Volume 101, Issue 6 -

The knowledge of the speciation of elements at trace and ultra-trace level, in biological and environmental media is essential to acquire a better understanding of the mechanisms of toxicity, transport and accumulation in which they are involved. Determining the speciation of an element in a given medium is challenging and requires the knowledge of different methodological approaches: the calculation approach and the experimental approach through the use of dedicated analytical and spectroscopic tools. In this framework, this mini-review reports the approaches to investigate the speciation of elements in biological and environmental media as well as the experimental techniques of speciation analysis, illustrated by recent examples. The main analytical and spectroscopic techniques to obtain structural, molecular, elemental and isotopic information are described. A brief overview of separation techniques coupled with spectrometric techniques is given. Imaging and micro-localisation techniques, which aim at determining the in situ spatial distribution of elements and molecules in various solid samples, are also presented. The last part deals with the development of micro-analytical systems, since they open crucial perspectives to speciation analysis for low sample amounts and analysis on field.

Solution thermodynamic studies were performed to characterize the coordination of U(VI) and Th(IV) by the multidentate hydroxypyridinonate chelating agents 3,4,3-LI(1,2-HOPO) and 5-LIO(Me-3,2-HOPO). Species distribution and conditional stability pM profiles were calculated based on the determined formation constants, showing that both ligands form extremely stable uranyl and thorium complexes in solution. At physiological pH, the formation of 1:1 ligand:uranyl complexes is favored for both ligands, while a 2:1 ligand:thorium complex is predominant with the tetradentate 5-LIO(Me-3,2-HOPO). Comparisons with functionally similar bidentate ligands as well as with the common actinide chelator diethylenetriamine pentaacetic acid emphasized the superior affinity for U(VI) and Th(IV) of both experimental compounds over a wide pH range. These analytical results corroborate the in vivo chelation efficacy of 3,4,3-LI(1,2-HOPO) and 5-LIO(Me-3,2-HOPO) and validate their selection for further development as therapeutic actinide decorporation agents.

The speciation of actinides in environmental or biological media is often difficult to assess because it involves complex media. We would like to report here on the properties of Np(IV) cation in the well known biological HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) buffer medium. HEPES has been targeted because the possibility to use this biological buffer in actinide toxicological studies presents several advantages although the possible effects of concentrated HEPES medium on the hydrolysis of the actinides (in particular at oxidation state +IV) has not been studied yet. A combination of spectrophotometric and EXAFS measurements at the Np  L III edge shows that stable hydrolyzed neptunium(IV) clusters are obtained between pH 2.5 and 4. In a second step, in order to better understand the reactivity of these hydrolysis species formed in HEPES, the effect of a strong chelating ligands such as the hydroxamic acid (HA) or desferrioxamine (DFO) siderophores has been also investigated using spectrophotometry and EXAFS. Upon addition of HA or DFO, the hydrolyzed clusters of Np(IV) are unstable and monomeric complexes are formed and yield Np environments that are very similar to that of crystallized Pu-DFOE complex [Al(H 2 O) 6 ][Pu(DFE)(H 2 O) 3 ] 2 (CF 3 SO 3 ) 5·10 H 2 O reported in the literature.

In this study, a thermodynamic model based on Mikulin and Sergievskii–Dannus equations to calculate activity coefficients in both organic and aqueous phases was used to model plutonium(IV), nitric acid and water extraction by 30% TBP in dodecane. The speciation of Pu(IV) in the organic phase was determined via extraction isotherms modeling. The best agreement between experimental and calculated plutonium distribution ratios was obtained by considering two Pu(IV) complexes in the organic phase: Pu(NO 3 ) 4 (TBP) 2 and Pu(NO 3 ) 4 (TBP) 3 .

Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to investigate the coordination environment of U(VI) in cementitious materials. The EXAFS measurements were carried out on U(VI)-doped samples prepared under varying conditions, such as samples from sorption, hydration and diffusion experiments, and using different cementitious materials, such as crushed hydrated hardened cement paste (HCP) and calcium silicate hydrates (C-S-H). The samples had U(VI) loadings ranging from 1700 μg/g to 45000 μg/g. Applying principal component analysis (PCA) on 13 EXAFS spectra (each spectra corresponding to aminimum of five different scans) of the low loading samples, one single species is obtained indicating a similar U(VI) coordination environment for both HCP and C-S-H samples. This result confirms that C-S-H phases control the uptake of U(VI) in the complex cement matrix. The coordination environment structure of this species is similar to a U(VI) surface complex or to U(VI) in uranyl silicate minerals (two axial O atoms at 1.82 ± 0.02 Å; four equatorial O atoms at 2.25 ± 0.01 Å; one Si atom at 3.10 ± 0.03 Å). At high U(VI) loading, PCA revealed a second U(VI) species, with a coordination environment similar to that of U(VI) in calcium uranate (two axial O atoms at 1.94 ± 0.04 Å; five equatorial O atoms at 2.26 ± 0.01 Å; four Ca atoms at 3.69 ± 0.05 Å and five U atoms at 3.85 ± 0.04 Å). This study suggest that, at low U(VI) loading, U(VI) is bound to C-S-H phases in HCP while at high U(VI) loading, the immobilization of U(VI) in cementitious materials is mainly controlled by the precipitation of a calcium uranate-type phase.

The solubility of uranium(VI) was determined in WIPP-relevant brines as a function of pC H + and ionic strength, in the absence of carbonate. Carbonate concentration was below 2 × 10 5  M, measured using the gas chromatography method. In the absence of carbonate, the uranium(VI) solubilities were about  × 10 6  M in GWB at pC H + ≥7 and about  × 10 8 – × 10 7  M in ERDA-6 brine at pC H + ≥8. Solubility of uranium(VI) was also measured in NaCl media at the same levels as in ERDA-6 brine. The data established a uranium solubility that was 10–100 times lower than published results from Diaz-Arocas and Grambow [13], and they are in good agreement with modeling results and other literature data [11,12]. In the absence of carbonate, hydrolysis was the main complexation and precipitation mechanisms for uranium(VI) solubility at high ionic strength and pC H + ≥7. However, the effect of borate complexation was noticeable at pC H + ∼8–9.