Adsorption of inorganic contaminants in ponded effluents from coal-fired power plants

Abstract

The objectives of this study were [1] to conduct some experimental tests of the surface precipitation adsorption model, and [2] to work on the development of a simple yet widely applicable approach to modelling adsorption of inorganic ions on oxide surfaces. The latter represents the first portion of our continuing effort to develop a data base for adsorption of inorganic contaminants in oxide suspensions.

An investigation of the kinetics of cadmium adsorption on hydrous ferric oxide at different initial adsorbate/adsorbent ratios was conducted as a partial test of the surface precipitation model. Kibetics of cadmium adsorption were observed to slow considerably as the adsorbate/adsorbent ratio was increased. The results confirm our hypothesis that adsorption kinetics should decrease as the adsorbate/adsorbent ratio is increased because of the shift from surface complexation to surface precipitation as the dominant adsorption mechanism.

A number of constant pH equilibrium adsorption experiments with cadmium and hydrous ferric oxide were conducted in order to verify the isotherm predicted by the surface precipitation model. A recently published, extensive isotherm for zinc adsorption on hydrous ferric oxide was also examined. This investigation revealed that cation adsorption isotherms exhibit adsorptive saturation at high adsorbate concentrations and that the smooth transition from adsorption to precipitation predicted by the surface precipitation model occurs above this saturation. To model these data, a two site-type model with surface precipitation on weak binding sites is needed.

As the first step in our effort to develop a data base for adsorption of inorganics, we reviewed available adsorption data and surface complexation models and identified a modelling approach capable of describing all existing data. The model that we propose is a two site surface complexation model with surface precipitation on weak binding sites, combined with the diffuse layer model for electrostatic corrections. For proton and anion binding, two site-types and surface precipitation will usually not be necessary - these refinements are included for accurate description of cation binding. The basic diffuse layer surface complexation model is thus the nucleus of the proposed universal model.