Selective capture of palladium by protonation-armed pyridine nitrogen in extreme water environments

Due to the hostile environment in which it is recovered, selective separation of palladium from the leachate of spent catalysts containing palladium is of considerable utility but faces substantial obstacles. This work designed and synthesized a bipyridine-based porous organic polymer (POP-BPy) via free radical polymerization. Meanwhile, its adsorption characteristics and the mechanism of adsorption in extreme environments were examined. In hostile conditions, the resultant POP-BPy display exceptional mechanical stability and environmental resistance. Impressively, POP-BPy has an excellent adsorption capacity of Pd(II) (248 mg/g) in harsh environments (aqua regia: AR; half-diluted aqua regia: AR"1 + 1"), and also had superior selectivity and rapid kinetics. The selectivity of POP-BPy for Pd(II) was verified in combination with relevant experiments (βPd(II)/Pt(II) = 36.19, βPd(II)/Pt(II) = 23.90). Furthermore, combining density functional theory (DFT) and molecular dynamics (MD) simulations, the mechanism of selectivity of POP-BPy for Pd(II) was revealed from static adsorption sites to dynamic adsorption processes in a holistic manner, which attributed to the strong attraction of protonated pyridine nitrogen to PdCl42- and site suitability. Furthermore, the application of POP-BPy to spent catalyst recovery resulted in up to 100% removal of Pd(II) from the actual leaching solution, and fixed bed column experiments can effectively treat 2688 bed volumes (L) of feed streams (10 mg/L) before reaching the breakthrough point (<1 mg/L). Overall, this work has opened up new avenues for the selective recycling of valuable metals in harsh environments.