Advances in Surface Complexation Modeling for Chromium Adsorption on Iron Oxide

Heavy metals presence in soil and groundwater is of utmost importance in terms of risk assessment as their toxicity affects natural systems. The ability of predicting the fate and transport of heavy metals in the environment relies on the efficient modeling of sorption. A major challenge of geochemical modeling is to predict sorption as a function of varying environmental conditions, which can be achieved using surface complexation models (SCM). Unlike empirical distribution factors, SCMs provide a more robust description of sorption reactions on mineral surfaces and can predict sorption as a function of pH, concentration and competing ions. Hexavalent chromium (Cr(VI)) or chromate is a common contaminant and its mobility and reactivity is affected by sorption on iron oxides. To elucidate the chromate sorption on ferrihydrite under various environmental conditions, it is important to construct a predictive model that can describe the adsorption behavior of chromate quantitatively taking into account the sorption mechanisms. This study presents a new 1-pK triple layer SCM based on the surface structure model put forth by Hiemstra (2013) and utilizing multiple surface species (monodentate and bidentate) for chromate sorption. Literature and experimental (batch isotherms and pH edge) data are employed for the calibration and validation of the SCM. Two fittings were performed, one using the literature and one using the experimental data. The model provided a very good fit for literature data with an initial concentration of 5 μM Cr(VI), especially at the low ferrihydrite suspension concentration, while it slightly underestimated adsorption at high ferrihydrite concentration and only in the pH range 7.5-8.5. Modeling of a pH edge obtained with 1 mM Cr(VI) initial concentration led to similar log Ks, however the fit was poorer above pH 6.5. The sensitivity analysis for the choice of surface species showed that this was due to the inclusion of the monodentate species, while the model with only bidentate contribution produced a much improved fit of the pH edge. Similarly, the isotherm at pH 4 is described better by the bidentate model, however the isotherm at pH 5.7 is closer to the monodentate model. Thus, the combined model accounting for both complexes is considered a better choice to capture a wider range of pH and chromate concentrations. Further spectroscopic investigation for chromate adsorption on ferrihydrite is needed to establish a more realistic model to predict adsorption.