Application of the Long-Period X-ray Standing Wave Technique to the Analysis of Surface Reactivity:  Pb(II) Sorption at α-Al₂O₃/Aqueous Solution Interfaces in the Presence and Absence of Se(VI)

A long-period X-ray standing wave (XSW) study of the reactivity of mineral surfaces in contact with metal(loid)-bearing aqueous solutions is presented. Experimental XSW fluorescence yield data for Pb(II) and Pb(II)/Se(VI) adsorption on α-Al 2 O 3 (0001) and -(11̄02) surfaces under in situ conditions are described, and the details of the data modeling approach are presented. XSW results for Pb(II) uptake (at [Pb] = 6−600 μM) on the (11̄02) and (0001) surfaces of α-Al 2 O 3 in the absence of Se(VI) confirm that the (11̄02) surface has an overall higher affinity for Pb(II) uptake than the (0001) surface at pH 4.5. The presence of Se(VI) at 600 μM appears to result in a significant diffuse-layer accumulation of Pb, likely because of a large negative surface charge on the α-Al 2 O 3 (11̄02) surface, but to exert only a small influence at the (0001) surface. The observed reactivity differences are discussed in terms of the surface structures of the hydrated α-Al 2 O 3 (0001) and -(11̄02) substrates.