First-principles simulation of arsenate adsorption on the (1 1¯ 2) surface of hematite

Recent experimental studies revealed an unprecedented bimodal distribution of arsenate at the hematite (1 1¯ 2) surface with a simultaneous adsorption of inner-sphere and outer-sphere complexes. In the present study, first-principles calculations based on density-functional theory were performed to provide detailed insights into the structural and electronic properties of such inner-sphere and outer-sphere adsorption complexes on two hydroxylated terminations of the hematite (1 1¯ 2) surface. For bidentate corner-sharing complexes, the predicted most stable adsorption configurations display interatomic distances in good agreement with EXAFS-derived data (i.e. As-Fe distances of ∼3.3 Å). Our calculations also suggest that edge-sharing bidentate complexes can form on ideal (1 1¯ 2) hematite surfaces and do not necessarily involve step edges. These edge-sharing complexes would display two As-Fe distances at about 2.85 and 3.45 Å, instead of the unique short As-Fe contribution that is usually considered for interpreting EXAFS data. For outer-sphere complexes, the predicted most favorable adsorption configurations indicate that strong hydrogen bonds as well as electrostatic forces stabilize arsenate at the hematite surface. Although not investigated here, the interfacial water structure may also contribute to stabilize further OS arsenate complexes.