Formation of CoAl layered double hydroxide on the boehmite surface and its role in tungstate sorption

Sorption of tungstate on boehmite (γ-AlOOH) is increased by co-sorption with Co²⁺ over the near-neutral pH range. Batch uptake experiments show up to a 3-fold increase in tungstate uptake over the range WO₄^2-=50-1000μmol/L compared to boehmite not treated with Co²⁺. Desorption experiments reveal a corresponding decrease in sorption reversibility for tungstate co-sorbed with Co²⁺. Reaction of boehmite with Co²⁺ results in the formation of CoAl layered double hydroxide (LDH), as confirmed by X-ray diffraction and X-ray absorption spectroscopy. Tungsten L₃-edge X-ray absorption near edge structure (XANES) reveals that W(VI) is octahedrally coordinated in all sorption samples, with polymeric tungstate species forming at higher tungstate concentrations. X-ray diffraction and X-ray absorption spectroscopy indicate that the mechanism for enhancement of tungstate uptake is the formation of surface complexes on boehmite at low tungstate concentrations, while exchange into the CoAl LDH becomes important at higher tungstate concentrations. The results provide a basis for developing strategies to enhance tungstate sorption and to limit its environmental mobility at near-neutral pH conditions.