Synergistic role of Cyanex 272 saponification and acetate buffering in selective Co(II)/Ni(II) separation via Green Emulsion Liquid Membrane (GELM)

Abstract

This study explores the selective extraction of cobalt and nickel ions from acidic solutions using a Green Emulsion Liquid Membrane (GELM) system. Corn oil was selected as a green diluent based on shake-out tests. The membrane phase was formulated by dissolving Cyanex 272 (as extractant) and a binary surfactant system (Span 80 and Tween 80) in corn oil, then emulsified with sulfuric acid (H₂SO₄) as the internal stripping agent. Partial saponification of Cyanex 272 enhanced extraction efficiency, while sodium acetate served as a buffering agent to improve selectivity. Equilibrium studies validated the extraction mechanism. An optimized surfactant blend of 4% v/v (80% Span 80, 20% Tween 80) provided superior emulsion stability. Key operational parameters were optimized, including extractant concentration (25% v/v, 30% saponified), sodium acetate concentration (1.5 M), feed pH (5), treatment ratio (5:1), stirring speed (200 rpm), time (20 min), phase ratio (2:3), and stripping agent (1 M H₂SO₄). Under these conditions, cobalt extraction reached 95.0%, with minimal nickel co-extraction (3.8%), yielding a high separation factor. The membrane phase was successfully recycled twice with minimal loss in performance, demonstrating the feasibility of this sustainable approach for selective metal separation.