Thermodynamic description of Tc(iv) solubility and carbonate complexation in alkaline NaHCO3 -Na2 CO3 -NaCl systems

Dalton Transactions : an International Journal of Inorganic Chemistry
A BaumannHorst Geckeis

Abstract

The solubility of99Tc(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 vali...Continue Reading

References

Jan 27, 2004·Environmental Science & Technology·Wayne W LukensKenneth R Ashley
Jun 22, 2005·Journal of Synchrotron Radiation·B Ravel, M Newville
Jul 13, 2006·The Journal of Physical Chemistry. a·Björn O RoosPer-Olof Widmark
Dec 17, 2009·Environmental Science & Technology·Isabelle AlliotMassoud Fattahi
Nov 13, 2015·Journal of Computational Chemistry·Francesco AquilanteRoland Lindh
May 7, 2016·Dalton Transactions : an International Journal of Inorganic Chemistry·Ezgi YalçıntaşHorst Geckeis
Jul 12, 2017·Environmental Science & Technology·Sarah A SaslowAlbert A Kruger

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