Interaction of copper and fulvic acid at the hematite-water interface

The influence of surface-bound fulvic acid on the sorption of Cu(II) to col loidal hematite particles was studied experimentally and the results were c ompared with model calculations based on the linear additivity assumption. In the first step, proton and Cu binding to colloidal hematite particles an d to purified fulvic acid was studied by batch equilibration and ion-select ive electrode titration experiments, respectively. The sorption data for th ese binary systems were modeled with a basic Stem surface complexation mode l for hematite and the NICA-Donnan model for fulvic: acid. In the second st ep, pH-dependent sorption of Cu and fulvic acid in ternary systems containi ng Cu, hematite, and fulvic acid in NaNO3 electrolyte solutions was investi gated in batch sorption experiments. Sorption of fulvic acid to the hematit e decreased with increasing pH (pH 3-10) and decreasing ionic strength (0.0 1-0.1 M NaNO3), while the presence of 22 muM Cu had a small effect on fulvi c acid sorption, only detectable at low ionic strength (0.01 M). Sorption o f Cu to the solid phase separated by centrifugation was strongly affected b y the presence of fulvic acid. Below pH 6, sorption of Cu to the solid phas e increased by up to 40% compared with the pure hematite. Above pH 6, the p resence of fulvic acid resulted in a decrease in Cu sorption due to increas ing concentrations of dissolved metal-organic complexes. At low ionic stren gth (0.01 M), the effects of fulvic acid on Cu sorption to the solid phase were more pronounced than at higher ionic strength (0.1 M). Comparison of t he experimental data with model calculations shows that Cu sorption in tern ary hematite-fulvic acid systems is systematically underestimated by up to 30% using the linear additivity assumption. Therefore, specific interaction s between organic matter and trace metal cations at mineral surfaces must b e taken into account when applying surface complexation models to soils or sediments which contain oxides and natural organic matter. Copyright (C) 20 01 Elsevier Science Ltd.