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.
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