TUNGSTEN ISOTOPIC FRACTIONATION DURING ADSORPTION TO BIRNESSITE

Tungsten is a metal that has been used in military and hunting ammunition as a substitute for lead, yet little is known about how tungsten contaminates 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. In order to better constrain the way in which tungsten migrates in water systems and interacts with soil, the adsorption mechanisms of tungsten to soil 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 in some cases this fractionation can be used to track the extent of the reactions in the field. Currently, little is known about whether and how tungsten fractionates 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). The experiment was setup by mixing varying amounts of a birnessite suspension with varying amounts of 5ppm 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. Isotopic analysis was performed on a Nu Plasma II MC-ICP-MS to determine whether and how tungsten fractionated during adsorption. Preliminary data show that tungsten has a small fractionation (∼0.3‰) which follows an equilibrium fractionation model, with lighter isotopes preferentially adsorbing to birnessite. Future experiments will include adsorption to different minerals and analysis of soils in which the timing and amount of tungsten input are well constrained. These results may help us analyze the extent to which adsorption reactions are attenuating tungsten migration in contaminant plumes.