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The green recycling of resources is an essential pathway toward achieving low-carbon water pollution control. Nanochannel-mediated confined separation processes serve as fundamental units for the resource recovery of various substances (including pollutants) in water. Correspondingly, elucidating the ion transport mechanisms based on nanoconfinement effects holds significant importance for the efficient and sustainable separation and resource utilization of metal ions in water. In recent years, Professor Pan Bingcai's research team has conducted extensive investigations into the fundamental principles and technological innovations of nanoconfinement effects in water treatment (PNAS 2019, 116, 6659; Environ. Sci. Technol. 2020, 54, 8509; Adv. Funct. Mater. 2020, 30, 1909014; Environ. Sci. Technol. 2021, 55, 665; Environ. Sci. Technol. 2024, 58, 826; Nat. Commun. 2024, 15, 917; Nat. Commun. 2024, 15, 2808). They have made significant advancements in the confined growth, confined adsorption, and confined catalysis of nanomaterials for water treatment. Recently, focusing on confined separation, the team developed a series of sub-nanometer channel membranes and elucidated how nanoconfined channel structures (Angew. Chem. Int. Ed. 2022, 61, e202115443) and channel charges (Nat. Commun. 2023, 14, 4907; Environ. Sci. Technol. 2024, 58, 6835) regulate ion transport and separation.
Currently, research on ion transport mechanisms mediated by nanoconfinement effects primarily focuses on how ion-channel interactions influence transport efficiency. However, real-world water treatment and resource recovery scenarios often involve multi-ion coexistence systems, and the impact of ion-ion interactions on the confined transport of target ions remains poorly understood. To address this gap, Dr. Xu Rongming and colleagues from the team discovered that polyamide-functionalized sub-nanometer channels can amplify interionic interactions, thereby significantly affecting ion transport rates and separation efficiencies. Within sub-nanometer confined channels, interactions between partially dehydrated ions are stronger than those in open systems. Cations transport in the form of "ion-pairs," with cations of lower hydration energy more readily pairing with anions, enabling rapid transport through the channels. In contrast, cations with higher hydration energy tend to bind more strongly with free water molecules within the channels, becoming restricted near cation-binding sites and exhibiting slower transport. Adjacent cation-anion binding sites within the confined channels promote the formation of "ion-pairs" and amplify the "ion-pairing" effect, thereby significantly enhancing the ion separation performance of sub-nanometer channels in real multi-ion scenarios. This research, titled "Regulate Ion Transport in Sub-nanochannel Membrane by Ion-pairing," was published in the prestigious academic journal Journal of the American Chemical Society (article link: //pubs.acs.org/doi/10.1021/jacs.5c02722). The team’s postdoctoral researcher, Xu Rongming, is the first author of the paper, with Professor Zhang Weiming, Professor Zhang Xiwang (University of Queensland), and Professor Pan Bingcai serving as corresponding authors. Co-authors include PhD student Yu Hang, Master's student Ren Jiachun, postdoctoral researcher Kang Yuan from Monash University, postdoctoral researcher Wang Zhuyuan from the University of Queensland, Professor Liu Zhe from the University of Melbourne, PhD student Feng Fan, Professor Peng Luming from the School of Chemistry and Chemical Engineering at Nanjing University, and PhD student Xia Xiaoli. The study was supported by the National Natural Science Foundation of China (U22A20403, 21925602), the ARC Future Fellowships (FT210100593) from the Australian Research Council, the Shanghai Tongji Gaotingyao Outstanding Young Doctoral Talent Scholarship, and the Ogano (Water Quality and Water Environment) Scholarship from the Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences.
