SURFACE COMPLEXATION OF ANTIMONY ON KAOLINITE

Antimony, a contaminant of emerging concern, belongs to the same group in the periodic table as arsenic (As). Although As has been extensively studied, geochemical fate of antimony - a similar oxyanion - in a variety of environments is largely unexplored. Kaolinite is an important, naturally occurring clay mineral in soils and aquifers that is known to control the fate of several contaminants via a multitude of geochemical processes, primarily adsorption. Here we report adsorption of antimony on kaolinite as a function of solution chemistry: initial antimony concentration, pH, ionic strength, and competing anions. A surface complexation modeling (SCM) approach was undertaken to understand potential mechanistic implications of sorption envelop data. In the SCM, a multicomponent additive approach, in which kaolinite is assumed to be a (1:1) mixture of quartz (=SiOH) and gibbsite (=AlOH), was tested. A diffuse layer model with least number of fitting parameters was used. Results indicated that ionic strength has minimal effect on antimony adsorption. For the lower initial antimony concentration (4.11 µM), the additive model with binuclear surface complexes on quartz and gibbsite showed a better fit at pH < 6, but somewhat under predicted the experimental data above pH 6. Fit of SCM to kaolinite adsorption envelop assuming the gibbsite surface forming binuclear surface complexes with antimony was better at pH > 6.5. At the higher initial antimony concentration (41.1 µM), the sorption envelop was of different shape than the lower load. Additive model, which considered binuclear surface complexes for quartz and gibbsite, resulted in over prediction of the adsorption data at pH > 3.5. However, additive model with binuclear surface complex on quartz and mononuclear surface complex on gibbsite showed excellent fit of data. Phosphate greatly influenced antimony adsorption on kaolinite at both low and high antimony loadings, indicating competition for available surface sites