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.

ESTIMATION OF EXPOSURE DOSES FOR THE SAFE MANAGEMENT OF NORM WASTE DISPOSAL.

Naturally occurring radioactive materials (NORM) wastes with different radiological characteristics are generated in several industries. The appropriate options for NORM waste management including disposal options should be discussed and established based on the act and regulation guidelines. Several studies calculated the exposure dose and mass of NORM waste to be disposed in landfill site by considering the activity concentration level and exposure dose. In 2012, the Korean government promulgated an act on the safety control of NORM around living environments to protect human health and the environment. For the successful implementation of this act, we suggest a reference design for a landfill for the disposal of NORM waste. Based on this reference landfill, we estimate the maximum exposure doses and the relative impact of each pathway to exposure dose for three scenarios: a reference scenario, an ingestion pathway exclusion scenario, and a low leach rate scenario. Also, we estimate the possible quantity of NORM waste disposal into a landfill as a function of the activity concentration level of U series, Th series and 40K and two kinds of exposure dose levels, 1 and 0.3 mSv/y. The results of this study can be used to support the establishment of technical bases of the management strategy for the safe disposal of NORM waste.

Microbial copper reduction method to scavenge anthropogenic radioiodine.

Unexpected reactor accidents and radioisotope production and consumption have led to a continuous increase in the global-scale contamination of radionuclides. In particular, anthropogenic radioiodine has become critical due to its highly volatile mobilization and recycling in global environments, resulting in widespread, negative impact on nature. We report a novel biostimulant method to effectively scavenge radioiodine that exhibits remarkable selectivity for the highly difficult-to-capture radioiodine of >500-fold over other anions, even under circumneutral pH. We discovered a useful mechanism by which microbially reducible copper (i.e., Cu(2+) to Cu(+)) acts as a strong binder for iodide-iodide anions to form a crystalline halide salt of CuI that is highly insoluble in wastewater. The biocatalytic crystallization of radioiodine is a promising way to remove radioiodine in a great capacity with robust growth momentum, further ensuring its long-term stability through nuclear I(-) fixation via microcrystal formation.

Modeling in-situ transport of uranine and colloids in the fracture network in KURT.

An in-situ dipole migration experiment was conducted using the conservative tracer uranine and latex colloids in KAERI (Korea Atomic Energy Research Institute) Underground Research Tunnel (KURT). The location and dimensions of the fractures between the two boreholes were estimated using the results of a borehole image processing system (BIPS) investigation, and the connectivity of the fractures was evaluated by a packer test. To investigate the flow and transport of uranine and colloids through an in-situ fracture network, a fracture network transport model was newly developed. The model consists of a series of one-dimensional advection-dispersion-matrix diffusion equations for each channel of the fracture network. Using the fracture network transport model, the most probable representation and the hydrologic parameters of the fracture network can be estimated by fitting the breakthrough of uranine. While the fracture network might not be unique, the representation chosen was adequate to describe the breakthrough of uranine and it represents a reasonable approach to modeling transport in the fracture network. An additional evaluation showed that the colloid transport in this study was influenced by filtration on the fracture surface rather than the enhancement of the colloid velocity. Overall, the model can explain successfully the in-situ experimental results of uranine and colloid transports through the fracture network.

Sorption and reduction of selenite on chlorite surfaces in the presence of Fe(II) ions.

The sorption and reduction of selenite on chlorite surfaces in the presence of Fe(II) ions were investigated as a function of pH, Se(IV) concentration, and Fe(II) concentration under an anoxic condition. The sorption of Se(IV) onto chlorite surfaces followed the Langmuir isotherm regardless of the presence of Fe(II) ions in the solution. The Se(IV) sorption was observed to be very low at all pH values when the solution was Fe(II)-free or the concentration of Fe(II) ions was as low as 0.5 mg/L. However, the Se(IV) sorption was enhanced at a pH > 6.5 when the Fe(II) concentration was higher than 5 mg/L because of the increased sorption of Fe(II) onto the chlorite surfaces. XANES (X-ray absorption near edge structure) spectra of the Se K-edge showed that most of the sorbed Se(IV) was reduced to Se(0) by Fe(II) sorbed onto the chlorite surfaces, especially at pH > 9. The combined results of field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) also showed that elemental selenium and goethite were formed and precipitated on the chlorite surfaces during the sorption of selenite. Consequently it can be concluded that Se(IV) can be reduced to Se(0) in the presence of Fe(II) ions by the surface catalytic oxidation of Fe(II) into Fe(III) and the formation of goethite at neutral and particularly alkaline conditions. Thus the mobility of selenite in groundwater is expected to be reduced by the presence of a relatively higher concentration of Fe(II) in subsurface environments.

Reactive transport of uranium with bacteria in fractured rock: model development and sensitivity analysis.

A numerical model for the reactive transport of uranium and bacteria in fractured rock was newly developed. The conceptual model consists of four phases (fracture, fracture surface, matrix pore, and matrix solid) and eight constituents (solutes in the fracture, on the fracture surface, on mobile bacteria, on immobile bacteria, in the rock matrix pores and on the rock matrix solids, and bacteria in the fracture and on the fracture surface). In addition to the kinetic sorption/desorption of uranium and bacteria, uranium reduction reaction accompanying with bacteria growth was considered in the reactive transport. The non-linear reactive transport equations were numerically solved using the symmetric sequential iterative scheme of the operator-splitting method. The transport and kinetic reaction modules in the developed model were separately verified, and the results were reasonably acceptable. From the sensitivity analysis, the uranium transport was generally more sensitive to the sorption rate rather than desorption rate of U(VI). Considering a uranium reduction reaction, bacteria could considerably retard the uranium transport no matter the uranium sorption/desorption rates. As the affinity of U(VI) onto the bacteria becomes higher than that onto a rock fracture surface, a biofilm effect, rather than a colloidal effect, of the bacteria becomes more influential on the uranium transport.

Effects of pH and anions on the sorption of selenium ions onto magnetite.

This study analyzes the influence of carbonate and silicate, which are generally abundant in granitic groundwater, on the sorption of selenium ions onto magnetite in order to understand the behaviors of selenium in a radioactive waste repository. Selenite was sorbed onto magnetite very well below pH 10, but silicate and carbonate hindered the sorption of selenite onto magnetite. On the other hand, little selenate was sorbed onto magnetite in neutral and weak alkaline solutions of 0.02 M NaNO(3) or NaClO(4), matching the ionic strength in a granitic groundwater, even though silicate or carbonate was not contained in the solutions. The surface complexation constants between selenite and magnetite were obtained by using a geochemical program, FITEQL 4.0, from the experimental data, and the formation of an inner-sphere surface complex such as =FeOSeO(2)(-) was suggested for the sorption of selenite onto magnetite from the diffuse double layer model calculation.

Biogenic formation and growth of uraninite (UO2).

Biogenic UO2 (uraninite) nanocrystals may be formed as a product of a microbial reduction process in uranium-enriched environments near the Earth's surface. We investigated the size, nanometer-scale structure, and aggregation state of UO2 formed by iron-reducing bacterium, Shewanella putrefaciens CN32, from a uranium-rich solution. Characterization of biogenic UO2 precipitates by high-resolution transmission electron microscopy (HRTEM) revealed that the UO2 nanoparticles formed were highly aggregated by organic polymers. Nearly all of the nanocrystals were networked in more or less 100 nm diameter spherical aggregates that displayed some concentric UO2 accumulation with heterogeneity. Interestingly, pure UO2 nanocrystals were piled on one another at several positions via UO2-UO2 interactions, which seem to be intimately related to a specific step in the process of growing large single crystals. In the process, calcium that was easily complexed with aqueous uranium(VI) appeared not to be combined with bioreduced uranium(IV), probably due to its lower binding energy. However, when phosphate was added to the system, calcium was found to be easily associated with uranium(IV), forming a new uranium phase, ningyoite. These results will extend the limited knowledge of microbial uraniferous mineralization and may provide new insights into the fate of aqueous uranium complexes.

Uranium and other trace elements' distribution in Korean granite: implications for the influence of iron oxides on uranium migration.

To understand trace radionuclide (uranium) migration occurring in rocks, a granitic batholith located at the Korea Atomic Energy Research Institute (KAERI) site was selected and investigated. The rock samples obtained from this site were examined using mineralogical methods, including scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The changes in the distribution pattern of uranium (U) and small amounts of trace elements, and the mineralogical textures affected by weathering, were examined. Based on the element distribution analyses, it was found that Fe2+ released from fresh biotite is oxidized in short geological time, forming amorphous iron oxides, such as ferrihydrite, around silicate minerals. In that case, the amorphous ferrihydrite does not show distinct adsorption for U. However, as it gradually crystallizes to goethite or hematite, the most U-rich phases were found to be associated with the secondary iron oxides having granular forms. This evidence suggests that the geological subsurface environment is favorable for the crystallized iron oxides to keep their structures more stable for a long time as compared with the amorphous phases. There is a possibility that the long residence of U which is in contact with the stable crystalline phases of iron may finally lead to the partial sequestration of U in their structure. Consequently, it seems that Fe-oxide crystallization can be a dominating mechanism for U uptake and controls long-term U transport in granites with low U contents.