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 H2SO4, while loaded Co and Ni are selectively stripped with concentrated H2SO4 and HCl, respectively. Structural analyses indicate that pa-BPP displaces water molecules in the inner coordination spheres of hydrated Ni2+ and Co2+ 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.