Highly selective recovery of palladium using innovative double-layer adsorptive membranes

Palladium (Pd) recovery from secondary resources is urgently needed to meet the rising demands for precious metals for sustainable development and environmental protection. To achieve the goal of simple, efficient, and selective recovery of precious metal Pd, we reported an innovative strategy that utilized a double-layer adsorptive membrane based on polydopamine (PDA) modification and polymethacryloxyethyltrimethyl ammonium chloride (PDMC) grafting. The adsorption results suggested that the adsorptive membrane exhibited an excellent adsorption capacity of 61.5 mg g-1 towards Pd(II) at pH 2.0 and the adsorption capacity remained stable after six cycles. Moreover, the adsorption processes of Pd(II) were fitted to pseudo second-order kinetic models and the Langmuir isotherm models. Density functional theory (DFT) simulation, and FTIR, XPS analysis indicated the adsorption mechanisms were ion exchange from the PDMC layer and chelation and ion exchange from the PDA layer. In the presence of other metal ions (Ni(II), Cu(II), Zn(II), Ca(II), Mg(II), K(I) and Na(I)), the separation factor of adsorptive membrane for Pd(II) was in the range of 50-529. The remarkable selectivity was due to the mechanism of the combination of electrostatic attraction and coordination interaction to Pd(II) while the electrostatic repulsion to other metal cations. Furthermore, membrane filtration results suggested that Pd(II) was effectively adsorbed and rejected and the leakage risk could be reduced by dual adsorption from the doublelayer membrane. This study not only offers an innovative strategy to precisely and effectively recover Pd(II) and other platinum group metals, but also provides valuable insights for the future implementation of adsorptive membranes in recycling low-concentration wastewater in an efficient and friendly way.