Development of tungsten stable isotope analysis for constraining sorption mechanisms and tracking the transport and fate of tungsten

Northern Arizona University, Flagstaff AZ

Investigators

Abstract

In this project funded by the Environmental Chemistry Program in the Chemistry Division at the National Science Foundation, Professor Laura Wasylenki of Indiana University (IU) develops a new way to assess how and to what extent sorption to mineral particles changes the mobility of toxic metals in contaminated settings. The new approach enables better prediction of the transport and fate for many metals, but the focus here is on tungsten (W), recently declared an "emerging contaminant of concern" by the US EPA. This approach takes advantage of variations in the stable isotope ratios of tungsten (e.g., W-183/W-182), which are highly sensitive to differences in the numbers, lengths, and strengths of chemical bonds involving the metal atoms. Stable isotope systematics are used to probe the molecular-scale interactions between dissolved W and porous media at realistic concentrations in the environment, but inaccessible to conventional spectroscopic techniques such as extended X-ray absorption fine structure (EXAFS) analysis. Concurrently, an innovative informational effort is launched at Indiana University to educate the campus community about where metals in everyday objects come from and what hazards and consequences are associated with taking those resources from the Earth. A series of signs are displayed on campus with quiz questions and QR (quadratic residue) code links that are developed by interns in IU's Office of Sustainability, with assistance from the Office of Science Outreach and Professor Wasylenki's research students. Simple sorption experiments and precise isotope analyses yield quantitative information about isotope fractionation during sorption of W onto four common sorbent phases. Studies are made first at high concentrations, where EXAFS provide information directly about coordination chemistry and sorption mechanisms, and then at field-relevant conditions, where isotopes may be the only way to constrain sorption mechanisms. Computational quantum mechanical modeling complement the experiments, isotope analyses, and EXAFS analyses. The modeling studies predict the expected isotope offsets between monomeric and polymeric species of W, which form in solution and as sorbed complexes. The work advances what is known about the environmental chemistry of tungsten and enables stable metal isotopes to be used as a tool for tracking transport and fate of metal contaminants.