TUNGSTEN STABLE ISOTOPES AS A PROBE OF ADSORPTION REACTION MECHANISMS

Tungsten (W) has been declared an “emerging contaminant of concern” by the US EPA [1], since it has been linked to leukemia clusters near mines in the western US [2], converts human osteoblasts to a tumorigenic form in vitro[3], and damages bone marrow DNA in mice [4]. Despite rapidly increasing production and use of tungsten and recent recognition of associated health hazards, remarkably little work has yet been conducted to understand the transport and fate of this heavy metal.

As dissolved W moves in soils or oxidizing groundwater, its mobility is likely governed primarily by adsorption to particles of Mn, Fe, and Al oxyhydroxides. In addition to how much W adsorbs, a crucial question is how exactly W bonds to particle surfaces and how stable it is in those chemical forms. Preliminary work [5,6] employed EXAFS to determine the structures of surface complexes, but unrealistically large concentrations of W are needed for that technique, especially at environmentally relevant pH, and speciation of W is known to vary strongly with concentration. A new tool is needed to discern adsorption mechanisms at field-relevant concentrations.

We anticipate that W stable isotope ratios are highly sensitive to changes in coordination number and W-O and W-metal bond distances that occur during adsorption reactions. Thus we are conducting experiments to determine W isotope systematics during adsorption to synthetic Mn and Fe oxyhydroxides, beginning at concentrations where adsorption mechanisms can be examined directly using EXAFS. Initial experiments indicate easily resolvable fractionations of Δ^183/182W ~ -0.4 ‰ (lighter W sorbed, equilibrium isotope effect). Once the relationships between isotope behavior and adsorption mechanisms are established, we can extend isotope experiments to field-relevant levels, since only 50 nanograms of W are needed for analysis. Eventually we hope to apply W isotopes as a tool for tracking the extent to which adsorption reactions are attenuating migration of W in contaminated settings.

[1] US EPA (2012) Technical Fact Sheet EPA 505-F-11-005. [2] Koutsospyros et al. (2006) J. Hazardous Mat. 136, 1. [3] Miller et al. (2001) Carcinogenesis 22, 115. [4] Kelly et al. (2013) Toxicol. Sciences 131, 434. [5] Hur & Reeder (2012) Amer. Chem. Soc. Abs. ENVR 487. [6] Sun & Bostick (2015) Chem. Geol. 417, 21.