Improved Pitzer activity model for Tc(iv) solubility and hydrolysis in the Tc(iv)–Na+–K+–Ca2+–Mg2+–H+–Cl−–OH−–H2O(l) system

Abstract

This study presents improved chemical, thermodynamic and Pitzer activity models for the system Tc(IV)–Na⁺–K⁺–Ca²⁺–Mg²⁺–H⁺–Cl⁻–OH⁻–H₂O(l) in the context of the German Thermodynamic Reference Database (THEREDA, https://www.thereda.de). The work is based on (i) the previous THEREDA release No. 10 for Technetium in 2016, (ii) the data selection in the second update book of the Nuclear Energy Agency-Thermochemical Database (NEA-TDB) project and (iii) available new solubility studies in dilute to concentrated NaCl, KCl, CaCl₂ and MgCl₂ solutions. Updated chemical and thermodynamic models are evaluated to include the hydrolysis species Tc₂O₃²⁺, TcO(OH)₂(aq), TcO(OH)₃⁻, Ca₃[TcO(OH)₅]³⁺ and Mg₃[TcO(OH)₅]³⁺. The model provides a new hydrolysis constant for TcO(OH)₃⁻ and new Pitzer activity coefficient for the interactions between TcO(OH)₃⁻ and Na⁺/K⁺. The updated model properly describes Tc(IV) solubility data in binary NaCl, KCl, CaCl₂ and MgCl₂ salt systems, as well as in selected mixed solutions (ternary, quaternary, quinary). The incorporation of the updated model in thermodynamic databases represents an important tool for source term estimations and speciation calculations of Tc(IV) under a variety of geochemical conditions of relevance in the context of nuclear waste disposal, with focus on high ionic strength conditions.