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

Abstract

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 persistent 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 kinetics 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.

Sorption of Molybdates and Tungstates on Functionalized Montmorillonites: Structural and Textural Features

“…Literature data reveal, that removal of tungstate using popular sorbents such as gibbsite, goethite or birnessite was significantly lower than the results obtained in this study (201, 174 and 24 mmol/kg, respectively) [4,7]. Previous studies also show, that for some sorbents, such as ferrihydrite, the sorption capacities obtained by different researchers differ significantly, form 120 mmol/kg to 420 mmol/kg [7,48]. It was observed that popular sorbents are also less effective than organically modified montmorillonite.…”

Influence of phosphate on tungstate sorption on hematite: A macroscopic and spectroscopic evaluation of the mechanism

“…One possible fate of added W (VI) is sorption by soil minerals. Consequently, many studies evaluated the sorption mechanisms of W (VI) on various soil minerals (Gustafsson, 2003;Xu et al, 2006Xu et al, , 2009Lorenz, 2009;Lorenz et al, 2011;Kashiwabara et al, 2013;Davantes and Lefevre, 2015;Sun and Bostick, 2015;Hur and Reeder, 2016;Rakshit et al, 2017). In general, W (VI) displays a high affinity for iron-and aluminum hydr (oxide) minerals (Gustafsson, 2003;Xu et al, 2009;Hur and Reeder, 2016).…”

Tungsten speciation and its geochemical behavior in geothermal water: A review

“…Kashiwabara et al [20] found that W shows a much larger adsorption on ferrihydrite than Mo. Sun and Bostick [21] examined that aqueous W speciation is of critical importance for regulating W adsorption on ferrihydrite. It is suggested that polyoxytungstates and heteropolytungstates adsorb to a lesser degree than tungstate onto ferrihydrite and presumably other minerals.…”