Abstract
The high-yield fission products 99Tc and 90Sr are found as problematic radioactive contaminants in groundwater at nuclear sites. Treatment options for radioactively contaminated land include bioreduction approaches, and this paper explores 99mTc and 90Sr behavior and stability under a range of biogeochemical conditions stimulated by electron donor addition methods. Dynamic column experiments with sediment from the Sellafield nuclear facility, completed at site relevant flow conditions, demonstrated that Fe(III)-reducing conditions had developed by 60 days. Sediment reactivity toward 99Tc was then probed using a 99mTc(VII) tracer at <10-10 mol L-1 and γ camera imaging showed full retention of 99mTc in acetate amended systems. Sediment columns were then exposed to selected treatments to examine the effects of different acetate amendment regimes and reoxidation scenarios over 55 days when they were again imaged with 99mTc. Here, partially oxidized sediments with no further electron donor additions remained reactive toward 99mTc under relevant groundwater O2 and NO3- concentrations over 55 days. Immobilization of 99mTc was highest where continuous acetate amendment had resulted in sulfate-reducing conditions. Interestingly, the sulfate reducing system showed enhanced Sr retention when stable Sr2+ was added continuously as a proxy for 90Sr. Overall, sediment reactivity was nondestructively imaged over an extended period to provide new information about dynamic iron and radionuclide biogeochemistry throughout realistic sediment redox cycling regimes.
Highlights
Bioremediation approaches show significant potential for the treatment of radioactively contaminated land
Microbes couple the oxidation of an electron donor to the reduction of available electron acceptors leading to increasingly reducing conditions
Biostimulation offers potential benefits at many nuclear sites where redox active radionuclides are present in groundwater by providing a low cost, low disturbance, deliverable technique to minimize offsite migration of contaminants
Summary
Bioremediation approaches show significant potential for the treatment of radioactively contaminated land. Bioreduction is a bioremediation technique applicable to key redox active radionuclides including technetium and typically involves stimulation of an in situ microbial community to develop reducing conditions in the subsurface. This promotes immobilization of radionuclide contaminants and shows promise, alongside other approaches, for the remediation of nuclear legacy sites.[1−5] To stimulate bioreduction, electron donors (typically simple organic compounds such as acetate or ethanol)[6,7] are injected into the subsurface to promote microbial growth and respiration. Redox active radionuclides, including Tc, Np, and U, which are soluble and mobile under oxic conditions as Tc(VII), Np(V), and U(VI) species, can be reduced to their less soluble, reduced forms under bioreducing conditions.[1,3,4,8,9] biostimulation offers potential benefits at many nuclear sites where redox active radionuclides are present in groundwater by providing a low cost, low disturbance, deliverable technique to minimize offsite migration of contaminants
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