Massoud Fattahi

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.

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.

Selenide retention onto pyrite under reducing conditions

Summary Pyrite (FeS2) is a mineral phase often present as inclusions in temperate soils. Moreover, it turns out to be a sorption sink for certain radionuclides in deep geological disposals. The present study was thus initiated to determine the capacity of pyrite to immobilize selenide (Se(-II)). Due to the fact that pyrite surface oxidizes readily, potentials were applied in order to minimise its surface evolution, and ensure the reducing conditions necessary for stabilizing Se(-II). The sorption experiments were carried out in NaCl electrolyte and were amperometrically controlled. After only several minutes of reaction, at least 97% of Se(-II) initially present in solution was disappeared. The Kd values vary from 7–65 L/g and the isotherm curve shows site saturation at higher initial selenide concentrations and no pH-dependence. By means of several spectroscopic techniques, the reaction mechanism was investigated. The XRD and in situ XANES results indicate the presence of Se(0) on pyrite surface, which explain the rapid disappearance of Se observed in the sorption experiments. Moreover, XPS results obtained from Se-reacted pyrite particles reveal cleavage of S–S bonding which resulted in formation of S2− on pyrite surface. Thus, we conclude that Se(-II) can be immobilized by pyrite via surface redox reaction: ≡FeS2 + HSe− ⇔ ≡FeS + Se(0) + HS−.

Effect of alpha radiolysis on doped UO2 dissolution under reducing conditions

Summary The aim of the present work is to quantify the influence of alpha radioactivity of 225Ac doped UO2 on the dissolution rate under reducing conditions at pH 6. The doped or undoped material was prepared by precipitation and the size of particles was about 3 nm. The total alpha activity of the doped material was about 2000 MBq g-1 UO2, about 4 times higher than that of 15 years old spent fuel. The solution was kept under reducing conditions during the experiment by permanent electrochemical reduction under inert atmosphere. The results showed that the dissolution rate of doped material was a function of alpha activity and thus a function of the dose.

Speciation of technetium and rhenium complexes by in situ XAS-electrochemistry

Summary A spectro-electrochemical cell was developed in order to study the speciation of radio-elements in thermodynamic unstable redox states using in situ XAS spectroscopy. This cell was used for the speciation of Re and Tc complexes in chloride media. Experiments on Re were carried out with the aim to validate the functionality of the experimental set-up. During electro-reduction of Re(VII) in HCl media, EXAFS and XANES studies were performed in order to reveal the formation of chloro-oxygenated compounds of Re(IV). The speciation of technetium in aqueous solutions of deep geological deposits for radioactive waste is important to predict its mobility under reducing conditions. XANES spectra showed that electro-reduction of Tc(VII) in chloride media leads to a position of K-edge absorption which agrees with a Tc(IV)/Tc(III) mixture.

Coprecipitation of thorium with UO2

Summary Coprecipitation may be a significant process in controlling radionuclide (RN) release during spent fuel dissolution/reprecipitation in a breached geological disposal site by water. The objective of this work was to study the coprecipitation of thorium with UO2 as host phase under reducing conditions (pure N2 and Eh<-100 mV/SHE) as a function of pH and [Th]/[U] initial concentration ratio. For pH < 4, the final concentration of Th was found to be considerably lower than expected if pure phase, Th(OH)4(s) was formed. Coprecipitation phenomena and formation of solid solutions (U,Th)O2+x(s) are suggested to be responsible for the lower solubility concentrations. For pH > 4, the final concentrations seems to be similar to pure phase solubility.

Electrochemical aspects of radiolytically enhanced UO2 dissolution

Summary Experiments were performed, irradiating UO2 colloids of 3 nm diameter by a 5 MeV alpha beam of a cyclotron. Both the dissolution rate of these colloids and the production rate of hydrogen peroxide have been measured. The experimentally measured corrosion rate of these colloids is very similar to the corrosion rates measured for bulk UO2 samples, indicating on the one hand that similar reaction mechanism prevail and on the other hand that the UO2 colloids are well representative for bulk UO2. The results were compared with two current models for radiolytical dissolution of spent fuel. Additionally a new coupling between water radiolysis reactions and spent fuel dissolution has been established. The coupling is not anymore based on rate constants for direct reaction of oxidants with UO2, but on the electrochemical coupling of anodic dissolution reactions with cathodic reduction of molecular and radical radiolysis products. The model not only describes well the experimental data, it also allows to predict both the establishment of corrosion potentials and of fractional reaction orders with respect to oxidant concentrations. Model results can also be compared with experimental data (corrosion potential ...) using UO2 as an electrode.

Condensation mechanisms of tetravalent technetium in chloride media

Summary The condensation mechanisms of tetravalent technetium in chloride media were studied in the pH range 0–1.5. A new dimer complex of Tc(IV) has thus been discovered, Tc2OCl104−. Spectroscopic and kinetics studies showed that the formation of this compound resulted from the condensation of TcCl5(H2O)−. An EXAFS study indicates that the dimer displays a [Tc-O-Tc]6+ structure. As the pH increases, UV-visible measurements showed a cyclization of [Tc-O-Tc]6+ into [Tc(μO)2Tc]4+ leading, in fine, to the precipitation of TcO2·x H2O. The aquation constant (Kaq) of TcCl62− into TcCl5(H2O)− and the dimerisation constant (logKdim) of TcCl5(H2O)− into Tc2OCl104− were determined to be 2.20±0.26 and 4.68±0.09, respectively.

The speciation of Tc(IV) in chloride solutions

The speciation of tetravalent technetium has been studied in HCl solutions ranging from 1.0 to 6.0 M. Tc(IV) exists in 6.0 M HCl as the well characterized TcCl62- chloride complex. In 1.0 M HCl, the chloride ligands of TcCl62- are slowly substituted by other ligands such as H2O, OH- or O2-. So TcClmOn(OH)p(H2O)q(4-m-2n-p)+ compounds may form with (m + n + p + q) = 6. Several speciation techniques such as UV-visible and Raman spectrometries, X-ray absorption spectroscopy and electrochemistry have enabled to get an insight into this problem. All the results have indicated two most likely species of Tc(IV) after aging for about ten days in 1.0 M HCl to be aquo-chlorocomplexes of technetium and not oxo-chorocomplexes: TcCl5 · H2O- and TcCl4 · 2 H2O.