Tungstate (VI) sorption on the hematite-water interface: Probing the mechanism using in situ spectroscopy coupled with macroscopic experiments

Tungsten (W) is a Group VI transition metal which possesses an array of unique properties suitable for a number of domestic and industrial applications, including hunting ammunition. Although W can enter the environment from anthropogenic sources, the dissolution of W-bearing rocks is also a source of environmental provenance. W has been declared a contaminant of emerging concern. This is primarily due to the suspected link between W and three childhood leukemia clusters in the western United States. Thus, it is increasingly important to understand the environmental chemistry of W and the mechanisms by which W sorbs to mineral surfaces. This work evaluates the W (VI) sorption mechanism on hematite, a common Fe (III) oxide mineral, as a function of various solution properties. In addition, the competitive effect of Group 15 oxyanion phosphate (P) on W (VI) retention was evaluated. A batch sorption study combined with in situ ATR-FTIR experiments were carried out with hematite and W (VI) under various solution properties. Results from macroscopic sorption experiments indicate that hematite has high affinity for W (VI). Variation in ionic strength had a negligible effect on W (VI) retention, thereby suggesting inner-sphere bonding. The results from ATR-FTIR experiments corroborate that W (VI) binds with hematite via inner-sphere bonding and that some of the surface complexes are polytungstate species at low pH values (pH 5.05 and pH 6). Results from macroscopic sorption envelope and in situ ATR-FTIR experiments indicate that W sorbs strongly at low pH (100% sorption through pH 7) and sorption decreases as pH rises above pH7 (approx. 50–66% sorption in pH 10 range, depending on ionic strength). Phosphorus (P) had a minimal effect on W (VI) sorption when P was added later. However, W (VI) sorption was weaker throughout the pH range when P was added first. ATR-FTIR analyses reveal that both P and W form inner-sphere bonds with hematite in a binary system. This study will help predict the fate of W (VI) in the environment