Proton interaction during phosphate adsorption onto goethite (alpha-FeOOH)

Abstract

Orthophosphate is a major pollutant in aqueous environments. Goethite (I?-FeOOH), a common iron oxide, has a strong capacity to adsorb phosphate. The adsorption reaction is often modeled by assuming a ligand exchange reaction with surface hydroxyl groups. However, there is some evidence that the surface reaction is more complex than a simple ligand exchange reaction. The objective of this study is to examine the possible binding forms of phosphate on goethite by evaluating the proton interaction and surface charge change during phosphate adsorption onto goethite. OHa?? release stoichiometry during the adsorption process increases with phosphate coverage, suggesting that different mechanisms may predominate at different phosphate loadings. One possible explanation is that surface binding changes from monodentate complexation to bidentate complexation with increasing surface phosphate coverage. The net OHa?? release accompanying this transformation could be best interpreted with a 2-pKa multisite model.