Lithium purification from aqueous solutions using bioinspired redox-active melanin membranes

The separation of lithium from magnesium ions in salt brines is an important step in producing raw lithium for prospective use in electrochemical storage systems. Liquid-liquid extraction of Mg2+ ions from Li+ ions is challenging because of comparable thermodynamic behavior in aqueous solutions. Removing Mg2+ ions from brines using consumable ion-exchange membranes is also a challenging prospect due to poor chemical selectivity and compromised sustainability. Here, we propose the use of redox-active catechols in the form of melanin pigments for selective removal of Mg2+ ions from aqueous solutions. Synthetic melanin films are oxidatively polymerized on stainless steel meshes from aqueous solutions of dopamine precursors to create electrochemically functionalized polydopamine membranes. The binding selectivity of redox-active catechol-bearing polydopamine melanin for Mg2+ ions in aqueous solutions was measured as a function of electrochemical cycling. The binding kinetics of Mg2+ ions to polydopamine pigments was measured. Mg2+ ions binding selectivity to polydopamine in mixed aqueous solutions containing both Li+ and Mg2+ ions were also measured. Additionally, the equilibrium binding concentrations of Mg2+ ions to polydopamine were achieved very rapidly (<1min). Redox-active polydopamine membranes therefore show promise as functional materials for the separation of Mg2+ and Li+ ions in aqueous solutions. (c) 2016 Society of Chemical Industry