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.     

Uranyl adsorption onto montmorillonite: Evaluation of binding sites and carbonate complexation

The fate and transport of uranium in contaminated soils and sediments may be affected by adsorption onto the surface of minerals such as montmorillonite. Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to investigate the adsorption of uranyl (UO₂²⁺) onto Wyoming montmorillonite. At low pH (∼4) and low ionic strength (10⁻³ M), uranyl has an EXAFS spectrum indistinguishable from the aqueous uranyl cation, indicating binding via cation exchange. At near-neutral pH (∼7) and high ionic strength (1 M), the equatorial oxygen shell of uranyl is split, indicating inner-sphere binding to edge sites. Linear-combination fitting of the spectra of samples reacted under conditions where both types of binding are possible reveals that cation exchange at low ionic strengths on SWy-2 may be more important than predicted by past surface complexation models of U(VI) adsorption on related montmorillonites. Analysis of the binding site on the edges of montmorillonite suggests that U(VI) sorbs preferentially to [Fe(O,OH)₆] octahedral sites over [Al(O,OH)₆] sites. When bound to edge sites, U(VI) occurs as uranyl-carbonato ternary surface complexes in systems equilibrated with atmospheric CO₂. Polymeric surface complexes were not observed under any of the conditions studied. Current surface complexation models of uranyl sorption on clay minerals may need to be reevaluated to account for the possible increased importance of cation exchange reactions at low ionic strengths, the presence of reactive octahedral iron surface sites, and the formation of uranyl-carbonato ternary surface complexes. Considering the adsorption mechanisms observed in this study, future studies of U(VI) transport in the environment should consider how uranium retardation will be affected by changes in key solution parameters, such as pH, ionic strength, exchangeable cation composition, and the presence or absence of CO₂.     

Contamination risk assessment of fresh groundwater using the distribution and chemical speciation of some potentially toxic elements in Calabar (southern Nigeria)

The computer program PHREEQC was used to determined the distribution, chemical speciation and mineral saturation indices in a fresh groundwater environment with limited mining activities in the adjoining areas. The aim was mainly to determine the potential risk of a coastal plain aquifer contamination by some potentially toxic elements. The results show that the elements Ba, Cd, Cu, Fe, Mn, Ni, Rb, Sr, and Zn are distributed as free metal ions. Arsenic is in the neutral form of H₃AsO₃ ^o, while three species of aluminium [Al³⁺, AlOH₂, Al(OH)₂ ⁺] dominate. The major species of uranium include UO₂CO₃, UO₂²⁺+, UO₂⁺, and UO₂OH⁺, respectively, in order of abundance. The groundwater is saturated with respect to alunite [KAl₃ (SO₄)₂ (OH)₆], basaluminite [Al₄ (OH)₁₀ SO₄], boehmite [Al(OH)], Cu metal (Cu), cuprous ferrite (CuFeO₂), diaspore [AlO(OH)], gibbsite [Al(OH)₃], goethite (FeOOH), hematite (Fe₂O₃), magnetite (Fe₃O₄) and uraninite (UO₂). Most of the species are not mobile under the prevailing pH (3.3 to 5.9) and Eh (7 to 158 mV) conditions. The mobile ones are very low in concentration and will be immobilized by precipitation of mineral phases. The study concludes that presently these species do not pose any risk to the aquifer.     

Chemical speciation of Cd,Cu, Pb,and Zn in mixed freshwater,seawater, and brine solutions

A theoretical model was developed to study the chemical speciation of the trace elements Zn, Cd, Cu and Pb aqueous solutions and their responses to variations in ionic strength and complexation. Two mixing solutions were investigated, a freshwater-seawater system and a freshwater-brine system. The brine was a calcium, sodium-chloride solution with a molal ionic strength of two. Trace element associations with the ligands OH^?, Cl^?, CO^2?₃, SO^2?₄, and HCO^?₃ were considered at pHs from 3.5 to 11.0 at 25°C. In general, the relative importance of the various ligand-trace element complexes can be predicted from a comparison of their stability constants. However, the effect of pH on the importance of a given complex is not readily apparent from the stability constants. Freshwater-seawater mixtures, as might be found in a totally mixed estuary, show that seawater composition is the dominant control on chemical complexing. Chloride complexing is similar for lead and zinc in the freshwater-brine mixtures. This similarity may account in part for the association of lead and zinc in strata-bound ore deposits.     

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.     

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).     

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.     

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.     

Solidsolution interaction: The effect of carbonate alkalinity on adsorbed thorium

Elevated activities of dissolved Th have been found in Soap Lake, an alkaline lake in Eastern Washington. Dissolved ²³²Th ranges from less than 0.001 to 4.9 dpm/L compared to about 1.3 × 10⁻⁵ dpm/ L in sea water. The enhanced activity in the lake coincides with an increase in carbonate alkalinity. Experiments were conducted to evaluate the effect of pH, ionic strength and carbonate alkalinity on Th adsorption on goethite. Thorium (10⁻¹³ M total) in the presence of 5.22 mg/L α-FeOOH and 0.1 M NaNO₃ has an adsorption edge from pH 2–5. At pH 9.0 ± 0.6 the percent Th absorbed on the solid began to decrease from 100% at 100 meq/L carbonate alkalinity and exhibited no adsorption above 300 meq/L. The experimental data were modeled to obtain the intrinsic adsorption equilibrium constants for Th hydrolysis species. These adsorption constants were incorporated in the model to interpret the observed effect of carbonate alkalinity on Th adsorption. There are two main effects of the alkalinity. To a significant degree the decrease in Th adsorption is due to competition of HCO⁻₃ and CO²⁻₃ ions for surface sites. Dissolved Th carbonate complexes also contribute to the increase of Th in solution.     

Fractionation of Cu and Zn isotopes during adsorption onto amorphous Fe(III) oxyhydroxide: Experimental mixing of acid rock drainage and ambient river water

Fractionation of Cu and Zn isotopes during adsorption onto amorphous ferric oxyhydroxide is examined in experimental mixtures of metal-rich acid rock drainage and relatively pure river water and during batch adsorption experiments using synthetic ferrihydrite. A diverse set of Cu- and Zn-bearing solutions was examined, including natural waters, complex synthetic acid rock drainage, and simple NaNO₃ electrolyte. Metal adsorption data are combined with isotopic measurements of dissolved Cu (⁶⁵Cu/⁶³Cu) and Zn (⁶⁶Zn/⁶⁴Zn) in each of the experiments. Fractionation of Cu and Zn isotopes occurs during adsorption of the metal onto amorphous ferric oxyhydroxide. The adsorption data are modeled successfully using the diffuse double layer model in PHREEQC. The isotopic data are best described by a closed system, equilibrium exchange model. The fractionation factors (α_soln-solid) are 0.99927 ± 0.00008 for Cu and 0.99948 ± 0.00004 for Zn or, alternately, the separation factors (Δ_soln-solid) are −0.73 ± 0.08‰ for Cu and −0.52 ± 0.04‰ for Zn. These factors indicate that the heavier isotope preferentially adsorbs onto the oxyhydroxide surface, which is consistent with shorter metal-oxygen bonds and lower coordination number for the metal at the surface relative to the aqueous ion. Fractionation of Cu isotopes also is greater than that for Zn isotopes. Limited isotopic data for adsorption of Cu, Fe(II), and Zn onto amorphous ferric oxyhydroxide suggest that isotopic fractionation is related to the intrinsic equilibrium constants that define aqueous metal interactions with oxyhydroxide surface sites. Greater isotopic fractionation occurs with stronger metal binding by the oxyhydroxide with Cu > Zn > Fe(II).     

ATR-FTIR spectroscopic characterization of coexisting carbonate surface complexes on hematite

The speciation of carbonate adsorbed to hematite in air-equilibrated aqueous solutions has been studied using ATR-FTIR spectroscopy. Samples were measured over a range of pH conditions, at 0.1 M NaCl and at low ionic strength, and in H₂O and D₂O solutions to permit a multispecies analysis of the data. Second-derivative analyses and fits to the spectra indicate the presence of two major and two minor surface-bound carbonate species. The two major complexes coexist at near-neutral pH and low ionic strength. One of these two complexes is relatively sensitive to ionic strength, being displaced at 0.1 M NaCl, whereas the other is not. Comparison of experimental to DFT/MO-calculated frequencies suggest these two major species to be (a) a monodentate binuclear inner-sphere carbonate surface complex, and (b) a fully or partially solvated carbonate (CO₃²⁻) species that is symmetry broken and appears to reside in the structured vicinal water layers at the hematite-water interface, retained by hydrogen bonding and/or other forces. Minor carbonate complexes include diffuse layer CO₃²⁻ and an unidentified inner-sphere species. Both of the dominant species observed here are likely to be significant controls of the surface charge and sorptive properties of Fe-oxides.     

The adsorption characteristics of δ-manganese dioxide: a collection of diffuse double layer constants for the adsorption of H+, Cu2+, Ni2+, Zn2+, Cd2+ and Pb2+

Thermodynamic data for all fate-determining processes are needed in order to predict the fate and transport of metals in natural systems. The surface complexation properties of a synthetic MnO₂, δ-MnO₂, have accordingly been investigated using glass electrode potentiometry. Experimental data were interpreted according to the surface complexation model in conjunction with the diffuse double layer model of the solid/solution interface. Adsorption constants were determined using the non-linear optimisation program FITEQL. Surface complexation parameters determined in this way were validated against results obtained from the literature. Best fits of alkalimetric titration data were obtained with a 2-site, 3 surface-species model of the δ-MnO₂ surface. Site concentrations of 2.23×10⁻³ mol g⁻¹ and 7.66×10⁻⁴ mol g⁻¹ were obtained. Corresponding logarithms of formation constants for the postulated surface species are −1.27 (≡XO⁻), −5.99 (≡YO⁻) and 3.52 (≡YOH₂⁺) at I=0.1 M. The surface speciation of δ-MnO₂ is dominated by ≡XO⁻ over the pH range investigated. Metal adsorption was modelled with surface species of the type ≡XOM⁺, ≡XOMOH, ≡YOM⁺, ≡YOMOH (M=Cu, Ni, Zn, Cd and Pb) and ≡XOM₂OH²⁺ (M=Pb). For Cu, Ni and Zn, titration data could be modelled with ≡XOM⁺, ≡XOMOH, ≡YOM⁺ and ≡YOMOH, whereas for Cd, ≡XOM⁺ and ≡YOM⁺ were sufficient. Lead data were best modelled by assuming the dinuclear species ≡XOM₂OH²⁺ to be the only surface species to form. Adsorption constants determined for Ni, Cu and Zn follow the Irving-Williams sequence. The model suggests an adsorption order of (Pb, Cu) > (Ni, Zn) > Cd. The discrepancy between model predictions and published adsorption results is similar to the variability observed in experimental results from different laboratories.     

The effect of cobalt ion on nucleation of calcium-carbonate polymorphs

Cobalt, like Mg, may cause the precipitation of aragonite rather than calcite in aqueous solutions due to the adsorption and crystal poisoning of calcite by a hydrated ion. Solutions containing NaCl and CaCl₂, having the ionic strength and Ca content of seawater (35‰ salinity), were spiked with known amounts of CoCl₂. Calcium carbonate was precipitated by the addition of 0.7 ml of 1 M Na₂CO₃. All experimental runs were made at 25°C, and all products were examined by X-ray diffraction. At low concentrations of Co (< 5·^?4M) calcite and vaterite formed. At concentrations from 5·10^?4 M to 2·10^?3M, the products consisted of combinations of calcite and vaterite; aragonite and calcite; aragonite and vaterite; calcite, vaterite and aragonite. In solutions of 3·10^?3M CoCl₂, most precipitates were aragonite with only one sample containing a small amount of calcite. All precipitates from 5·10^?3M CoCl₂ solutions either contained aragonite or were amorphous. Solutions with concentrations of 1 · 10^?2M CoCl₂ produced only amorphous precipitates. All precipitates contained an amorphous violet phase, assumed to be basic cobaltous carbonate (2CoCO₃·Co(OH)₂·H₂O).     

Agricolaite, a new mineral of uranium from J��chymov, Czech Republic

The new mineral agricolaite, a potassium uranyl carbonate with ideal formula K₄(UO₂)(CO₃)₃, occurs in vugs of ankerite gangue in gneisses in the abandoned Giftkiesstollen adit at Jáchymov, Czech Republic. The name is after Georgius Agricola (1494?C1555), German scholar and scientist. Agricolaite occurs as isolated equant irregular translucent grains to 0.3?mm with yellow color, pale yellow streak, and vitreous luster. It is brittle with uneven fracture and displays neither cleavage nor parting. Agricolaite is non-fluorescent. Mohs hardness is ~4. It is associated with aragonite, brochantite, posnjakite, malachite, rutherfordine, and ??pseudo-voglite??. Experimental density is higher than 3.3?g.cm^?3, Dcalc is 3.531?g.?cm^?3. The mineral is monoclinic, space group C2/c, with a 10.2380(2), b 9.1930(2), c 12.2110(3) ?, ?? 95.108(2)°, V 1144.71(4) ?³, Z?=?4. The strongest lines in the powder X-ray diffraction pattern are d(I)(hkl): 6.061(55)(002), 5.087(57)(200), 3.740(100)(202), 3.393(43)(113), 2.281(52)(402). Average composition based on ten electron microprobe analyses corresponds to (in wt.%) UO₃ 48.53, K₂O 31.49, CO₂(calc) 22.04 which gives the empirical formula K_3.98(UO₂)_1.01(CO₃)_3.00. The crystal structure was solved from single-crystal X-ray diffraction data and refined to R ₁?=?0.0184 on the basis of the 1,308 unique reflections with F _o?>?4??F _o. The structure of agricolaite is identical to that of synthetic K₄(UO₂)(CO₃)₃ and consists of separate UO₂(CO₃)₃ groups organized into layers parallel to (100) and two crystallographically non-equivalent sites occupied by K⁺ cations. Both the mineral and its name were approved by the IMA-CNMNC.     

X-ray absorption fine structure determination of pH-dependent U-bacterial cell wall interactions

X-ray absorption fine structure (XAFS) measurements was used at the U L3-edge to directly determine the pH dependence of the cell wall functional groups responsible for the absorption of aqueous UO₂²⁺ to Bacillus subtilis from pH 1.67 to 4.80. Surface complexation modeling can be used to predict metal distributions in water-rock systems, and it has been used to quantify bacterial adsorption of metal cations. However, successful application of these models requires a detailed knowledge not only of the type of bacterial surface site involved in metal adsorption/desorption, but also of the binding geometry. Previous acid-base titrations of B. subtilis cells suggested that three surface functional group types are important on the cell wall; these groups have been postulated to correspond to carboxyl, phosphoryl, and hydroxyl sites. When the U(VI) adsorption to B. subtilis is measured, observed is a significant pH-independent absorption at low pH values (<3.0), ascribed to an interaction between the uranyl cation and a neutrally charged phosphoryl group on the cell wall. The present study provides independent quantitative constraints on the types of sites involved in uranyl binding to B. subtilis from pH 1.67 to 4.80. The XAFS results indicate that at extremely low pH (pH 1.67) UO₂²⁺ binds exclusively to phosphoryl functional groups on the cell wall, with an average distance between the U atom and the P atom of 3.64 ± 0.01 Å. This U-P distance indicates an inner-sphere complex with an oxygen atom shared between the UO₂²⁺ and the phosphoryl ligand. The P signal at extremely low pH value is consistent with the UO₂²⁺ binding to a protonated phosphoryl group, as previously ascribed. With increasing pH (3.22 and 4.80), UO₂²⁺ binds increasingly to bacterial surface carboxyl functional groups, with an average distance between the U atom and the C atom of 2.89 ± 0.02 Å. This U-C distance indicates an inner-sphere complex with two oxygen atoms shared between the UO₂²⁺ and the carboxyl ligand. The results of this XAFS study confirm the uranyl-bacterial surface speciation model.     

Uranium(6+) sorption on montmorillonite: Experimental and surface complexation modeling study

Sorption interactions with montmorillonite and other clay minerals in soils, sediments, and rocks are potentially important mechanisms for attenuating the mobility of U(6+) and other radionuclides through the subsurface environment. Batch experiments were conducted (in equilibrium with atmospheric % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D% aebbfv3ySLgzGueE0jxyaibaiiYdd9qrFfea0dXdf9vqai-hEir8Ve% ea0de9qq-hbrpepeea0db9q8as0-LqLs-Jirpepeea0-as0Fb9pgea% 0lrP0xe9Fve9Fve9qapdbaqaaeGacaGaaiaabeqaamaabaabcaGcba% acbiGaiWiG-bfadaWgaaWcbaacbaGaa43qaiaa+9eadaWgaaqaaiaa% +jdaaWqabaaaleqaaaaa!400D!\[P_{CO_2 } \])to determine the effects of varying pH (2 to 9), solid-mass to solution-volume ratio (M/V = 0.028 to 3.2 g/L), and solution concentration (2 × 10^?7 and 2 × 10^?6 M ²³³U) on U(6+) sorption on SAz-1 montmorillonite. The study focused on U(6+) surface complexation on hydroxylated edge sites as the sorption mechanism of interest because it is expected to be the predominant sorption mechanism at pHs typical of natural waters (pH ≈6 to ≈9). Thus, the experiments were conducted with a 0.1 M NaNO₃ matrix to suppress ion-exchange between U(6+) in solution and interlayer cations. The results show that U(6+) sorption on montmorillonite is a strong function of pH, reaching a maximum at near-neutral pH (≈6 to ≈6.5) and decreasing sharply towards more acidic or more alkaline conditions. A comparison of the pH-dependence of U(6+) sorption with that of U(6+) aqueous speciation indicates a close correspondence between U(6+) sorption and the predominance field of U(6+)-hydroxy complexes. At high pH, sorption is inhibited due to formation of aqueous U(6+)-carbonate complexes. At low pH, the low sorption values indicate that the 0.1 M NaNO₃ matrix was effective in suppressing ion-exchange between the uranyl (UO₂ ²⁺) species and interlayer cations in montmorillonite. At pH and carbonate concentrations typical of natural waters, sorption of U(6+) on montmorillonite can vary by four orders of magnitude and can become negligible at high pH. The experimental results were used to develop a thermodynamic model based on a surface complexation approach to permit predictions of U(6+) sorption at differing physicochemical conditions. A Diffuse-Layer model (DLM) assuming aluminol (>AlOH^?) and silanol (>SiOH^?) edge sites and two U(6+) surface complexation reactions per site effectively simulates the complex sorption behavior observed in the U(6+)-H₂O-CO₂-montmorillonite system at an ionic strength of 0.1 M and pH > 3.5. A comparison of model predictions with data from this study and from published literature shows good agreement and suggests that surface complexation models based on parameters derived from a limited set of data could be useful in extrapolating radionuclide sorption over a range of geochemical conditions. Such an approach could be used to support transport modeling by providing a better alternative to the use of constant K _d s in transport calculations.     

Thorium sorption in the marine environment: Equilibrium partitioning at the hematite/water interface,sorption/desorption kinetics and particle tracing

Thorium(IV) sorption onto hematite (-Fe₂O₃) was examined as a function of pH and ionic strength. Sorption behaved Langmuirian over an eleven order of magnitude range in adsorption densities, : 10^–12 to 10^–1 moles Th sorbed per mole hematite sites, indicating that the overall free energy of Th adsorption is independent of adsorption density. Modeling of Th sorption was conducted with the Triple Layer Model of Davis and Leckie; reactions considered included solution-phase hydroxy and carbonato complexes of thorium, and carbonate/hematite surface complexes. The entire Th sorption isotherm can be modeled with a single surface complex formation reaction

Biogenic uraninite precipitation and its reoxidation by iron(III) (hydr)oxides: A reaction modeling approach

One option for immobilizing uranium present in subsurface contaminated groundwater is in situ bioremediation, whereby dissimilatory metal-reducing bacteria and/or sulfate-reducing bacteria are stimulated to catalyze the reduction of soluble U(VI) and precipitate it as uraninite (UO₂). This is typically accomplished by amending groundwater with an organic electron donor. It has been shown, however, that once the electron donor is entirely consumed, Fe(III) (hydr)oxides can reoxidize biogenically produced UO₂, thus potentially impeding cleanup efforts. On the basis of published experiments showing that such reoxidation takes place even under highly reducing conditions (e.g., sulfate-reducing conditions), thermodynamic and kinetic constraints affecting this reoxidation are examined using multicomponent biogeochemical simulations, with particular focus on the role of sulfide and Fe(II) in solution. The solubility of UO₂ and Fe(III) (hydr)oxides are presented, and the effect of nanoscale particle size on stability is discussed. Thermodynamically, sulfide is preferentially oxidized by Fe(III) (hydr)oxides, compared to biogenic UO₂, and for this reason the relative rates of sulfide and UO₂ oxidation play a key role on whether or not UO₂ reoxidizes. The amount of Fe(II) in solution is another important factor, with the precipitation of Fe(II) minerals lowering the Fe⁺² activity in solution and increasing the potential for both sulfide and UO₂ reoxidation. The greater (and unintuitive) UO₂ reoxidation by hematite compared to ferrihydrite previously reported in some experiments can be explained by the exhaustion of this mineral from reaction with sulfide. Simulations also confirm previous studies suggesting that carbonate produced by the degradation of organic electron donors used for bioreduction may significantly increase the potential for UO₂ reoxidation through formation of uranyl carbonate aqueous complexes.