Activating inert cobalt sites via interfacial nitrogen vacancies in Co3O4@g-C3N4 heterojunctions for selective boron recovery from hypersaline brines.

Selective boron capture from hypersaline brines remains challenging because existing adsorbents generally suffer from three unresolved limitations: low metal-site activity toward neutral boric acid, poor anti-interference performance in highly saline matrices, and an unclear molecular-level activation mechanism. Here, we report an interfacial N-vacancy-engineered Co3O4@g-C3N4 heterojunction (CoCN) for selective boron recovery, constructed by coupling ZIF-67-derived porous Co3O4 with nitrogen-vacancy-rich ultrathin g-C3N4 nanosheets. Batch adsorption experiments, real-brine tests, and density functional theory (DFT) calculations collectively show that interfacial N vacancies induce local charge redistribution and render adjacent Co sites more electron-deficient, thereby enhancing their Lewis acidity toward boric acid. This defect-regulated interfacial activation changes boric acid adsorption from thermodynamically unfavorable on the pristine interface (ΔG = +1.41 eV) to favorable on the defective interface (ΔG = -4.28 eV), promoting Co-O-B bond formation. As a result, the optimized CoCN-5 delivers a boron adsorption capacity of 128.5 mg g-1 at 25 °C within 120 min, markedly higher than that of pristine Co3O4, and achieves boron uptakes of 81 and 79 mg g-1 in real LaGuoCuo and ChaErHan salt-lake brines, respectively, while maintaining high selectivity against major coexisting ions.