Microbial impacts on 99mTc migration through sandstone under highly alkaline conditions relevant to radioactive waste disposal

Abstract

Geological disposal of intermediate level radioactive waste in the UK is planned to involve the use of cementitious materials, facilitating the formation of an alkali-disturbed zone within the host rock. The biogeochemical processes that will occur in this environment, and the extent to which they will impact on radionuclide migration, are currently poorly understood. This study investigates the impact of biogeochemical processes on the mobility of the radionuclide technetium, in column experiments designed to be representative of aspects of the alkali-disturbed zone. Results indicate that microbial processes were capable of inhibiting Tc migration through columns, and X-ray radiography demonstrated that extensive physical changes had occurred to the material within columns where microbiological activity had been stimulated. The utilisation of organic acids under highly alkaline conditions, generating H and CO, may represent a mechanism by which microbial processes may alter the hydraulic conductivity of a geological environment. Column sediments were dominated by obligately alkaliphilic H-oxidising bacteria, suggesting that the enrichment of these bacteria may have occurred as a result of H generation during organic acid metabolism. The results from these experiments show that microorganisms are able to carry out a number of processes under highly alkaline conditions that could potentially impact on the properties of the host rock surrounding a geological disposal facility for intermediate level radioactive waste.

Biogeochemical Cycling of 99Tc in Alkaline Sediments

“…To explore the biogeochemistry of technetium under anoxic conditions at pH 10, microcosms containing well-characterized, calcite-dominated sediments and surface waters from a high pH lime working site in the U.K. were setup (SI Table 1). ,, …”

“…Throughout, selected experiments were run at elevated Tc concentrations (9 × 10 –4 mol L –1 ), which allowed direct speciation of Tc to be measured using X-ray absorption spectroscopy. Additionally, select experiments were spiked with pertechnetate using the medical isotope 99m Tc at ∼10 –11 mol L –1 to quantify radionuclide behavior in reducing systems at very low Tc concentrations using γ camera imaging. ,,, Collectively, our experiments demonstrate the biogeochemical behavior of Tc across a wide concentration range (10 –11 to 10 –4 mol L –1 ) under oxic and anoxic conditions in alkaline (pH 10) sediments, giving important insights into the behavior of a risk-driving radionuclide under conditions relevant to an evolving GDF environment.…”

“…Sediment and surface water (pH 11.8) were collected from a well-characterized area at Harpur Hill Buxton, Derbyshire, that had been contaminated for decades by high pH generating spoil from a legacy lime works. ,,, The sediments at the site have a circumneutral to alkaline pH (pH 12.8 maximum) and contain high calcium and silicate concentrations, analogous to a cementitious radioactive waste repository . Sediment and surface waters were transferred into sterile containers and stored at 4 °C in darkness prior to use.…”

“…Consequently, the potential for microbial processes in deep geological disposal is becoming increasingly recognized . In terms of biogeochemical impacts on radionuclide behavior, studies show the potential for the microbially mediated reduction of Fe­(III) and also U­(VI) and Np­(V) at elevated pH. − A select number of studies with Tc have investigated Tc­(VII) bioreduction processes under alkaline conditions in both pure culture and sediment consortia. ,,, However, there is a paucity of information on Tc biogeochemistry in complex, elevated pH systems relevant to ILW disposal. Geological disposal facilities may also be subject to reoxidation events by oxygenated groundwater influx or due to nitrate present within ILW wasteforms .…”

The Bioreduction of Selenite under Anaerobic and Alkaline Conditions Analogous to Those Expected for a Deep Geological Repository System

“…The previous results suggested that S. bentonitica would decrease the solubility, and hence the mobility, of Se in the environment surrounding DGRs. Only few papers, however, describe the impact of microbial processes on radionuclide mobility under alkaline conditions analogous to the DGR system [18,19].…”