Abstract
Globally, the need for radioactive waste disposal and contaminated land management is clear. Here, gaining an improved understanding of how biogeochemical processes, such as Fe(III) and sulfate reduction, may control the environmental mobility of radionuclides is important. Uranium (U), typically the most abundant radionuclide by mass in radioactive wastes and contaminated land scenarios, may have its environmental mobility impacted by biogeochemical processes within the subsurface. This study investigated the fate of U(VI) in an alkaline (pH ∼9.6) sulfate-reducing enrichment culture obtained from a high-pH environment. To explore the mobility of U(VI) under alkaline conditions where iron minerals are ubiquitous, a range of conditions were tested, including high (30 mM) and low (1 mM) carbonate concentrations and the presence and absence of Fe(III). At high carbonate concentrations, the pH was buffered to approximately pH 9.6, which delayed the onset of sulfate reduction and meant that the reduction of U(VI)(aq) to poorly soluble U(IV)(s) was slowed. Low carbonate conditions allowed microbial sulfate reduction to proceed and caused the pH to fall to ∼7.5. This drop in pH was likely due to the presence of volatile fatty acids from the microbial respiration of gluconate. Here, aqueous sulfide accumulated and U was removed from solution as a mixture of U(IV) and U(VI) phosphate species. In addition, sulfate-reducing bacteria, such as Desulfosporosinus species, were enriched during development of sulfate-reducing conditions. Results highlight the impact of carbonate concentrations on U speciation and solubility in alkaline conditions, informing intermediate-level radioactive waste disposal and radioactively contaminated land management.
Highlights
Uranium (U) is a radionuclide of global importance due to its use within the nuclear industry, its presence as a significant component of many radioactive wastes, and its occurrence at many radioactively contaminated land sites
For the biogenic sulfidation experiment, enrichment cultures were set up under sulfate-reducing conditions using an enrichment from an alkaline legacy lime working sediment as the inoculum
All active microbial cultures darkened throughout the duration of the experiment, consistent with the development of reducing reduction proceeded at a faster rate under low carbonate conditions compared with that under the high carbonate conditions
Summary
Uranium (U) is a radionuclide of global importance due to its use within the nuclear industry, its presence as a significant component of many radioactive wastes, and its occurrence at many radioactively contaminated land sites. Many proposed intermediate-level waste (ILW) GDF systems involve the use of cement as a significant proportion of both the wasteform and, in some cases, the backfill. In many ILW disposal designs, an alkaline chemically disturbed zone (CDZ) is expected to form in the near-field of a GDF due to the reaction of high-pH groundwater, which has passed through cement, with the surrounding host rock.[2,7] The CDZ is expected to partition many radionuclides (including U) to the solid phase, via Received: May 5, 2021 Revised: August 17, 2021 Accepted: August 17, 2021
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