Direct Quantification of Ion Partitioning and Diffusion Resistances in Reverse Osmosis Membranes via Electrochemical Impedance Spectroscopy

Polyamide (PA) reverse osmosis (RO) membranes are crucial for water desalination and purification, where salt ion transport is governed by partitioning and diffusion through the PA film. Despite extensive research, decoupling these two steps and quantifying their relative contributions remain challenging due to the lack of reliable characterization methods. Here, we develop a rapid, reproducible electrochemical impedance spectroscopy (EIS) protocol incorporating advanced electrical equivalent circuits to directly quantify partitioning and diffusion resistance. Its validity is verified through membrane filtration experiments and activation energy analysis. Our findings reveal that diffusion dominates ion transport resistance, with values 4.5 to 6.0 times higher than partitioning resistance across diverse monovalent cations. However, we discovered a critical concentration-dependent behavior where partitioning resistance becomes increasingly significant at lower electrolyte concentrations, eventually equaling diffusion resistance near 0.1 mM. We also uncovered that the anomalously low rejection of NH₄⁺ of RO membranes stemmed from significantly reduced diffusion resistance, likely due to moderate hydrogen-bonding interactions with membrane pores or its tetrahedral geometry. This quantitative insight into transport resistance mechanisms establishes new design principles for next-generation RO membranes, enabling tailored strategies for applications ranging from high-salinity desalination to the removal of low-concentration micropollutants.