A surface complexation model for sulfate and selenate on iron oxides consistent with spectroscopic and theoretical molecular evidence

Abstract

Sulfate and selenate adsorption on iron oxides are important reactions in natural systems under a very wide range of pH values, ionic strengths, and electrolyte compositions. Under such conditions, spectroscopic and theoretical calculations have demonstrated the potential importance of a variety of surface species. Understanding the variations in the surface speciation of these oxyanions is fundamental to prediction of their partitioning between minerals and aqueous solutions. In the present study, published experimental spectroscopic and theoretical molecular evidence of the identity of sulfate/selenate surface species are integrated with a surface complexation model consistent with a wide variety of experimental adsorption, surface titration, and proton coadsorption data to define the surface speciation of sulfate and selenate on iron oxides under a wide range of conditions. The analysis was carried out with the extended triple layer model (ETLM) taking into account the electrostatics of water dipole desorption during ligand exchange reactions. On seven out of eight goethites studied, sulfate and selenate surface reactions can be represented by the formation of a monodentate-mononuclear inner-sphere and a bidentate-binuclear outer-sphere (or H-bonded) species according to > FeOH + H + + AO 4 2 - = > FeOAO 3 - + H 2 O and 2 > FeOH + 2 H + + AO 4 2 - = ( > FeOH 2 + ) 2 _ AO 4 2 - respectively, where A stands for S or Se. The model predicted changes in the proportions of the species with pH, ionic strength and surface coverage consistent with independently derived experimental evidence from in situ Raman, ATR-FTIR and EXAFS studies. In contrast to goethite, the ETLM analysis of sulfate and selenate adsorption on hydrous ferric oxide (HFO) required an additional outer-sphere (or H-bonded) surface species represented by, > FeOH + 2 H + + AO 4 2 - = > FeOH 2 + _ HAO 4 - Equilibrium constants for sulfate and selenate adsorption based on site-occupancy standard states ( K θ ) for > FeOAO 3 - and ( > FeOH 2 + ) 2 _ AO 4 2 - on HFO are systematically higher than those on goethite, indicating that HFO has a greater affinity for sulfate and selenate than goethite.