TOWARDS W ISOTOPE SIGNATURES AS A TRACER OF ENVIRONMENTAL CONTAMINATION: FRACTIONATION DURING SORPTION TO MN OXYHYDROXIDE

Tungsten is a metal that has been used in ammunition as a substitute for lead and is commonly found in coal and coal fly 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 (Sheppard et al. 2007). 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. A major control on how a contaminant interacts with water and soil is adsorption to clays and other minerals. In order to better constrain the way in which tungsten migrates in water systems and interacts with soil, the adsorption of tungsten to minerals commonly found in soil and aquifers needs 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 were 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 (Rayleigh or equilibrium). Birnessite (MnOx) is a common mineral in soil and has a large adsorption capacity, making it an ideal mineral 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 then left on a shaker for 24 hours before being filtered to separate the adsorbed and aqueous 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 analyze the extent to which adsorption reactions are attenuating tungsten migration in contaminant plumes.