Origin of tungsten and geochemical controls on its occurrence and mobilization in shallow sediments from Fallon, Nevada, USA
2020
Hobson, Chad | Kulkarni, Harshad V. | Johannesson, Karen H. | Bednar, Anthony | Tappero, Ryan | Mohajerin, T Jade | Sheppard, Paul R. | Witten, Mark L. | Hettiarachchi, Ganga M. | Datta, Saugata
Tungsten (W) occurrence and speciation was investigated in sediments collected from Fallon, Nevada where previous studies have linked elevated W levels in human body fluids to an unusual cluster of childhood leukemia cases. The speciation of sedimentary W was determined by μ-XRF mapping and μ-XANES. The W content of the analyzed surface sediments ranged between 81 and 25,908 mg/kg, which is significantly higher than the W content in deeper sediments which ranged from 37 to 373 mg/kg at 30 cm depth. The μ-XANES findings reveal that approximately 20–50% of the total W in the shallow sediment occurs in the metallic form (W⁰); the rest occurs in the oxide form (WⱽᴵO₃). Because W⁰ does not occur naturally, its elevated concentrations in surface sediments point toward a possible local anthropogenic origin. The oxidation of metallic W⁰ with meteoric waters likely leads to the formation of WⱽᴵO₃. The chief water-soluble W species was identified as WO₄²⁻ by chromatographic separation and speciation modeling. These results led us to postulate that W⁰ particles from a currently unknown but local source(s) is (are) deposited onto the soils and/or surface sediments. The W⁰ in interaction with meteoric water is oxidized to WⱽᴵO₃, and as these sediment-water interactions progress, WO₄²⁻ is formed in the water at pH ∼7. Under pH < 7, and sufficient W concentrations, tungstate tends to polymerize, and polymerized species are less likely to adsorb onto sediments. Polymerized species have lower affinity than monomers, which leads to enhanced mobility of W.
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