A consistent model for surface complexation on birnessite (-MnO2) and its application to a column experiment

Available surface complexation models for birnessite required the inclusion of bidentate bonds or the adsorption of cation-hydroxy complexes to accoun t for experimentally observed H+/Mm+ exchange. These models contain inconsi stencies and therefore the surface complexation on birnessite was re-examin ed. Structural data on birnessite indicate that sorption sites are located on three oxygens around a vacancy in the octahedral layer. The three oxygen s together carry a charge of -2, i.e., constitute a doubly charged sorption site. Therefore a new surface complexation model was formulated using a do ubly charged, diprotic, sorption site where divalent cations adsorbing via inner-sphere complexes bind to the three oxygens. Using the diprotic site c oncept we have remodeled the experimental data for sorption on birnessite b y Murray (1975) using the surface complexation model of Dzombak and Morel ( 1990). Intrinsic constants for the surface complexation model were obtained with the non-linear optimization program PEST in combination with a modifi ed version of PHREEQC (Parkhurst, 1995). The optimized model was subsequent ly tested against independent data sets for synthetic birnessite by Balistr ieri and Murray (1982) and Wang et al. (1996). It was found to describe the experimental data well. Finally the model was tested against the results o f column experiments where cations adsorbed on natural MnO2 coated sand. In this case as well, the diprotic surface complexation model gave an excelle nt description of the experimental results. Copyright (C) 1999 Elsevier Sci ence Ltd.