RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Complex systems, simulating natural conditions like in groundwater, have rarely been studied, since measuring and in particular, modeling of such systems is very challenging. In this paper, the adsorption of the oxyanions of As(III) and As(V) on goethite has been studied in presence of various inorganic macro-elements (Mg²⁺, Ca²⁺, PO₄³⁻, CO₃²⁻). We have used ‘single-,’ ‘dual-,’ and ‘triple-ion’ systems. The presence of Ca²⁺ and Mg²⁺ has no significant effect on As(III) oxyanion (arsenite) adsorption in the pH range relevant for natural groundwater (pH 5–9). In contrast, both Ca²⁺ and Mg²⁺ promote the adsorption of PO₄³⁻. A similar (electrostatic) effect is expected for the Ca²⁺ and Mg²⁺ interaction with As(V) oxyanions (arsenate). Phosphate is a major competitor for arsenate as well as arsenite. Although carbonate may act as competitor for both types of As oxyanions, the presence of significant concentrations of phosphate makes the interaction of (bi)carbonate insignificant. The data have been modeled with the charge distribution (CD) model in combination with the extended Stern model option. In the modeling, independently calculated CD values were used for the oxyanions. The CD values for these complexes have been obtained from a bond valence interpretation of MO/DFT (molecular orbital/density functional theory) optimized geometries. The affinity constants (log K) have been found by calibrating the model on data from ‘single-ion’ systems. The parameters are used to predict the ion adsorption behavior in the multi-component systems. The thus calibrated model is able to predict successfully the ion concentrations in the mixed 2- and 3-component systems as a function of pH and loading. From a practical perspective, data as well as calculations show the dominance of phosphate in regulating the As concentrations. Arsenite (As(OH)₃) is often less strongly bound than arsenate (AsO₄³⁻) but arsenite responses less strongly to changes in the phosphate concentration compared to arsenate, i.e., δlogc_As(III)/δlogc_PO4 ≈ 0.4 and δlogc_As(V)/δlogc_PO4 ≈ 0.9 at pH 7. Therefore, the response of As in a sediment on a change in redox conditions will be variable and will depend on the phosphate concentration level.