Differential Pair Distribution Function Study of the Structure of Arsenate Adsorbed on Nanocrystalline γ-Alumina

Structural information is important for understanding surface adsorption mechanisms of contaminants on metal (hydr)oxides. In this work, a novel technique was employed to study the interfacial structure of arsenate oxyanions adsorbed on gamma-alumina nanoparticles, namely, differential pair distribution function (d-PDF) analysis of synchrotron X-ray total scattering. The d-PDF is the difference of properly normalized PDFs obtained for samples with and without arsenate adsorbed, otherwise identically prepared. The real space pattern contains information on atomic pair correlations between adsorbed arsenate and the atoms on gamma-alumina surface (Al, O, etc.). PDF results on the arsenate adsorption sample on gamma-alumina prepared at 1 mM As concentration and pH 5 revealed two peaks at 1.66 angstrom and 3.09 angstrom, corresponding to As-O and As-Al atomic pair correlations. This observation is consistent with those measured by extended X-ray absorption fine structure (EXAFS) spectroscopy, which suggests a first shell of As-O at 1.69 +/- 0.01 angstrom with a coordination number of similar to 4 and a second shell of As-Al at similar to 3.13 +/- 0.04 angstrom with a coordination number of similar to 2. These results are in agreement with a bidentate binuclear coordination environment to the octahedral Al of gamma-alumina as predicted by density functional theory (DFT) calculation.