Skierszkan, Elliott K.Robertson, Jared M.Lindsay, Matthew B. J.Stockwell, Justin S.Dockrey, John W.Das, SoumyaWeis, DominiqueBeckie, Roger D.Mayer, K. Ulrich2019-11-082019-11-082019-04-18Skierszkan, E.K., Robertson, J.M., Lindsay, M.B.J., Stockwell, J.S., Dockrey, J.W., Das, S., Weis, D., Beckie, R.D., & Mayer, K.U., (2019). Tracing molybdenum attenuation in mining environments using molybdenum stable isotopes. Environmental Science and Technology 53, 5678–5686. https://doi.org/10.1021/acs.est.9b007660013-936X1520-5851http://hdl.handle.net/10388/12445Molybdenum contamination is a concern in mining regions worldwide. Better understanding of processes controlling Mo mobility in mine wastes is critical for assessing potential impacts and developing water-quality management strategies associated to this element. Here, we used Mo stable isotope (δ98/95Mo) analyses to investigate geochemical controls on Mo mobility within a tailings management facility (TMF) featuring oxic and anoxic environments. These isotopic analyses were integrated with X-ray absorption spectroscopy, X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and aqueous chemical data. Dissolved Mo concentrations were inversely correlated with δ98/95Mo values such that enrichment of heavy Mo isotopes in solution reflected attenuation processes. Inner-sphere complexation of Mo(VI) with ferrihydrite was the primary driver of Mo removal and was accompanied by a circa 1 ‰ isotope fractionation. Limited Mo attenuation and isotope fractionation was observed in Fe(II)- and Mo-rich anoxic TMF seepage, while attenuation and isotope fractionation were greatest during discharge and oxidation of this seepage after discharge into a pond where Fe-(oxyhydr)oxide precipitation promoted Mo sorption. Overall, this study highlights the role of sorption onto Fe-(oxyhydr)oxides in attenuating Mo in oxic environments, a process which can be traced by Mo isotope analyses.mine wastemolybdenumisotopesgroundwateriron (oxyhydr)oxidesArticle10.1021/acs.est.9b00766