Separation of Ni2+ and Co2+ Using Sorbitol–Stearic Acid-Derived Bio-Based Solvent: A Sustainable Solvent Extraction Approach

The separation of Ni²⁺ and Co²⁺ is challenging due to their similar physicochemical properties. In industrial processes, solvent extraction is widely used for their refinement; however, the excessive use of petroleum-derived solvents poses significant environmental concerns. To address this, a greener approach was explored in this study by utilizing a bio-based solvent synthesized from sorbitol and stearic acid. The resulting solvent, SSA2, was prepared via Fischer esterification and employed as the organic phase in solvent extraction experiments. After 5 hours of reaction, the acid value (AV) of SSA2 decreased roughly 87%, indicated ester bond formation and the reduction of carboxylic acid groups, confirming successful synthesis. FTIR analysis confirmed the presence of ester bonds in SSA2, which are absent in the starting materials, sorbitol and stearic acid. The separation performance of SSA2 for Ni²⁺ and Co²⁺ was assessed by measuring the distribution coefficient (KD) and separation factor (SF) values as functions of contact time and aqueous phase pH. SSA2 demonstrated higher selectivity towards Co²⁺, achieving a maximum SFCo2+/Ni2+ of 6.20 at pH 5. The highest KD values recorded were 75.6 and 12.2 for Co²⁺ and Ni²⁺ respectively.