Analysis of the Cu2+-soil fulvic acid complexation by anodic stripping voltammetry using an electrostatic model

An electrostatic model has been used in the analysis of the fulvic acid-copper ion complexation with the aim of estimating the contribution of the electrostatic effect to metal binding and calculating the intrinsic complexation parameters in solutions with a dissolved organic carbon (DOC) analogous to that in natural media such as aqueous environments and soil solution. For this purpose solutions of a soil extracted fulvic acid (FA) at concentrations 30, 40, 50, and 60 mg L-1 and ionic strengths 0.001, 0.005, 0.01, 0.05, and 0.1 were titrated at pH 6.5 with copper ion by using differential pulse anodic stripping voltammetry. By means of a 1:1 nonelectrostatic model conditional complexation parameters (apparent stability constant and complexation capacity) were calculated, and their dependence on the FA concentration and ionic strength was demonstrated. Consideration of a second binding site and assumption of bidentate binding sites do not significantly affect the results. The application of a 1:1 electrostatic model with the geometric parameters derived from a previous study of the proton binding reaction of the fulvic acid allowed us for the obtention of the intrinsic stability constants which do not exhibit any dependence on the FA concentration nor on the ionic strength. Comparison of the apparent and intrinsic stability constants shows a significant contribution of the electrostatic effect on the copper complexation.