Immobilization of heavy metals via two parallel pathways during in-situ bioremediation
Florida State University
Bioreduction is being actively investigated as an effective strategy for subsurface remediation and long-term management of DOE sites contaminated by metals and radionuclides (i.e. U(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. There is evidence from DOE field experiments suggesting 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, we used selenite, Se(IV), instead of U(VI) in the lab since 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 has been well understood, the second pathway has not been widely studied. Another unique aspect of our work is that we investigate 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 donor in the presence of background sulfate. We characterized the microbial and chemical reaction products using a advanced microscopic and spectroscopic methods. We also developed a continuum-scale reactive transport model to that was able to simulate this experiment.
Our work demonstrates that engineering remediation of metal contaminated sites via electron-donor addition can lead to secondary and abiotic reactions that can immobile metals, in addition to previously studied biotic reactions. The improved understanding of selenite immobilization and the improved model can help to better design in situ bioremediation processes for groundwater contaminated by selenite or other contaminants (e.g., uranium(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. 2015. Immobilization of selenite via two parallel pathways during in situ bioremediation. Environ Sci Technol. 49:4543-4550; doi:10.1021/es506107r
Electron microscope image of mixed bacteria culture observe in the experiment with a schematic illustration of the biotic and abiotic pathway for immobilization of selenium following biostimulation by addition of electron donor.