Uranium retention in a Callovo-Oxfordian clay rock formation: From laboratory-based models to in natura conditions.

For the performance assessment of radioactive waste disposal, it is critical to predict the mobility of radionuclides in the geological barrier that hosts it. A key challenge consists of assessing the transferability of current knowledge on the retention properties deduced from model systems to in natura situations. The case of the redox-sensitive element uranium in the Callovo-Oxfordian clay formation (COx) is presented herein. Extensive experimental work was carried out with respect to parameters affecting uranium speciation (pH, PCO2, [Ca] and redox potential) with illite, COx clay fraction and raw COx claystone. The "bottom-up" approach implemented, with illite and montmorillonite as reactive phases, quantitatively explains the adsorption results of U(VI) and U(IV) on COx. While retention is high for U(IV) (Rd∼104 L kg-1), it remains very low for U(VI) (Rd∼4 L kg-1) due to the formation of soluble ternary Ca(Mg)-U(VI)-carbonate complexes. The applicability of the sorption model was then assessed by comparing predictive analyses with data characterizing the behavior of naturally-occurring U (<3 mg kg-1). The COx clay phase is the largest reservoir of naturally-occurring U (∼65%) but only a small fraction appears to be adsorbed (∼1%). Under representative site conditions (especially with respect to reducing conditions), we have concluded that ternary U(VI) complexes control U speciation in solution while U(IV) surface species dominate U adsorption, with Rd values > 70 L kg-1.

New insights into the accessibility of native cellulose to environmental contaminants toward tritium behavior prediction.

Tritium speciation and behavior in the environment directly rely on accessible OH groups of organic molecules and their hydrogen exchangeability properties. As one of the most widespread biomolecule, cellulose role in reducing the exchange capacity of the hydrogen atom has been previously highlighted experimentally in various environmental matrices. In this paper, a robust and reliable T/H gas-solid isotopic exchange procedure has been implemented to assess the OH group accessibility of native celluloses with an increasing degree of crystallinity. A linear relationship was found between hydroxyl reactivity and the crystallinity index (CrI) of native celluloses, as determined by the analysis of their crystalline structure from XRD characterization. The application of the obtained linear experimental model to cellulosic materials was then evaluated and an acceptable minimum value of 12% for the CrI parameter on environmental matrices could thus be established. The authors have therefore proposed an environmental matrices relevant and efficient analytical process in order to determine the accessibility of native cellulose hydroxyl groups to tritium in the environment from a single and quick sample characterization procedure.

A combined DGT - DET approach for an in situ investigation of uranium resupply from large soil profiles in a wetland impacted by former mining activities.

An in situ methodology combining DET and DGT probes was applied in a wetland soil, downstream of a former uranium mine (Rophin), to evaluate metal resupply by calculating the R ratio (R = [U]DGT/[U]pore water) from a high resolution and large (75 cm) soil profile. Our study confirms its applicability in soil layers with varying properties; only soil layers with low water content or coarse texture appear to be limiting factors. For soil profiles, DET provides new insights of the distribution of Uranium as soluble species (free ions, small inorganic complexes, …) along the pore water profile, whereas DGT highlights the presence of other "DGT labile" species. The pairing of DET and DGT, plus the calculation of the R, highlights two U behaviors in combining results from red-ox sensitive elements (Mn, Fe). First, in the organic topsoil layer, an increase in [U]DET and [U]DGT at 3-4 cm reflects the desorption of U probably trapped onto Fe- and Mn-oxohydroxides in a DGT-labile form. However, the resupply from soil to pore water is close to a diffusion only case (R < 0.2) meaning that a portion of U is certainly tightly bound by OM in soil as non-labile species. Second, a peak in [U]DGT perfectly corresponding to the former mine deposit layer signifies the presence of U under DGT-labile species. Moreover, a maximum R value of 0.87 demonstrates the near complete resupply of U from a labile fraction in this layer, as opposed to other elements like Pb.

Non-intrusive and reliable speciation of organically bound tritium in environmental matrices.

Measurements of tritium in its various forms within the environment and especially in organic matter are keys to improving the current understanding of its environmental behavior and distribution. Validated or standardized analytical procedures for tritium determination methods have now been developed for several forms of tritium in environmental samples, yet an analytical lack remains regarding the quantifications of exchangeable and non-exchangeable forms of organically bound tritium (OBT) fractions. The present work therefore aims to provide a means of developing a standardized method for OBT fraction determination by evaluating the robustness and relevance of two methods (intrusive and non-intrusive methods) developed for non-exchangeable OBT quantification on a broad panel of pertinent environmental matrices. The validity and reliability of a non-intrusive method has thus been confirmed through a robust comparative study. Moreover, its relevance for standardization purposes is discussed, while the fundamental weakness of the conventional and most widespread method is highlighted and directly quantified for the first time in relying on many demonstrated biases.

Cellulose, proteins, starch and simple carbohydrates molecules control the hydrogen exchange capacity of bio-indicators and foodstuffs.

Over the past several years, it has become increasingly acknowledged that Organically Bound Tritium (OBT) is the most pertinent tritium form for understanding its behavior and distribution within the biosphere. The fate of tritium actually depends on the accessibility and exchangeability of hydrogen atoms for isotopic exchanges in natural organic matter, especially in widespread biomass biomolecules like carbohydrates or proteins. The present work is therefore aimed at providing a means for improving the knowledge of tritium speciation and distribution on environmental matrices by evaluating the impact of molecular structure of various carbohydrate molecules on OBT behavior. We are thus proposing to assess the exchange capacities of hydrogen from a gas-solid isotopic exchange methodology in wheat grains, water-milfoil and apple environmental matrices using starch, cellulose/proteins and simple carbohydrates as their respective main constituents. For wheat grains, a good agreement was obtained between experimental and theoretical values as a result of the predominantly simple molecular structure of starch. For both water-milfoil and apple, the disparities between experimental and theoretical values showed the occurrence of the buried form of tritium, correlated with the 3D molecular complexity of their main constituents. The key role played by these determinant constituents on hydrogen exchange capacity could thus be experimentally demonstrated on several environmental matrices. These distinct hydrogen exchange capacities were then proven to exert an influence on the NE-OBT distribution on environmental matrix constituents, in yielding critical information to better the understanding of tritium distribution and behavior in the environment.

An integrated approach combining soil profile, records and tree ring analysis to identify the origin of environmental contamination in a former uranium mine (Rophin, France).

Uranium mining and milling activities raise environmental concerns due to the release of radioactive and other toxic elements. Their long-term management thus requires a knowledge of past events coupled with a good understanding of the geochemical mechanisms regulating the mobility of residual radionuclides. This article presents the results on the traces of anthropic activity linked to previous uranium (U) mining activities in the vicinity of the Rophin tailings storage site (Puy de Dôme, France). Several complementary approaches were developed based on a study of the site's history and records, as well as on a radiological and chemical characterization of soil cores and a dendrochronology. Gamma survey measurements of the wetland downstream of the Rophin site revealed a level of 1050 nSv.h-1. Soil cores extracted in the wetland showed U concentrations of up to 1855 mg.kg-1, which appears to be associated with the presence of a whitish silt loam (WSL) soil layer located below an organic topsoil layer. Records, corroborated by prior aerial photographs and analyses of 137Cs and 14C activities, suggest the discharge of U mineral particles while the site was being operated. Moreover, lead isotope ratios indicate that contamination in the WSL layer can be discriminated by a larger contribution of radiogenic lead to total lead. The dendroanalysis correlate U emissions from Rophin with the site's history. Oak tree rings located downstream of the site contain uranium concentrations ten times higher than values measured on unaffected trees. Moreover, the highest U concentrations were recorded not only for the operating period, but more surprisingly for the recent site renovations as well. This integrated approach corroborates that U mineral particles were initially transported as mineral particles in Rophin's watershed and that a majority of the deposited uranium appears to have been trapped in the topsoil layer, with high organic matter content.

Flow period influence on uranium and trace elements release in water from the waste rock pile of the former La Commanderie uranium mine (France).

Uranium mining activities expose uranium ore and mine tailings to the surface environment, where the release of radionuclides is facilitated by weathering at rates exceeding those typically found in nature. Therefore, close to former uranium mining sites, radionuclides and especially uranium concentrations in water may surpass local background levels. The methodology proposed herein, entails coupling, gamma-ray mapping, water sampling and chemical analyses including DGT (Diffusive Gradient in Thin Film) measurements, provides new insights into describing the environment of the La Commanderie site (France). Gamma-ray mapping allows identifying water seepage, output from a waste rock pile, as a potential pathway for radionuclides into the environment. Water seepage monitoring has shown: a low pH value (4.2), high sulfate content (179 mg.L-1) and high uranium concentrations of up to 436 µg.L-1. These recordings indicate that an acid mining drainage (AMD) process is occurring inside or under the oxidized parts of the waste rock pile. Monitoring data over three flow periods revealed the release of the highest uranium concentrations during a high-flow period downstream of the site, which is compliant with local regulations. The AMD process is also responsible for the release of significant amounts of Fe, Mn and As within the immediate environment in both dissolved and particulate forms. Changes in dissolved oxygen concentration and redox potential during low flow periods, modify the speciation of Fe (in AMD waters) which acts as a scavenger for other elements such as As, Mn and U. The use of DGT under environmental conditions, and specifically AMD waters, seems to be relevant in comparison to filtered spot water sampling strategies. Moreover, based on DGT measurements, the dissolved part of the released uranium is considered as labile with concentrations above the environmental standards for freshwater organisms.

A new rapid protocol for 226Ra separation and preconcentration in natural water samples using molecular recognition technology for ICP-MS analysis.

A new rapid protocol for 226Ra separation and preconcentration in natural water samples was developed before its determination by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). For this purpose, the commercially available Ra specific resin AnaLig® Ra-01 was used. This resin shows a high selectivity for radium in a large range of acid concentrations and no affinity or possible elution of 226Ra interfering elements. The distribution coefficients of Ra and other elements over a wide range of acid (HCl and HNO3) concentrations were obtained. Due to the high radium selectivity, the new developed protocol uses only 50 mg of dry resin and its performance was evaluated using 100 mL of three natural waters with different ionic strengths, spiked with a known quantity of 226Ra. Radium was successfully separated and preconcentrated yielding recoveries ranging between 72% and 86%. In parallel with the characterisation of the resin sorption properties, a detailed study of polyatomic interferences was performed on our ICP-MS allowing to identify the prominent elements favouring interferences at m/z = 226. Furthermore, a 226Ra sensitivity comparison between different ICP-MS instruments and configurations was done in order to determine high sensitivity conditions for radium analysis.

Towards speciation of organically bound tritium and deuterium: Quantification of non-exchangeable forms in carbohydrate molecules.

An original methodology to quantitatively explore exchangeability of hydrogen isotopes in carbohydrate molecules is proposed. To access the speciation of organically bound hydrogen isotopes, isotopic exchanges were carried out under a soft path regime in the vapor phase at 20 °C with set (D,T/H) vapor pressure ratios. When steady states were reached, the fraction of exchangeable hydrogen of microcrystalline cellulose, alpha-cellulose and wheat grains were obtained and ranged from 13 to 31% (versus a theoretical value of 30%). In cellulose, and more specifically in microcrystalline cellulose, the molecular hydrogen bonds as well as the different conformations of the network seemed to decrease the hydroxyl groups of glucose units available for isotopic exchange. On the contrary, the assumed enzymatic hydrolysis of the constitutive molecules of wheat starch into low-molecular weight carbohydrate molecules enhanced the exchangeable pool. An average value of the activity between non-exchangeable organically bound tritium (NE-OBT) and non-exchangeable organically bound hydrogen was calculated for wheat grains, (TH)NE  = 0.55 ±â€¯0.03 Bq.g-1 of hydrogen atoms.

DTPA complexation of bismuth in human blood serum.

The in vivo(212)Pb/(212)Bi generator is promising for application in targeted alpha therapy (TAT) of cancer. One main limitation of its therapeutic application is due to potential release of (212)Bi from the radioconjugate upon radioactive decay of the mother nuclide (212)Pb, potentially leading to irradiation of healthy tissue. The objective of the present work is to assess whether the chelate CHX-A''-DTPA (N-(2-aminoethyl)-trans-1,2-diaminocyclohexane-N,N',N''-pentaacetic acid) bound to a biological carrier molecule may be able to re-complex released (212)Bi under in vivo conditions to limit its translocation from the target site. CHX-A''-DTPA was bound to bovine gamma globulin (BGG) to mimic a model conjugate and the stability of the Bi-CHX-A''-DTPA-BGG conjugate was studied in blood serum by ultrafiltration. TRLFS experiments using Cm(III) as a fluorescent probe demonstrated that linking CHX-A''-DTPA to BGG does not affect the coordination properties of the ligand. Furthermore, comparable stability constants were observed between Bi(III) and free CHX-A''-DTPA, BGG-bound CHX-A''-DTPA and DTPA. The complexation constants determined between Bi(III) and the chelate molecules are sufficiently high to allow ultra trace amounts of the ligand to efficiently compete with serum transferrin controlling Bi(III) speciation in blood plasma conditions. Nevertheless, CHX-A''-DTPA is not able to complex Bi(III) generated in blood serum because of the strong competition between Bi(III) and Fe(II) for the ligand. In other words, CHX-A''-DTPA is not "selective" enough to limit Bi(iii) release in the body when applying the (212)Pb/(212)Bi in vivo generator.

Characterization of At- species in simple and biological media by high performance anion exchange chromatography coupled to gamma detector.

Astatine is a rare radioelement belonging to the halogen group. Considering the trace amounts of astatine produced in cyclotrons, its chemistry cannot be evaluated by spectroscopic tools. Analytical tools, provided that they are coupled with a radioactive detection system, may be an alternative way to study its chemistry. In this research work, high performance anion exchange chromatography (HPAEC) coupled to a gamma detector (γ) was used to evaluate astatine species under reducing conditions. Also, to strengthen the reliability of the experiments, a quantitative analysis using a reactive transport model has been done. The results confirm the existence of one species bearing one negative charge in the pH range 2-7.5. With respect to the other halogens, its behavior indicates the existence of negative ion, astatide At(-). The methodology was successfully applied to the speciation of the astatine in human serum. Under fixed experimental conditions (pH 7.4-7.5 and redox potential of 250 mV) astatine exists mainly as astatide At(-) and does not interact with the major serum components. Also, the method might be useful for the in vitro stability assessment of (211)At-labeled molecules potentially applicable in nuclear medicine.

Astatine standard redox potentials and speciation in acidic medium.

A combined experimental and theoretical approach is used to define astatine (At) speciation in acidic aqueous solution and to answer the two main questions raised from literature data: does At(0) exist in aqueous solution and what is the chemical form of At(+III), if it exists. The experimental approach considers that a given species is characterized by its distribution coefficient (D) experimentally determined in a biphasic system. The change in speciation arising from a change in experimental conditions is observed by a change in D value. The theoretical approach involves quasi-relativistic quantum chemistry calculations. The results show that At at the oxidation state 0 cannot exist in aqueous solution. The three oxidation states present in the range of water stability are At(-I), At(+I), and At(+III) and exist as At(-), At(+), and AtO(+), respectively, in the 1-2 pH range. The standard redox potentials of the At(+)/At(-) and AtO(+)/At(+) couples have been determined, the respective values being 0.36 +/- 0.01 and 0.74 +/- 0.01 V vs NHE.

Sorption of Eu(III) on attapulgite studied by batch, XPS, and EXAFS techniques.

The effects of pH, ionic strength, and temperature on sorption of Eu(III) on attapulgite were investigated in the presence and absence of fulvic acid (FA) and humic acid (HA). The results indicated that the sorption of Eu(III) on attapulgite was strongly dependent on pH and ionic strength, and independent of temperature. In the presence of FA/HA, Eu(III) sorption was enhanced at pH < 4, decreased at pH range of 4-6, and then increased again at pH > 7. The X-ray photoelectron spectroscopy (XPS) analysis suggested that the sorption of Eu(III) might be expressed as is identical to X3Eu0, is identical to S(w)OHEu3+, and is identical to SOEu-OOC-/HA in the ternary Eu/HN/attapulgite system. The extended X-ray absorption fine structure (EXAFS) analysis of Eu-HA complexes indicated that the distances of d(Eu-O) decreased from 2.415 to 2.360 angstroms with increasing pH from 1.76 to 9.50, whereas the coordination number (N) decreased from approximately 9.94 to approximately 8.56. Different complexation species were also found for the different addition sequences of HA and Eu(III) to attapulgite suspension. The results are important to understand the influence of humic substances on Eu(III) behavior in the natural environment.

Quantitative description and local structures of trivalent metal ions Eu(III) and Cm(III) complexed with polyacrylic acid.

The trivalent metal ion (M(III)=Cm, Eu)/polyacrylic acid (PAA) system was studied in the pH range between 3 and 5.5 for a molar PAA-to-metal ratio above 1. The interaction was studied for a wide range of PAA (0.05 mg L(-1)-50 g L(-1)) and metal ion concentrations (2x10(-9)-10(-3) M). This work aimed at 3 goals (i) to determine the stoichiometry of M(III)-PAA complexes, (ii) to determine the number of complexed species and the local environment of the metal ion, and (iii) to quantify the reaction processes. Asymmetric flow-field-flow fractionation (AsFlFFF) coupled to ICP-MS evidenced that size distributions of Eu-PAA complexes and PAA were identical, suggesting that Eu bound to only one PAA chain. Time-resolved laser fluorescence spectroscopy (TRLFS) measurements performed with Eu and Cm showed a continuous shift of the spectra with increasing pH. The environment of complexed metal ions obviously changes with pH. Most probably, spectral variations arose from conformational changes within the M(III)-PAA complex due to pH variation. Complexation data describing the distribution of complexed and free metal ion were measured with Cm by TRLFS. They could be quantitatively described in the whole pH-range studied by considering the existence of only a single complexed species. This indicates that the slight changes in M(III) speciation with pH observed at the molecular level do not significantly affect the intrinsic binding constant. The interaction constant obtained from the modelling must be considered as a mean interaction constant.

Modeling the complexation properties of mineral-bound organic polyelectrolyte: an attempt at comprehension using the model system alumina/polyacrylic acid/M (M=Eu, Cm, Gd).

This paper contributes to the comprehension of kinetic and equilibrium phenomena governing metal ion sorption on organic-matter-coated mineral particles. Sorption and desorption experiments were carried out with Eu ion and polyacrylic acid (PAA)-coated alumina colloids at pH 5 in 0.1 M NaClO(4) as a function of the metal ion loading. Under these conditions, M interaction with the solid is governed by sorbed PAA (PAA(ads)). The results were compared with spectroscopic data obtained by time-resolved laser-induced fluorescence spectroscopy (TRLFS) with Cm and Gd. The interaction between M and PAA(ads) was characterized by a kinetically controlled process: after rapid metal adsorption within less than 1 min, the speciation of complexed M changed at the particle surface till an equilibrium was reached after about 4 days. At equilibrium, one part of complexed M was shown to be not exchangeable. This process was strongly dependent on the ligand-to-metal ratio. Two models were tested to explain the data. In model 1, the kinetically controlled process was described through successive kinetically controlled reactions that follow the rapid metal ion adsorption. In model 2, the organic layer was considered as a porous medium: the kinetic process was explained by the diffusion of M from the surface into the organic layer. Model 1 allowed a very good description of equilibrium and kinetic experimental data. Model 2 could describe the data at equilibrium but could not explain the kinetic data accurately. In spite of this disagreement, model 2 appeared more realistic considering the results of the TRLFS measurements.

Study of the interaction of Ni2+ and Cs+ on MX-80 bentonite; effect of compaction using the "capillary method".

The goal of the paper is to assess the applicability of sorption models to describe the retention of contaminants on clay materials, both in dispersed and compacted states. A batch method is used to characterize the sorption equilibria between Cs, Ni, and MX-80 bentonite for solid-to-liquid ratios varying from 0.5 to 4200 kg/m3. For compacted bentonite (dry density of 1100 kg/m3), a new method is presented where the material compaction is performed in PEEK capillaries. Sorption edges and isotherms were measured in the presence of a synthetic groundwater. A model considering cation exchange reactions with interlayer cations and surface complexation reactions with edge sites was used for the dispersed state. Montmorillonite was shown to be the dominant interacting phase in MX-80 bentonite. The applicability of the model to compacted bentonite was tested. The results indicate that under conditions where the cation exchange mechanism is dominant, there is no difference between the dispersed and compacted states. For the degree of compaction studied, all exchange sites are available for sorption. For Ni, when surface complexation is the dominant sorption mechanism, a decrease of the Kd values by a factor of about 3 was observed (pH 7-8, trace concentrations). This could be explained quantitatively by a diminution of the conditional interaction constant between Ni and the edge surface site in the compacted state. One consequence of this decrease is that the contribution of the organic matter content of MX-80 bentonite to the total sorption becomes significant.

Comparison of complexed species of Eu in alumina-bound and free polyacrylic acid: a spectroscopic study.

The speciation of Eu complexed with polyacrylic acid (PAA) and alumina-bound PAA (PAA(ads)) was studied at pH 5 in 0.1 M NaClO(4). Structural parameters were obtained from (7)F(0) -->(5)D(0) excitation spectra measured by laser-induced fluorescence spectroscopy as well as from Eu L(III)-edge extended X-ray absorption fine structure (EXAFS) spectra. The coordination mode was also investigated by infrared spectroscopy. To elucidate the nature of the complexed species, Eu-acetate complexes were used as references. The spectroscopic techniques show that two carboxylate groups with 2-3 (EuPAA) and 4-5 (EuPAA(ads)) water molecules are coordinated to Eu in the first coordination sphere. For EuPAA(ads), the coordination between carboxylate groups and Eu appears to be bidendate. A similar coordination is probable for EuPAA but the EXAFS data indicate a slightly distorted coordination. The results show that the degree of freedom of carboxylate groups is not the same for free or adsorbed PAA. For PAA, the degree of freedom is constrained by the flexibility of the methylene chain. When PAA is adsorbed on alumina, the polymer chains cannot any more be treated as independent chains. One may rather assume formation of aggregates that form an organic layer at the mineral surface presenting a complex arrangement of carboxylate groups.

Interaction of Eu(III)/Cm(III) with alumina-bound poly(acrylic acid): sorption, desorption, and spectroscopic studies.

This paper contributes to the comprehension of kinetic and equilibrium phenomena governing trace metal ion sorption on organic matter coated mineral particles. Sorption and desorption experiments were carried out with trivalent metal ions M(III) (M = Eu, Cm) and poly(acrylic acid) (PAA)-coated alumina colloids at pH 5 in 0.1 M NaClO4. Under these conditions, M(III) interaction with the solid is governed by sorbed PAA. The results were compared with spectroscopic data obtained by time-resolved laser-induced fluorescence spectroscopy (TRLFS). Within less than 30 s, a state of local equilibrium is reached between M(III) and adsorbed poly(acrylic acid). M(II) bound to the organic-mineral surface and to dissolved PAA keeps five water molecules in its first hydration sphere. Interaction of M(III) with alumina-bound PAA appears to be strongerthan with dissolved PAA. With increasing contact time, a change of the metal ion speciation occurs at the organic-mineral surface. This change is explained quantitatively by kinetically controlled reactions, which succeed a rapid local equilibrium. The experimental findings suggest, in agreement with model calculations, that a part of the initially sorbed M(III) is slowly converted to a kinetically stabilized species, thereby losing water molecules from the first coordination sphere as indicated by TRLFS. This species might be assigned as a ternary Al2O3-M(III)-PAA complex. The second part of the initially bound M(II) appears to experience as well kinetically controlled reactions, however, without showing changes in the first coordination sphere. We assume that the kinetic stabilization is the consequence of rearrangement processes of the PAA at the alumina surface.

Transfer of Eu (III) associated with polymaleic acid to Bacillus subtilis.

The aim of this study is to contribute to the understanding of the distribution of Eu(III) between dissolved organic matter and microorganisms, and to investigate the effect of competitive ions such as Ca(+2) on adsorption properties. Polymaleic acid (PMA), is used as synthetic organic matter, having similar properties as natural fulvic acid, and Bacillus subtilis is chosen as microorganism. A double labeling method was used: [14C]MPA and 152Eu to quantify the behavior of the various components. Preliminary experiments showed that the adsorption of polymaleic acid onto Bacillus subtilis was negligible at pH=5 in 0.15mol/l of NaCl. In the absence of Ca(+2), the transfer of Eu(III) from PMA to B. subtilis could be described by a simple empirical model based on data obtained from sorption isotherms on the reference systems Eu(III)/PMA and Eu(III)/B. subtilis. In the presence of Ca(+2), the transfer was increased. The hypothesis that Ca(+2) ions acted as a bridging agent between PMA and the bacteria was proposed.

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.