Bernd Grambow

Study of the interaction between europium (III) and Bacillus subtilis: fixation sites, biosorption modeling and reversibility

In order to elucidate the underlying mechanisms involved in the biosorption of metal ions, potentiometric titrations, complexation studies, and time-resolved laser-induced fluorescence spectroscopy (TRLFS) measurements were used to characterize the interaction between Eu(III) and Bacillus subtilis. The reversibility of the interaction between Eu(III) and Bacillus subtilis was studied by a cation-exchange technique using the Chelex resin. For complexation studies in the presence of 0.15 mol/l of NaCl, the metal ion, the biomass, concentrations and the pH were varied. The adsorption data were quantified by a surface complexation model without electrostatic term. The data on the Eu(III)/B.subtilis system at pH 5 were satisfactorily described by one site at which Eu(III) was bound through one carboxylic function of the bacteria. With increasing pH, another site should be considered, involving a phosphate-bound environment. This was partially confirmed by time-resolved laser-induced fluorescence spectroscopy. In addition to this, it was evidenced that the site availability was dependent on the nature of the cation, i.e., a proton or Eu(III). Finally, it was shown that, at pH 5, the Eu(III)/Bacillus subtilis equilibrium was reversible.

SORPTION AND REDUCTION OF URANIUM(VI) ON IRON CORROSION PRODUCTS UNDER REDUCING SALINE CONDITIONS

Pertinent to the behaviour of carbon steel based nuclear waste packages in saline geological environments, sorption and reduction of U(VI) on real container corrosion products has been studied in an Ar/8%H2 flushed glove box. A carbon steel was corroded in MgCl2-brine at elevated temperatures. U(VI) was added and the redox states both of uranium and iron and their distribution among solid and liquid phases were investigated. The corrosion products initially consisted to more than 97% of hydrous Fe(II) oxides containing CI and Mg, but always some magnetite was present. In the course of the sorption step, the amount of magnetite increased. Reaction products buffer pH values of the system in a very narrow neutral range. Less than 1% of iron or uranium were found in colloidal state and already after one day, more than 98% of initially dissolved uranium was found associated with the immobile phases. A reciprocal relationship of solution concentrations of uranium with the nominal mass of magnetite was found. Behaviour of uranium species was rationalized in terms of Eh/pH diagrams. Reduction of hexavalent to tetravalent uranium was observed but to a much less pronounced extent than expected from thermodynamical considerations.

Surface layers on a borosilicate nuclear waste glass corroded in MgCl2 solution

Surface layers on the French borosilicate nuclear waste glass, R7T7, corroded in MgCl2 solution were studied to determine the composition, structure and stability of crystalline phases. The characteristics of the phases constituting the surface layer varied with the parameter SV × t, the glass surface area (S) to solution volume (V) ratio, times time (t). At low SV × t values ( 98% of the neodymium released from the glass were precipitated in the surface layer. In the 463 day experiment, 86% of the neodymium in the surface layer was in solid solution with powellite, the rest with saponite. Uranium was contained exclusively in saponite. High SV ratios, typical of disposal conditions for vitrified high-level radioactive waste, favor retention of actinides in fairly insoluble corrosion products. Observation of similar corrosion products on natural glasses as on nuclear waste glasses lend support to the hypothesis that the host phases for actinides observed in the laboratory are stable over geological periods of time.

Empirical Dissolution Rate Law for the Glass R7T7 Contacting Halite- and Silica-Saturated Brines

We report on the time dependence of release of glass constituents during static dissolution experiments with the COGEMA glass R7T7 in saline MgCl 2 and NaCl dominated solutions at temperatures between 110 and 190°C. The experiments were performed at high S/V values to ensure silica saturation almost from the start of the tests. The results show a square root of time dependence indicating diffusion as rate-controlling with apparent diffusion coefficients similar to that of water diffusion in alkali silicate, borosilicate glasses or long-term weathered obsidians.

Anoxic corrosion of various high burnup spent fuel samples

Abstract To assess the long-term performance of spent fuel in a repository in saline environments, the effect of iron and sample dimension on the corrosion behavior and radionuclide release from high burnup fuel was studied. Additionally, the release contributions of the highly burned fuel rim and of α-recoil transport mechanism was evaluated. Results show that spent fuel is thermodynamically unstable even under reducing conditions (presence of Fe) but the dissolution rates are very slow. Slow dissolution rates are also encountered with powdered fuel, probably resulting from depletion of radiolytic reactants.

The effect of high power ultrasound on an aqueous suspension of graphite

Ultrasound treatment was used to study the decrease of the granulometry of graphite, due to the cavitation, which allows the erosion by separating grains. At a smaller scale, cavitation bubble implosion tears apart graphite sheets as shown by HRTEM, while HO(*) and H(*) radicals produced from water sonolysis, generate oxidative and reductive reactions on these sheet fragments. Such reactions form smaller species, e.g. dissolved organic matter. The methodology proposed is very sensitive to unambiguously identifying the in situ composition of organic compounds in water. The use of the atmospheric pressure chemical ionization (APCI) Fourier transform mass spectrometry (FTMS) technique minimizes the perturbation of the organic composition and does not require chemical treatment for analysis. The structural features observed in the narrow range (m/z<300) were mainly aromatic compounds (phenol, benzene, toluene, xylene, benzenediazonium, etc.), C(4)-C(6) alkenes and C(2)-C(10) carboxylic acids. Synthesis of small compounds from graphite sonication has never been reported and will probably be helpful to understand the mechanisms involved in high energy radical reactions.

XAS study of technetium(IV) polymer formation in mixed sulphate/chloride media

Summary X-ray absorption spectroscopy has been used to establish polymer formation of Tc(IV) in aqueous solutions of Na+SO42− and Na+Cl−/SO42−. As the molybdenum chemistry show similarities to that of technetium, we used MoO2 as a reference to model our technetium species. Fitting of TcO2·xH2O with this model led to a good correlation with the literature data: (Tc-Tc=2.53 Å, Tc-O=1.87-1.98 Å). In aqueous solution, some polymers are formed regardless to the nature of the media composition: Tc-Tc=2.50 ± 0.02 Å. The general structure is in agreement with a first coordination shell containing 6 O. The modelling shows that, in the first coordination shell, there is no chloride ligand. The observed geometries are close to those found for TcO2·xH2O, hence the unknown aqueous species must be considered as a precursor of the solid technetium dioxide. Combination of these results with XANES led to attribute TcnIVOp(4n-2p)+(H2O)q with n>2 to the species.

Solubility Equilibria in the U(VI)-Ca-K-Cl-H2O System: Transformation of Schoepite into Becquerelite and Compreignacite

Schoepite type phases, (U03 · xH20(s)), have been reported as corrosion products of spent fuel in long-term leaching experiments under oxidizing conditions. This paper describes the transformation of such a phase into two different phases: (a) becquerelite, CaU6019 · 11H20(S), and (b) compreignacite, K2U6019 • 1 1 H 2 0 ( S ) . It has been proven that the uranyl hydroxide solid transforms spontaneously into these two phases (a) and (b), at room temperature after a short contact time in the presence of Ca and/or K ions, respectively. Analysis of the solution data indicates that equilibrium was reached and also, the predominance of the hydrolysis reaction vs. the chloride complexation of the uranyl ion at pH values higher than 4.5, even at the high chloride concentration. The solubility products of the two phases were determined in l m CaCl2 and l m KCl solutions, respectively. The values were extrapolated to the reference state 1 = 0 . The effect of calcium and potassium concentrations in natural waters on the formation of possible secondary phases of uranium(VI) is assessed in the light of these new data.

Sorption of Cs, Ni, Pb, Eu(III), Am(III), Cm, Ac(III), Tc(IV), Th, Zr, and U(IV) on MX 80 bentonite: An experimental approach to assess model uncertainty

A multi-site surface complexation/ion exchange model for dispersed MX 80 bentonite has been calibrated, considering the dissolution properties of the constituting mineral assemblage, for sorption of a large number of radionuclides, using experimental data from the present study together with well constrained literature data. Emphasis was on tri- and tetravalents actinides and fission products and reducing groundwater compositions.

Precipitation of technetium by subsurface sulfate-reducing bacteria

Summary To study the interaction between Tc and subsurface bacteria, we conducted batch experiments with soil and groundwater or sterilized deionized water. The system water/soil was amended with lactate and phosphate for bacterial growth. Nitrate and sulfate were added to stimulate the growth of indigenous denitrifying and sulfate-reducing bacteria. During denitrification Tc-concentration did not change with time. In the presence of sulfate-reducing bacteria, Tc-concentrations decreased in reacted waters which could be attributed to Tc(VII) reduction and precipitation of TcO2 and/or TcS2. Coprecipitation with newly formed iron sulfide is expected to contribute to Tc removal. Additional experiments with U and Tc showed that these elements were simultaneously reduced by sulfate-reducing bacteria. This work shows that 1) subsurface mixed cultures of denitrifying bacteria do not remove Tc from solution, this is different from uranium and 2) sulfate-reducing bacteria reduce and remove Tc from aqueous solutions and thus in situ bioremediation of subsurface waters and soils may be possible with such ubiquitous bacteria.