Project d 2.31 rfactor download
(2) Furthermore, a range of carbonate concentrations (∼0.2–12 mM from both natural and engineered sources) (8−10) are expected in the latter stage of evolution of a cementitious GDF environment from sources such as biodegradation and, in some cases, in groundwaters. (2,7) The CDZ is expected to partition many radionuclides (including U) to the solid phase, via precipitation and adsorption to mineral surfaces, thereby immobilizing potential contaminants. Furthermore, 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. Here, iron (oxyhydr)oxide minerals may be present from both engineered and natural sources, including the corrosion of steel canisters and rock. (6) Additionally, 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. Uranium speciation may be altered by microbial processes that can influence redox behavior (2−5) and thereby induce changes in chemical form, such as dissolved or colloidal U. (1) As a result, U will be present in radioactive wastes emplaced within the deep subsurface, with its environmental fate significantly controlled by its speciation. Currently, the globally favored management pathway for higher activity radioactive wastes containing U and other radionuclides is via an engineered geological disposal facility (GDF), which is intended to prevent the release of harmful quantities of radionuclides to the surface environment over geological time scales.
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. 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. In addition, sulfate-reducing bacteria, such as Desulfosporosinus species, were enriched during development of sulfate-reducing conditions. Here, aqueous sulfide accumulated and U was removed from solution as a mixture of U(IV) and U(VI) phosphate species. This drop in pH was likely due to the presence of volatile fatty acids from the microbial respiration of gluconate. Low carbonate conditions allowed microbial sulfate reduction to proceed and caused the pH to fall to ∼7.5. 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. 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). This study investigated the fate of U(VI) in an alkaline (pH ∼9.6) sulfate-reducing enrichment culture obtained from a high-pH environment.
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. 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. Globally, the need for radioactive waste disposal and contaminated land management is clear.