New insights into U(VI) sorption onto montmorillonite from batch sorption and spectroscopic studies at increased ionic strength.

The influence of ionic strength up to 3 mol kg-1 (background electrolytes NaCl or CaCl2) on U(VI) sorption onto montmorillonite was investigated as function of pHc in absence and presence of CO2. A multi-method approach combined batch sorption experiments with spectroscopic methods (time-resolved laser-induced fluorescence spectroscopy (TRLFS) and in situ attenuated total reflection Fourier-transform infrared spectroscopy (ATR FT-IR)). In the absence of atmospheric carbonate, U(VI) sorption was nearly 99% above pHc 6 in both NaCl and CaCl2 and no significant effect of ionic strength was found. At lower pH, cation exchange was strongly reduced with increasing ionic strength. In the presence of carbonate, U(VI) sorption was reduced above pHc 7.5 in NaCl and pHc 6 in CaCl2 system due to formation of aqueous UO2(CO3)x(2-2x) and Ca2UO2(CO3)3 complexes, respectively, as verified by TRLFS. A significant ionic strength effect was observed due to the formation of Ca2UO2(CO3)3(aq), which strongly decreases U(VI) sorption with increasing ionic strength. The joint analysis of determined sorption data together with literature data (giving a total of 213 experimental data points) allowed to derive a consistent set of surface complexation reactions and constants based on the 2SPNE SC/CE approach, yielding log K°≡SSOUO2+ = 2.42 ± 0.04, log K°≡SSOUO2OH = -4.49 ± 0.7, and log K°≡SSOUO2(OH)32- = -20.5 ± 0.4. Ternary uranyl carbonate surface complexes were not required to describe the data. With this reduced set of surface complexes, an improved robust sorption model was obtained covering a broad variety of geochemical settings over wide ranges of ionic strengths and groundwater compositions, which subsequently was validated by an independent original dataset. This model improves the understanding of U(VI) retention by clay minerals and enables now predictive modeling of U(VI) sorption processes in complex clay rich natural environments.

A comprehensive study of the sorption mechanism and thermodynamics of f-element sorption onto K-feldspar.

The mobility of heavy metal contaminants and radionuclides in the environment is directly controlled by their interactions with charged mineral surfaces, hence an assessment of their potential toxicity, e.g. in the context of radioactive waste disposal sites, requires understanding of sorption processes on the molecular level. Here, we investigate the sorption of a variety of rare earth elements (REE) and trivalent actinides (Am, Cm) on K-feldspar using batch sorption, time-resolved laser-induced fluorescence spectroscopy (TRLFS), and a surface complexation model. Initially, a reliable pKa for K-feldspar's surface deprotonation reaction was determined as 2.5 ± 0.02 by column titration experiments, in excellent agreement with a measured pHIEP of 2.8. Batch sorption experiments over a broad range of experimental conditions in terms of mineral grain size, pH, [M3+], ionic radius, solid/liquid ratio, ionic strength, and equilibration procedures were carried out to quantify macroscopic retention. The trivalent d-block element Y, early, mid, and late lanthanides (La, Eu, Nd, Lu), as well as two minor actinides (Am, Cm) were used for batch sorption experiments and showed similar pH dependent uptake behavior, underlining their chemical analogy. In parallel, spectroscopic investigations provided insight into surface speciation. Cm TRLFS spectra indicate the formation of three inner-sphere sorption complexes with increasing hydrolysis. Additionally, a ternary K-feldspar/Cm/silicate complex was found for pH > 10, and batch and spectroscopic data at low pH (<4) point to small amounts of outer sphere sorption complexes. Based on TRLFS data, batch sorption, and titration data, a generic geochemical sorption model was developed, that describes sorption edges for all investigated M3+/K-feldspar systems satisfactorily. The derived stability constants for the binary sorption complexes (logK1-4 = -3.6, -7.7, -11.5, and -17.4, respectively) could successfully be used to reproduce literature data. The stability constants obtained for the surface complexes were included into the database for the Smart Kd-concept, which will further improve the safety assessment of potential repositories for radioactive waste.

Complexation of Trivalent Lanthanides (Eu) and Actinides (Cm) with Aqueous Phosphates at Elevated Temperatures.

In this study, the complexation of Eu(III) and Cm(III) with aqueous phosphates was investigated using laser-induced luminescence spectroscopy. Experiments at 25 °C and different ionic strengths (0.6-3.1 mol·L-1 NaClO4) established the formation of EuH2PO42+ and CmH2PO42+. From the conditional stability constants, the respective values at infinite dilution as well as the ε(Me(H2PO4)2+;ClO4-) (Me = Eu or Cm) ion interaction coefficients (using the specific ion interaction theory - SIT) were derived. Further experiments (at constant ionic strength of 1.1 mol·L-1) showed that upon increasing the temperature (25-80 °C), the formation of both EuH2PO42+ and CmH2PO42+ was favored. Using the van't Hoff equation, the molal enthalpy Δ R H m° and molal entropy Δ R S m° of these reactions were derived, corroborating an endothermic and entropy driven complexation process. This work contributes to a better understanding of the coordination chemistry of both trivalent lanthanides and actinides with phosphate ions.

Smart Kd-values, their uncertainties and sensitivities - Applying a new approach for realistic distribution coefficients in geochemical modeling of complex systems.

One natural retardation process to be considered in risk assessment for contaminants in the environment is sorption on mineral surfaces. A realistic geochemical modeling is of high relevance in many application areas such as groundwater protection, environmental remediation, or disposal of hazardous waste. Most often concepts with constant distribution coefficients (Kd-values) are applied in geochemical modeling with the advantage to be simple and computationally fast, but not reflecting changes in geochemical conditions. In this paper, we describe an innovative and efficient method, where the smart Kd-concept, a mechanistic approach mainly based on surface complexation modeling, is used (and modified for complex geochemical models) to calculate and apply realistic distribution coefficients. Using the geochemical speciation code PHREEQC, multidimensional smart Kd-matrices are computed as a function of varying (or uncertain) environmental conditions. On the one hand, sensitivity and uncertainty statements for the distribution coefficients can be derived. On the other hand, smart Kd-matrices can be used in reactive transport (or migration) codes (not shown here). This strategy has various benefits: (1) rapid computation of Kd-values for large numbers of environmental parameter combinations; (2) variable geochemistry is taken into account more realistically; (3) efficiency in computing time is ensured, and (4) uncertainty and sensitivity analysis are accessible. Results are presented exemplarily for the sorption of uranium(VI) onto a natural sandy aquifer material and are compared to results based on the conventional Kd-concept. In general, the sorption behavior of U(VI) in dependence of changing geochemical conditions is described quite well.

Spectroscopic evidence for selenium(IV) dimerization in aqueous solution.

The aqueous speciation of selenium(iv) was elucidated by a combined approach applying quantum chemical calculations, infrared (IR), Raman, and (77)Se NMR spectroscopy. The dimerization of hydrogen selenite (HSeO3(-)) was confirmed at concentrations above 10 mmol L(-1) by both IR and NMR spectroscopy. Quantum chemical calculations provided the assignment of vibrational bands observed to specific molecular modes of the (HSeO3)2(2-) ion. The results presented will provide a better understanding of the chemistry of aqueous Se(iv) which is of particular interest for processes occurring at mineral/water interfaces.

The influence of biofilms on the migration of uranium in acid mine drainage (AMD) waters.

The uranium mine in Königstein (Germany) is currently in the process of being flooded. Huge mass of Ferrovum myxofaciens dominated biofilms are growing in the acid mine drainage (AMD) water as macroscopic streamers and as stalactite-like snottites hanging from the ceiling of the galleries. Microsensor measurements were performed in the AMD water as well as in the biofilms from the drainage channel on-site and in the laboratory. The analytical data of the AMD water was used for the thermodynamic calculation of the predominance fields of the aquatic uranium sulfate (UO(2)SO(4)) and UO(2)(++) speciation as well as of the solid uranium species Uranophane [Ca(UO(2))(2)(SiO(3)OH)(2)∙5H(2)O] and Coffinite [U(SiO(4))(1-x)(OH)(4x)], which are defined in the stability field of pH>4.8 and Eh<960 mV and pH>0 and Eh<300 mV, respectively. The plotting of the measured redox potential and pH of the AMD water and the biofilm into the calculated pH-Eh diagram showed that an aqueous uranium(VI) sulfate complex exists under the ambient conditions. According to thermodynamic calculations a retention of uranium from the AMD water by forming solid uranium(VI) or uranium(IV) species will be inhibited until the pH will increase to >4.8. Even analysis by Energy-filtered Transmission Electron Microscopy (EF-TEM) and electron energy loss spectroscopy (EELS) within the biofilms did not provide any microscopic or spectroscopic evidence for the presence of uranium immobilization. In laboratory experiments the first phase of the flooding process was simulated by increasing the pH of the AMD water. The results of the experiments indicated that the F. myxofaciens dominated biofilms may have a substantial impact on the migration of uranium. The AMD water remained acid although it was permanently neutralized with the consequence that the retention of uranium from the aqueous solution by the formation of solid uranium species will be inhibited.

TRLFS: analysing spectra with an expectation-maximization (EM) algorithm.

A new approach for fitting statistical models to time-resolved laser-induced fluorescence spectroscopy (TRLFS) spectra is presented. Such spectra result from counting emitted photons in defined intervals. Any photon can be described by emission time and wavelength as observable attributes and by component and peak affiliation as hidden ones. Understanding the attribute values of the emitted photons as drawn from a probability density distribution, the model estimation problem can be described as a statistical problem with incomplete data. To solve the maximum likelihood task, an expectation-maximization (EM) algorithm is derived and tested. In contrast to the well known least squares method, the advantage of the new approach is its ability to decompose the spectrum into its components and peaks using the revealed hidden attributes of the photons as well as the ability to decompose a background-superimposed spectrum into the exploitable signal of the fluorescent chemical species and the background. This facilitates new possibilities for evaluation of the resulting model parameters. The simultaneous detection of temporal and spectral model parameters provides a mutually consistent description of TRLFS spectra.

Evaluation and ranking of restoration strategies for radioactively contaminated sites.

An international project, whose aim was the development of a transparent and robust method for evaluating and ranking restoration strategies for radioactively contaminated sites (RESTRAT), was carried out under the Fourth Framework of the Nuclear Fission Safety Programme of the EU. The evaluation and ranking procedure used was based on the principles of justification and optimisation for radiation protection. A multi-attribute utility analysis was applied to allow for the inclusion of radiological health effects, economic costs and social factors. Values of these attributes were converted into utility values by applying linear utility functions and weighting factors, derived from scaling constants and expert judgement. The uncertainties and variabilities associated with these utility functions and weighting factors were dealt with by a probabilistic approach which utilised a Latin Hypercube Sampling technique. Potentially relevant restoration techniques were identified and their characteristics determined through a literature review. The methodology developed by this project has been illustrated by application to representative examples of different categories of contaminated sites; a waste disposal site, a uranium tailing site and a contaminated freshwater river.