Abstract
The stimulation of bacterial activities that convert hexavalent uranium, U(VI), to tetravalent uranium, U(IV), appears to be feasible for cost-effective remediation of contaminated aquifers. However, U(VI) reduction typically results in the precipitation of U(IV) particles less than 5 nanometers in diameter, except for environmental conditions enriched with iron. Because these tiny particles are mobile and susceptible to oxidative dissolution after the termination of nutrient injection, in situ bioremediation remains to be impractical. Here we show that U(IV) nanoparticles of coffinite (U(SiO4)1−x(OH)4x) formed in fracture-filling calcium carbonate in a granitic aquifer. In situ U-Pb isotope dating demonstrates that U(IV) nanoparticles have been sequestered in the calcium carbonate for at least 1 million years. As the microbiologically induced precipitation of calcium carbonate in aquifer systems worldwide is extremely common, we anticipate simultaneous stimulation of microbial activities for precipitation reactions of calcium carbonate and U(IV) nanoparticles, which leads to long-term sequestration of uranium and other radionuclides in contaminated aquifers and deep geological repositories.
Highlights
As calcium carbonate minerals are stable over geological time scales[14,15], the stimulation of heterotrophic activities in anaerobic aquifers coupled to enzymatic uranium reduction might result in the in situ removal of uranium nanoparticles embedded in calcium carbonate
To demonstrate a new form of uranium immobilization in nature, we investigated fracture-filling calcium carbonate formed in a sulfate-reducing granitic aquifer at the vicinity of a uranium ore deposit[16,17]
A well-developed fracture-filling vein of calcium carbonate in granite was collected from a 200-m deep horizontal borehole (07MI07) at the Mizunami Underground Laboratory (MIU) in Japan
Summary
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