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Radiochimica Acta

International Journal for chemical aspects of nuclear science and technology

Editor-in-Chief: Qaim, Syed M.


IMPACT FACTOR 2018: 1.339

CiteScore 2018: 1.20

SCImago Journal Rank (SJR) 2018: 0.333
Source Normalized Impact per Paper (SNIP) 2018: 0.720

Online
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2193-3405
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Volume 103, Issue 1

Issues

Redox chemistry of Tc(VII)/Tc(IV) in dilute to concentrated NaCl and MgCl2 solutions

Ezgi Yalçıntaş / Xavier Gaona / Andreas C. Scheinost
  • Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf, Germany
  • Rossendorf Beamline (ROBL) at ESRF, Grenoble, France
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Taishi Kobayashi / Marcus Altmaier / Horst Geckeis
Published Online: 2014-11-12 | DOI: https://doi.org/10.1515/ract-2014-2272

Abstract

The redox behaviour of Tc(VII)/Tc(IV) was investigated within the pHc range 2–14.6 in (0.5 M and 5.0 M) NaCl and (0.25 M, 2.0 M and 4.5 M) MgCl2 solutions in the presence of different reducing agents (Na2S2O4, Sn(II), Fe(II)/Fe(III), Fe powder) and macroscopic amounts of Fe minerals (magnetite, mackinawite, siderite: S/L = 20–30 g L1). In the first group of samples, the decrease of the initial Tc concentration (1 · 105 M, as Tc(VII)) indicated the reduction to Tc(IV) according to the chemical reaction TcO4 + 4H++ 3e ↔ TcO2 · 1.6H2O(s) + 0.4H2O. Redox speciation of Tc in the aqueous phase was further confirmed by solvent extraction. A good agreement is obtained between the experimentally determined Tc redox distribution and thermodynamic calculations based on NEA–TDB (Nuclear Energy Agency, Thermochemical Database) and ionic strength corrections by SIT or Pitzer approaches. These observations indicate that experimental pHc and Eh values in buffered systems can be considered as reliable parameters to predict the redox behaviour of Tc in dilute to highly concentrated NaCl and MgCl2 solutions. Eh of the system and aqueous concentration of Tc(IV) in equilibrium with TcO2 · 1.6H2O(s) are strongly affected by elevated ionic strength, especially in the case of 4.5 M MgCl2 solutions. In such concentrated brines and under alkaline conditions (pHc = pHmax ∼ 9), kinetics play a relevant role and thermodynamic equilibrium for the system Tc(IV)(aq) ↔ Tc(IV)(s) was not attained from oversaturation conditions within the timeframe of this study (395 days). Tc(VII) is reduced to Tc(IV) by magnetite, mackinawite and siderite suspensions at pHc = 8 – 9 in concentrated NaCl and MgCl2 solutions. Sorption is very high in all cases (Rd ≥ 103 L kg1), although Rd values are significantly lower in 4.5 M MgCl2 solutions. XANES (X-ray absorption near edge spectroscopy) evaluation of these samples confirms that Tc(VII) is reduced to Tc(IV) by Fe(II) minerals also in concentrated NaCl and MgCl2 brines.

Keywords: Technetium; redox reactions; salt brines; thermodynamics; Fe(II) minerals; XANES

About the article

Accepted: 2014-08-11

Received: 2014-04-01

Published Online: 2014-11-12

Published in Print: 2015-01-28


Citation Information: Radiochimica Acta, Volume 103, Issue 1, Pages 57–72, ISSN (Online) 2193-3405, ISSN (Print) 0033-8230, DOI: https://doi.org/10.1515/ract-2014-2272.

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