Tungstate sorption mechanisms on boehmite: Systematic uptake studies and X-ray absorption spectroscopy analysis

Mechanisms of tungstate sorption on the mineral boehmite (γ-AlOOH) were studied using batch uptake experiments and X-ray absorption spectroscopy. Batch uptake experiments over the pH range 4-8 and [W]=50-2000μM show typical oxyanion behavior, and isotherm experiments reveal continued uptake with increasing tungstate concentration without any clear uptake maximum. Desorption experiments showed that sorption is irreversible at pH 4 and partly reversible at pH 8. Tungsten L₁- and L₃-edge XANES spectroscopy indicates that all sorbed tungstates are octahedrally coordinated, even though the dominant solution species at pH 8 is a tetrahedral monotungstate. Tungsten L₃-edge EXAFS analysis shows that sorbed tungstate occurs as polymeric form(s), as indicated by the presence of corner- and edge-sharing of distorted tungstate octahedra. The occurrence of polymeric tungstate on the surface at pH 8 indicates that sorption is accompanied by polymerization and a coordination change from tetrahedral (in solution) to distorted octahedral (on the surface). The strong tendency for tungstate polymerization on boehmite can explain the continued uptake without an apparent maximum in sorption, and the limited desorption behavior. Our results provide the basis for a predictive model of tungstate uptake by boehmite, which can be important for understanding tungstate mobility, toxicity, and bioavailability.