Sorption of Cu²⁺ and Zn²⁺ at Na-montmorillonite: Analysis by means of affinity spectra

Abstract

The sorption isotherms of Cu²⁺ and Zn²⁺ at Na-montmorillonite are determined electrochemically by the combination of DPASV and DPP and analyzed for the first time by affinity spectra. To obtain these distribution functions an inverse integral transformation is performed numerically by the constrained regularization technique CONTIN with the integral adsorption equation, employing a smoothing regularizer.

Affinity spectra are obtained with a main process (>94%) characterized by a mean pseud-Langmuir sorption coefficient of log K=4.23 and 4.08 for the binding of Zn²⁺ and Cu²⁺, respectively. The exchange coefficient for Zn²⁺ is calculated as log K _ex=−0.14. The affinity spectrum for Cu²⁺ is broader than for Zn²⁺, and expresses a considerable tailing to the low affinity range. This indicates a greater heterogeneity, probably caused by precipitation of copper hydroxides at higher concentrations of Cu²⁺. Additionally, pH-dependent investigations are presented, indicating no pH-dependent binding sites for Cu²⁺ and Zn²⁺ at montmorillonite.