Eastern Section Meeting

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Tungsten Isotopic Fractionation During Adsorption to Birnessite


Tungsten is a metal that is commonly found in coal ash, a known environmental contaminant, yet little is known about how tungsten contamination affects soil and water systems, nor about the effects of tungsten poisoning on humans and wildlife. Between 1997 and 2002 there were 16 reported cases of childhood leukemia near Fallon, Nevada. These cases were linked to high levels of tungsten in airborne particles and drinking water, and as a result tungsten has become a contaminant of interest. Adsorption to clays and other minerals is a major control on how a contaminant interacts with soil and water. In order to better constrain the way in which tungsten migrates in water systems and interacts with soil, the adsorption mechanisms of tungsten need to be understood.

Recent research has led to the discovery of metal isotope fractionation during reactions that are relevant to the transport and immobilization of heavy metals and that this fractionation can be used to track the extent of the reactions. Little is known about tungsten isotope fractionation while adsorbing to minerals, such as birnessite, that are commonly found in soil. The goals of this experiment are to determine if there is a measurable fractionation when tungsten adsorbs to synthetic birnessite, to quantify the amount of fractionation, and to determine the manner in which it fractionates. Birnessite is a common mineral in soil and has a large adsorption capacity, making it ideal to use for this study. The experiment was set up by mixing birnessite suspension with a 5 ppm tungsten solution and fixing the mixture to a pH of approximately 8. These samples were left on a shaker for 24 hours before being filtered to separate the solid and liquid fractions. Preliminary isotopic analysis data show that tungsten has a small fractionation (~0.3‰), which is consistent with an equilibrium isotope effect, with lighter isotopes preferentially adsorbing to birnessite. These results may help us understand how tungsten contamination caused by coal ash migrates through the environment and how adsorption reactions affect this migration.