Quantifying Technetium and Strontium Bioremediation Potential in ...

Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX-XXX

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Quantifying Technetium and Strontium Bioremediation Potential in Flowing Sediment Columns Clare L. Thorpe,†,⊥ Gareth T. W. Law,†,‡ Jonathan R. Lloyd,† Heather A. Williams,§ Nick Atherton,∥ and Katherine Morris*,† †

Research Centre for Radwaste Disposal and Williamson Research Centre for Molecular Environmental Science, School of Earth and Environmental Sciences, The University of Manchester, Manchester M13 9PL, United Kingdom ‡ Centre for Radiochemistry Research, School of Chemistry, The University of Manchester, Manchester M13 9PL, United Kingdom § Nuclear Medicine Centre, Manchester Royal Infirmary, Manchester M13 9WL, United Kingdom ∥ Sellafield Ltd., Land Quality, Sellafield, Seascale, Cumbria CA20 1PG, United Kingdom S Supporting Information *

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 0.5 mm which were removed by hand picking. Grains were generally coated with clay-sized iron oxides. X-ray fluorescence confirmed that the sediment comprised Si (30.4 wt %), Al (9.4 wt %), Fe (5.0 wt %), K (3.3 wt %), Mg (1.3 wt %), Na (1.2 wt %), Ti (0.4 wt %), Ca (0.4 wt %), Mn (0.1 wt %), and Sr at 102 ppm. The total iron was approximately 890 mmol kg−1 and between 80 and 100 mmol kg−1 of the sediment Fe(III) was extractable using a 1 h 0.5 N HCl digestion, an indicator of bioavailable Fe(III).51 This suggested ∼10% of the total iron in the Sellafield sediment was readily bioavailable consistent with other studies on Sellafield near surface materials.14,15,31 The total organic carbon content of the soils was determined as 0.13 ± 0.01% using a LECO CR-412 Carbon Analyzer. After equilibration with synthetic groundwater for 24 h, the sediment pH was 7.2, bracketing typical on site pH values (pH 5−8).12 B

DOI: 10.1021/acs.est.7b02652 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology Table 1. Experimental Details column

continuous Sr2+ addition

input 0−60 days

A B C D E

12μmol L−1 12 μmol L−1 12 μmol L−1 12 μmol L−1 12 μmol L−1

no acetate amendment 3 mmol L−1 acetate 3 mmol L−1 acetate 3 mmol L−1 acetate 3 mmol L−1 acetate

99m

F

12 μmol L−1

3 mmol L−1 acetate

99m

61−65 days Tc 99m Tc 99m Tc 99m Tc 99m Tc

decay decay decay decay decay

Tc decay

input 65−115 days no acetate amendment 3 mmol L−1 acetate 3 mmol L−1 acetate in 5 day pulses every 20 days no further amendment, 0.3 mmol L−1nitrate, 0.25 mmol L−1 oxygen no further amendment, 0.3 mmol L−1nitrate, oxygen purged influent (0.31 mmol L−1) no further amendment, 10 mmol L−1nitrate, 0.25 mmol L−1 oxygen

Tc (∼7 MBq in 1 mL; 3.5 × 10−13 mols) and the flow restarted to image 99mTc behavior under flow conditions. After imaging for 12 h, a radioactive decay period was necessary to allow the 99mTc (half-life 6 h) to decay to levels that would allow safe handling; here, the columns were capped and stored without pumping at 4 °C in the dark for 5 days. After decay, the columns were transported back to the laboratory where flow was reinstigated under different experimental regimes for a further 50 days. During this postbioreduction period, a range of different treatments (Table 1) were undertaken to examine 99m Tc reactivity: (A) the nonacetate amended groundwater system representing a natural attenuation control; (B) the system with continual acetate amendment; (C) a bioreduced system where acetate was then pulsed (5 days acetate additions, 15 days no acetate additions); (D) a bioreduced system with no further acetate amendment; (E) a bioreduced system where air was then bubbled into the synthetic groundwater (0.31 mM O2); and (F) a bioreduced system where nitrate was then added at elevated levels. Sampling and Geochemical Analysis. The column influent and effluent were monitored at regular intervals during experiments. In the effluent, pH and the concentrations of acetate, SO42−, NO2−, NO3−, and Fe(II), were measured to track the progress of terminal electron accepting processes and total Fe, Mn, Al, Ca, Sr, and Mg to assess changes in sediment geochemistry. Eh and pH were measured immediately using calibrated electrodes (Denver-Basic). Porewater NO2−, Mn, and Fe(II) were measured spectrophotometrically.54−56 Acetate, SO42−, NO3− and in tracer tests, Br− were measured by ion chromatography on samples stored at 4 °C prior to analysis. Cation concentrations (total Mg, Al, Fe, Mn, Sr, and Ca) were measured by ICP-AES on acidified (2% HNO3) samples. The dissolved O2 concentration of the influent synthetic groundwater was periodically measured using the Winkler titration.57 At experiment end points (115 days), columns were extruded and sampled under a N2 atmosphere at 2 cm intervals and bioavailable Fe(II) as a proxy for Fe(III)-reduction was measured. Here, sediment samples were digested in 0.5 N HCl for 1 h and aqueous Fe(II) (filtered