The hydrophobic deep eutectic solvent in selective co-extraction of nickel and cobalt from deep-sea polymetallic nodule leaching Solution: Efficiency and Mechanistic insights

This study introduces a sustainable and environmentally friendly process for selective co-extraction of nickel and cobalt from deep-sea polymetallic nodule (DPN) leachate, establishing the foundation for green DPN utilization. A hydrophobic deep eutectic solvent (HDES) comprising decanoic acid (DA), alpha-hydroxyoxime (M 63), and 4-Pyridinecarboxylate (4PC) was synthesized for Ni and Co extraction, with systematic optimization of the unit operation and investigation of extraction mechanisms. Experimental results showed that under optimal conditions, after two-stage multistage countercurrent extraction and three-stage multistage countercurrent stripping, total recovery rates of Ni and Co reached 93.12 % and 98.58 %, respectively. The strip solution contains 16.73 g/L Ni, 3.99 g/L Co, and 3.37 g/L Mn (Ca < 0.02 g/L; Mg < 0.002 g/L), making it suitable for direct use in preparing ternary battery cathode precursors. Machine learning via the MARS model enabled on-demand customization and adjustment of HDES component ratios for diverse complex solution systems. Mechanistic studies combining FT-IR spectroscopy, UV-Vis spectroscopy, H-1 NMR, slope analysis, and quantum chemical calculations revealed the extraction mechanism. Ni and Co form hexacoordinate complexes Ni(DA)(2)(M 63)(2) and Co(DA)(2)(M 63)(2) in the organic phase. This work provides insights for developing eco-friendly HDES systems to improve selective co-extraction of Ni and Co from complex solutions while reducing environmental pollution.