A predictive model (ETLM) for arsenate adsorption and surface speciation on oxides consistent with spectroscopic and theoretical molecular evidence

Abstract

The nature of adsorbed arsenate species for a wide range of minerals and environmental conditions is fundamental to prediction of the migration and long-term fate of arsenate in natural environments. Spectroscopic experiments and theoretical calculations have demonstrated the potential importance of a variety of arsenate surface species on several iron and aluminum oxides. However, integration of the results of these studies with surface complexation models and extrapolation over wide ranges of conditions and for many oxides remains a challenge. In the present study, in situ X-ray and infrared spectroscopic and theoretical molecular evidence of arsenate (and the analogous phosphate) surface speciation are integrated with an extended triple layer model (ETLM) of surface complexation, which takes into account the electrostatic work associated with the ions and the water dipoles involved in inner-sphere surface complexation by the ligand exchange mechanism. Three reactions forming inner-sphere arsenate surface species 2 > SOH + H 3 AsO 4 0 = ( > SO ) 2 AsO 2 - + H + + 2 H 2 O 2 > SOH + H 3 AsO 4 0 = ( > SO ) 2 AsOOH + 2 H 2 O and > SOH + H 3 AsO 4 0 = > SO AsO 2 2 - + 2 H + + H 2 O were found to be consistent with adsorption envelope, adsorption isotherm, proton surface titration and proton coadsorption of arsenate on hydrous ferric oxide (HFO), ferrihydrite, four goethites, amorphous aluminum oxide, α-Al 2O 3, β-Al(OH) 3, and α-Al(OH) 3 up to surface coverages of about 2.5 μmol m −2. At higher surface coverages, adsorption is not the predominant mode of arsenate sorption. The four goethites showed a spectrum of model arsenate surface speciation behavior from predominantly binuclear to mononuclear. Goethite in the middle of this spectrum of behavior (selected as a model goethite) showed predicted changes in arsenate surface speciation with changes in pH, ionic strength and surface coverage very closely consistent with qualitative trends inferred in published in situ X-ray and infrared spectroscopic studies of arsenate and phosphate on several additional goethites. The model speciation results for arsenate on HFO, α- and β-Al(OH) 3 were also consistent with X-ray and molecular evidence. The equilibrium constants for arsenate adsorption expressed in terms of site-occupancy standard states show systematic differences for different solids, including the model goethite. The differences can be explained with the aid of Born solvation theory, which enables the development of a set of predictive equations for arsenate adsorption equilibrium constants on all oxides. The predictive equations indicate that the Born solvation effect for mononuclear species is much stronger than for binuclear species. This favors the development of the mononuclear species on iron oxides such as HFO with high values of the dielectric constant relative to aluminum oxides such as gibbsite with much lower dielectric constants. However, on hematite and corundum, with similar dielectric constants, the predicted surface speciations of arsenate are similar: at lower pH values and/or higher surface coverages, binuclear species are predicted to predominate, at higher pH values and/or lower surface coverages, mononuclear species predominate.