| Tungsten, previously considered to be non-toxic, is now associated with serious toxicological and environmental effects, possibly including 16 childhood leukemia cases near Fallon, NV, from 1997--2001 (CDC, 2003; Koutsospyros, 2006; Strigul, 2010). As a result, controls on tungsten's environmental transport and fate, such as sorption reactions, are now of critical interest. As is the case for other sorbates, speciation and coordination chemistry likely dictate the extent to which tungsten is immobilized by sorption. Unfortunately some techniques often used to study sorption mechanisms cannot be applied at field-relevant conditions for tungsten. Heavy metal isotopes, however, are sensitive to coordination chemistry, and thus analysis of tungsten stable isotopes may help unlock the mechanisms which govern tungsten sorption reactions. This study aims to determine the magnitude and manner in which tungsten isotopes fractionate during sorption to birnessite and ferrihydrite at pH ~5 and ~8, and in doing so, provide insight into tungsten sorption reactions that occur within contaminated aquifers. Tungsten isotope fractionation (Delta 183/182W) ranged from 0.28 +/- 0.07‰ to 0.47 +/- 0.06‰ (1sd), with lighter isotopes preferentially sorbing. In every case, tungsten fractionation followed an equilibrium isotope trend, indicating that tungsten sorption reactions are easily reversible, and that isotope behavior is governed by thermodynamics, rather than kinetics. In the context of the few recent spectroscopic studies of sorbed tungsten, our results also indicate that tungsten coordination is changing from tetrahedral in solution to octahedral on the mineral surface and that this change is consistent even as pH, tungsten concentration, and sorbent are varied. |
