Gert Bernhard

Uranyl(VI) carbonate complex formation: Validation of the Ca2UO2(CO3)3(aq.) species

We recently discovered a neutral dicalcium uranyl tricarbonate complex, Ca2UO2(CO3)3(aq.), in uranium mining related waters [1]. We are now reporting a further validation of the stoichiometry and the formation constant of this complex using two analytical approaches with time-resolved laser-induced fluorescence spectroscopy (TRLFS) species detection: i) titration of a non-fluorescent uranyl tricarbonate complex solution with calcium ions, and quantitative determination of the produced fluorescent calcium complex via TRLFS; and ii) variation of the calcium concentration in the complex by competitive calcium complexation with EDTA4-. Slope analysis of the log (fluorescence intensity) versus log[Ca2+] with both methods have shown that two calcium ions are bound to form the complex Ca2UO2(CO3)3(aq.). The formation constants determined from the two independent methods are: i) logβ°213=30.45±0.35 and ii) logβ°213=30.77±0.25. A bathochrome shift of 0.35 nm between the UO2(CO3)34- complex and the Ca2UO2(CO3)3(aq.) complex is observed in the laser-induced photoacoustic spectrum (LIPAS), giving additional evidence for the formation of the calcium uranyl carbonate complex. EXAFS spectra at the LII and LIII-edges of uranium in uranyl carbonate solutions with and without calcium do not differ significantly. A somewhat better fit to the EXAFS of the Ca2UO2(CO3)3(aq.) complex is obtained by including the U-Ca shell. From the similarities between the EXAFS of the Ca2UO2(CO3)3(aq.) species in solution and the natural mineral liebigite, we conclude that the calcium atoms are likely to be in the same positions both in the solution complex and in the solid. This complex influences considerably the speciation of uranium in the pH region from 6 to 10 in calcium-rich uranium-mining-related waters.

Chlorite dissolution in the acid pH-range: A combined microscopic and macroscopic approach

Abstract The dissolution of chlorite with intermediate Fe-content was studied macroscopically via mixed flow experiments as well as microscopically via atomic force microscopy (AFM). BET surface area normalized steady state dissolution rates at 25 °C for pH 2 to 5 vary between 10−12 and 10−13 mol/m2.s. The order of the dissolution reaction with respect to protons was calculated to be about 0.29. For pH 2 to 4, chlorite was found to dissolve non-stoichiometrically, with a preferred release of the octahedrally coordinated cations. The additional release of octahedrally coordinated cations may be due to the transformation of chlorite to interstratified chlorite/vermiculite from the grain edges inward. In-situ atomic force microscopy performed on the basal surfaces of a chlorite sample, which has been preconditioned at pH 2 for several months, indicated a defect controlled dissolution mechanism. Molecular steps with height differences which correspond to the different subunits of chlorite, e.g. TOT sheet and brucite like layer, originated at surface defects such or compositional inhomogenities or cracks, which may be due to the deformation history of the chlorite sample. In contrast to other sheet silicates, at pH 2 nanoscale etch pits occur on the chlorite basal surfaces within flat terraces terminated by a TOT-sheet as well as within the brucite like layer. The chlorite basal surface dissolves layer by layer, because most of the surface defects are only expressed through single TOT or brucite-like layers. The defect controlled dissolution mechanism favours dissolution of molecular steps on the basal surfaces compared to dissolution of the grain edges. At pH 2 the dissolution of the chlorite basal surface is dominated by the retreat of 14 A steps, representing one chlorite unit cell. The macroscopic and microscopic chlorite dissolution rates can be linked via the reactive surface area as identified by AFM. The reactive surface area with respect to dissolution consists of only 0.2% of the BET-surface area. A dissolution rate of 2.5 × 10−9 mol/m2s was calculated from macroscopic and microscopic dissolution experiments at pH 2, when normalized to the reactive surface area.

Sorption behavior of U(VI) on phyllite: experiments and modeling.

The sorption of U(VI) onto low-grade metamorphic rock phyllite was modeled with the diffuse double layer model (DDLM) using the primary mineralogical constituents of phyllite, i.e. quartz, chlorite, muscovite, and albite, as input components, and as additional component, the poorly ordered Fe oxide hydroxide mineral, ferrihydrite. Ferrihydrite forms during the batch sorption experiment as a weathering product of chlorite. In this process, Fe(II), leached from the chlorite, oxidizes to Fe(III), hydrolyses and precipitates as ferrihydrite. The formation of ferrihydrite during the batch sorption experiment was identified by Mössbauer spectroscopy, showing a 2.8% increase of Fe(III) in the phyllite powder. The ferrihydrite was present as Fe nanoparticles or agglomerates with diameters ranging from 6 to 25 nm, with indications for even smaller particles. These Fe colloids were detected in centrifugation experiments of a ground phyllite suspension using various centrifugal forces. The basis for the successful interpretation of the experimental sorption data of uranyl(VI) on phyllite were: (1) the determination of surface complex formation constants of uranyl with quartz, chlorite, muscovite, albite, and ferrihydrite in individual batch sorption experiments, (2) the determination of surface acidity constants of quartz, chlorite, muscovite, and albite obtained from separate acid-base titration, (3) the determination of surface site densities of quartz, chlorite, muscovite, and albite evaluated independently of each other with adsorption isotherms, and (4) the quantification of the secondary phase ferrihydrite, which formed during the batch sorption experiments with phyllite. The surface complex formation constants and the protolysis constants were optimized by using the experimentally obtained data sets and the computer code FITEQL. Surface site densities were evaluated from adsorption isotherms at pH 6.5. The uranyl(VI) sorption onto phyllite was accurately modeled with these newly determined constants and parameters of the main mineralogical constituents of phyllite and the secondary mineralization phase ferrihydrite. The modeling indicated that uranyl sorption to ferrihydrite clearly dominates uranyl sorption, showing the great importance of secondary iron phases for sorption studies.

Humic colloid-borne migration of uranium in sand columns

Column experiments were carried out to investigate the influence of humic colloids on subsurface uranium migration. The columns were packed with well-characterized aeolian quartz sand and equilibrated with groundwater rich in humic colloids (dissolved organic carbon (DOC): 30 mg dm(-3)). U migration was studied under an Ar/1% CO2 gas atmosphere as a function of the migration time, which was controlled by the flow velocity or the column length. In addition, the contact time of U with groundwater prior to introduction into a column was varied. U(VI) was found to be the dominant oxidation state in the spiked groundwater. The breakthrough curves indicate that U was transported as a humic colloid-borne species with a velocity up to 5% faster than the mean groundwater flow. The fraction of humic colloid-borne species increases with increasing prior contact time and also with decreasing migration time. The migration behavior was attributed to a kinetically controlled association/dissociation of U onto and from humic colloids and also a subsequent sorption of U onto the sediment surface. The column experiments provide an insight into humic colloid-mediated U migration in subsurface aquifers.

Study of uranyl(VI) malonate complexation by time-resolved laser-induced fluorescence spectroscopy (TRLFS)

Summary The uranyl(VI) malonate complex formation was studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS) at pH 4 and an ionic strength of 0.1 M NaClO4. The uranium concentration was 5 × 10−6 M at ligand concentrations from 1 × 10−5 to 1 × −2 M. The measured fluorescence lifetimes of the 1:1 and 1:2 uranyl(VI) malonate complexes are 1.24 ± 0.02 µs and 6.48 ± 0.02 µs, respectively. The fluorescence lifetime of the uranyl(VI) ion is 1.57 ± 0.06 µs in 0.1 M perchloric media. The main fluorescence bands of the malonate complexes show a bathochromic shift compared to the uranyl(VI) ion and are centered at 494 nm, 515 nm and 540 nm for the 1:1 complexes and at 496 nm, 517 nm and 542 nm for the 1:2 complex. The spectra of the individual uranyl(VI) malonate complexes were calculated using a multi exponential fluorescence decay function for each intensity value at each wavelength, covering the entire wavelength range. Stability constants were determined for the complexes UO2C3H2O4°(aq) and UO2(C3H2O4)22− from results of spectra deconvolution using a least square fit algorithm (logβ1° = 4.48 ± 0.06, logβ2° = 7.42 ± 0.06 or logK2° = 2.94 ± 0.04). The results are compared with literature values obtained by potentiometric measurements.

Sorption of uranium(VI) onto phyllite

Abstract The sorption of U(VI) on phyllite and on its main mineral constituents, muscovite, quartz, chlorite, and albite feldspar was studied in individual batch experiments at ambient pressure from pH 3.5 to 9.5. The ionic strength was held constant at 0.1 M (NaClO 4 solution). An uranium concentration of 1×10 −6 M, a size fraction of 63–200 μm and a solid solution ratio of 0.5 g/40 ml were used. The study showed that sorption of uranium on the various mineral surfaces had its maximum at near neutral pH range. It further showed that the maximum amount of uranium sorbed onto each individual mineral was different and ranged from 48% of the initially added uranium for quartz, 58% for albite, 70% for muscovite and chlorite, to 97% for phyllite. From this, we conclude that none of the main mineral constituents of phyllite is dominating the sorption behaviour of uranium onto phyllite, but instead an additional component, minor in mass and volume, significantly influences the uranium sorption onto phyllite. Based on a 3% increase in the Fe(III) concentration during the batch experiments, detected by Mossbauer spectroscopy, and the formation of a slight reddish–brownish colour of the investigated powder at the end of the batch experiments, we expect that this additional component is the iron mineral ferrihydrite. Its formation is due to alteration reactions occurring in the course of the batch experiments. We have modelled our U(VI) sorption results onto phyllite with the Diffuse Double Layer Model assuming the formation of surface complexes between the ferrihydrite surface and aqueous uranium. With this model we were able to describe our uranium sorption data on phyllite.

URANIUM(VI) SULFATE COMPLEXATION STUDIED BY TIME-RESOLVED LASER-INDUCED FLUORESCENCE SPECTROSCOPY (TRLFS)

The use of the ΟΡΟ technique in laser induced spectroscopic instrumentation is a new and improved spectroscopic technique. Solid crystals for the tunable laser allows one to perform laser spectroscopy over a wide wavelength range without the use of a dye laser. We have demonstrated that the laser output of the ΟΡΟ-system (in our case 270 nm) is suitable for time-resolved laser-induced fluorescence measurements. The uranyl sulfate complexation was investigated for the first time by fluorescence measurements. Compared to spectrophotometric methods, lower uranyl concentrations can be investigated and therefore measurements could be made in seepage waters of uranium mine tailing piles. The measured lifetimes of the 1:1, 1:2, and 1:3 uranyl sulfate complexes are 4.3 + / 0.5 μβ, 11.0 + / 1.0 με and 18.8 + / 1.0 μβ, respectively. The main fluorescence bands of the sulfate complexes are centered at 498 nm, 515 nm and 538 nm and no significant shift of the fluorescence maxima was found between the three complexes. The complex formation constants of the first two uranyl sulfate complexes at an ionic strength of 0.2 M were measured as log βι(ο.2Μ) = 2.42 and log β2(0 2 M) 3.30, respectively. The complex formation constants for the three uranyl sulfate complexes at an ionic strength of 1 M were measured as log β1(1οΜ) = 1-88, log β2.(, 0M) = 2.9 and log β3(ι.0Μ) = 3.2, respectively. A speciation diagram was calculated from the results of the time resolved measurements.

Interaction of uranium(VI) with various modified and unmodified natural and synthetic humic substances studied by EXAFS and FTIR spectroscopy

Abstract The complexation of uranium(VI) by humic acids (HAs) and fulvic acids (FAs) was studied to obtain information on the binding of uranium(VI) onto functional groups of humic substances. For this, various natural and synthetic HAs were chemically modified resulting in HAs with blocked phenolic OH groups. Both from the original and from the modified humic substances, solid uranyl humate complexes were prepared at pH 2. FTIR and extended X-ray absorption fine structure (EXAFS) spectroscopy were applied to study the chemical modification process of humic substances, to study the structure of uranyl humate complexes and to evaluate the effect of individual functional groups of humic substances (carboxylic and phenolic OH groups) on the complexation of uranyl ions. The results confirmed the predominant blocking of phenolic OH groups in the modified HAs. These modified HAs are suitable model substances to study the role of phenolic OH groups of HAs in dependence on pH. By EXAFS spectroscopy, identical structural parameters were determined for all uranyl humates. Axial UO bond distances of 1.78 A were determined. In the equatorial plane approximately five oxygen atoms were found at a mean distance of 2.39 A. The blocking of phenolic OH groups of HAs did not change the near-neighbor surrounding of uranium(VI) in uranyl humate complexes. Thus, the results confirmed that predominantly HA carboxylate groups are responsible for binding of uranyl ions and that the influence of phenolic OH groups is insignificant under the applied experimental conditions. The carboxylate groups are monodentate coordinated to uranyl ions.

Uranium(VI) sorption onto kaolinite in the presence and absence of humic acid

We studied the U(VI) sorption onto kaolinite in batch experiments in the absence and presence of humic acid (HA) under different experimental conditions: [U]0 = 1 × 10-6 M or 1 × 10-5 M, [HA]0=10 or 50 mg/L, I=0.1 M or 0.01 M NaClO4, pH=3–10, CO2 or N2 atmosphere. The study showed that the U(VI) sorption onto kaolinite is influenced by pH, CO2 and HA presence. In the absence of CO2, the U(VI) uptake increases with increasing pH value up to pH 6. Above pH 6 it remains unchanged. Because of the formation of negatively charged uranyl carbonate complexes, the decrease in the U(VI) sorption onto the negative surface of kaolinite was observed above pH 8 in the presence of CO2. In presence of HA, the adsorption of U(VI) closely follows the adsorption of HA. In the acidic pH range the U(VI) uptake is enhanced compared to the system without HA due to the formation of additional binding sites for U(VI) coming from HA adsorbed onto kaolinite. The formation of aqueous uranyl-humate complexes reduces the U(VI) sorption in the near neutral pH range. The enhancement of the U(VI) concentration from 1 × 10-6 M to 1 × 10-5 M results in the shift of the sorption pH edge by one pH unit to higher pH values. The ionic strength has only a slight influence on the U(VI) sorption onto kaolinite, whereas the HA sorption shows a dependence on the ionic strength.

EXAFS investigation of uranium(VI) complexes formed at Bacillus cereus and Bacillus sphaericus surfaces

Uranium(VI) complex formation at vegetative cells and spores of Bacillus cereus and Bacillus sphaericus was studied using uranium LII-edge and LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. A comparison of the measured equatorial U-O distances and other EXAFS structural parameters of uranyl species formed at the Bacillus strains with those of the uranyl structure family indicates that the uranium is predominantly bound as uranyl complexes with phosphoryl residues.