Structural evolution of Cr(III) polymeric species at the γ-Al2O3/water interface

We have characterized the structure of the predominant Cr(III) species present at the gamma -Al2O3/water interface as a function of equilibration time and Cr(III) surface loading using X-ray absorption fine structure (XAFS) spectroscopy. The spectroscopic measurements span two distinct time periods of Cr(III) uptake. During the initial period (<2 h), which is characterized by rapid uptake, Cr(III) monomers form inner-sphere complexes on the <gamma>-Al2O3 surface by bonding to at least two surface functional groups. During the second period (ranging from 2 h to 1 week), significant quantities of Cr(III) continue to be removed from solution, but sorption proceeds at a greatly reduced rate. The XAFS spectra collected during the period of slow uptake show an increase in scattering contributions from neighboring Cr(III) atoms with increasing equilibration time. The inferred structural changes are consistent with a progression from hydroxo-bridged Cr(III) dimers to higher-order polymers. In combination with spectroscopic evidence, which shows that monomeric Cr(III) species are the only significant reactants in solution, the observed evolution of Cr(III) surface species suggests that chemical bonding between adsorbed Cr(III) ions and gamma -Al2O3 surface functional groups enhances Cr(III) polymerization. The proposed reaction sequence has important implications for surface complexation modeling used to predict Cr(III) partitioning by hydroxide sorbents in wastewater treatment and contaminant migration scenarios.