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Europium / TRLFS / Clay minerals / Surface complexation / Ternary complexesSummary. The surface sorption process of Eu(III) onto smectite and kaolinite was investigated by time-resolved laser fluorescence spectroscopy (TRLFS) in the trace concentration range. The experiments were performed in 0.025 M and 0.45 M NaClO 4 . The sorption process of Eu(III) onto smectite was obtained by TRLFS under atmospheric conditions and in absence of CO 2 . The pH was varied between 3.5 and 9 at a fixed metal ion concentration of 3.3 × 10 −6 mol/L Eu(III). At low pH (< 4) the metal ion keeps its complete hydration sphere indicating outer-sphere complexation. With increasing pH the formation of an inner-sphere Eu(III) surface complex was observed. The differences in the spectra and the fluorescence emission lifetimes of the surface sorbed Eu(III) in presence and absence of carbonate indicate the formation of ternary clay/Eu(III)/carbonate complexes. The different europium/clay surface complexes were characterized by their fluorescence emission spectra ( 5 D 0 → 7 F 1 / 5 D 0 → 7 F 2 intensity ratio) and their fluorescence emission lifetime.

For long-term performance assessment of nuclear waste repositories knowledge concerning interactions of actinides with mineral surfaces is imperative. The mobility and bioavailability of released radionuclides is strongly dependent on sorption/desorption processes onto mineral surfaces. Therefore it is necessary to characterize the surface species formed and to elucidate the reaction mechanisms involved. The high fluorescence spectroscopic sensitivity of Cm(III) has attracted our interest regarding the complexation process of Cm(III) onto smectite and kaolinite as a model system for the sorption of trivalent actinides in the trace concentration range. We conclude that at low pH Cm(III) is sorbed onto kaolinite and smectite as an outer-sphere complex and retains its complete primary hydration sphere. With increasing pH inner-sphere adsorption onto kaolinite and smectite occurs via the aluminol edge sites. The same evolution of the Cm(III)-clay surface species as a function of pH was observed for both minerals. Starting at a pH > or = 5 we observe the formation of a [triple bond]Al-O-Cm2+(H2O)5 surface complex, which is replaced by a second species at higher pH. The second surface complex may be a monodentate [triple bond]Al-O-Cm+(OH)(H2O)4 species or bidentate [triple bond](Al-O)2-Cm+(H2O)5 species. The Cm(III)/clay surface complexes are characterized bytheir emission spectra (peak maxima at 598.8 and 603.3 nm) and their fluorescence lifetime (both 110 +/- 7 micros). An important result in view of the mobility and bioavailability of radionuclides is that no incorporation of Cm(III) into the bulk clay structure was observed.

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The interactions between uranium and four metalloproteins (Apo-HTf, HSA, MT and Apo-EqSF) were investigated using fluorescence quenching measurements. The combined use of a microplate spectrofluorometer and logarithmic additions of uranium into protein solutions allowed us to define the fluorescence quenching over a wide range of [U]/[Pi] ratios (from 0.05 to 1150) at physiologically relevant conditions of pH. Results showed that fluorescence from the four metalloproteins was quenched by UO(2)(2+). Stoichiometry reactions, fluorescence quenching mechanisms and complexing properties of metalloproteins, i.e. binding constants and binding sites densities, were determined using classic fluorescence quenching methods and curve-fitting software (PROSECE). It was demonstrated that in our test conditions, the metalloprotein complexation by uranium could be simulated by two specific sites (L(1) and L(2)). Results showed that the U(VI)-Apo-HTf complexation constant values (log K(1)=7.7, log K(2)=4.6) were slightly higher than those observed for U(VI)-HSA complex (log K(1)=6.1, log K(2)=4.8), U(VI)-MT complex (log K(1)=6.5, log K(2)=5.6) and U(VI)-Apo-EqsF complex (log K(1)=5.3, log K(2)=3.9). PROSECE fitting studies also showed that the complexing capacities of each protein were different: 550 moles of U(VI) are complexed by Apo-EqSF while only 28, 10 and 5 moles of U(VI) are complexed by Apo-HTf, HSA and MT, respectively.

International audienceThe aim of the study presented here is to determine the impact of short- and medium-term transformations (0–3 years) of the soil organic matter (SOM) on the major processes and parameters that enable or inhibit selenite, Se(+IV), transfers between the soil components (solid, liquid or gaseous). Three types of soil of similar mineralogical origin but containing diverse quantities and qualities of SOM were first contaminated with Se(+IV) and incubated at 28°C. Soils were sampled throughout the incubation period to characterise the mobility of Se (batch and soil column experiments) and also its fractionation within the soil compartments (selective extractions and size-density fractionation). The following are the main results obtained within the first month of incubation. (a) Selenium was partly volatilized during soil incubation (<0.1%), (b) Se extracted with CaCl2 (5×10−4 M) was equally small for the three soil samples (∼1–5%), suggesting that Se was strongly sorbed on the solid phase and (c) at least 10% of Se was associated to the particulate organic matter $${\left( {{\text{POM}}_{{ > {\text{50}}\mu {\text{m}}}} } \right)},$$whereas 60% of Se was extracted with soil humic substances. These results suggested that both SOM quantity and quality played a significant role in selenium retention. Furthermore, comparison between experimental and predicted variations of CO2 fluxes (due to C mineralisation) and soil biomasses are presented. By this way, we estimated the capacity of the RothC model as an experimental gauging tool in the prediction of C turnover on a laboratory scale

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Although uranium (U) is naturally found in the environment, soil remediation programs will become increasingly important in light of certain human activities. This work aimed to identify U(VI) detoxification mechanisms employed by a bacteria strain isolated from a Chernobyl soil sample, and to distinguish its active from passive mechanisms of interaction. The ability of the Microbacterium sp. A9 strain to remove U(VI) from aqueous solutions at 4 °C and 25 °C was evaluated, as well as its survival capacity upon U(VI) exposure. The subcellular localisation of U was determined by TEM/EDX microscopy, while functional groups involved in the interaction with U were further evaluated by FTIR; finally, the speciation of U was analysed by TRLFS. We have revealed, for the first time, an active mechanism promoting metal efflux from the cells, during the early steps following U(VI) exposure at 25 °C. The Microbacterium sp. A9 strain also stores U intracellularly, as needle-like structures that have been identified as an autunite group mineral. Taken together, our results demonstrate that this strain exhibits a high U(VI) tolerance based on multiple detoxification mechanisms. These findings support the potential role of the genus Microbacterium in the remediation of aqueous environments contaminated with U(VI) under aerobic conditions.