The effect of calcium on aqueous uranium(VI) speciation and adsorption to ferrihydrite and quartz

Recent studies of uranium(VI) geochemistry have focused on the potentially important role of the aqueous species, CaUO₂(CO₃)₃²⁻ and Ca₂UO₂(CO₃)₃⁰(aq), on inhibition of microbial reduction and uranium(VI) aqueous speciation in contaminated groundwater. However, to our knowledge, there have been no direct studies of the effects of these species on U(VI) adsorption by mineral phases. The sorption of U(VI) on quartz and ferrihydrite was investigated in NaNO₃ solutions equilibrated with either ambient air (430 ppm CO₂) or 2% CO₂ in the presence of 0, 1.8, or 8.9 mM Ca²⁺. Under conditions where the Ca₂UO₂(CO₃)₃⁰(aq) species predominates U(VI) aqueous speciation, the presence of Ca in solution lowered U(VI) adsorption on quartz from 77% in the absence of Ca to 42% and 10% at Ca concentrations of 1.8 and 8.9 mM, respectively. U(VI) adsorption to ferrihydrite decreased from 83% in the absence of Ca to 57% in the presence of 1.8 mM Ca. Surface complexation model predictions that included the formation constant for aqueous Ca₂UO₂(CO₃)₃⁰(aq) accurately simulated the effect of Ca²⁺ on U(VI) sorption onto quartz and ferrihydrite within the thermodynamic uncertainty of the stability constant value. This study confirms that Ca²⁺ can have a significant impact on the aqueous speciation of U(VI), and consequently, on the sorption and mobility of U(VI) in aquifers.     

Geochemical evolution of uraniferous soda lakes in Eastern Mongolia

Extremely high concentrations of uranium (U) were discovered in shallow, groundwater-fed hyperalkaline soda lakes in Eastern Mongolia. A representative groundwater sample in this area is dilute and alkaline, pH = 7.9, with 10 mM TIC and 5 mM Cl⁻. In contrast, a representative lake water sample is pH ~ 10 with TIC and Cl⁻ each more than 1,000 mM. Groundwater concentrations of U range from 0.03 to 0.43 μM L⁻¹. Lake water U ranges from 0.24 to >62.5 μM, possibly the highest naturally occurring U concentrations ever reported in surface water. Strontium isotopes ⁸⁷Sr/⁸⁶Sr varied in groundwaters from 0.706192 to 0.709776 and in lakes ⁸⁷Sr/⁸⁶Sr varied from 0.708702 to 0.709432. High concentrations of U, Na, Cl⁻, and K correlate to radiogenic Sr in lake waters suggesting that U is sourced from local Cretaceous alkaline rhyolites. Uranium-rich groundwaters are concentrated by evaporation and U(VI) is chelated by CO₃⁻² to form the highly soluble UO₂(CO₃)₃⁻⁴. Modeled evaporation of lakes suggests that a U-mineral phase is likely to precipitate during evaporation.     

Interaction of U(VI) with Äspö diorite: A batch and in situ ATR FT-IR sorption study

Pristine diorite drill cores, obtained from the Äspö Hard Rock Laboratory (HRL, Sweden), were used to study the retention properties of fresh, anoxic crystalline rock material towards the redox-sensitive uranium. Batch sorption experiments and spectroscopic methods were applied for this study. The impact of various parameters, such as solid-to-liquid ratio (2–200 g/L), grain size (0.063–0.2 mm, 0.5–1 mm, 1–2 mm), temperature (room temperature and 10 °C), contact time (5–108 days), initial U(VI) concentration (3 × 10⁻⁹ to 6 × 10⁻⁵ M), and background electrolyte (synthetic Äspö groundwater and 0.1 M NaClO₄) on the U(VI) sorption onto anoxic diorite was studied under anoxic conditions (N₂). Comparatively, U(VI) sorption onto oxidized diorite material was studied under ambient atmosphere (pCO₂ = 10^−3.5 atm). Conventional distribution coefficients, K_d, and surface area normalized distribution coefficients, K_a, were determined. The K_d value for the U(VI) sorption onto anoxic diorite in synthetic Äspö groundwater under anoxic conditions by investigating the sorption isotherm amounts to 3.8 ± 0.6 L/kg which corresponds to K_a = 0.0030 ± 0.0005 cm (grain size 1–2 mm). This indicates a weak U sorption onto diorite which can be attributed to the occurrence of the neutral complex Ca₂UO₂(CO₃)₃(aq) in solution. This complex was verified as predominating U species in synthetic Äspö groundwater by time-resolved laser-induced fluorescence spectroscopy (TRLFS). Compared to U sorption at room temperature under anoxic conditions, U sorption is further reduced at decreased temperature (10 °C) and under ambient atmosphere. The U species in aqueous solution as well as sorbed on diorite were studied by in situ time-resolved attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy. A predominant sorbing species containing a UO₂(CO₃)₃⁴⁻ moiety was identified. The extent of U sorption onto diorite was found to depend more on the low sorption affinity of the Ca₂UO₂(CO₃)₃(aq) complex than on reduction processes of uranium.     

X-ray absorption spectroscopy identifies calcium-uranyl-carbonate complexes at environmental concentrations

Current research on bioremediation of uranium-contaminated groundwater focuses on supplying indigenous metal-reducing bacteria with the appropriate metabolic requirements to induce microbiological reduction of soluble uranium(VI) to poorly soluble uranium(IV). Recent studies of uranium(VI) bioreduction in the presence of environmentally relevant levels of calcium revealed limited and slowed uranium(VI) reduction and the formation of a Ca-UO₂-CO₃ complex. However, the stoichiometry of the complex is poorly defined and may be complicated by the presence of a Na-UO₂-CO₃ complex. Such a complex might exist even at high calcium concentrations, as some UO₂-CO₃ complexes will still be present. The number of calcium and/or sodium atoms coordinated to a uranyl carbonate complex will determine the net charge of the complex. Such a change in aqueous speciation of uranium(VI) in calcareous groundwater may affect the fate and transport properties of uranium. In this paper, we present the results from X-ray absorption fine structure (XAFS) measurements of a series of solutions containing 50 μM uranium(VI) and 30 mM sodium bicarbonate, with various calcium concentrations of 0-5 mM. Use of the data series reduces the uncertainty in the number of calcium atoms bound to the UO₂-CO₃ complex to approximately 0.6 and enables spectroscopic identification of the Na-UO₂-CO₃ complex. At nearly neutral pH values, the numbers of sodium and calcium atoms bound to the uranyl triscarbonate species are found to depend on the calcium concentration, as predicted by speciation calculations.     

Experimental study and modeling of the sorption of uranium(VI) onto olivine-rock

The sorption of U(VI) onto the surface of olivine has been experimentally investigated at 25 °C under an air atmosphere as a function of pH, solid surface to volume ratio and total U concentration. Sorption has been observed to decrease as the extent of carbonate complexation of U(VI) in solution increases, which is attributed to the competition between aqueous and solid ligands for the coordination of U. The experimental results have been interpreted by means of two different approaches: (1)a semi-empirical model, exemplified by the application of a Langmuir isotherm and (2) a non-electrostatic thermodynamic surface complexation model which includes the formation of the surface species: >SO–UO₂⁺ and >SO–UO₂(OH). The following stability constants for these species have been determined from the thermodynamic analysis: K(>SO–UO₂⁺)=289±71 and K(>SO–UO₂(OH))=(3.4±0.4)×10⁻⁶. The comparison of the sorption of U onto olivine with granites of different origin indicate that the use of this mineral as additive to the backfill of deep high level nuclear waste repositories could retard the migration of U from the repository to the geosphere.     

Impact of Alkaline Earth Metals on Aqueous Speciation of Uranium(VI) and Sorption on Quartz

The effect of Mg-, Ca-, and Sr–Uranyl-Carbonato complexes with respect to sorption on quartz was studied by means of batch experiments with U(VI) concentration of 0.126 × 10⁻⁶ M in the presence and absence of Mg, Ca, and Sr (each 1 mM) at pH from 6.5 to 9. In the absence of alkaline earth elements, 90% of the U(VI) sorbed on the quartz surface at all pH. In the presence of Mg, Ca, and Sr, the sorption of U(VI) on quartz decreased to 50, 10, and 30%, respectively. Sorption kinetics of U(VI) on quartz is faster in the absence of alkaline earth elements and reached equilibrium after 12 h, whereas in the presence of Mg, Ca and Sr, the kinetics of U(VI) sorption on quartz is pH dependent and attained equilibrium after 24 h. Aqueous speciation calculations for alkaline earth uranyl carbonates were carried out by using PHREEQC with the Nuclear Energy Agency thermodynamic database (NEA_2007) by adding constants for MUO₂(CO₃)₃²⁻ and M₂UO₂(CO₃)₃⁰ (M = Ca, Mg, Sr). This study reveals that alkaline earth elements can have a significant effect on the aqueous speciation of U(VI) under neutral to alkaline pH conditions and subsequently sorption behavior and mobility of U(VI) in aqueous environments.     

Effects of aqueous complexation on reductive precipitation of uranium by Shewanella putrefaciens

We have examined the effects of aqueous complexation on rates of dissimilatory reductive precipitation of uranium by Shewanella putrefaciens. Uranium(VI) was supplied as sole terminal electron acceptor to Shewanella putrefaciens (strain 200R) in defined laboratory media under strictly anaerobic conditions. Media were amended with different multidentate organic acids, and experiments were performed at different U(VI) and ligand concentrations. Organic acids used as complexing agents were oxalic, malonic, succinic, glutaric, adipic, pimelic, maleic, citric, and nitrilotriacetic acids, tiron, EDTA, and Aldrich humic acid. Reductive precipitation of U(VI), resulting in removal of insoluble amorphous UO₂ from solution, was measured as a function of time by determination of total dissolved U. Reductive precipitation was measured, rather than net U(VI) reduction to U(IV), to assess overall U removal rates from solution, which may be used to gauge the influence of chelation on microbial U mineralization. Initial linear rates of U reductive precipitation were found to correlate with stability constants of 1:1 aqueous U(VI):ligand and U(IV):ligand complexes. In the presence of strongly complexing ligands (e.g., NTA, Tiron, EDTA), UO₂ precipitation did not occur. Our results are consistent with ligand-retarded precipitation of UO₂, which is analogous to ligand-assisted solid phase dissolution but in reverse: ligand exchange with the U⁴⁺ aquo cation acts as a rate-limiting reaction moderating coordination of water molecules with U⁴⁺, which is a necessary step in UO₂ precipitation. Ligand exchange kinetics governing dissociation rates of ligands from U(VI)-organic complexes may also influence overall UO₂ production rates, although the magnitude of this effect is unclear relative to the effects of U(IV)-organic complexation. Our results indicate that natural microbial-aqueous systems containing abundant organic matter can inhibit the formation of biogenic amorphous UO₂.     

Surface complexation of U(VI) on goethite (α-FeOOH)

Sorption of U(VI) to goethite is a fundamental control on the mobility of uranium in soil and groundwater. Here, we investigated the sorption of U on goethite using EXAFS spectroscopy, batch sorption experiments and DFT calculations of the energetics and structures of possible surface complexes. Based on EXAFS spectra, it has previously been proposed that U(VI), as the uranyl cation , sorbs to Fe oxide hydroxide phases by forming a bidentate edge-sharing (E2) surface complex, >Fe(OH)₂UO₂(H₂O)_n. Here, we argue that this complex alone cannot account for the sorption capacity of goethite (α-FeOOH). Moreover, we show that all of the EXAFS signal attributed to the E2 complex can be accounted for by multiple scattering. We propose that the dominant surface complex in CO₂-free systems is a bidentate corner-sharing (C2) complex, (>FeOH)₂UO₂(H₂O)₃ which can form on the dominant {101} surface. However, in the presence of CO₂, we find an enhancement of UO₂ sorption at low pH and attribute this to a (>FeO)CO₂UO₂ ternary complex. With increasing pH, U(VI) desorbs by the formation of aqueous carbonate and hydroxyl complexes. However, this desorption is preceded by the formation of a second ternary surface complex (>FeOH)₂UO₂CO₃. The three proposed surface complexes, (>FeOH)₂UO₂(H₂O)₃, >FeOCO₂UO₂, and (>FeOH)₂UO₂CO₃ are consistent with EXAFS spectra. Using these complexes, we developed a surface complexation model for U on goethite with a 1-pK model for surface protonation, an extended Stern model for surface electrostatics and inclusion of all known UO₂-OH-CO₃ aqueous complexes in the current thermodynamic database. The model gives an excellent fit to our sorption experiments done in both ambient and reduced CO₂ environments at surface loadings of 0.02-2.0 wt% U.     

Characterization of uranium-contaminated sediments from beneath a nuclear waste storage tank from Hanford, Washington: Implications for contaminant transport and fate

The concentration and distribution of uranium (U) in sediment samples from three boreholes recovered near radioactive waste storage tanks at Hanford, Washington, USA, were determined in detail using bulk and micro-analytical techniques. The source of contamination was a plume that contained an estimated 7000 kg of dissolved U that seeped into the subsurface as a result of an accident that occurred during filling of tank BX-102. The desorption character and kinetics of U were also determined by experiment in order to assess the mobility of U in the vadose zone. Most samples contained too little moisture to obtain quantitative information on pore water compositions. Concentrations of U (and contaminant phosphate—P) in pore waters were therefore estimated by performing 1:1 sediment-to-water extractions and the data indicated concentrations of these elements were above that of uncontaminated “background” sediments. Further extraction of U by 8 N nitric acid indicated that a significant fraction of the total U is relatively immobile and may be sequestered in mobilization-resistant phases. Fine- and coarse-grained samples in sharp contact with one another were sub-sampled for further scrutiny and identification of U reservoirs. Segregation of the samples into their constituent size fractions coupled with microwave-assisted digestion of bulk samples showed that most of the U contamination was sequestered within the fine-grained fraction. Isotope exchange (²³³U) tests revealed that ∼51% to 63% of the U is labile, indicating that the remaining fund of U is locked up in mobilization-resistant phases. Analysis by Micro-X-ray Fluorescence and Micro-X-ray Absorption Near-Edge Spectroscopy (μ-XRF and μ-XANES) showed that U is primarily associated with Ca and is predominately U(VI). The spectra obtained on U-enriched “hot spots” using Time-Resolved Laser-Induced Fluorescence Spectroscopy (TRLIFS) provide strong evidence for uranophane-type [Ca(UO₂)₂(SiO₃OH)₂(H₂O)₅] and uranyl phosphate [Ca(UO₂)₂(PO₄)₂(H₂O)_10-12] phases. These data show that disseminated micro-precipitates can form in narrow pore spaces within the finer-grained matrix and that these objects are likely not restricted to lithic fragment environments. Uranium mobility may therefore be curtailed by precipitation of uranyl silicate and phosphate phases, with additional possible influence exerted by capillary barriers. Consequently, equilibrium-based desorption models that predict the concentrations and mobility of U in the subsurface matrix at Hanford are unnecessarily conservative.     

Experimental determination of uranium (IV) speciation in HF solutions at 500°C and 1000 bar

Uraninite solubility in HF solutions (0.0001–0.5 m) was experimentally studied at 500°C, 1000 bar, and hydrogen fugacity corresponding to the Ni/NiO buffer. It was shown that the predominant U(IV) species in aqueous solution are U(OH)₄⁰, U(OH)₃F⁰, and U(OH)₂ F₂⁰. Using the results of uraninite solubility measurement, the Gibbs free energies of the uranium (IV) species were calculated at 500°C and 1000 bar (kJ/mol): −986.55 for UO₂(aq), −1712.42 for U(OH)₃F⁰, −1755.53 for U(OH)₂F₂⁰, and the equilibrium constants of the uraninite solubility in water and HF solutions were estimated: UO₂(κ) = UO₂(aq), which is similar to UO₂(cr) + 2H₂O = U(OH)₄⁰, pK₀ = 6.64; UO₂(cr) + HF⁰ + H₂O = U(OH)₃F⁰, K₁ = 0.0513; UO₂(cr) + 2HF⁰ = U(OH)₂F₂⁰K₂ = 7.00 × 10⁻⁴. Approximate values K₃ = 5.75 × 10⁻³ and K₄ = 6.7 × 10⁻² were obtained for equilibria UO₂(cr) + 4HF⁰ =UF₄⁰ + 2H₂O and UO₂(cr) + 4HF = UF₄⁰ + 2H₂O. Maximum observed in the uranium concentration curve as a function of HF concentration can be explained by the decrease (to < 1) of activity coefficient ratio of HF⁰ to U(OH)₃F⁰ with increasing HF concentrations.     

Use of mineral/solution equilibrium calculations to assess the potential for carnotite precipitation from groundwater in the Texas Panhandle,USA

This study investigated the potential for the uranium mineral carnotite (K₂(UO₂)₂(VO₄)₂·3H₂O) to precipitate from evaporating groundwater in the Texas Panhandle region of the United States. The evolution of groundwater chemistry during evaporation was modeled with the USGS geochemical code PHREEQC using water-quality data from 100 groundwater wells downloaded from the USGS National Water Information System (NWIS) database. While most modeled groundwater compositions precipitated calcite upon evaporation, not all groundwater became saturated with respect to carnotite with the system open to CO₂. Thus, the formation of calcite is not a necessary condition for carnotite to form. Rather, the determining factor in achieving carnotite saturation was the evolution of groundwater chemistry during evaporation following calcite precipitation. Modeling in this study showed that if the initial major-ion groundwater composition was dominated by calcium-magnesium-sulfate (>70 precent Ca + Mg and >50 percent SO₄ + Cl) or calcium-magnesium-bicarbonate (>70 percent Ca + Mg and <70 percent HCO₃ + CO₃) and following the precipitation of calcite, the concentration of calcium was greater than the carbonate alkalinity (2mCa⁺² > mHCO₃⁻ + 2mCO₃⁻²) carnotite saturation was achieved. If, however, the initial major-ion groundwater composition is sodium-bicarbonate (varying amounts of Na, 40–100 percent Na), calcium-sodium-sulfate, or calcium-magnesium-bicarbonate composition (>70 percent HCO₃ + CO₃) and following the precipitation of calcite, the concentration of calcium was less than the carbonate alkalinity (2mCa⁺² < mHCO₃^- + 2mCO₃⁻²) carnotite saturation was not achieved. In systems open to CO_2, carnotite saturation occurred in most samples in evaporation amounts ranging from 95 percent to 99 percent with the partial pressure of CO₂ ranging from 10^−3.5 to 10^−2.5 atm. Carnotite saturation occurred in a few samples in evaporation amounts ranging from 98 percent to 99 percent with the partial pressure of CO₂ equal to 10^−2.0 atm. Carnotite saturation did not occur in any groundwater with the system closed to CO₂.     

Chemical factors controlling the solubility of uraninite and their significance in the genesis of unconformity-related uranium deposits

A quantitative evaluation of the solubility of uraninite (UO₂) in aqueous solutions under hydrothermal conditions was made using previously reported thermodynamic data, so as to inquire into the controlling factors for Canadian unconformity-type ore mineralization as observed in the Athabasca uranium field. The results of solubility calculations suggest that uranyl carbonate complexes, such as UO₂CO ₃ ^o , UO₂(CO₃) ₂ ^2- and UO₂(CO₃) ₃ ^4- , predominate under relatively oxidizing and slightly acidic-alkaline conditions and that the uranyl chloride complex, UO₂Cl⁺ is dominant under acidic conditions. These features are predicted at temperatures up to 200 °C over reasonable ranges of CO₂ pressure (Pco₂) and salinity. Consequently, the physico-chemical parameters, such as oxygen activity (ao₂), and pH are regarded as the most important factors controlling uraninite solubility. Judging from the paragenetic sequences observed in most unconformity-type uranium deposits in the Athabasca district, appreciable decreases in the above variables are postulated to have occurred in the stage of principal uranium deposition. Such changes would be due to fluid-mixing phenomenon accompanied by the diagenetic-hydrothermal activity (Hoeve and Quirt 1987).     

Hydrochemical characteristics of shallow groundwater in aquifer containing uranyl phosphate minerals, in the Köprübaşı (Manisa) area, Turkey

The concentrations of uranium, iron and the major constituents were determined in groundwater samples from aquifer containing uranyl phosphate minerals (meta-autunite, meta-torbernite and torbernite) in the Köprüba?? area. Groundwater samples from wells located at shallow depths (0.5–6 m) show usually near neutral pH values (6.2–7.1) and oxidizing conditions (Eh = 119–275 mV). Electrical conductivity (EC) values of samples are between 87 and 329 μS/cm^?1. They are mostly characterized by mixed cationic Ca dominating bicarbonate types. The main hydrogeochemical process is weathering of the silicates in the shallow groundwater system. All groundwater in the study area are considered undersaturated with respect to torbernite and autunite. PHREEQC predicted UO₂(HPO₄) ₂ ^2? as the unique species. The excellent positive correlation coefficient (r = 0.99) between U and PO₄ indicates the dissolved uranium in groundwater would be associated with the dissolution of uranyl phosphate minerals. The groundwater show U content in the range 1.71–70.45 μg/l but they are mostly lower than US EPA (2003) maximum contaminant level of 30 μg/l. This low U concentrations in oxic groundwater samples is attributed to the low solubility of U(VI) phosphate minerals under near neutral pH and low bicarbonate conditions. Iron closely associated with studied sediments, were also detected in groundwater. The maximum concentration of Fe in groundwater samples was 2837 μg/l, while the drinking water guidelines of Turkish (TSE 1997) and US EPA (2003) were suggested 200 and 300 μg/l, respectively. Furthermore, iron and uranium showed a significant correlation to each other with a correlation coefficient (r) of 0.94. This high correlation is probably related to the iron-rich sediments which contain also significant amounts of uranium mineralization. In addition to pH and bicarbonate controlling dissolution of uranyl phosphates, association of uranyl phosphates with iron (hydr) oxides seems to play important role in the amount of dissolved U in shallow groundwater.     

Uptake of uranium and trace elements in pyrite (FeS2) suspensions

Pyrite dissolution and interaction with Fe^(II), Co^(II), Eu^(III) and U^(VI) have been studied under anoxic conditions by solution chemistry and spectroscopic techniques. Aqueous data show a maximal cation uptake above pH 5.5. Iron (II) uptake can explain the non-stoichiometric [S]_aq/[Fe]_aq ratios often observed during dissolution experiments. Protonation data corrected for pyrite dissolution resulted in a proton site density of 9 ± 3 sites nm⁻². Concentration isotherms for Eu^(III) and U^(VI) sorption on pyrite indicate two different behaviours which can be related to the contrasted redox properties of these elements. For Eu^(III), sorption can be explained by the existence of a unique site with a saturation concentration of 1.25 × 10⁻⁶ mol g⁻¹. In the U^(VI) case, sorption seems to occur on two different sites with a total saturation concentration of 4.5 × 10⁻⁸ mol g⁻¹. At lower concentration, uranium reduction occurs, limiting the concentration of dissolved uranium to the solubility of UO₂(s).Scanning electron microscopy and micro-Raman spectrometry of U^(VI)-sorbed pyrite indicate a heterogeneous distribution of U at the pyrite surface and a close association with oxidized S. X-ray photoelectron spectroscopy confirms the partial reduction of U and the formation of a hyperstoichiometric UO_2+x(s). Our results are consistent with a chemistry of the pyrite surface governed not by Fe^(II)-bound hydroxyl groups, but by S groups which can either sorb cations and protons, or sorb and reduce redox-sensitive elements such as U^(VI).     

Identification of the uranium speciation in an underground acid mine drainage environment

The subsurface acid mine drainage (AMD) environment of an abandoned underground uranium mine in Königstein/Saxony/Germany, currently in the process of remediation, is characterized by low pH, high sulfate concentrations and elevated concentrations of heavy metals, in particular uranium. Acid streamers thrive in the mine drainage channels and are heavily coated with iron precipitates. These precipitates are biologically mediated iron precipitates and related to the presence of Fe-oxidizing microorganisms forming copious biofilms in and on the Fe-precipitates. Similar biomineralisations were also observed in stalactite-like dripstones, called snottites, growing on the gallery ceilings.The uranium speciation in these solutions of underground AMD waters flowing in mine galleries as well as dripping from the ceiling and forming stalactite-like dripstones were studied by time resolved laser-induced fluorescence spectroscopy (TRLFS). The fluorescence lifetime of uranium species in both AMD water environments were best described with a mono-exponential decay, indicating the presence of one major species. The detected positions of the emission bands and by comparing it in a fingerprinting procedure with spectra obtained for acid sulfate reference solutions, in particular Fe(III) - SO₄²⁻ - UO₂²⁺ reference solutions, indicated that the uranium speciation in the AMD environment of Königstein is dominated in the pH range of 2.5-3.0 by the highly mobile aquatic uranium sulfate species UO₂SO_4(aq) and formation of uranium precipitates is rather unlikely as is retardation by sorption processes. The presence of iron in the AMD reduces the fluorescence lifetime of the UO₂SO_4(aq) species from 4.3 μs, found in iron-free uranium sulfate reference solutions, to 0.7 μs observed in both AMD waters of Königstein and also in the iron containing uranium sulfate reference solutions.Colloids were not observed in both drainage water and dripping snottite water as photon correlation spectroscopy analyses and centrifugation experiments at different centrifugal accelerations between 500g and 46000g revealed. Thus transport and uranium speciation at the investigated AMD sites is neither influenced by U(IV) or U(VI) eigencolloids nor by uranium adsorbed on colloidal particles.This study shows that TRLFS is a suitable spectroscopic technique to identify the uranium speciation in bulk solutions of AMD environments.     

Uranyl acetate speciation in aqueous solutions—an XAS study between 25°C and 250°C

Speciation of uranium (VI) in acetate solutions between 25 and 250°C, at pH values between 1.8 and 3.8 and acetate/uranium (Ac/U) ratios of 0.5 to 100 has been investigated using uranium L_III-edge X-ray absorption spectroscopy. With increasing pH the UO₂(Ac)₂⁰ species becomes more important than UO₂(Ac)⁺ species, which is predominant below pH 2. It remains the dominant species as pH is further increased to 3.8 at an Ac/U ratio of 20. Decrease in U-O_eq bond distance and coordination number with increasing solution age indicates that steric/kinetic factors are important and that equilibrium is attained slowly in this system with initial acetate coordination to the uranyl ion being monodentate or pseudo-bridging before slow conversion to bidentate chelation. Acetate coordination to the uranyl ion appears to decrease as temperature is increased from room temperature to ∼100°C before increasing in solutions of Ac/U > 2. For solutions where Ac/U ≤ 2 at pH 2.1, there is no evidence for uranyl acetate speciation at low temperatures, but at elevated temperature bidentate uranyl-acetate ion-pairing is evident. The existence of the uranyl acetate species in the temperature range 200 to 240°C demonstrates the importance of including acetate and other organic ligands in models of uranium transport at elevated temperatures.     

A surface structural model for ferrihydrite II: Adsorption of uranyl and carbonate

The adsorption of uranyl (UO₂²⁺) on ferrihydrite has been evaluated with the charge distribution (CD) model for systems covering a very large range of conditions, i.e. pH, ionic strength, CO₂ pressure, U(VI) concentration, and loading. Modeling suggests that uranyl forms bidentate inner sphere complexes at sites that do not react chemically with carbonate ions. Uranyl is bound by singly-coordinated surface groups present at particular edges of Fe-octahedra of ferrihydrite while another set of singly-coordinated surface groups may form double-corner bidentate complexes with carbonate ions. The uranyl surface speciation strongly changes in the presence of carbonate due to the specific adsorption of carbonate ions as well as the formation of ternary uranyl-carbonate surface complexes. Data analysis with the CD model suggests that a uranyl tris-carbonato surface complex, i.e. (UO₂)(CO₃)₃⁴⁻, is formed. This species is most abundant in systems with a high pH and carbonate concentration. This finding differs significantly from previous interpretations made in the literature. At high pH and low carbonate concentrations, as can be prepared in CO₂-closed systems, the model suggests the additional presence of a ternary uranyl-monocarbonato complex. The binding mode (type A or type B complex) is uncertain. At high uranyl concentrations, uranyl polymerizes at the surface of ferrihydrite giving, for instance, tris-uranyl surface complexes with and without carbonate. The similarities and differences between U(VI) adsorption by goethite and ferrihydrite are discussed from a surface structural point of view.     

Uranium co-precipitation with iron oxide minerals

In oxidizing environments, the toxic and radioactive element uranium (U) is most soluble and mobile in the hexavalent oxidation state. Sorption of U(VI) on Fe-oxides minerals (such as hematite [α-Fe₂O₃] and goethite [α-FeOOH]) and occlusion of U(VI) by Fe-oxide coatings are processes that can retard U transport in environments. In aged U-contaminated geologic materials, the transport and the biological availability of U toward reduction may be limited by coprecipitation with Fe-oxide minerals. These processes also affect the biological availability of U(VI) species toward reduction and precipitation as the less soluble U(IV) species by metal-reducing bacteria.To examine the dynamics of interactions between U(VI) and Fe oxides during crystallization, Fe-oxide phases (containing 0.5 to 5.4 mol% U/(U + Fe)) were synthesized by means of solutions of U(VI) and Fe(III). Wet chemical (digestions and chemical extractions) and spectroscopic techniques were used to characterize the synthesized Fe oxide coprecipitates after rinsing in deionized water. Leaching the high mol% U solids with concentrated carbonate solution (for sorbed and solid-phase U(VI) species) typically removed most of the U, leaving, on average, about 0.6 mol% U. Oxalate leaching of solids with low mol% U contents (about 1 mol% U or less) indicated that almost all of the Fe in these solids was crystalline and that most of the U was associated with these crystalline Fe oxides. X-ray diffraction and Fourier-transform infrared (FT-IR) spectroscopic studies indicate that hematite formation is preferred over that of goethite when the amount of U in the Fe-oxides exceeds 1 mol% U (∼4 wt% U). FT-IR and room temperature continuous wave luminescence spectroscopic studies with unleached U/Fe solids indicate a relationship between the mol% U in the Fe oxide and the intensity or existence of the spectra features that can be assigned to UO₂²⁺ species (such as the IR asymmetric υ₃ stretch for O = U = O for uranyl). These spectral features were undetectable in carbonate- or oxalate-leached solids, suggesting solid phase and sorbed U(VI)O₂²⁺ species are extracted by the leach solutions. Uranium L₃-edge x-ray absorption spectroscopic (XAFS) analyses of the unleached U-Fe oxide solids with less than 1 mol% U reveal that U(VI) exists with four O atoms at radial distances of 2.19 and 2.36 Å and second shell Fe at a radial distance at 3.19 Å.Because of the large ionic radius of UO₂²⁺ (∼1.8 Å) relative to that of Fe³⁺ (0.65 Å), the UO₂²⁺ ion is unlikely to be incorporated in the place of Fe in Fe(III)-oxide structures. Solid-phase U(VI) can exist as the uranyl [U(VI)O₂²⁺] species with two axial U-O double bonds and four or more equatorial U-O bonds or as the uranate species (such as γ-UO₃) without axial U-O bonds. Our findings indicate U⁶⁺ (with ionic radii of 0.72 to 0.8 Å, depending on the coordination environment) is incorporated in the Fe oxides as uranate (without axial O atoms) until a point of saturation is reached. Beyond this excess in U concentration, precipitating U(VI) forms discrete crystalline uranyl phases that resemble the uranyl oxide hydrate schoepite [UO₂(OH)₂·2H₂O]. Molecular modeling studies reveal that U⁶⁺ species could bond with O atoms from distorted Fe octahedra in the hematite structure with an environment that is consistent with the results of the XAFS. The results provide compelling evidence of U incorporation within the hematite structure.     

Competition between enzymatic and abiotic reduction of uranium(VI) under iron reducing conditions

Reduction of U(VI) under iron reducing conditions was studied in a model system containing the dissimilatory metal-reducing bacterium Shewanella putrefaciens and colloidal hematite. We focused on the competition between direct enzymatic uranium reduction and abiotic reduction of U(VI) by Fe(II), catalyzed by the hematite surface, at relatively low U(VI) concentrations (< 0.5 μM) compared to the concentrations of ferric iron (> 10 mM). Under these conditions surface catalyzed reduction by Fe(II), which was produced by dissimilatory iron reduction, was the dominant pathway for uranium reduction. Reduction kinetics of U(VI) were identical to those in abiotic controls to which soluble Fe(II) was added. Strong adsorption of U(VI) at the hematite surface apparently favored the abiotic pathway by reducing the availability of U(VI) to the bacteria. In control experiments, lacking either hematite or bacteria, the addition of 45 mM dissolved bicarbonate markedly slowed down U(VI) reduction. The inhibition of enzymatic U(VI) reduction and abiotic, surface catalyzed U(VI) reduction by the bicarbonate amendments is consistent with the formation of aqueous uranyl-carbonate complexes. Surprisingly, however, more U(VI) was reduced when dissolved bicarbonate was added to experimental systems containing both bacteria and hematite. The enhanced U(VI) reduction was attributed to the formation of magnetite, which was observed in experiments. Biogenic magnetite produced as a result of dissimilatory iron reduction may be an important agent of uranium immobilization in natural environments.     

Speciation and Precipitation of Uranium Complexes in Hydrothermal Solutions Related to Granite—type Uranium Deposits

Uranium-bearing hydrothermal solutions during the stage of ore deposition are weakly alkaline and of the Ca^2 -Na^ /HCO3^- -F^- type.UO2(CO3)2^2- and UO2F4^-, are dominant in the hydrothermal solutions with respect to their activity.Wall-rock hydrothermal alterations ,temperature and pressure drop and the reducing capability of rock assemblage (Δeh) led to a decrease in Eh of the hydrothermal solutions and an increase in Eh at which uranium began precipitating.Therefore,the mechanism of uranium precipitation is essentially the reduction of uranium complexes.The granite-type uranium deposits are the most important type of uranium resources in China.Discussions will be made in this paper concerning the hydrothermal speciation and precipitation mech-anisms of uranium complexes in the light of fluid inclusion and geological data from some major de-posits of this type in South China.