Retention of 99mtc at ultra-trace levels in flowing column experiments – insights into bioreduction and biomineralization for remediation at nuclear facilities

Abstract

The behavior of technetium at ultra-trace (<10−10 mol l−1) concentrations in bioreducing sediment column experiments was investigated using 99mTc γ-camera imaging. Four flowing sediment columns were biostimulated for varying periods of time, using acetate as an electron donor, such that on the day of imaging they represented oxic, early metal-reducing, Fe(III)-reducing and sulfate-reducing conditions. Prior to imaging, columns were spiked with 9.6 MBq of 99mTc(VII)O4−, which is relevant to the 99Tc mass concentrations observed at nuclear facilities, run under site relevant flow conditions, and imaged using γ-camera imaging at 20 min intervals over 12h. In the oxic column 99mTc behaved as a conservative tracer and did not interact significantly with the sediment. In all of the biostimulated columns 99mTc associated with the sediment although the spike was least mobile in the order sulfate < Fe(III)-reducing < early metal-reducing columns, confirming higher reactivity for 99mTc under increasingly reducing conditions. Columns were destructively sampled after 99mTc decay (5 days storage), and 0.5 N HCl extractable Fe as Fe(II) was measured at 2 cm intervals along the column length. The Fe(II) measured in all electron donor amended columns was present in stoichiometric excess to the 99mTc. Geochemical modelling and aqueous geochemical data from the different biostimulation treatments suggested that the elevated pH observed in the more reduced columns lead to increased sorbed and mineral associated Fe(II), both stronger reductants for Tc(VII) than aqueous Fe(II). In the sulfate reduction column the presence of FeS lead to the fastest rates of 99mTc immobilization. The results are the first to show the variable but significant retention of 99mTc at ultra-trace levels relevant to conditions at many nuclear sites in a range of biostimulated sediment columns and present a positive outlook for the treatment of 99Tc contaminated groundwater through in-situ biostimulation.