Effects of Phosphate Competition on Arsenate Binding to Aluminum Hydroxide Surfaces

Aluminum hydroxide plays a substantial role in regulating the environmental fate and transport of arsenate and phosphate. These minerals display multiple types of surface reactive sites with distinct adsorption affinities. As arsenate and phosphate coexist in many natural systems, a detailed understanding of their interactive adsorption behaviors on aluminum hydroxides and the underlying binding mechanisms is thus required to develop predictive models of their fate and mobility. In this study, we explore how phosphate affects arsenate adsorption onto the aluminum hydroxide polymorphs gibbsite and bayerite and develop a unified surface complexation model (SCM) to predict arsenate adsorption under diverse conditions parameterized only from noncompetitive uptake data. Macroscopic studies show that both oxyanions behave similarly in individual adsorption experiments. The addition of phosphate reduces arsenate adsorption on both minerals with a greater effect observed on gibbsite. Extended X-ray absorption fine structure spectroscopy reveals that similar arsenate species occur on both minerals and are unchanged in the presence of phosphate. This indicates that competitive adsorption does not affect arsenate surface speciation. These results demonstrate that arsenate and phosphate primarily interact on aluminum hydroxide surfaces via site competition. Notably, their similar pKa values and adsorption affinities minimize modification of adsorption behavior from surface electrostatic effects. The SCMs demonstrate that multiple arsenate surface species are needed to reproduce the observed competitive uptake data, in apparent conflict with the spectroscopic results that reveal no change in coordination at different surface coverages. This indicates that the molecular-scale drivers of differences in surface complex stability are not only from distinct local coordination, instead other factors, such as hydrogen bonding among the adsorbates, surface oxygens, and interfacial water or different surface complexes coordinating to functional groups with distinct connections to the underlying mineral structure, may also play an important role.