Dual Synergistic/Antagonistic Mechanism of Protonated-Amino Bidentate Ligand for Selective Separation of Nickel and Cobalt under High Acidity

The efficient separation of Ni and Co is crucial for their short-process recovery but remains challenging due to the complex leachate compositions. Herein, a bidentate pyridine-pyrazole ligand featuring a protonated-amino group (pa-BPP) was synthesized for the selective extraction of Ni and Co from acidic polymetallic solutions. Combined with dinonyl naphthalenesulfonic acid (DNNS), pa-BPP exhibits a unique synergistic effect on Ni and Co extraction under high acidity, while demonstrates antagonism toward impurities including Mn, Mg, Al, Ca, Zn, and Fe. Remarkably, this system achieves near-quantitative Ni extraction (>99%) and high Co extraction (>80%) at pH < 1.5, while maintaining minimal impurities coextraction except for Zn. The coextracted Zn is readily scrubbed with dilute H 2 SO 4, while loaded Co and Ni are selectively stripped with concentrated H 2 SO 4 and HCl, respectively. Structural analyses indicate that pa-BPP displaces water molecules in the inner coordination spheres of hydrated Ni 2+ and Co 2+ ions, enabling their selective separation over impurities. Intermolecular interaction analyses disclose that hydrogen bonding between DNNS and pa-BPP delivers the antagonistic effect on suppressing impurity coextraction. This dual synergistic/antagonistic mechanism facilitates the direct separation of Ni and Co under high acidity, demonstrating significant potential for short-process hydrometallurgy of strategic resources.