Thermodynamic Model for the Solubility of TcO₂· xH₂O(am) in the Aqueous Tc(IV) – Na⁺ – Cl⁻ – H⁺ – OH⁻ – H₂O System

Abstract

Solubility studies of TcO₂· xH₂O(am) have been conducted as a function of H⁺ concentration from 1 × 10^{− 5} to 6 M HCl and as a function of chloride concentration from 1 × 10^{− 3} to 5 M NaCl. These experiments were conducted under carefully controlled reducing conditions such that the preponderance of Tc present in solution is in the reduced oxidation state and was determined to be Tc(IV) by XANES analysis. The aqueous species and solid phases were characterized using a combination of techniques including thermodynamic analyses of solubility data, XRD, and XANES, EXAFS, and UV-vis spectroscopies. Chloride was found to significantly affect Tc(IV) concentrations through (1) the formation of Tc(IV) chloro complexes [i.e., TcCl₄(aq) and TcCl₆ ^{2 −}] and a stable compound [data suggests this compound to be TcCl₄(am)] in highly acidic and relatively concentrated chloride solutions, and (2) its interactions with the positively charged hydrolyzed Tc(IV) species in solutions of relatively low acidity and high chloride concentrations. A thermodynamic model was developed that included hitherto unavailable chemical potentials of the Tc(IV)–chloro species and Pitzer ion-interaction parameters for Tc(IV) hydrolyzed species with bulk electrolyte ions used in this study. The thermodynamic model presented in this paper is consistent with the extensive data reported in this study and with the reliable literature data and is applicable to a wide range of H⁺ and Cl⁻ concentrations and ionic strengths.