Solubility, complexation and thermodynamics of the Tc(IV)–isosaccharinic acid system: Trends in the M(IV) series

Isosaccharinic acid (HISA, or ISA in its deprotonated form) is the main degradation product of cellulose under alkaline conditions. It can form strong complexes with radionuclides and other toxic metal ions, eventually enhancing their mobility in the context of nuclear waste repositories and other environmental systems. ⁹⁹Tc is a redox-sensitive, long-lived fission product produced in high yield in nuclear reactors. The solubility of ⁹⁹Tc(IV) was investigated in 0.5 M NaCl‒NaISA‒NaOH solutions with 6 ≤ pH_m ≤ 12.5 and 10^-6 M ≤ [ISA] ≤ 0.2 M. Complete chemical and thermodynamic models were derived on the basis of solubility data, (pe + pH_m) measurements, redox speciation, and solid phase characterization. These models include the previously unreported aqueous complexes TcO(OH)(ISA)₂^‒ and TcO(OH)₂(ISA)₂^2-. In spite of the small size and high polarizability of the Tc⁴⁺ metal ion, the Tc(IV)-ISA complexes described in this work are significantly weaker than other ISA complexes formed with larger M⁴⁺ metal ions, i.e., Zr, Pu and U. This unexpected behavior can be possibly explained by the strong hydrolysis of Tc(IV) and corresponding stabilization of the TcO²⁺ moiety, which does not occur for other M(IV) systems. Thermodynamic data derived in this work can be implemented in geochemical calculations of relevance in the context of nuclear waste disposal and other environmental applications.