Aquatic chemistry of technetium in the presence of inorganic and organic ligands under repository-relevant conditions

Technetium is the lightest element of the periodic table whose isotopes are all radioactive. ${}^{99}$Tc is formed as a fission product of ${}^{235}$U and ${}^{239}$Pu in nuclear reactors, and is found in spent nuclear fuel with a relatively high abundance of ≈ 6%. Due to the long half-life of the ${}^{99}$Tc isotope (t${}_{1/2}$ = 2.13x10${}^5$ years) and the remarkably different chemical behavior of its main oxidation states (+IV and +VII) in aqueous solutions, an accurate knowledge of its redox properties, solubility and complexation with inorganic and organic ligands is of special relevance in the context of the nuclear waste disposal safety case. The influence of different inorganic and organic ligands relevant to nuclear waste disposal on the behavior of Tc in aqueous solutions has been previously investigated for a number of systems, which mainly focused on the inorganic ligands hydroxide, chloride, and carbonate. The interaction of Tc with organic ligands was investigated for a few cases, although most of the available studies are of qualitative character and do not provide a thermodynamic description of these systems. This is likely due to the possible role of organic ligands in stabilizing less conventional oxidation states of Tc (e.g. +V or +VI), thus challenging the definition of correct chemical models for these systems. In this context, the present PhD work aims at a quantitative and mechanistic description of the interaction of Tc with selected ligands, i.e. sulfate, sulfide and gluconate. A final chapter also explores the analogies and differences between Tc and Re in terms of redox chemistry and solubility of the +IV oxidation state. Due to their similar electronic configurations (Re: [Xe]4f${}^{14}$ 5d${}^5$ 6s${}^2$; Tc: [Kr] 4d${}^5$ 5s${}^2$), Re is often considered as inactive analogue of Tc with similar chemical behavior. These systems are investigated using a combined approach that includes classical wet-chemistry methods (i.e. under- and oversaturation solubility experiments), advanced spectroscopic techniques (e.g. L${}_3$-and K-edge Tc XAFS) and theoretical calculations. The redox couple sulfate / sulfide is of great relevance in environmental systems and has also direct effects on underground repositories for nuclear waste. Despite this, the number of experimental studies investigating the interaction of these components with Tc under repository-relevant conditions is very limited, and no thermodynamic description for these systems is available to date. For this reason, the interaction of Tc with sulfate was first investigated in reducing, dilute to concentrated NaCl-Na${}_2$SO${}_4$ solutions and concentrated MgCl${}_2$-MgSO${}_4$ and CaCl${}_2$-CaSO${}_4$ solutions in acidic to hyperalkaline pH-conditions. Dedicated experimental efforts in combination with thermodynamic calculations using the Pitzer activity model targeted the quantification of the correction factors A${}_m$ in pure Na${}_2$SO${}_4$ as well as mixed NaCl-Na${}_2$SO${}_4$ solutions, which are essential for the accurate determination of the pH${}_m$ values in systems with I${}_m$ ≥ 0.1 m. Solubility data, (pe + pH) measurements as well as XANES results confirm the predominance of Tc(IV) in the aqueous and solid phases under the investigated conditions. EXAFS data confirm the predominance of TcO${}_2$(am, hyd) as solid phase controlling the solubility of Tc above pH${}_m$ ˃ 1.5. On the other hand, the formation of a Tc(IV)-O-Cl solid phase is observed for samples in concentrated NaCl-Na${}_2$SO${}_4$ solutions and lower pH${}_m$ values. Solubility data obtained for Tc(IV) in the mixed chloride-sulfate systems are in line with data previously reported for analogue (sulfate-free) chloride systems. Although the formation of small fractions of Tc(IV)-sulfate complexes cannot be completely ruled-out, these results support that under repository-relevant conditions sulfate can easily be outcompeted by hydrolysis, and thus has a minor impact on the solubility of TcO${}_2$(am, hyd). Following the study with sulfate, the impact of sulfide on the solubility an aqueous speciation of Tc was investigated from over- and undersaturation conditions in alkaline to hyperalkaline NaCl-Na${}_2$S solutions. In contrast to the sulfate system, investigated samples from oversaturation conditions (with [Tc(VII)]${}_0$ = 10${}^{-5}$ M) in aqueous systems containing 0.001 – 0.1 M Na${}_2$S show a significant decrease of the initial Tc concentration. This decrease in solubility is accompanied by the formation of a brownish-black precipitate in some of the investigated samples. Above pH${}_m$ ≈ 12, the concentration of Tc decreases well below the solubility defined by TcO${}_2$(am, hyd), thus supporting the formation of a secondary phase beyond the known Tc(IV) hydrous oxide. XPS measurements hint towards the predominance of a Tc-S compound, which is proposed to control the solubility of Tc in these systems. Below pH${}_m$ ≈ 12, measured Tc concentrations are clearly above the solubility of TcO${}_2$(am, hyd). This could be explained by the formation of stable Tc(IV)-sulfide aqueous complexes, or possibly due to slow precipitation kinetics involving the formation of metastable Tc-S colloids, as previously described in the literature. Gluconate is a polyhydroxocarboxylic acid found as additive in different cement preparations. It has been described to strongly complex hard Lewis acids such as actinides or lanthanides, but information on the type and stability of the possible complexes forming with technetium is very limited. The interaction of Tc with gluconate was investigated from under- and oversaturation conditions in reducing, alkaline to hyperalkaline systems characteristic of cementitious environments. Solubility samples were prepared in 0.1–5.0 M NaCl solutions with 9 ≤ pH${}_m$ ≤ 14 and 10${}^{\text{–}4}$ M ≤ [GLU]${}_{tot}$ ≤ 0.5 M. Reducing conditions were chemically set with Na${}_2$S${}_2$O${}_4$, Sn(II), hydrazine or Fe powder, except for a limited number of samples which remained unbuffered. The solubility of TcO${}_2$(am, hyd) is clearly enhanced in the presence of gluconate compared to gluconate-free systems. The experimental solubility data obtained in the pH${}_m$ range of 10–14 provide indirect evidence for the change in the aqueous speciation, which is dominated by TcO(OH)${}_2$(aq) and TcO(OH)${}_3$${}^{\text{–}}$ under reducing conditions but in absence of gluconate. Tc L${}_3$-edge XANES and K-edge EXAFS measurements of selected samples support the predominance of Tc(IV)-GLU aqueous species under the very reducing conditions defined by Sn(II), but the predominance of a Tc(V)-GLU complex in the absence of Sn(II). Preliminary chemical and thermodynamic models were derived for the system Tc(IV)-GLU based on the combination of solubility data, solid phase characterization and spectroscopic observations. These models allow geochemical calculations for this system under conditions relevant for nuclear waste disposal. The last topic in this PhD work targeted the solubility and redox behavior of rhenium as possible non-radioactive chemical analogue of technetium. Experiments were performed from under- and oversaturation conditions in dilute NaCl solutions containing Sn(II) or Na${}_2$S${}_2$O${}_4$ as reducing agents. The results of this work are compared to previous studies conducted with technetium under analogous conditions. The solubility of Re(IV) under weakly acidic to hyperalkaline conditions shows similar trends as those previously described for Tc(IV), with the corresponding predominance of the hydrolysis species ReO(OH)${}_2$(aq) and ReO(OH)${}_3$${}^{\text{–}}$. No evidence was obtained for the formation of cationic hydrolysis species of Re(IV) in acidic solutions, which however are expected to form in more acidic solutions or at higher ionic strength conditions. Compared to Tc(VII), Re(VII) is reluctant to reduction in the presence of Sn(II) except at very low (pH${}_m$ = 1) and very high pH values (pH${}_m$ = 12.8). The dissimilar behavior is attributed to the lower E° values for the reduction of M(VII) to M(IV) in the case of Re. The observed decrease of Re concentration at pH${}_m$ = 1 and 12.8 is tentatively attributed to the co-precipitation with SnO${}_2$. These results highlight that the use of Re as a non-radioactive chemical analogue of Tc must be considered with precaution, especially for those system involving redox transitions. This work provides new data and improved fundamental understanding on the solubility, redox speciation, and complexation behavior of technetium under conditions relevant for nuclear waste disposal. New insights on the chemical analogies between technetium and rhenium have been attained based on systematic wet-chemistry experiments. Chemical and thermodynamic models derived in this work can be implemented in thermodynamic databases and used in geochemical calculations under a variety of boundary conditions.