A NEW APPROACH FOR MODELING THE REACTIONS OF OXYANIONS AT THE MINERAL-WATER INTERFACE

Major constituents of natural waters that can form adsorbed oxyanions, such as carbonate, sulfate, and silica compete for sites on mineral surfaces with other oxyanions of contaminant interest, such as selenite, selenate, arsenite, arsenate, chromate, nitrate and perchlorate. Furthermore, many of these oxyanions form two or more types of adsorbed surface species (e.g. inner- vs. outer-sphere), the proportions of which vary with environmental variables such as pH, ionic strength and surface loading. In order to describe these interactions quantitatively, it is essential to use a comprehensive surface complexation approach. The extended triple-layer model (ETLM) is able to do this with a new treatment of inner-sphere anion adsorption. When oxyanions adsorb by a ligand exchange mechanism, the release of one or more water dipoles coordinated to a metal at the surface involves electrostatic work which has previously been neglected in surface complexation models. Taking this effect into account enables the ETLM to closely fit oxyanion adsorption and surface protonation and proton coadsorption in the presence of oxyanions using inner- and outer-sphere species consistent with in situ spectroscopic and theoretical molecular evidence. Based on these results, the ETLM can then be used to make independent predictions of oxyanion surface speciation as a function of pH, ionic strength and surface loading. The predictions are in agreement with spectroscopic results. For arsenite and arsenate, the equilibrium constants for surface species vary in a systematic way from one oxide to another consistent with Born solvation theory which enables predictions for all oxides. Together with previous results for oxides in 1:1 and 2:1 electrolytes, the oxyanion results enable ETLM predictions of surface charge and surface chemical reactions for all the constituents of natural waters with contaminants present.