COMPARISON OF SURFACE COMPLEXATION MODELING APPROACHES TO DESCRIBE CR(VI) ADSORPTION on γ-ALUMINA IN THE PRESENCE AND ABSENCE OF CO2

Cr(VI) is a common industrial accessory element, with usage ranging from the production of stainless steel to leather tanning. Cr(VI) is known to be a genotoxic carcinogen, and has been classified as a priority contaminant by the DOE. Under oxidizing conditions, chromate (CrO₄^-2) and hydrogen chromate (HCrO₄^-) are the dominant aqueous Cr species. Under some conditions, these anions sorb significantly to solids, necessitating the development of surface complexation models to better understand the threat posed by Cr(VI) contaminant plumes and to accurately predict Cr(VI) movement in the environment. In this study, chromate adsorption was measured as a funtion of pH, ionic strength (0.001 to 0.1 M), sorbate/sorbent ratio (10^-4 to 10^-5 M Cr with 5 g/L solid), and pCO₂ concentrations (0, atmospheric, and 2.5%). γ-alumina was chosen as a sorbent because it has a high zero point of charge. Furthermore, because it contains both tetrahedrally and octahedrally-coordinated Al, it is a useful aluminosilicate analog. Adsorption edges were measured by titrating 500 mL batch experiments from low to high pH, removing 10 mL aliquots at ~0.5 pH intervals. Aliquots were equilibrated for a further 4 hrs, centrifuged, and the pH of the supernatant remeasured. Chromate sorbed was calculated by measuring supernatant concentrations colorimetrically with matrix-matched standards, and was observed to decrease with increasing ionic strength, pH and pCO₂. A diffuse layer surface model (DLM), a triple layer model (TLM), and a constant capacitance model (CCM) are used with FITEQL to extract stability constants for chromate and carbonate sorption, testing a variety of reaction stoichiometries. Goodness-of-fit (V(Y)) are assessed for best fit and median stability constants. The models with the lowest V(Y) from each model type are compared to identify the most robust model.