Quantifying Technetium and Strontium Bioremediation Potential in Flowing Sediment Columns

The high-yield fission products 99 Tc and 90 Sr are found as problematic radioactive contaminants in groundwater at nuclear sites. Treatment options for radioactively contaminated land include bioreduction approaches, and this paper explores 99m Tc and 90 Sr 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 99 Tc was then probed using a 99m Tc(VII) tracer at <10 -10 mol L -1 and γ camera imaging showed full retention of 99m Tc 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 99m Tc. Here, partially oxidized sediments with no further electron donor additions remained reactive toward 99m Tc under relevant groundwater O 2 and NO 3 - concentrations over 55 days. Immobilization of 99m Tc was highest where continuous acetate amendment had resulted in sulfate-reducing conditions. Interestingly, the sulfate reducing system showed enhanced Sr retention when stable Sr 2+ was added continuously as a proxy for 90 Sr. 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.