Volume 93, Issue 11 - Chemical Speciation

Transport methods, and especially capillary electrophoresis, are shown to bring with a high simplicity and efficiency useful complementary data to solve some speciation problems. Their application in this aim is particularly interesting in the case of actinides or solutions resulting from the radioactive waste management. In the present article, three examples are proposed to illustrate this point. In the first one, the open-end capillary method is applied to point out the hydrolyzed species of Pa(V) over a wide pH range. The difficulties overcome in this case are related to the very low concentration and specific properties of the element under consideration. The second illustration concerns the complexation of uranyl by the anionic forms of some long-lived fission products (MO x n− , with M = I and Se). For the study of these systems, capillary electrophoresis appears to be a method particularly simple and efficient. The last example deals with the quantification of selenate and selenite ions in the presence of nitrate. Capillary electrophoresis is found to be especially appropriate to the simultaneous determination of these two anionic species in such media.

By introducing a preliminary in-capillary preconcentration step, CE-ICP/MS was applied to trace metal speciation in proteins. Emphasis was put on metal/protein interactions where CE-ICP/MS is demonstrated to provide informations concerning metal binding and metal exchanges occuring in proteins. Preliminary studies on the fate of several metals in the presence of a mixture of standard proteins were performed. Cadmium was shown to bind mainly to albumin and ceruloplasmin. Cobalt and nickel were detected only in carbonic anhydrase and ceruloplasmin in the mixture of proteins but also in albumin when injected alone.

A robust and efficient interface between a capillary electrophoresis (CE) and an ICP-MS for actinide speciation studies was developed. This interface was made of two stainless steel T-shape pieces connected to the ICP-MS through a PFA-50 nebulizer. Fast separations (typically in less that 15 min) were obtained. The performances of the technique in terms of chemical separations carried out by the capillary electrophoresis and in terms of detection limits were investigated. Concerning the detection limit of the CE-ICP-MS system for plutonium, it was determined as 5 × 10 −10 mol L −1 or 9 × 10 −18 mol under our injection conditions. The coupling enables to separate at least three (III, V and VI) of the four plutonium oxidation states which can exist in aqueous solutions and to monitor oxidation and reduction processes.

This paper describes the ability of ion pairing-reversed phase chromatography coupled to UV detector to determine an experimental speciation of cobalt in the binary Co(II)-citrate system. A method has been developed to investigate aqueous Co(II)-citrate samples at physiological pH as a function of time, by varying metal-to-ligand ratios, concentration and temperature. Solid 1:1 dimeric and 1:2 monomeric Co(II)-citrate complexes previously described have been synthesised and their analysis by IP-RPC-UV allowed the assignation of the solution state separated species. In solution state and at Co(II) concentration of 5×10 -2 M, 1:2 Co(II)-citrate monomeric species are predominant forms, while 1:1 mono and dimeric complexes are the minor species. At Co(II) concentration lower than 5 mM, only species of 1:1 stoichiometry are involved in the aqueous cobalt distribution. The monitoring of the formed species as a function of time allows highlighting the high inertness of this system. At low concentration ranges, electrospray ionisation-mass spectrometry experiments were carried out to support the chromatographic data. This study is the first one establishing a link among synthetic cobalt-citrate complexes and species existing in solution.

Our purpose was to investigate the ability of Synechocystis sp . PCC 6803, a photosynthetic prokaryote isolated from fresh water, to resist, incorporate and reduce the oxidized forms of selenium including selenite and selenate, the major selenium species present in aquatic systems. Selenium speciation and the chemical intermediates during selenium transformation were determined by X-ray absorption near edge structure (XANES) spectroscopy. The possible internalisation pathways involving selenium and the metabolic fate of selenate and selenite were examined. Selenate metabolism seemed to proceed via the sulfate reduction pathway resulting in the formation of the R-Se-H, R-Se-R and R-Se-Se-R species. The transformation of selenate to toxic amino acids may explain the high sensitivity of Synechocystis to selenate. Several mechanisms of selenium reduction seem to compete during selenite assimilation. A specific mechanism may transform internalised selenite into selenide and, subsequently induce the biosynthesis of selenoproteins. A non-specific mechanism may interfere with thiols, such as glutathione in the cell cytoplasm, or with proteins in the periplasm of the bacteria, notably thioredoxins. Several hypotheses concerning the complex transformation of selenium in Synechocystis could therefore be proposed.

Uranium bioavailability and toxicity are closely linked to the metal's speciation in solution. However in biological fluids or in media classically used for cell culture – and subsequently for in vitro cell exposure –, uranium is rarely present as free-ion since these media contain non-negligible concentrations of potential ligands such as phosphate and bicarbonate but also co-ions such as calcium which can cause U(VI) complexes precipitation. The chemical form of uranium that is internalized in cells and interferes with biological processes is of major concern. Uranium toxicity and accumulation were evaluated in vitro on NRK-52 E cells, model for rat renal proximal tubule. Uranium intracellular accumulation begins after 12 h exposure to 600 μM U(VI); toxicity appears as soon as cells accumulated 25 to 30 mg U/g protein. Modification of uranium speciation in the exposure medium induces great changes in toxicity and cell accumulation. Comparison of toxicity and accumulation results to theoretical uranium speciation, calculated with the J-Chess computer program, shows that free-ion concentration can not explain the total uranium intracellular accumulation. Low molecular weight U(VI) complexes, such as UO 2 (CO 3 ) 3 4− but also UO 2 PO 4 − could be implicated in U(VI) cellular accumulation and toxicity.

In order to better understand the mechanisms of actinide uptake by specific biomolecules, it is essential to explore the intramolecular interactions between the cation and the protein binding site. Although this has long been done for widely investigated transition metals, very few studies have been devoted to complexation mechanisms of actinides by active chelation sites of metalloproteins. In this field, X-ray absorption spectroscopy has been extensively used as a structural and electronic metal cation probe. The two examples that are presented here are related to two metalloproteins in charge of iron transport and storage in eukaryote cells: transferrin and ferritin. U(VI)O 2 2+ , Np(IV) and Pu(IV) have been selected because of their possible role as contaminant from the geosphere.

Plutonium has no known biological utility, yet it has the potential to interact with bacterial cellular and extracellular structures that contain metal-binding groups, to interfere with the uptake and utilization of essential elements, and to alter cell metabolism. These interactions can transform plutonium from its most common forms, solid, mineral-adsorbed, or colloidal Pu(IV), to a variety of biogeochemical species that have much different physico-chemical properties. Organic acids that are extruded products of cell metabolism can solubilize plutonium and then enhance its environmental mobility, or in some cases facilitate plutonium transfer into cells. Phosphate- and carboxylate-rich polymers associated with cell walls can bind plutonium to form mobile biocolloids or Pu-laden biofilm/mineral solids. Bacterial membranes, proteins or redox agents can produce strongly reducing electrochemical zones and generate molecular Pu(III/IV) species or oxide particles. Alternatively, they can oxidize plutonium to form soluble Pu(V) or Pu(VI) complexes. This paper reviews research on plutonium-bacteria interactions and closely related studies on the biotransformation of uranium and other metals.

An intercomparison exercise was led in order to study the impact of thermodynamic database (TDB) on the speciation of americium(III) in biochemical media. A first exercice was led, considering only the americium(III) species in solution. Even with different data sets, all the exercises are in relatively good agreement, concluding to successive complexation of the radioelement by phosphate (pH range from 0 to 3), citrate (pH range from 3 to 8) and hydroxo-carbonate (pH range from 8 to 14). Nevertheless, the speciation of americium differs for each pH range from one work to an other. These results are still observed when the speciation calculation includes the formation of solids. Nevertheless, some workers did not integrate in their data set the formation constant of Am(PO 4 ). This solid is known to be very insoluble in typical biological media. All the calculations where Am(PO 4 ) was not in the TDB presented a speciation where americium is solvated (mostly under citrate forms). When the formation of Am(PO 4 ) is considered, the actinide is quantitatively present under this solid species. This work is a representative example of the impact of the thermodynamic data used in speciation exercises. Some exercices, using uncompleted TDB had wrong results and led to the conclusion of the necessity in the use of expertised and extended TDB.

Within the scope of the OECD Nuclear Energy Agency (NEA) Thermochemical Data Base Project (TDB) a comprehensive review of selected organic ligands has been carried out by the authors. The selected ligands are oxalate, citrate, ethylenediaminetetraacetate (edta) and α-isosaccharinate (isa), and the elements considered in the review are U, Np, Pu, Am, Tc, Ni, Se and Zr, as well as the necessary basic data concerning protonation of the ligands and interactions with the major competing elements Na, K, Mg and Ca. This review on organic ligands showed that the pragmatic ionic strength correction procedure, the Specific ion Interaction Theory (SIT), chosen as the default method for all NEA TDB reviews, can be applied successfully also to organic ligands. The SIT interaction parameters derived from ligand protonation data for different media, e.g. NaCl and KCl, pass the consistency test when applied to other systems evaluated in the organics review, e.g. solubility data. Hence, the thermodynamic constants selected in this NEA TDB organics review can be used with some confidence in real world applications, provided that SIT is used in the speciation calculations.