A New Surface Complexation Model for Chromate Adsorption on Ferrihydrite

Surface complexation models (SCMs) provide a mechanistic and thermodynamicallybased description of sorption reactions on mineral surfaces and can thus be used in geochemical modelling to predict contaminant sorption under a variety of geochemical conditions. In the last years, substantial effort has been put into obtaining multiple lines of evidence for the types of complexes formed on mineral surfaces and refining reactions and assumptions in SCMs using such information. A large number of studies have been performed to investigate the sorption behaviour of oxyanions (carbonate, sulphate, phosphate, arsenate and others) on iron oxides such as goethite, hematite and ferrihydrite. Surprisingly, there are very few studies conducted for chromate (CrO4), despite its prevalence as a soil and groundwater contaminant. Only two SCMS for chromate sorption on ferrihydrite were identified, dating back to the late 80s and early 90s, when no insight into mechanisms was available. A recent study that utilized spectroscopy and computational chemistry observed that chromate forms inner-sphere monodentate and bidentate complexes, the relative distribution of which depends on solution pH, surface loading and ionic strength. Accordingly, this study presents a new SCM that utilized the observed mechanisms for formulation and calibration of the sorption reactions. The Charge-Distribution Multisite Complexation (CD MUSIC) modelling framework was adopted and a recent analysis for the surface structure of ferrihydrite was considered. Specifically, only one type of singlycoordinated oxygens are thought capable of forming inner-sphere complexes with chromate, while both singlyand triply-coordinated oxygens can interact with electrolyte ions (Na, Cl, NO3). Additionally, the influence of carbonate was considered, assuming an inner-sphere bidentate complex based on previous studies. The results showed that substantial improvement in the fit between model and experimental data was obtained using the new formulation compared to an older study that only considered outer-sphere complex formation for chromate. The fit was very good for a wide range of chromate concentrations (5 μΜ to 1 mM), pH (5-10) and ferrihydrite concentrations (0.077 to 2 g/L). Modeling the effect of carbonate proved more challenging. The model under-predicted carbonate sorption at pH<6 and also under-predicted chromate sorption in the presence of carbonate at pH<7. It is considered that the two phenomena are related and that an improvement in the carbonate sorption model is necessary to improve modeling of chromate sorption on ferrihydrite in the presence of carbonate. Additional data on carbonate sorption of ferrihydrite are required to this end.