Electrochemical Analysis of the Heavy Metal/Humic Acid Interaction

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Abstract
Heavy metal ion titrations of solutions of humic acid (HA) and polyacrylic acid (PAA) at different degrees of neutralization have been studied by normal pulse polarography and conductometry. The systems Cd,Pb,Zn/HA and Cd,Zn/PAA appeared to be polarographically labile. As the diffusion coefficients of the polymeric metal complexes are very small, a mean diffusion coefficient of the reducible heavy metal ions controls the polarographic current, and is to be used to calculate binding constants of the complex formation. The apparent binding constants for the Cd,Pb,Zn/HA systems are strongly dependent on the degree of neutralization. From comparison with corresponding results for the heavy metal binding to the synthetic polycarboxylic acid, it is concluded that polyelectrolyte effects play an important role in the interaction of heavy metals with humic acid. This finding is supported by the conductometric results. Values of the intrinsic binding constants for Cd,Pb,Zn/HA are close to those for the binding with monocarboxylic acids.