Surface complexation modeling of Cr(VI) adsorption at the goethite-water interface.

In this study, a charge distribution multisite surface complexation model (CD-MUSIC) for adsorption of chromate onto goethite was carefully developed. The adsorption of Cr(VI) on goethite was firstly investigated as a function of pH, ionic strength and Cr(VI) concentration. Results showed that an inner-sphere complexation mechanism was involved because the retention of Cr(VI) was little influenced by ionic strength. Then two surface species: a bidentate complex (≡Fe2O2CrOOH) and a monodentate complex (≡FeOCrO3(-3/2)), which is constrained by prior spectroscopic evidence were proposed to fit the macroscopic adsorption data. Modeling results showed that the bidentate complex was found to be the dominant species at low pH, whereas, with increasing pH, monodentate species became more pronounced. The model was then verified by prediction of competitive adsorption of chromate and phosphate at various ratios and ionic strengths. The model successfully predicted the inhibition of chromate with the presence of phosphate, suggesting phosphate has higher affinity to goethite surface than Cr(VI). Results showed that the model developed in this study for Cr(VI) onto goethite was applicable for various conditions. It is a useful supplement for the surface complexation model database for oxyanions onto goethite surfaces.