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 99m Tc g-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 99m Tc(VII)O 4 ¡ , which is relevant to the 99 Tc mass concentrations observed at nuclear facilities, run under site relevant flow conditions, and imaged using g-camera imaging at 20 min intervals over 12h. In the oxic column 99m Tc behaved as a conservative tracer and did not interact significantly with the sediment. In all of the biostimulated columns 99m Tc 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 99m Tc under increasingly reducing conditions. Columns were destructively sampled after 99m Tc 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 99m Tc. 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 99m Tc immobilization. The results are the first to show the variable but significant retention of 99m Tc 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 99 Tc contaminated groundwater through in-situ biostimulation.

Metaschoepite Dissolution in Sediment Column Systems—Implications for Uranium Speciation and Transport

“…Prior to use in experiments, the sediment was sealed and stored at 4 °C for <2-months under a normal atmosphere. Autoclaved synthetic groundwater representative of the Sellafield region, , was used in the experiments (Table S1).…”

“…A peristaltic pump (Watson-Marlow 205-U) was used to pump synthetic groundwater into the base of the columns, with effluents then collected from the top of the columns. The groundwater pumping rate was 1.8 ± 0.2 mL h –1 (1.14 × 10 –6 m s –1 ) …”

“…For the oxic treatment, an air bubbler was used to maintain O 2 in the groundwater; for the electron-donor amended treatment, 1 mM (total) of acetate and lactate (50:50) was added, and the groundwater was sparged with N 2 (30 min L –1 ). Prior work with these sediments has shown that similar electron-donor addition readily promotes microbially mediated bioreduction. ,, Throughout the experiment, the pH of the influent groundwaters was held between 7.2 and 7.4 via HCl addition. In the groundwater reservoirs, the concentrations of NO 3 – , SO 4 2– , and electron-donors (when present) remained within 5% of the starting concentrations between weekly groundwater reservoir exchanges.…”

“…The groundwater pumping rate was 1.8 ± 0.2 mL h −1 (1.14 × 10 −6 m s −1 ). 38 Two groundwater treatments were used: (i) oxic and (ii) electron-donor amended. For the oxic treatment, an air bubbler was used to maintain O 2 in the groundwater; for the electrondonor amended treatment, 1 mM (total) of acetate and lactate (50:50) was added, and the groundwater was sparged with N 2 (30 min L −1 ).…”

Microbial interaction with and tolerance of radionuclides: underlying mechanisms and biotechnological applications

“…In addition, phosphate mineral formation by several yeast strains may involve an organic source of phosphorus such as glycerol 2-phosphate or phytic acid, in the presence of soluble U (Liang et al, 2016). Biomineralization processes with other radionuclides, such as caesium, strontium, technetium, plutonium or neptunium, have also been described (Macaskie et al, 1994;Macaskie and Basnakova, 1998;Hallberg and Ferris, 2004;Mitchell and Ferris, 2005;Thorpe et al, 2016, Cleary et al, 2019.…”