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Issue 12, 2018
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Thermodynamic description of Tc(IV) solubility and carbonate complexation in alkaline NaHCO3–Na2CO3–NaCl systems

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Abstract

The solubility of 99Tc(IV) was investigated in dilute to concentrated carbonate solutions (0.01 M ≤ Ctot ≤ 1.0 M, with Ctot = [HCO3] + [CO32−]) under systematic variation of ionic strength (I = 0.3–5.0 M NaHCO3–Na2CO3–NaCl–NaOH) and pHm (−log[H+] = 8.5–14.5). Strongly reducing conditions (pe + pHm ≈ 2) were set with Sn(II). Carbonate enhances the solubility of Tc(IV) in alkaline conditions by up to 3.5 log10-units compared to carbonate-free systems. Solvent extraction and XANES confirmed that Tc was kept as +IV during the timeframe of the experiments (≤ 650 days). Solid phase characterization performed by XAFS, XRD, SEM-EDS, chemical analysis and TG-DTA confirmed that TcO2·0.6H2O(am) controls the solubility of Tc(IV) under the conditions investigated. Slope analysis of the solubility data in combination with solid/aqueous phase characterization and DFT calculations indicate the predominance of the species Tc(OH)3CO3 at pHm ≤ 11 and Ctot ≥ 0.01 M, for which thermodynamic and activity models are derived. Solubility data obtained above pHm ≈ 11 indicates the formation of previously unreported Tc(IV)–carbonate species, possibly Tc(OH)4CO32−, although the likely formation of additional complexes prevents deriving a thermodynamic model valid for this pHm-region. This work provides the most comprehensive thermodynamic dataset available for the system Tc4+–Na+–Cl–OH–HCO3–CO32−–H2O(l) valid under a range of conditions relevant for nuclear waste disposal.

Graphical abstract: Thermodynamic description of Tc(iv) solubility and carbonate complexation in alkaline NaHCO3–Na2CO3–NaCl systems

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Publication details

The article was received on 19 Jan 2018, accepted on 21 Feb 2018 and first published on 02 Mar 2018


Article type: Paper
DOI: 10.1039/C8DT00250A
Citation: Dalton Trans., 2018,47, 4377-4392
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    Thermodynamic description of Tc(IV) solubility and carbonate complexation in alkaline NaHCO3–Na2CO3–NaCl systems

    A. Baumann, E. Yalçıntaş, X. Gaona, R. Polly, K. Dardenne, T. Prüßmann, J. Rothe, M. Altmaier and H. Geckeis, Dalton Trans., 2018, 47, 4377
    DOI: 10.1039/C8DT00250A

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