Platte River (Colorado,DOC = 2.6 mg·L-1, SUVA254 = 2.4 L/mg·m) using a two-column array of XAD-8 and XAD-4 resins. The three majorisolated fractions of DOM, which accounted for 57% ofthe bulk DOM, were characterized using a variety of analyticaltools. Proton and copper binding properties were studiedfor each fraction. The main objective of this work wasto compare the structural and chemical characteristics ofthe isolated fractions and test models describing DOMreactivity toward metal ions. The characterization workshowed significant structural differences between the threeisolated fractions of DOM. The hydrophobic acid fraction(i.e., humic substances isolated from the XAD-8 resin) gavethe largest C/H, C/O, and C/N ratios and aromatic carboncontent among the three isolated fractions. The transphilicacid (TPHA) fraction (“transphilic” meaning fraction ofintermediate polarity isolated from the XAD-4 resin) wasfound to incorporate the highest proportion of polysaccharides,whereas the transphilic neutral (TPHN) fraction wasalmost entirely proteinaceous. The gradual increase ofthe charge with pH for the three DOM fractions is mostlikely caused by a large distribution of proton affinity constantsfor the carboxylic groups, as well as a second type ofgroup more generally considered to be phenolic. In the caseof the DOM fraction enriched in proteinaceous material(i.e., TPHN fraction), the results showed that the aminogroups are responsible for the charge reversal. For lowcopper concentrations, nitrogen-containing functional groupssimilar to those of amino acids are likely to be involvedin complexation, in agreement with previously publisheddata.
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