Anaerobic Dissolution Rates of U(IV)-Oxide by Abiotic and Nitrate-Dependent Bacterial Pathways

Area of Science:

• Environmental Geochemistry
• Microbial Geochemistry
• Nuclear Waste Management

Background:

• The stability of reduced uranium (U(IV)) solid phases in anaerobic environments is critical for nuclear waste disposal and understanding natural uranium cycling.
• Reactivity of U(IV) solids is influenced by chemical oxidants and microbial activity, particularly in subsurface aquifers.
• Nitrate and nitrite are common redox-active species in groundwater that can impact uranium speciation and mobility.

Purpose of the Study:

• To investigate the mechanisms and rates of biogenic uranium dioxide (UO2(s)) dissolution under anaerobic conditions.
• To compare the efficacy of chemical oxidants (nitrate, nitrite) versus microbial oxidation by *Thiobacillus denitrificans*.
• To elucidate the role of microbial U(IV) oxidation in the mobilization of reduced uranium solids.

Main Methods:

• Flow-through column experiments conducted under strictly anaerobic conditions.
• Utilized biogenic, noncrystalline UO2(s) as the substrate.
• Employed chemical oxidants (nitrate, nitrite) and wild-type and mutant strains of *Thiobacillus denitrificans*.
• Analyzed post-reaction solids using X-ray absorption spectroscopy (XAS).

Main Results:

• Dissolution rates of UO2(s) were 5-10 times higher with nitrite compared to nitrate alone.
• Microbially mediated UO2(s) dissolution by *T. denitrificans* with nitrate yielded rates comparable to abiotic nitrite dissolution.
• XAS analysis confirmed the formation of soluble U(VI) species, not a solid U(VI) phase.
• Surface detachment of oxidized U(VI) was identified as the rate-determining step.

Conclusions:

• Nitrate-reducing microorganisms, through nitrite production and direct enzymatic catalysis, significantly enhance U(IV) oxidation and mobilization.
• The formation of soluble U(VI) species, rather than secondary solid phases, facilitates uranium release.
• These findings highlight the potential for microbial activity to increase uranium mobility in contaminated aquifers and geological repositories.