March 3, 2015
Bioreduction is being actively investigated as an effective strategy for subsurface remediation and long-term management of Department of Energy (DOE) sites contaminated by metals and radionuclides [i.e., uranium (VI)]. These strategies require manipulation of the subsurface, usually through injection of chemicals (e.g., electron donor) which mix at varying scales with the contaminant to stimulate metal reducing bacteria. Evidence from DOE field experiments suggests that mixing limitations of substrates at all scales may affect biological growth and activity for U(VI) reduction.
To study the effects of mixing on U(VI) reduction, researchers used selenite, Se(IV), instead of U(VI) in the lab because Se(IV) is easier to handle and microbial reduction of Se(IV) and U(VI) is similar in that two immobilization pathways are involved. In one pathway, the soluble contaminant [Se(IV) or U(VI)] is biologically reduced to a solid [Se0 or U(IV)]. In the other pathway, sulfate, which is commonly present in groundwater, is first biologically reduced to sulfide; this product then abiotically reacts with the soluble contaminant [Se(IV) or U(VI)] to form a solid [selenium sulfide or U(IV)]. While the first pathway is well understood, the second pathway has not been widely studied. Another unique aspect of this study is that researchers investigated mixing and reaction in a microfluidic flow cell with realistic pore geometry and flow conditions that mimic the transverse-mixing dominated reaction zone along the margins of a selenite plume undergoing bioremediation due to injected electron donors in the presence of background sulfate. Microbial and chemical reaction products were characterized using advanced microscopic and spectroscopic methods. A continuum-scale reactive transport model also was developed to simulate this experiment.
Results demonstrate that engineering remediation of metal-contaminated sites via electron-donor addition can lead to secondary and abiotic reactions that can immobilize metals, in addition to previously studied biotic reactions. The improved understanding of selenite immobilization as well as the improved model can help in the design of in situ bioremediation processes for groundwater contaminated by selenite or other contaminants [e.g., U(IV)] that can be immobilized via similar pathways.
Tang, Y., C. J. Werth, R. A. Sanford, R. Singh, K. Michelson, M. Nobu, W. Liu, and A.J. Valocchi. “Immobilization of selenite via two parallel pathways during in situ bioremediation.” Environmental Science & Technology 49(7), 4543–50. [DOI:10.1021/es506107r].
Tang, Y., C.J. Werth, R.A. Sanford, R. Singh, K. Michelson, M. Nobu, W. Liu, and A.J. Valocchi. 2015. Immobilization of selenite via two parallel pathways during in situ bioremediation. Environ Sci Technol. 49:4543-4550; doi:10.1021/es506107r