Cu 2+ -Soil Fulvic Acid Binding: Effect of pH and Concentration

We studied the effect of pH on the binding of Cu²⁺ to a soil fulvic acid in solutions containing dissolved organic carbon at concentrations within ranges typical of freshwaters and soil solutions. Solutions of soil extracted fulvic acid (FA) at concentrations of 15, 45, 55, and 80 mg L^-1, ionic strength 0.005 M, and pH values of 5.0, 5.5, 6.0, 6.5, and 7.0 were titrated with copper ion, using differential pulse anodic stripping voltammetry. The results showed that at all FA concentrations studied, cation binding increased with pH, whereas the effect of FA concentration was only observed at the lowest concentration. Use of a double layer model allowed us to attribute the effect of pH to the electrostatic interactions resulting from the increased negative charge generated by a higher degree of ionization of fulvic acid molecules.