Professors Zhiwei Wang and Li Wang from our institute, in collaboration with Professor Menachem Elimelech of Rice University, have published a research article titled “Deciphering co-ion and counterion transport in polyamide desalination membranes reveals ion selectivity mechanisms” in Science Advances. This study focuses on the ion transport behavior in polyamide desalination membranes. By combining experimental analysis and molecular dynamics simulations (MD), the research team demonstrated that electrostatic interactions between salt ions and the membrane suppressed co-ion partitioning while accelerating their diffusion. The study reveals the fundamental mechanism underlying ion selectivity in polyamide membranes. It reveals that the partitioning behavior of co-ions (ions with the same charge as the membrane) at the membrane interface plays a key role in determining overall salt selectivity. This discovery provides a new theoretical framework for the design of highly selective ion separation membranes and supports the advancement of green technologies such as selective resource recovery and lithium extraction.

Reverse osmosis (RO) and nanofiltration (NF) membrane technologies are widely used in water treatment and resource recovery. However, the mechanism of ion-ion selectivity has long been debated. Ion-ion selectivity refers to the membrane’s ability to selectively separate different ions (e.g., Li⁺/Mg²⁺). The traditional solution-diffusion model fails to distinguish the respective contributions of ion partitioning and diffusion, making it difficult to explain the selective transport mechanisms of different ions through membranes. Due to their inherent negative charge, polyamide membranes induce complex coupling effects between co-ion and counterion partitioning at the interface and diffusion within the membrane. Decoupling these mechanisms and elucidating the principles of ion-ion selectivity remains a major challenge in membrane-based resource recovery.

Through a combination of Quartz Crystal Microbalance (QCM) experiments, electrochemical membrane resistance measurements, MD simulations, and density functional theory (DFT) calculations, the research team elucidated the ion selectivity mechanisms in polyamide membranes. QCM results indicated that the negative charge of polyamide membranes, via the Donnan effect, significantly suppresses co-ion partitioning while enhancing counterion partitioning. However, as salt concentration increases, the co-ion partition coefficient also increases due to electrostatic screening effects. Compared to RO membranes, NF membranes have larger pore sizes and higher surface charge densities, resulting in significantly higher co-ion partitioning coefficients.

The study designed an experimental system involving coupled ion-exchange membranes, allowing for the first-time independent measurement of co-ion and counterion diffusion coefficients within the membrane. Results showed that the negatively charged pore walls accelerate co-ion diffusion within the membrane while impeding counterion diffusion. For instance, SO₄²⁻ exhibited a higher diffusion coefficient than F⁻ at low concentrations due to strong electrostatic repulsion, though this trend reversed at higher concentrations.

DFT calculations revealed that the binding energy order between ions and the polyamide membrane interface is Na⁺ > Cl⁻ > F⁻ > SO₄²⁻, consistent with partitioning experiment results. MD simulations showed that the transmembrane energy barrier for SO₄²⁻ (5.2 kcal mol^−¹) is significantly higher than that of Cl⁻ (3.9 kcal mol^−¹), explaining its low permeability. Selective partitioning of co-ions is the key factor determining overall membrane selectivity. For example, SO₄²⁻, due to its high charge and strong hydration energy, has a much lower partition coefficient than Cl⁻, resulting in high sulfate rejection by NF membranes.

MD simulations further confirmed that the binding energy sequence of ions at the polyamide interface is Na⁺ > Cl⁻ > F⁻ > SO₄²⁻, aligning with experimental findings. Simulated diffusion coefficients within the membrane were found to be SO₄²⁻ > F⁻ > Cl⁻ > Na⁺, validating the experimental data. Moreover, the simulations indicated that ion diffusion coefficients within the membrane depend on the magnitude of transmembrane energy coefficients.

Finally, filtration experiments using NF membranes were conducted to evaluate the salt permeability coefficients and selectivity for different salts. Results revealed a strong correlation between membrane salt selectivity and co-ion partitioning selectivity, highlighting that selective co-ion partitioning is the dominant mechanism determining membrane-based salt separation performance.

Ph.D. candidate Yun Guo is the first author of the paper. Professors Zhiwei Wang, Menachem Elimelech, and Li Wang are co-corresponding authors. Other contributors include Dr. Jinlong He from Sichuan University, Junwei Zhang (Ph.D. student at Yale University), and Meng Sheng (M.S. student, at the College of Environmental Science and Engineering, Tongji University). The study was supported by the National Natural Science Foundation of China and the Fundamental Research Funds for the Central Universities.

Article Link: https://www.science.org/doi/10.1126/sciadv.adu8302