Thermodynamic model of Ni(II) solubility, hydrolysis and complex formation with ISA

The solubility of β-Ni(OH)₂(cr) was investigated at T=(22±2) C in the absence and presence of α-isosaccharinic acid (ISA), the main degradation product of cellulose under alkaline pH conditions. Batch solubility experiments were performed from undersaturation conditions under inert gas (Ar) atmosphere. Solubility experiments in the absence of ISA were conducted in 0.5 and 3.0 M NaCl-NaOH solutions at 7.5 ≤ pH_m ≤ 13 (with pH_m = -log₁₀[H⁺]). XRD analyses of selected solid phases collected after completing the solubility experiments (∼300 days) confirmed that β-Ni(OH)₂(cr) remains as solid phase controlling the solubility of Ni(II) in all investigated conditions. Based on the slope analysis (log₁₀[Ni] vs. pH_m) of the solubility data and solid phase characterization, the equilibrium reactions β-Ni(OH)₂(cr)+2 H⁺ <=> Ni²⁺+2 H₂O(l) and β-Ni(OH)₂(cr) <=> Ni(OH)₂(aq) were identified as controlling the solubility of Ni(II) within the investigated pH_m region. The conditional equilibrium constants determined from the solubility experiments at different ionic strengths were evaluated with the specific ion interaction theory (SIT). In contrast to the current thermodynamic selection in the NEA-TDB, solubility data collected in the present work does not support the formation of the anionic hydrolysis species Ni(OH)₃^- up to pH_m ≤ 13.0. Solubility experiments in the presence of ISA were conducted in 0.5 M NaCl-NaOH-NaISA solutions with 0.01 M ≤ [NaISA] ≤ 0.2 M and 9 ≤ pH_m ≤ 13. XRD analyses confirmed that β-Ni(OH)₂(cr) is also the solid phase controlling the solubility of Ni(II) in the presence of ISA. Solubility data of all investigated systems can be properly explained with chemical and thermodynamic models including the formation of the complexes NiOHISA(aq), Ni(OH)₂ISA^- and Ni(OH)₃ISA^2-. The reported data confirm the low solubility (<10^-7 M) of Ni(II) in hyperalkaline pH conditions representative of cementitious environments (10 ≤ pH ≤ 13), which increases to up to 10^-5 M in the presence of 0.2 M NaISA. These results significantly improve source term estimations for Ni(II) in environments relevant for the disposal of low and intermediate level radioactive waste (L/ILW). The chemical and thermodynamic models derived in this work can be implemented in geochemical models/calculations, and provide further confidence in the safety analysis of repositories for the disposal of L/ILW.