The oxygen concentration and inherent Fe in bentonite have a significant influence on the Se(IV) sorption process. In this study, the sorption of selenite on natural bentonite was investigated using a batch experiment method, and the distribution coefficient (Kd) values were obtained in the pH range from 2.0 to 10.0 under oxic/anoxic conditions. The Kd values always reached a maximum value at a pH of 4 under oxic conditions and at a pH of 8 under anoxic conditions; meanwhile, the Kd value under anoxic conditions was larger than the value under oxic conditions, especially in regard to the maximum Kd values. The oxygen conditions have a significant influence on the ratio of redox-sensitive Fe2+/Fe3+, which was closely related to the difference in the Kd values under both oxic/anoxic conditions. Ferric selenite and green rust could be responsible for the maximum Kd values under oxic/anoxic conditions. In the leaching experiments, we found that the Fe2+ in bentonite could replace Mg2+ and Al3+ in the octahedral sheet. Spectroscopy methods, such as Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) were used to characterize the surface properties of the samples after reaction. Overall, this study shows that the addition of Fe2+-containing materials into backfill/buffer materials under anoxic condition could enhance the sorption of 79Se(IV).

•The sorption of HA on Beishan granite (BsG) is strongly pH and ionic strength-dependent.•Parts of HA tend to enter into the pores formed by close-packing of BsG particles.•Precipitation, ligand exchange–surface complexation and electrostatic interaction may dominate the sorption behaviors of HA on BsG.•The colloidal properties of HA under certain conditions may significantly affect its sorption interactions with BsG.To explore the sorption phenomenon and mechanism between humic acid (HA) and Beishan granite (BsG), the influence of factors such as pH, contact time, ionic strength, temperature, HA concentration and solid-to-liquid ratio were investigated using the batch technique. The sorption process was significantly affected by pH and ionic strength. The maximum adsorption capacity of HA on BsG was approximately 0.9493 mg g−1 at pH = 6.7 and T = 305 K, and the thermodynamic parameters indicated that the sorption process was spontaneous and endothermic. The SEM–EDS results indicated the presence of a certain amount of HA on BsG after sorption with HA. The results of FT-IR spectroscopy and the change in pH demonstrated that ligand exchange–surface complexation may be an important mechanism. BET analysis showed that parts of the HA tended to enter the pores formed by close-packing of BsG particles, and albite and muscovite might have played a role in the sorption process, as indicated by the XRD analysis. Overall, it is speculated that the mechanisms governing the sorption behaviors of HA on BsG are precipitation, ligand exchange–surface complexation and electrostatic interaction. Moreover, the colloidal properties of HA under certain conditions may also significantly affect its sorption interactions with BsG.

To explore the possible application of graphene oxide (GO) as a suitable adsorbent for the removal of U(VI) in solutions, sorption of uranium on GO was investigated as a function of pH, ionic strength, foreign ions and U(VI) concentrations using a batch technique. The results showed that the sorption of U(VI) was strongly pH-dependant and the influences of foreign ions were not significant. The sorption isotherm can be described by the Langmuir model, and the thermodynamic parameters indicated that the sorption process was endothermic and spontaneous. The results suggest that GO is a promising adsorbent for U(VI).

We report an ammonium uranyl formate framework of formula [(C2H5)4N][U2O4(HCOO)5], prepared by using components of tetraethylammonium, uranyl, and formate. The compound possesses a layered structure of anionic uranyl–formate wavy sheets and intercalated (C2H5)4N+ cations. The sheet consists of pentagonal bipyramidal uranyl cations connected by equatorial anti–anti and anti–syn HCOO– bridges, and it has a topology of 33·43·54 made of edge-sharing square and triangle grids. The high-temperature (HT) phase belongs to the chiral but nonpolar tetragonal space group P4̅21m. In the structure, one HCOO– is 2-fold disordered, showing a flip motion between the two anti–syn orientations. On cooling, this flip motion slowed and finally froze, leading to a phase transition at ∼200 K. The low-temperature (LT) structure is monoclinic and polar in space group P21; the cations shift, and the layers slide. Especially, the concerted and net shifts of the ammonium cations toward the −b direction, with respect to the anionic sheets, result in an estimated spontaneous polarization of 0.86 μC cm–2 in LT. The phase transition is thus para- to ferro-electric, in Aizu notation 4̅2mF2, accompanied by significant, anisotropic dielectric anomalies, with a quite significant thermal hysteresis. Variable-temperature luminescent spectroscopy and differential scanning calorimetry confirmed the transition and provided further information. The structure–property relationship is established.

Reductive precipitation is an effective method of attenuating the mobility of uranium (U) in subsurface environments. The reduction of U(VI) by synthetic and naturally occurring pyrite was investigated at pH 3.0–9.5. In contrast to thermodynamic calculations that were used to predict UO2(s) precipitation, a mixed U(IV) and U(VI) product (e.g., U3O8/U4O9/U3O7) was only observed at pH 6.21–8.63 and 4.52–4.83 for synthetic and natural pyrite, respectively. Under acidic conditions, the reduction of UO22+ by surface-associated Fe2+ may not be favored because the mineral surface is nearly neutral or not negative enough. At high pH, the sorption of negatively charged U(VI) species is not favored on the negatively charged mineral surface. Thus, the redox reaction is not favored. Trace elements generally contained within the natural pyrite structure can affect the reactivity of pyrite and lead to a different result between the natural and synthetic pyrite. Because UO2(s) is extremely redox-sensitive toward U(VI), the observed UO2+x(s) phase reduction product indicates a surface reaction that is largely controlled by reaction kinetics and pyrite surface chemistry. These factors may explain why most laboratory experiments have observed incomplete U(VI) reduction on Fe(II)-bearing minerals.

Crystallizations in uranyl-containing ionic liquids yielding crystals {X}a{[UO2]b[Y]c} with selected anions and cations (X = [Bmim]+ for Y = Cl–, NO3–, and SCN–; Y = Cl– for X = [Emim]+, [Emmim]+, [Bmim]+, and [Bmmim]+; Emim = l-ethyl-3-methyl-imidazolium, Emmim = l-ethyl-2,3-dimethylimidazolium, Bmim = l-butyl-3-methylimidazolium, and Bmmim = l-butyl-2,3-dimethylimidazolium) were performed herein. Through standard crystallographic analyses, Hirshfeld surface analyses, and multiple characterization techniques, compounds with common cations/anions were investigated. For compounds [Bmim]2[(UO2)2(μ-OH)2(NO3)4] (1), [Bmim]3[UO2(NCS)5] (2), and [Bmim]2[UO2Cl4] (3TT and 3TG), common [Bmim]+ cations with different conformations were studied with respect to packing and interactions (for 1). The coordinated [(UO2)2(μ-OH)2(NO3)4]2–, [UO2(NCS)5]3–, and [UO2Cl4]2– anions that have historically been related to nuclear fuel cycles were demonstrated with respect to geometry and distortion. For compounds with common [UO2Cl4]2– anions [Emim]2[UO2Cl4] (4), [Emmim]2[UO2Cl4] (5), [Bmmim]2[UO2Cl4] (6), 3TT, and 3TG, observed interionic interactions that have been previously impeded by limited structural information were discussed fully in relation to different cations and temperatures. Moreover, multistep phase transformations of 2, which have been undefined in solution studies, have been identified through differential scanning calorimetry analyses and polarizing optical microscopy. The polymorph transformations between 3TT and 3TG in solution, as controlled by uranyl concentration, were studied using optical microscopy and powder X-ray diffraction. The thermal stability, IR/Raman, and UV–vis/luminescence spectra of these compounds were also investigated.

This work presents the first report of a method for simultaneous speciation of inorganic selenium and iodine using high-performance liquid chromatography coupled with inductively coupled plasma mass spectrometry (HPLC-ICP-MS). Baseline separation of selenite, selenate, iodide and iodate within a single chromatographic run of 10 min was achieved with the optimized mobile phase of 3 mM ammonium citrate, 25 mM ammonium perchlorate and 2% (v/v) methanol at pH 8.5. The mass spectrometric operating conditions were optimized to detect selenium and iodine simultaneously. Potential matrix effects were addressed and corrected using the standard addition method, and the recoveries (95.7–103%) were satisfactory. The repeatability and intermediate precision were satisfactory (the coefficients of variation were all less than 5% and 8%, respectively). The obtained detection limits were 22 ng L−1 and 23 ng L−1 for selenite and selenate, respectively, and 4.3 ng L−1 and 5.4 ng L−1 for iodide and iodate, respectively, which allows for the speciation of selenium and iodine in groundwater at the ultra-trace level. Groundwater samples collected from a candidate site for the Chinese high-level radioactive waste repository were analyzed. Within the three different groundwater samples, selenate was observed to be the dominant Se species; in addition, the iodide concentration increased and the iodate concentration decreased with increasing sampling depth. With its satisfactory sensitivity and accuracy, this method provides a promising tool for studying the environmental behaviors of selenium and iodine, thereby supporting safety assessments of high-level radioactive waste repositories.

The effects of pH, counter ions and temperature on the adsorption of U(VI) on Beishan granite (BsG) were investigated in the presence and absence of fulvic acid (FA) and humic acid (HA). The adsorption edge of U(VI) on BsG suggested that U(VI) adsorption was mainly controlled by ion exchange and outer-sphere complexation at low pH, whereas inner-sphere complex was the dominant adsorption species in the pH range of 4.0–9.0. Above pH 9.0, Na2U2O7 might play an important role in the rise of U(VI) adsorption again. Counter ions such as Cl−, SO42− and PO43− can provoke U(VI) adsorption on BsG to some extent, which was directly correlated to the complexing ability of U(VI)–ligand. More noticeably, the large enhancement of U(VI) adsorption in the presence of phosphate can be attributed to the ternary complex formation (BsG–PO4–UO2), precipitation ((UO2)3(PO4)2(s)) and secondary phase (Na-autunite). Both FA and HA can slightly increase U(VI) adsorption at low pH, whereas they strongly inhibited U(VI) adsorption at high pH range. Artificial synthesized granite (AsG) prepared in the laboratory is impossible to use as an analogue of natural granite because of the large difference in the adsorption and surface properties.

The mobility and bioavailability of selenium is a major health and environmental issue and a main concern for geological disposal of high-level radioactive waste. Chemically and/or microbially mediated oxidation of insoluble Se-bearing particulate, such as iron selenides, to dissolved and mobile phases controls the transport and distribution of Se in the environment. The oxidation of ferroselite (FeSe2) by ferric iron was investigated in anoxic conditions. The redox reaction can be represented by: FeSe2 + 2Fe3+ = 2Se0 + 3Fe2+. Kinetic studies indicated that the reaction can be described by second-order rate law, with rate constants of 0.49±0.01, 0.85±0.02, 1.84±0.04, and 3.29±0.13 L mol−1 s−1 at pH 1.62, 1.87, 2.23, and 2.49, respectively. The positive correlation between reaction rate and pH implies that diffusion of Fe3+ oxidant to the mineral surface is the rate-determining step. The strong reactivity of FeSe2 towards Fe3+ suggests that ferric iron may play a significant role in FeSe2 oxidation process (e.g., by Se4+, O2, etc.) and Se0 should be the first reaction product. Also, it was shown that the reduction rate of Fe3+ or Se4+ by pyrite (FeS2) can be significantly increased in the presence of FeSe2, suggesting a stronger reactivity of FeSe2 compared with pyrite. The results obtained extend our knowledge about the subtle interaction between Se, pyrite and iron selenides in the environment, and give insight into the transfer of selenium from iron selenides to bio-available selenium (i.e., selenite and selenate) in the Se-rich environment.

We present here two systems of [dabcoH2]2+–uranyl–oxalate and [pipH2]2+–uranyl–oxalate in which [dabcoH2]2+ and [pipH2]2+ are cations of doubly protonated 1,4-diazabicyclo-[2.2.2]-octane (dabco) and piperazine (pip), respectively. Each system yielded two different crystals and showed the reversible crystal-to-crystal transformations between them in aqueous solutions, controlled by the ratio of reactants or building blocks in the reaction systems. The four compounds in pairs are [dabcoH2][UO2(C2O4)2(H2O)]·2H2O (dabco1) and [dabcoH2][(UO2)2(C2O4)3(H2O)2]·2H2O (dabco2), and [pipH2][UO2(C2O4)2(H2O)]·4H2O (pip1) and [pipH2][(UO2)2(C2O4)3(H2O)2]·2H2O (pip2). Besides the cations and lattice water, dabco1 and pip1 contain mononuclear anions of [UO2(C2O4)2(H2O)]2–, whereas dabco2 and pip2 possess dinuclear anions of [(UO2)2(C2O4)3(H2O)2]2–, and in all structures, the uranium ion shows a pentagonal bipyramid environment made up of equatorial oxalate, water, and apical oxygen. The needle crystals of dabco1 belong to the chiral space group P6522. In the structure, the [UO2(C2O4)2(H2O)]2– anions form anionic helixes along the 65 axis and they are further linked by N–H···O hydrogen bonds between the interhelix [dabcoH2]2+ cation and oxalates of the anion. Disorder of lattice water and the [dabcoH2]2+ cation is observed in dabco1. The thin plate crystals of pip1 in space group P1̅ possess a lamellar structure, with dense layers of [pipH2]2+·2[UO2(C2O4)2(H2O)]2– pillared by other crystallographically unique [pipH2]2+ cations, and the structure contains lattice water forming branched zigzag water chains. Block crystals of dabco2 and pip2, in monoclinic space groups C2/c and P21/c, respectively, are of similar layer-like structures, possessing the dinuclear [(UO2)2(C2O4)3(H2O)2]2– anion with one tetradentate oxalate bridge. The anions and ammonium cations form chains by N–H···O hydrogen bonds between them and then further form stacked layers via O–H···O hydrogen bonds among the coordinated and lattice water and the oxalate ligands. The ratio of diammonium/uranyl/oxalate in the reaction systems controlled the final outcomes and the occurrence of transformation, no transformation, and reverse transformation. The thermal stability; UV–vis, IR, and Raman spectra; and luminescence were also investigated.

In terms of pre-safety assessment of a potential site for high-level radioactive wastes disposal in China, the geochemical behavior of key radionuclides which tend to be released from the repository must be thoroughly investigated. 99Tc is a long-lived fission product with appreciable productivity in nuclear fuel, and Tc (+7) has unlimited solubility in near-field geochemical environments. In this study, the effects of ionic strength and humic acid on 99TcO4− sorption and diffusion in Beishan granite were investigated with through-diffusion and batch sorption experiments. Results indicated that the effective diffusion coefficients (De) of 99TcO4− in Beishan granite varied from 1.07 × 10−12 to 1.28 × 10−12 m2/s without change with ionic strength, while the distribution coefficients (Kd) negatively correlated with ionic strength of the rock/water system. This study also indicates that there is no evident influence of humic acid concentration on the diffusion behavior of 99TcO4− in Beishan granite, due to the limited interaction between humic acid and 99TcO4−.

The adsorption and migration behavior of a radionuclide in geological media heavily depends on its chemical forms in a given chemical environment. In order to predict the temporal and spatial distribution of radionuclides around a disposal site when its canister is damaged, it is necessary to develop coupled chemical speciation-solute transport models and relevant software. For that reason, we wrote a new chemical speciation program CHEMSPEC. In this paper, the principles and structure of CHEMSPEC are briefly described, and the strategy and algorithms that were used in this code are interpreted in some detail, such as the measures adopted to prevent divergence in iteratively solving the mass balance equations, the “predictor-corrector” algorithm for calculation of the number and quantities of solid species formed, and the alternate use of “freezing” and “defreezing” oxidation states in handling of co-existent redox and precipitation equilibria. Four examples are given to illustrate CHEMSPEC’s features and capabilities.

The concentration of radon in an underground research facility (URF) was measured by setting up 12 sampling points in the URF and with 3 different measurement methods. All the methods were calibrated in the radon laboratory of the No. 6 Institute of Nuclear Industry. The accumulation of radon in the URF was observed before a ventilation system was applied. The reduction of radon concentration in the URF by 1-hour ventilation was also observed. Experimental result indicates that the concentration of radon in the URF increased from 15 to 50 Bq·m−3 in 5 days without ventilation, and decreased to less than 10 Bq·m−3 with 1-hour ventilation. Applying the average working time of 4 hours per day of the workers in the URF, the additional effective dose is 0.75 msv·y−1 when 1 hour ventilation is applied before entering the URF and 13 mSv·y−1 without ventilation. These figures strongly suggest that for the health of the workers, ventilation in such underground research facilities is needed.

•The diffusion of 75SeO32− in crushed BsG was slowest at approximately pH 5.•The Da values under aerobic conditions were nearly one order of magnitude lower than those under anaerobic conditions.•Fe(II)-bearing mineralogical components can be responsible for the great difference in Da values.•SeIV can be reduced to Se0 by BsG under anaerobic conditions.The diffusion of selenite (labeled with 75Se) in compacted Beishan granite (BsG) was investigated using the in-diffusion capillary method at pH values from ∼2.0 to ∼11.0 under oxic and anoxic conditions. The results indicate that the apparent diffusion coefficient (Da) values of selenite in BsG always reached the minimum at approximately pH 5. Unexpectedly, the Da values under oxic conditions are nearly one order of magnitude lower than those under the anoxic conditions. Further characterization reveals the existence of redox-sensitive Fe(II)-containing components, which can be responsible for the great difference in Da values. Fe(2p) X-ray photoelectron spectroscopy (XPS) results show that more Fe(III)-oxyhydroxide coating is formed on the granite’s surface under aerobic conditions than is formed under anaerobic conditions. Correspondingly, Se(3d) spectra indicate that more selenium is sorbed under oxic conditions, and the sorbed amount always reached the maximum at pH values from ∼4 to ∼5. A linear combination fit of X-ray absorption near edge structure (XANES) spectroscopy data revealed that Se(0) was formed under anoxic condition and that selenite preferred to form inner-sphere complexes with Fe(III)-oxyhydroxide. Overall, this study indicates that natural Fe-bearing minerals can greatly attenuate selenite diffusion and the retardation would be enhanced under aerobic conditions.Download full-size image

Molecular dynamics (MD) simulations of uranyl species adsorption in montmorillonite pores in 0, 0.05 and 0.10 M NaCl were carried out to investigate the influence of internal electrolyte concentration on the uptake amount, species distribution and electrical double-layer (EDL) structures. Most cases revealed that the β- and d-planes of the adsorbates are located 4.3–4.6 Å and 5.5–5.8 Å from the clay planes, respectively. However, based on the split carbonate peaks, an exception, the left peaks near the clay surface for 0.10 M NaCl, formed more uranyl-(bi)carbonate complexes than the other peaks. Under this condition, each peak and minimum of cations shifted slightly farther away from the clay plane. For this outlying case, the charge density profile confirmed the charge inversion of carbonates near like-charged surfaces. Collectively, the simulations revealed the subtle influence of the internal NaCl concentration (0–0.05 M) on the EDL structure, uptake amount and species distribution. In particular, a threshold concentration (0.10 M NaCl in this study) for charge inversion within the β-plane may exist. Under this condition, a pronounced change in the EDL structure occurs, which in turn causes a dramatic alteration in the uranyl species adsorption relative to lower electrolyte concentrations.

The effects of pH, counter ions and temperature on the adsorption of U(VI) on Beishan granite (BsG) were investigated in the presence and absence of fulvic acid (FA) and humic acid (HA). The adsorption edge of U(VI) on BsG suggested that U(VI) adsorption was mainly controlled by ion exchange and outer-sphere complexation at low pH, whereas inner-sphere complex was the dominant adsorption species in the pH range of 4.0–9.0. Above pH 9.0, Na2U2O7 might play an important role in the rise of U(VI) adsorption again. Counter ions such as Cl−, SO42− and PO43− can provoke U(VI) adsorption on BsG to some extent, which was directly correlated to the complexing ability of U(VI)–ligand. More noticeably, the large enhancement of U(VI) adsorption in the presence of phosphate can be attributed to the ternary complex formation (BsG–PO4–UO2), precipitation ((UO2)3(PO4)2(s)) and secondary phase (Na-autunite). Both FA and HA can slightly increase U(VI) adsorption at low pH, whereas they strongly inhibited U(VI) adsorption at high pH range. Artificial synthesized granite (AsG) prepared in the laboratory is impossible to use as an analogue of natural granite because of the large difference in the adsorption and surface properties.

This work presents the first report of a method for simultaneous speciation of inorganic selenium and iodine using high-performance liquid chromatography coupled with inductively coupled plasma mass spectrometry (HPLC-ICP-MS). Baseline separation of selenite, selenate, iodide and iodate within a single chromatographic run of 10 min was achieved with the optimized mobile phase of 3 mM ammonium citrate, 25 mM ammonium perchlorate and 2% (v/v) methanol at pH 8.5. The mass spectrometric operating conditions were optimized to detect selenium and iodine simultaneously. Potential matrix effects were addressed and corrected using the standard addition method, and the recoveries (95.7–103%) were satisfactory. The repeatability and intermediate precision were satisfactory (the coefficients of variation were all less than 5% and 8%, respectively). The obtained detection limits were 22 ng L−1 and 23 ng L−1 for selenite and selenate, respectively, and 4.3 ng L−1 and 5.4 ng L−1 for iodide and iodate, respectively, which allows for the speciation of selenium and iodine in groundwater at the ultra-trace level. Groundwater samples collected from a candidate site for the Chinese high-level radioactive waste repository were analyzed. Within the three different groundwater samples, selenate was observed to be the dominant Se species; in addition, the iodide concentration increased and the iodate concentration decreased with increasing sampling depth. With its satisfactory sensitivity and accuracy, this method provides a promising tool for studying the environmental behaviors of selenium and iodine, thereby supporting safety assessments of high-level radioactive waste repositories.