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.
Shale gas exploitation relies on hydraulic fracturing, which often involves a range of chemical additives in the injection fluid. However, relatively little is known about how these additives influence fractured shale microbial communities.
Natural gas is recovered from shale formations by hydraulic fracturing, a process known to create microbial ecosystems in the deep subsurface. Microbial communities that emerge in fractured shales include organisms known to degrade fracturing fluid additives and contribute to corrosion of well infrastructure. In order to limit these negative microbial processes, it is essential to constrain the source of the microorganisms responsible. Previous studies have identified a number of potential sources, including fracturing fluids and drilling muds, yet these sources remain largely untested. Here, we apply high pressure experimental approaches to assess whether the microbial community in synthetic fracturing fluid made from freshwater reservoir water can withstand the temperature and pressure conditions of hydraulic fracturing and the fractured shale environment. Using cell enumerations, DNA extraction and culturing, we show that the community can withstand high pressure or high temperature alone, but the combination of both is fatal. These results suggest that initial freshwater-based fracturing fluids are an unlikely source of microorganisms in fractured shales. These findings indicate that potentially problematic lineages, such as sulfidogenic strains of Halanaerobium that have been found to dominate fractured shale microbial communities, likely derive from other input sources into the down well environment, such as drilling muds.
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