Effects of tungstate polymerization on tungsten(VI) adsorption on ferrihydrite

- Tungsten(VI) polyoxoanions adsorb to ferrihydrite less than the monomeric tungstates.
- Due to slow depolymerization, W(VI) equilibrium adsorption can take weeks.
- Fundamental data are presented to explain the unexpectedly high W solubility.
- Continued efforts are needed to comprehend the environmental fate of W.

Tungsten (W) is expected to adsorb to soil minerals, but does not appear to do so in many environments. To explain this behavior, adsorption experiments were performed over a variety of W(VI) solution compositions with different extents of W polymerization, on ferrihydrite. Tungsten adsorption was more extensive at circumneutral pHs and in systems with lower W(VI) concentrations, conditions under which tungstate rather than polytungstate was stable or the transformation to polytungstates was inhibited. Polytungstates are not as particle-reactive as tungstate monomers. Silicate and phosphate suppressed W(VI) adsorption. At high pHs, their impact resulted from competitive adsorption; at pH 4-7, suppressed adsorption was attributed to incorporation of tungstate into polyoxometalates (POMs). Systems containing phosphotungstate (a model POM) exhibited limited adsorption initially, but adsorption increased over time as phosphotungstate depolymerized. The structure of adsorbed W(VI) was examined using X-ray absorption spectroscopy, which suggested that tungstates often represented the bulk of adsorbed W(VI), even when POMs were the primary species in solution. Many polytungstates and POMs are metastable; the slow transformations between POMs and tungstate could affect adsorption over extended periods. Adsorption kinetic data confirmed that W(VI) adsorption could take more than 2 months to reach equilibrium. The results suggest that tungstate polymerization can significantly decrease W adsorption, and thereby potentially mobilize it in the environment.