Key Takeaways & Executive Findings
- •• • Mg2+ permeation rate drops from 3.16 × 10−3 to 3.2 × 10−5 mol m−2 h−1 (two orders of magnitude) upon adding 0.2 M KCl, demonstrating reversible ion-pairing-based gating with industrial relevance for selective ion recovery. • • The MLM–γ-PGA membrane maintains structural integrity after prolonged salt exposure, indicating robust stability for continuous operation in harsh feed streams. • • The membrane achieves K+/Mg2+ selectivity rivalling leading ion-separation membranes, as benchmarked in Fig. 1k, offering a competitive alternative for lithium/magnesium separation in brine processing. • • Dynamic manipulation of ion-ion interactions via external ionic stimuli enables reversible modulation of Mg2+ permeation over multiple cycles, providing a tunable separation mechanism without membrane modification.
Abstract
Selective ion transport in spatial confinement is central to environmental and energy sustainability, including desalination, energy conversion, and resource recovery. While biomimetic nanochannel membranes often rely on size exclusion or fixed ion-channel interactions, real-world separation systems involve multicomponent ionic mixtures where ion-ion interactions, such as dynamic pairing and competition, play a decisive yet underexplored role. Pan and coworkers address this gap by developing a functionalized membrane enabling dynamic manipulation of ion-ion interactions triggered by external ionic stimuli, achieving precise and reversible control over target ion transport. The membrane is constructed from aligned MXene nanosheets functionalized with γ-PGA through covalent and hydrogen bonding, establishing well-defined spatial confinements and adjacent amino and carboxyl groups that serve as anion-cation binding sites. This design enhances ion-pair formation, yielding ion selectivity and stimulus-responsive performance rivalling biological channels. Notably, when K+ and Mg2+ co-permeate, anions preferentially associate with K+ within the nanochannel, disrupting Mg2+ transport despite Mg2+'s higher affinity for channel walls. The MLM–γ-PGA membrane exhibits uniform channel architecture and remarkable aqueous stability. Ion transport characterization using a U-shaped diffusion cell with chloride salt solutions shows that with 0.2 M MgCl2 feed, Mg2+ permeation rate is 3.16 × 10−3 mol m−2 h−1. Introducing 0.2 M KCl suppresses Mg2+ permeation by two orders of magnitude to 3.2 × 10−5 mol m−2 h−1, with full reversibility over multiple cycles. This work highlights the potential of exploiting ion-ion interactions in nanoconfinement for precision separation.
1. Introduction
Industrial separation of ionic species, particularly the selective extraction of monovalent ions from divalent ones, remains a critical bottleneck in brine processing, lithium recovery, and water treatment. Conventional membranes rely on size exclusion or fixed charge interactions, which often suffer from trade-offs between selectivity and permeability, and are inadequate for multicomponent feeds where ion-ion interactions dominate. The inability to dynamically control ion transport in real time limits process flexibility and efficiency.
This work introduces a paradigm shift by exploiting ion association in nanoconfinement. The MLM–γ-PGA membrane, composed of aligned MXene nanosheets functionalized with γ-PGA, provides a platform where ion-pairing can be manipulated by external ionic stimuli. Specifically, the presence of K+ induces preferential anion association, effectively blocking Mg2+ permeation. This mechanism offers a reversible and precise control over ion transport, addressing the bottleneck of static selectivity and enabling adaptive separation processes.
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Gang Lu, Yan Zhao, Bart Van der Bruggen (2026). Ion Association in Nanoconfinement for Precision Separation. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3868-y
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Frequently Asked Questions
What is the mechanism behind the selective suppression of Mg2+ permeation in the presence of K+?
In the MLM–γ-PGA nanochannel, γ-PGA provides amino and carboxyl groups that act as binding sites. When K+ is introduced, anions (Cl−) preferentially associate with K+ rather than Mg2+, as evidenced by radial distribution functions. This selective ion pairing disrupts the transport of Mg2+ by reducing its effective mobility and interaction with the channel, leading to a two-order-of-magnitude decrease in permeation rate.
How reversible is the ion transport modulation upon cyclic introduction and removal of K+?
The modulation is fully reversible over multiple cycles, as shown in Fig. 1e. The Mg2+ permeation rate returns to its original value when K+ is removed, indicating that the ion-pairing mechanism is dynamic and does not cause permanent changes to the membrane structure.
What is the structural stability of the MLM–γ-PGA membrane under prolonged salt exposure?
The membrane maintains its structural integrity after prolonged salt exposure, as confirmed by cross-sectional STEM and HR-STEM images (Fig. 1b, c). This stability is crucial for practical applications where membranes are subjected to continuous operation in saline environments.
How does the K+/Mg2+ selectivity of MLM–γ-PGA compare to other leading ion-separation membranes?
As shown in Fig. 1k, the K+/Mg2+ selectivity of MLM–γ-PGA is comparable to or exceeds that of other leading membranes reported in the literature, making it a competitive candidate for applications such as lithium extraction from brines where Mg2+ is a major impurity.
What are the potential scalability challenges for the MLM–γ-PGA membrane?
While the membrane shows excellent performance in lab-scale diffusion cells, scalability would require uniform functionalization of large-area MXene membranes and maintaining the subnanochannel architecture. The use of γ-PGA, a biodegradable polymer, may also raise cost and stability considerations for industrial-scale production.
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