Quantifying Technetium and Strontium Bioremediation Potential in Flowing Sediment Columns.

The high-yield fission products Tc-99 and Sr-90 are found as problematic radioactive contaminants in groundwater at nuclear sites. Treatment options for radioactively contaminated land include bioreduction approaches, and this paper explores Tc-99m and Sr-90 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 Tc-99 was then probed using a Tc-99m(VII) tracer at <10(-10) mol L-1 and gamma camera imaging showed full retention of Tc-99m 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 Tc-99m. Here, partially oxidized sediments with no further electron donor additions remained reactive toward Tc-99m under relevant groundwater O-2 and NO3- concentrations over 55 days. Immobilization of Tc-99m 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 Sr-90. 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.