Identifying the origin and fate of dissolved U in the Boeun aquifer based on microbial signatures and C, O, Fe, S, and U isotopes.

The uranium inventory in the Boeun aquifer is situated near an artificial reservoir (40-70 m apart) intended to supply water to nearby cities. However, toxic radionuclides can enter the reservoir. To determine the U mobility in the system, we analyzed groundwater and fracture-filling materials (FFMs) for environmental tracers, including microbial signatures, redox-sensitive elements and isotopes. In the site, U mass flux ranged from only 9.59 × 10-7 µg/L/y to 1.70 × 10-4 µg/L/y. The δ18O-H2O and 14C signatures showed that groundwater originated mainly from upland recharges and was not influenced by oxic surface water. We observed U accumulations (∼157 mg/kg) in shallow FFMs and Fe enrichments (∼226798 mg/kg) and anomalies in the 230Th/238U activity ratio (AR), 230Th/234U AR, δ56Fe and δ57Fe isotopes, suggesting that low U mobility in shallow depths is associated with a Fe-rich environment. At shallow depths, anaerobic Fe-oxidizers, Gallionella was prevalent in the groundwater, while Acidovorax was abundant near the U ore deposit depth. The Fe-rich environment at shallow depths was formed by sulfide dissolution, as demonstrated by δ34S-SO4 and δ18O-SO4 distribution. Overall, the Fe-rich aquifer including abundant sulfide minerals immobilizes dissolved U through biotic and abiotic processes, without significant leaching into nearby reservoirs.

Potential of indigenous bacteria driven U(VI) reduction under relevant deep geological repository (DGR) conditions.

Understanding the biogeochemical U redox processes is crucial for controlling U mobility and toxicity under conditions relevant to deep geological repositories (DGRs). In this study, we examined the microbial reduction of aqueous hexavalent uranium U(VI) [U(VI)aq] by indigenous bacteria in U-contaminated groundwater. Three indigenous bacteria obtained from granitic groundwater at depths of 44-60 m (S1), 92-116 m (S2), and 234-244 m (S3) were used in U(VI)aq bioreduction experiments. The concentration of U(VI)aq was monitored to evaluate its removal efficiency for 24 weeks under anaerobic conditions with the addition of 20 mM sodium acetate. During the anaerobic reaction, U(VI)aq was precipitated in the form of U(IV)-silicate with a particle size >100 nm. The final U(VI)aq removal efficiencies were 37.7%, 43.1%, and 57.8% in S1, S2, and S3 sample, respectively. Incomplete U(VI)aq removal was attributed to the presence of a thermodynamically stable calcium uranyl carbonate complex in the U-contaminated groundwater. High-throughput 16S rRNA gene sequencing analysis revealed the differences in indigenous bacterial communities in response to the depth, which affected to the U(VI)aq removal efficiency. Pseudomonas peli was found to be a common bacterium related to U(VI)aq bioreduction in S1 and S2 samples, while two SRB species, Thermodesulfovibrio yellowstonii and Desulfatirhabdium butyrativorans, played key roles in the bioreduction of U(VI)aq in S3 sample. These results indicate that remediation of U(VI)aq is possible by stimulating the activity of indigenous bacteria in the DGR environment.

Uranium sequestration in fracture filling materials from fractured granite aquifers.

The migration of the uranium (U) in high-level radioactive waste that is held in deep geological repositories via fractures in deep granite aquifers is a serious safety concern, thus, this study investigates the effect of fracture filling materials designed to mitigate these concerns. Geochemical analysis was conducted on granite rock core and groundwater samples collected from boreholes located in granite areas. Sequential extraction tests on fracture filling material (FFM) samples were also conducted. The rock core samples were classified as two-mica granite that had uranium (U) content ranging from 1900 to 22,100 µg/kg with an arithmetic mean of 8500 µg/kg. The total U concentration in the FFM samples was found to range from 790 to 80,781 µg/kg. The U in the FFM samples was mainly associated with a carbonate phase that made up from 29.9 to 100% of the total U in the FFM. The U fraction of carbonate phase was closely correlated with the Ca fraction. U associated with crystalline inorganic FFM constituents (e.g, clay minerals and metal oxyhydroxides) was also found in FFM samples in fractions ranging from 21.1 to 70.1%. U in FFM is mainly incorporated via Ca-carbonate, which might have not been formed in modern groundwater, but the time and temperature during formation are unknown. In addition, the Fe, Si, Al, Ca, K, and U levels were found to be well correlated with each other, suggesting that U can also become geochemically associated with crystalline clay minerals or Fe-oxyhydroxides.

Chlorite alteration in aqueous solutions and uranium removal by altered chlorite.

Chlorite alteration and the U removal capacity of altered chlorite were investigated. Batch kinetic dissolution tests using clinochlore CCa-2 were conducted for 60days in aqueous solutions of various pHs and ionic strengths. Batch sorption tests using these altered chlorite samples were conducted for 48h with natural groundwater containing 3.06×10-6M U. Chlorite dissolution was influenced more by pHo than by the ionic strength of the solution. TEM analysis revealed Fe(oxy)hydroxide aggregates in the solid residue from the batch dissolution test with 0.1M NaClO4 solution at pHo=10. The U removal capacity of the reacted chlorite samples at pHo=6-10 was higher than that of the reacted chlorite samples at pHo=3. The degree of dissolution of chlorite samples reacted at pHo=3-8 was inversely proportional to the U removal capacity, but that of chlorite samples reacted at pHo=10 was proportional to the U removal capacity. The positive correlation between the U removal capacity and degree of chlorite dissolution at pHo=10 might be due to the formation of Fe-containing secondary minerals and changes in the reactive sites.

A microfluidic approach to water-rock interactions using thin rock sections: Pb and U sorption onto thin shale and granite sections.

The feasibility of using microfluidic tests to investigate water-rock (mineral) interactions in fractures regarding sorption onto thin rock sections (i.e., shale and granite) of lead (Pb) and uranium (U) was evaluated using a synthetic PbCl2 solution and uranium-containing natural groundwater as fluids. Effluent composition and element distribution on the thin rock sections before and after microfluidic testing were analyzed. Most Pb removal (9.8mg/cm2) occurred within 3.5h (140 PVF), which was 74% of the total Pb removal (13.2mg/cm2) at the end of testing (14.5h, 560 PVF). Element composition on the thin shale sections determined by µ-XRF analysis indicated that Pb removal was related primarily to Fe-containing minerals (e.g., pyrite). Two thin granite sections (biotite rich, Bt-R and biotite poor, Bt-P) exhibited no marked difference in uranium removal capacity, but a slightly higher amount of uranium was removed onto the thin Bt-R section (266µg/cm2) than the thin Bt-P section (240µg/cm2) within 120h (4800 PVF). However, uranium could not be detected by micro X-ray fluorescence (µ-XRF) analysis, likely due to the detection limit. These results suggest that microfluidic testing on thin rock sections enables quantitative evaluation of rock (mineral)-water interactions at the micro-fracture or pore scale.

Effect of Cr(VI) concentration on gas and particle production during iron oxidation in aqueous solutions containing Cl- ions.

Zero-valent iron (ZVI) is commonly used as a medium in permeable reactive barriers (PRBs) because of its high reducing ability. The generation of H2 gas in PRBs, however, can decrease the permeability of PRBs and reduce the contact area between the PRB and contaminated groundwater. This study investigated the effect of the initial Cr(VI) concentration ([Cr(VI)init]) in aqueous solutions containing Cl- ions on the generation of H2 gas. ZVI chips were reacted in reactors with 0.5-M NaCl solutions with [Cr(VI)init] ranging between 51 and 303 mg/L. The initial pH was set at 3. The oxidation of ZVI chips by Cr(VI) in aqueous solutions containing Cl- ions produced H2 gas and particles (Fe(III)-Cr(III)(oxy)hydroxides). The Cr(VI) removal from aqueous solutions increased as the [Cr(VI)init] increased, as did H2 gas generation. The positive effect of [Cr(VI)init] on H2 gas generation might be due to an increase in the redox potential gradient as [Cr(VI)init] increases. This increased gradient would enhance H+ ion penetration through the passive film (Fe(III)-Cr(III)(oxy)hydroxides), which formed on the ZVI surface, by diffusion from the solution to pits beneath the passive film.

Characterization of redox processes in shallow groundwater of Owens Dry Lake, California.

Redox status of shallow groundwaters (1-3 m depths) at Owens Dry Lake was studied to help guide mitigation efforts for attenuating dust generation from the dry lakebed. Redox conditions were characterized by Eh, oxidative capacity (OXC), and terminal electron accepting processes (TEAPs) as well as examining the energetics of TEAPs. Groundwater chemistry related to redox status was determined by major solute concentrations, dissolved gases (O2, H2, CH4), aqueous redox species (NO3, Mn2+, Fe2+/ Fe3+, SO4(2-)/HS-, DOC), and major redox sensitive components in the solid phase (extractable Fe/Mn). All of these measures of redox status indicate that sulfate reduction is the major process regulating redox conditions in most shallow groundwaters of Owens Dry Lake. Dissolved sulfate concentrations were regulated primarily by evaporation resulting in increasing concentrations as water migrates from the shoreline (<1 mM) to the center (up to 417 mM) of the dry lakebed. Eh values were generally in the range of -240 to -170 mV. The oxidative capacity demonstrates the dominant contribution of sulfate to OXC. The dominance of sulfate restricts further redox development, such as methanogenesis. Dissolved H2 concentrations ranged from 0.5 to 7.8 nM. According to the empirically defined H2 ranges, sulfate reduction was the most predominant TEAP. Moreover, thermodynamic calculations of TEAPs for H2 utilization support favorable energetics for both sulfate reduction and methanogenesis. The calculated available energy yield for sulfate reduction in the shallow groundwater of Owens Dry Lake was higher than other systems due to the high sulfate concentration.