Ion Valence Difference Governs Membrane Charge Preference in Nanofiltration-Based Radionuclide Separation

Selective separation of radionuclides with varying environmental hazards is critical for mitigating the disposal burden of high-level radioactive wastewater. Although positively charged (PC) membranes are generally considered effective for cation pair separation, the applicability of negatively charged (NC) membranes for separating cations with large valence differences remains unresolved. In this study, Ag+/Zr4+ and Ag+/Sr2+ were used as representative radionuclides with large and small valence differences, respectively, to evaluate the separation performance and underlying mechanisms of PC and NC membranes. Notably, NC membranes maintained near-complete rejection of Zr4+ while simultaneously exhibiting higher Ag+ permeability, thereby achieving superior Ag+/Zr4+ selectivity relative to PC membranes. DFT calculations and MD simulations revealed that strong ion–membrane interactions facilitated preferential partitioning of Zr4+ at the NC membrane interface, whereas differences in hydration energy governed the competitive transport of Ag+ and Zr4+. Furthermore, Zr4+ accumulated at the membrane surface induced incomplete dehydration of Ag+, reducing interfacial transport barriers and promoting Ag+ permeation. Comparative analysis of Ag+/Zr4+ and Ag+/Sr2+ systems further demonstrated that ion valence difference governs the preference of membrane charges: NC membranes are preferred for M+/M4+ separations, whereas PC membranes are preferred for M+/M2+ separations, which was further validated by Li+/Mg2+ and Li+/Ti4+ separations. These findings clarify the mechanistic distinction between co-ion and counterion competition, providing guidance for the selection of membrane charge types for energy-efficient radionuclide separation and selective resource recovery beyond the nuclear industry.