Fractionation of dissolved organic matter affected by polyvalent metal cations

The purpose of the study was to evaluate the influence of polyvalent cations known to form complexes with natural organic substances on the operational fractionation of dissolved organic matter (DOM) using XAD-I adsorber resin. Dissolved organic matter solutions from a forest floor were treated with increasing concentrations of polyvalent metal cations (Ca2+, Al3+, Fe3+) at different pH levels. Then the concentrations of total dissolved organic carbon (DOC) and the distribution between hydrophilic and hydrophobic DOC were determined. The concentrations of total DOC decreased slightly when the C/metal ratio was less than 10, especially for Al and Fe. Hydrophilic DOC increased and hydrophobic DOC decreased with increasing concentrations of metal cations. Effects increased in the order Ca < Al < Fe and were more pronounced at low DOC concentrations and high pH values. The reason for the reduction of the DOC concentrations seemed to be the formation of insoluble metal-DOM complexes, while soluble metal-DOM complexes may induce an alteration of the distribution between hydrophilic and hydrophobic DOG. Thus, the polyvalent cations and their concentration need to be considered when DOM fraction distributions, determined with XAD-S resin, of different waters are compared, especially at low DOC contents and high pH. (C) 1998 Elsevier Science Ltd. All rights reserved.