Structure and stability of arsenate adsorbed on α-Al2O3 single-crystal surfaces investigated using grazing-incidence EXAFS measurement and DFT calculation

Direct characterization of contaminants on single-crystal planes is required because the specific adsorption characteristics on different exposed crystal planes constitute their actual behavior at water–mineral interfaces in aquifers. Here, the structure and stability of arsenate on α-Al2O3 (0001) and ( 11 2 ¯ 0 ) surfaces were characterized by using a combination of grazing-incidence extended X-ray absorption fine structure (GI-EXAFS) spectra and periodic density functional theory (DFT) calculation. The combined results indicated that arsenate was mainly adsorbed as inner-sphere monodentate and bidentate complexes on both surfaces, but the orientational polar angles on the (0001) surface were commonly 10–20° greater than that on the ( 11 2 ¯ 0 ) surface. The DFT calculation showed that the large polar angle was more favorable for arsenate stabilized on the alumina surfaces. Based on the spectroscopic and computational data, the dominant bonding modes of arsenate on the two crystal planes of α-Al2O3 were identified as bidentate binuclear structures, and the (0001) surface displayed a stronger affinity toward arsenate.