Thermodynamic description of the Ni(II)-citrate system in alkaline, dilute to concentrated NaCl solutions. Formation of quaternary complexes with Ca

The impact of citrate (cit) on the solubility of Ni(II) was comprehensively investigated in alkaline 0.1–3.0 M NaCl–NaOH–Na 3 cit solutions in the absence and presence of 0.02 mol dm −3 CaCl 2 . Experiments were conducted at a temperature of (22 ± 2)°C under inert gas (Ar) atmosphere. Thermodynamic equilibrium was approached from undersaturation conditions using β-Ni(OH) 2 (cr). The characterization by X-ray diffraction of selected solid phases after completing the solubility experiments (∼360 days, depending upon experimental series) confirmed that β–Ni(OH) 2 (cr) controls the solubility of Ni(II) in all investigated systems, both in the absence and presence of Ca. In Ca-free systems, the observed increase in the solubility of Ni(II) is attributed to the formation of binary and ternary Ni(II)–OH–cit aqueous complexes. The use of thermodynamic data currently selected within the Thermochemical Database project of the Nuclear Energy Agency (NEA-TDB) clearly underestimates the solubility of Ni(II) in the hyperalkaline pH m regime of relevance in cementitious systems. Taking as anchoring point the species currently selected in the NEA-TDB (Ni(cit) – and Ni(cit) 2 4− ) and based on slope analysis of solubility data (log 10 [Ni] vs. pH m and log 10 [Ni] vs. log 10 [cit]), the predominance of the complexes Ni(H –1 cit) 2– , Ni(OH)(H –1 cit) 3– , Ni(cit)(H –1 cit) 5– and Ni(H –1 cit) 2 6− at pH m ≥ 10 is proposed, with H –1 cit corresponding to a citrate ligand with deprotonated alcohol group. The increase in solubility observed in the presence of Ca can only be explained claiming the formation of a quaternary complex, i.e. , Ca[Ni(OH)(H –1 cit)] – . Based on this chemical model and the fit of solubility data, thermodynamic and SIT activity models were derived for the system Na + –Ca 2+ –Ni 2+ –Cl – –OH – –cit 3– –H 2 O(l). These models can be implemented in thermodynamic databases and geochemical calculations, allowing for the first time an accurate description of Ni(II) solubility and aqueous speciation in citrate-containing cementitious systems. • Comprehensive solubility study on the Ca–Ni(II)-citrate system in alkaline pH. • Stoichiometry of ternary and quaternary complexes Ca–Ni(II)–OH-citrate identified. • Chemical, thermodynamic and SIT activity models derived for the investigated system. • Accurate quantitative description of Ni retention in cement systems with citrate.