Uranium(VI) sorption to hematite in the presence of humic acid

A long-standing problem in aquatic geochemistry has been the incorporation of natural organic matter (NOM) into speciation models. The general effect of NOM on metal ion sorption by particles has been understood for some time, and significant progress has been made in elucidating some of the details of the role of NOM through the use of surrogate organic acids such as citric acid. However, a gap exists between the general observations that have been made of NOM behavior and the inclusion of NOM in surface chemical models for metal ion sorption. In this paper, we report on the results of a study on the sorption of U(VI) by hematite in the absence and presence of Suwannee river humic acid (HA) and over a range of other system conditions (e.g., pH, I). Essential HA characteristics (e.g., its acid/base, metal binding, and surface chemical properties) were “captured” by representing the HA as an assembly of monoprotic acids with assumed pK values and without explicit correction for electrostatic effects. The ternary system (hematite/HA/U(VI)) was simulated through the combination of the binary submodels (i.e., CO₃²⁻/hematite, U(VI)/HA, U(VI)/hematite, and HA/hematite) with model constants fixed at the values determined from simulations of the respective experimental systems. However, the “summed-binary” approach undersimulated experimental results, and the ternary system model required the postulation of two ternary surface (Type A) complexes composed of the uranyl ion, hematite surface sites, and the model ligands comprising the HA. Consideration of the HA in this manner permitted the simulation of HA effects on U(VI) sorption by hematite over a range of solution conditions using a general 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.     

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.     

Uranium (VI) sorption on colloidal magnetite under anoxic environment: experimental study and surface complexation modelling

Magnetite is one of the most important end member of iron corrosion products under a reducing environment; therefore, it may be one of the first products interacting with radionuclides in a radioactive waste disposal after the canister failure.Nanocrystalline magnetite was synthetised in the laboratory and its main physico-chemical properties (microstructure, surface area, surface charge) were analysed. The stability of the oxide was also investigated under the experimental conditions used in sorption studies. The sorption behaviour of U^VI onto magnetite was analysed under O₂- and CO₂-free conditions in a wide range of pH, ionic strengths and radionuclide concentrations.The uranyl binding to magnetite is characterised by a sorption edge between pH 4 and 5.5, and sorption was found to be independent on the electrolyte concentration, which indicates the formation of inner sphere complexes. The sorption isotherms showed a linear behaviour up to the saturation of the sorption sites with a Langmuir-type behaviour.One of the aims of this work was to find the simplest model capable to reproduce the experimental data. Sorption data were fitted using a classical approximation (diffuse double layer model), considering only one type of surface site and evaluating two different options: the first one involving two different monodentate complexes, and the second one a single binuclear bidentate complex. A highly satisfactory fit of the experimental data was obtained by both approaches in the range of the experimental conditions investigated.     

Uranium(VI) sorption on iron oxides in Hanford Site sediment: Application of a surface complexation model

Sorption of U(VI) on Hanford fine sand (HFS) with varying Fe-oxide (especially ferrihydrite) contents showed that U(VI) sorption increased with the incremental addition of synthetic ferrihydrite into HFS, consistent with ferrihydrite being one of the most reactive U(VI) sorbents present in natural sediments. Surface complexation model (SCM) calculations for U(VI) sorption, using only U(VI) surface-reaction constants obtained from U(VI) sorption data on freshly synthesized ferrihydrite at different pHs, were similar to the measured U(VI) sorption results on pure synthetic ferrihydrite and on HFS with high contents of ferrihydrite (5 wt%) added. However, the SCM prediction using only U(VI) sorption reactions and constants for synthetic ferrihydrite overestimated U(VI) sorption on the natural HFS or HFS with addition of low amounts of added ferrihydrite (1 wt% added). Over-predicted U(VI) sorption was attributed to reduced reactivity of natural ferrihydrite present in Hanford Site sediments, compared to freshly prepared synthetic ferrihydrite. Even though the SCM general composite (GC) approach is considered to be a semi-quantitative estimation technique for contaminant sorption, which requires systematic experimental data on the sorbent–sorbate system being studied to obtain credible SCM parameters, the general composite SCM model was still found to be a useful technique for describing U(VI) sorption on natural sediments. Based on U(VI) batch sorption results, two simple U(VI) monodentate surface species, SO_UO₂HCO₃ and SO_UO₂OH on ferrihydrite and phyllosillicate in HFS, respectively, can be successfully used to describe U(VI) sorption onto Hanford Site sediment contacting varying geochemical solutions.     

Uranium(VI) adsorption and surface complexation modeling onto vadose sediments from the Savannah River Site

Uranium U(VI) adsorption was measured as function of pH (3–10) on goethite, kaolinite, quartz, two binary mixtures of goethite and kaolinite, and a vadose zone sediment collected on The Department of Energy’s Savannah River Site (SRS), the clay mineral fraction of which is composed largely of kaolinite and goethite. Diffuse-layer surface complexation models were parameterized using the code PEST together with PHREEQC to fit U(VI) sorption data for the pure goethite, kaolinite, and quartz. U(VI) adsorption on kaolinite and goethite was modeled as the formation of two bidentate U(VI) complexes at mineral edge sites on a variable charge site. U(VI) adsorption on quartz was described using a one-site diffuse-layer with the formation of bidentate complex on a variable charge site. These models were used to predict U(VI) adsorption on the binary sorbent mixtures and the SRS sediment using a simple component-additivity approach. In general, the predicted adsorption edges were in good agreement with measured data, with statistically similar goodness of fit compared to that obtained for the pure mineral systems.     

Surface complexation modeling of Co(II) adsorption on mixtures of hydrous ferric oxide, quartz and kaolinite

Co sorption was measured as a function of pH, ionic strength (0.001-0.1 M NaNO₃) and sorbate/sorbent ratio on pure quartz, HFO and kaolinite and on binary and ternary mixtures of the three solids. Sorption data measured for the pure solids were used to derive internally-consistent diffuse layer surface complexation model (DLM) stability constants for Co sorption. Co sorption on HFO could be adequately modeled over a broad range of ionic strengths and sorbate/sorbent ratios with a two variable-charge site model. Fits based on a single variable-charge site model were reasonable, but were improved by using ionic-strength dependent stability constants. A single variable-charge site model with an additional permanent ion exchange site produced the best fit to Co edges measured on kaolinite over a range of ionic strength and sorbate/sorbent ratios. These DLM fits were also improved by using ionic-strength dependent stability constants. The DLM approach could not adequately describe the slope of Co sorption edges on quartz. This study demonstrates that for accurate prediction of Co sorption over wide ranges of ionic strength and sorbate/sorbent ratio, the DLM may require ionic-strength dependent stability constants. DLM stability constants for Co sorption derived for the pure solids were used to predict sorption as a function of pH and solid concentration on binary and ternary mixtures of the three solids. Discrepancies between predictions and measurements were quantitatively similar to those observed for the pure mineral systems. Thus, a simple component additivity approach provides useful predictions of metal sorption in the mixed solid systems.     

Approaches to surface complexation modeling of Uranium(VI) adsorption on aquifer sediments

Uranium(VI) adsorption onto aquifer sediments was studied in batch experiments as a function of pH and U(VI) and dissolved carbonate concentrations in artificial groundwater solutions. The sediments were collected from an alluvial aquifer at a location upgradient of contamination from a former uranium mill operation at Naturita, Colorado (USA). The ranges of aqueous chemical conditions used in the U(VI) adsorption experiments (pH 6.9 to 7.9; U(VI) concentration 2.5 · 10⁻⁸ to 1 · 10⁻⁵ M; partial pressure of carbon dioxide gas 0.05 to 6.8%) were based on the spatial variation in chemical conditions observed in 1999-2000 in the Naturita alluvial aquifer. The major minerals in the sediments were quartz, feldspars, and calcite, with minor amounts of magnetite and clay minerals. Quartz grains commonly exhibited coatings that were greater than 10 nm in thickness and composed of an illite-smectite clay with occluded ferrihydrite and goethite nanoparticles. Chemical extractions of quartz grains removed from the sediments were used to estimate the masses of iron and aluminum present in the coatings. Various surface complexation modeling approaches were compared in terms of the ability to describe the U(VI) experimental data and the data requirements for model application to the sediments. Published models for U(VI) adsorption on reference minerals were applied to predict U(VI) adsorption based on assumptions about the sediment surface composition and physical properties (e.g., surface area and electrical double layer). Predictions from these models were highly variable, with results overpredicting or underpredicting the experimental data, depending on the assumptions used to apply the model. Although the models for reference minerals are supported by detailed experimental studies (and in ideal cases, surface spectroscopy), the results suggest that errors are caused in applying the models directly to the sediments by uncertain knowledge of: 1) the proportion and types of surface functional groups available for adsorption in the surface coatings; 2) the electric field at the mineral-water interface; and 3) surface reactions of major ions in the aqueous phase, such as Ca²⁺, Mg²⁺, HCO₃⁻, SO₄²⁻, H₄SiO₄, and organic acids. In contrast, a semi-empirical surface complexation modeling approach can be used to describe the U(VI) experimental data more precisely as a function of aqueous chemical conditions. This approach is useful as a tool to describe the variation in U(VI) retardation as a function of chemical conditions in field-scale reactive transport simulations, and the approach can be used at other field sites. However, the semi-empirical approach is limited by the site-specific nature of the model parameters.     

Surface phase transformations,surface complexation and solubilities of hydroxyapatite in the absence/presence of Cd(II) and EDTA

The removal of Cd from aqueous solutions by hydroxyapatite (HAP) was investigated with and without EDTA being present. Batch experiments were carried out using synthetic hydroxyapatite with Ca/P 1.57 and a specific surface area of 37.5 m²/g in the pH range 4–9 (25 °C; 0.1 M KNO₃). The surface composition of the solid phases were analysed by X-ray Photoelectron Spectroscopy (XPS). The surface layer of HAP was found to undergo a phase transformation with a (Ca + Cd)/P atomic ratio of 1.4 and the involvement of an ion exchange process (Ca²⁺ ↔ Cd²⁺). The amount of Cd removed from the solution increased with increasing pH, reaching ≈100% at pH 9. In the presence of EDTA Cd removal was reduced due to the formation of [CdEDTA]²⁻ in solution. The solubility of HAP increases in the presence of EDTA at pH values above 5, mainly due to the formation of [CaEDTA]²⁻. In contrast to this, the solubility was found to decrease in the presence of Cd²⁺ and CdEDTA²⁻. Using XPS the formation of a Cd-enriched HAP surface was found, which was interpreted as the formation of a solid solution of the general composition: Ca_8.4-xCd_x(HPO_4)1.6(PO_4)4.4(OH_)0.4${\text{Ca}}_{8.4-x}{\text{Cd}}_x({\text{HPO}}_4{)}_{1.6}({\text{PO}}_4{)}_{4.4}(\text{OH}{)}_{0.4}$.     

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.     

Surface complexation modeling of Cu(II) adsorption on mixtures of hydrous ferric oxide and kaolinite

Background  

The application of surface complexation models (SCMs) to natural sediments and soils is hindered by a lack of consistent models and data for large suites of metals and minerals of interest. Furthermore, the surface complexation approach has mostly been developed and tested for single solid systems. Few studies have extended the SCM approach to systems containing multiple solids.     

Surface complexation and precipitate geometry for aqueous Zn(II) sorption on ferrihydrite: II. XANES analysis and simulation

X-ray absorption near-edge spectroscopy (XANES) analysis of sorption complexes has the advantages of high sensitivity (10- to 20-fold greater than extended X-ray absorption fine structure [EXAFS] analysis) and relative ease and speed of data collection (because of the short k-space range). It is thus a potentially powerful tool for characterization of environmentally significant surface complexes and precipitates at very low surface coverages. However, quantitative analysis has been limited largely to “fingerprint” comparison with model spectra because of the difficulty of obtaining accurate multiple-scattering amplitudes for small clusters with high confidence.In the present work, calculations of the XANES for 50- to 200-atom clusters of structure from Zn model compounds using the full multiple-scattering code Feff 8.0 accurately replicate experimental spectra and display features characteristic of specific first-neighbor anion coordination geometry and second-neighbor cation geometry and number. Analogous calculations of the XANES for small molecular clusters indicative of precipitation and sorption geometries for aqueous Zn on ferrihydrite, and suggested by EXAFS analysis, are in good agreement with observed spectral trends with sample composition, with Zn-oxygen coordination and with changes in second-neighbor cation coordination as a function of sorption coverage. Empirical analysis of experimental XANES features further verifies the validity of the calculations. The findings agree well with a complete EXAFS analysis previously reported for the same sample set, namely, that octahedrally coordinated aqueous Zn²⁺ species sorb as a tetrahedral complex on ferrihydrite with varying local geometry depending on sorption density. At significantly higher densities but below those at which Zn hydroxide is expected to precipitate, a mainly octahedral coordinated Zn²⁺ precipitate is observed. An analysis of the multiple scattering paths contributing to the XANES demonstrates the importance of scattering paths involving the anion sublattice. We also describe the specific advantages of complementary quantitative XANES and EXAFS analysis and estimate limits on the extent of structural information obtainable from XANES analysis.     

Surface charge density on silica in alkali and alkaline earth chloride electrolyte solutions

The surface charge density of colloidal SiO₂ (Aerosil 380) was measured in alkali chloride (0.067 and 0.20 M LiCl, NaCl, and KCl) and alkaline earth chloride (0.067 M MgCl₂, CaCl₂, SrCl₂, BaCl₂) solutions. Measurements were conducted at 25°C by potentiometric titrations using the constant ionic medium method in a CO₂-free system. The experimental design measured surface charge for solutions with constant ionic strength as well as constant cation concentration. Alkali chloride solutions promote negative surface charge density in the order LiCl < NaCl < KCl to give the “regular” lyotropic behavior previously reported. In contrast, the alkaline earth chloride solutions exhibit a reversed lyotropic trend with increasing crystallographic radius where increasing negative charge is promoted in the order BaCl₂ < SrCl₂ < CaCl₂ < MgCl₂.The origin of the opposing affinity trends is probed by testing the hypothesis that this reversal is rooted in the differing solvent structuring characteristics of the IA and IIA cations at the silica-water interface. This idea arises from earlier postulations that solvent structuring effects increase entropy through solvent disordering and these gains must be much greater than the small, positive enthalpy associated with electrostatic interactions. By correlating measured charge density with a proxy for the solvent-structuring ability of cations, this study shows that silica surface charge density is maximized by those electrolytes that have the strongest effects on solvent structuring. We suggest that for a given solid material, solvation entropy has a role in determining the ionic specificity of electrostatic interactions and reiterate the idea that the concept of lyotropy is rooted in the solvent-structuring ability of cations at the interface.     

Surface complexation and precipitate geometry for aqueous Zn(II) sorption on ferrihydrite I: X-ray absorption extended fine structure spectroscopy analysis

“Two-line” ferrihydrite samples precipitated and then exposed to a range of aqueous Zn solutions (10⁻⁵ to 10⁻³ M), and also coprecipitated in similar Zn solutions (pH 6.5), have been examined by Zn and Fe K-edge X-ray absorption spectroscopy. Typical Zn complexes on the surface have Zn-O distances of 1.97(.02) Å and coordination numbers of about 4.0(0.5), consistent with tetrahedral oxygen coordination. This contrasts with Zn-O distances of 2.11(.02) Å and coordination numbers of 6 to 7 in the aqueous Zn solutions used in sample preparation. X-ray absorption extended fine structure spectroscopy (EXAFS) fits to the second shell of cation neighbors indicate as many as 4 Zn-Fe neighbors at 3.44(.04) Å in coprecipitated samples, and about two Zn-Fe neighbors at the same distance in adsorption samples. In both sets of samples, the fitted coordination number of second shell cations decreases as sorption density increases, indicating changes in the number and type of available complexing sites or the onset of competitive precipitation processes. Comparison of our results with the possible geometries for surface complexes and precipitates suggests that the Zn sorption complexes are inner sphere and at lowest adsorption densities are bidentate, sharing apical oxygens with adjacent edge-sharing Fe(O,OH)₆ octahedra. Coprecipitation samples have complexes with similar geometry, but these are polydentate, sharing apices with more than two adjacent edge-sharing Fe(O,OH)₆ polyhedra. The results are inconsistent with Zn entering the ferrihydrite structure (i.e., solid solution formation) or formation of other Zn-Fe precipitates. The fitted Zn-Fe coordination numbers drop with increasing Zn density with a minimum of about 0.8(.2) at Zn/(Zn + Fe) of 0.08 or more. This change appears to be attributable to the onset of precipitation of zinc hydroxide polymers with mainly tetrahedral Zn coordination. At the highest loadings studied, the nature of the complexes changes further, and a second type of precipitate forms. This has a structure based on a brucite layer topology, with mainly octahedral Zn coordination. Amorphous zinc hydroxide samples prepared for comparison had a closely similar local structure. Analysis of the Fe K-edge EXAFS is consistent with surface complexation reactions and surface precipitation at high Zn loadings with little or no Fe-Zn solid solution formation. The formation of Zn-containing precipitates at solution conditions two or more orders of magnitude below their solubility limit is compared with other sorption and spectroscopic studies that describe similar behavior.     

Impact of phosphate on U(VI) immobilization in the presence of goethite

Past mining, processing, and waste disposal activities have left a legacy of uranium-contaminated soil and groundwater. Phosphate addition to subsurface environments can potentially immobilize U(VI) in-situ through interactions with uranium at mineral-water interfaces. Phosphate can induce the precipitation of low solubility U(VI)-phosphates, and it may enhance or inhibit U(VI) adsorption to iron(III) (oxy)hydroxide surfaces. Such surfaces may also facilitate the heterogeneous nucleation of U(VI)-phosphate precipitates. The interactions among phosphate, U(VI), and goethite (α-FeOOH) were investigated in a year-long series of experiments at pH 4. Reaction time, total U(VI), total phosphate, and the presence and absence of goethite were systematically varied to determine their effects on the extent of U(VI) uptake and the dominant uranium immobilization mechanism. Dissolved U(VI) and phosphate concentrations were interpreted within a reaction-based modeling framework that included dissolution-precipitation reactions and a surface complexation model to account for adsorption. The best available thermodynamic data and past surface complexation models were integrated to form an internally consistent framework. Additional evidence for the uptake mechanisms was obtained using scanning electron microscopy and X-ray diffraction. The formation and crystal growth of a U(VI)-phosphate phase, most likely chernikovite, UO₂HPO₄·4H₂O_(s), occurred rapidly for initially supersaturated suspensions both with and without goethite. Nucleation appears to occur homogeneously for almost all conditions, even in the presence of goethite, but heterogeneous nucleation was likely at one condition. The U(VI)-phosphate solids exhibited metastability depending on the TOTU:TOTP ratio. At the highest phosphate concentration studied (130 μM), U(VI) uptake was enhanced due to the likely formation of a ternary surface complex for low (∼1 μM) to intermediate (∼10 μM) TOTU concentrations and to U(VI)-phosphate precipitation for high TOTU (∼100 μM) concentrations. For conditions favoring precipitation, the goethite surface acted as a sink for dissolved phosphate that resulted in higher dissolved U(VI) concentrations relative to goethite-free conditions. Based on the total uranium and available sorption sites, a critical phosphate concentration between 15 μM and 130 μM was required for preferential precipitation of uranium phosphate over U(VI) adsorption.     

Surface and complexation effects on the rate of Mn(II) oxidation in natural waters

Montmorillonite, kaolinite, goethite, and particulate and soluble natural organic materials influence the rate of Mn(II) oxidation. While surfaces accelerate the reaction, apparently by bonding Mn²⁺ in a manner which fulfills the requirements of the transition state, soluble organic materials retard the reaction by complexing the oxidizable species. It is doubtful whether particulate matter would influence the oxidation process under natural loading conditions since 50–500 mg l^?1quantities are required to produce measurable changes in the reaction rate. Complexation by humic materials, however, might be expected to reduce the rate of oxidation by an amount proportional to the dissolved organic carbon concentration. Oxidation followed by precipitation is predicted to be an important mechanism for Mn²⁺ removal in oceanic waters. The situation is less predictable in lake waters.     

铀-邻氯苯基荧光酮-吐温-80显色体系的研究和应用

研究了U(Ⅵ)与邻氯苯基荧光酮(o-Cl-PF)的显色反应条件:在非离子表面活性剂(吐温-80)存在下,pH 8.0三乙醇胺(TEA)-HCl缓冲介质中形成紫红色配合物,最大吸收波长在560mn,ε=1.48×10~5 L·mol~(-1)·cm~(-1),含U量在0～1.2μg/ml范围内符合比耳定律。结合D_(235)型强碱性阴离子交换树脂分离,方法可用于矿石中痕量U的测定。     

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.