Synergistic Aqueous Biphasic Systems: A New Paradigm for the “One-Pot” Hydrometallurgical Recovery of Critical Metals

An acidic aqueous biphasic system (AcABS), in which the inorganic salt component of a traditional aqueous biphasic system (ABS) is replaced by the inorganic acid inherently present in typical hydrometallurgical leachate solution, is shown to selectively separate cobalt from nickel, a separation relevant to the recycling of nickel metal hydride (NiMH) batteries. To overcome the limitation of electrodeposition in the presence of high acid concentration, a mixed ABS–AcABS approach is developed in which HCl is partially substituted by addition of a predictable amount of NaCl. This synergistic ABS–AcABS retains the metal extraction efficiency of AcABS while diminishing the acid concentration required to induce phase separation as well as its distribution to the ionic-liquid-rich phase. Selective deposition of cobalt in the presence of coextracted manganese impurities was achieved in AcABS, ABS, and ABS–AcABSs. The morphology and composition of the obtained deposits as well as the Faradaic efficiency of the pro...