RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

This report contains new laboratory data and equilibrium constants for key reactions needed to predict the geochemical behavior of Cr in soils and groudwater. These new data include 1) the solubility of BaCrO₄(c) and Ba(S,Cr)O₄(c) solid solutions, 2) the Fe(II)-mediated reduction of Cr(VI) to Cr(III), and 3) Cr(VI) adsorption by kaolinite and Al-substituted goethite. The new mechanistic data along with the previously developed data have improved the understanding of reactions that limit Cr(III) and Cr(VI) mobility in soils and groundwater. Experiments show that both BaCrO₄(c) and Ba(S,Cr)O₄(c) solid solution form rapidly under the geochemical conditions expected in the environment. Solubility products were experimentally determined for BaCrO₄(c) and for Ba(S,Cr)O₄(c) at several mole fractions of Cr(VI). The Ba(S,Cr)O₄(c) solid solution is of special importance because it maintains lower aqueous Cr(VI) concentrations than BaCrO₄(c) and is likely to form and control Cr(VI) mobility in the utility environment, where Ba/sup 2/plus// levels are controlled by BaSO₄(c). The ferrous ion (Fe(II)) was found to be a rapid and effective reductant for Cr(VI) to Cr(III). This redox transformation is important in predicting migration of chromium species because the resulting Cr(III) concentrations in groundwater are kept low by the solubility of (Cr,Fe)(OH)₃. Chromium(VI) adsorption was measured on kaolinite and Al-substituted goethite. Both solids adsorb Cr(VI) only under acidic and near-neutral conditions. The observed Cr(VI) adsorption by both solids was described with a multiple-site surface complexation model and binding constants for single-phase oxides.