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