Modeling Lead Adsorption on Clays by Models with and without Electrostatic Terms

Abstract

The research describes the adsorption of heavy metals (lead) on clay minerals (kaolinite) as function of different experimental conditions. The lead adsorption was investigated as function of pH and ionic strength at several initial lead concentrations. The adsorption of proton and sodium (obtained by titration curves and negative surface charge measurements, respectively) was also investigated as function of pH. All experimental variables were simultaneously accounted for by using the multivariate nonlinear regression of data, according to different models including or not including electrostatic interactions. The adsorption of lead, proton, and sodium on the clay was successfully described by a three-layer electrostatic model considering two discrete types of adsorption sites (with fixed and variable charge, respectively) and the additional effect of the heterogeneity of each site (in terms of continuous distribution of the affinity constants). Confident estimates were obtained for the site concentrations (adsorption maxima) and the heterogeneity parameters. The sites with fixed and variable charge were found to be 40 and 60% of the total, respectively. Both of them contribute to the lead adsorption in proportions which highly depend on the pH. The heterogeneity parameters of both sites have low values which underline the high influence of the continuous distribution of the affinity constants. The diffuse layer potential was also well estimated. It depends mainly on the ionic strength and it is quite different from the measured zeta potential (about one order of magnitude more negative). To consider simultaneously all experimental variables by a multivariate approach appeared to be very important for an accurate modeling. If only the lead adsorption was taken into account at a single ionic strength, the electrostatic effects could be neglected with no practical difference on the calculated behavior.