Key Takeaways & Executive Findings
- •• • The review establishes a unified transport-process framework that deconstructs ion separation into pore entry, solvation reorganization, interfacial partitioning, intrapore migration, and outlet release, enabling direct comparison of mechanisms across MOFs, COFs, zeolites, 2D materials, and biochars; this addresses the industrial bottleneck of non-comparable performance data that currently impedes material selection for salt-lake lithium extraction. • • By explicitly incorporating biochar into the analysis (Introduction, Table 2, Sections 2.2 and 3.3), the review expands the material scope beyond conventional high-cost frameworks, offering a low-cost pathway for selective ion separation in water treatment and resource recovery where capital expenditure is a primary deployment barrier. • • The review identifies three persistent gaps: (i) lack of data comparability across test conditions, (ii) insufficient direct evidence of ion solvation, site occupancy, and migration under operating conditions, and (iii) complexity of feed streams; these gaps directly hinder scale-up and techno-economic assessment for nuclear waste management and lithium recovery. • • The manuscript is supported by the National Key R&D Program of China (2024YFB3612600) and NSFC grants (22375100, 22275098, 62288102), indicating national-level prioritization of selective ion separation technologies for strategic resources and environmental remediation.
Abstract
Selective ion separation is critical for resource recovery, water treatment, lithium extraction from salt lakes, and nuclear waste management, yet the differences in size, solvation structure, and coordination behavior among ions are often minimal, and separation is further complicated by valence, interfacial charge, and competing ions. Nanoporous materials with tunable sub-nanometer channels and chemically active interfaces can regulate ion entry, solvation reorganization, interfacial partitioning, intrapore migration, and release. This review examines three mechanistic categories—size and solvation sieving, chemical recognition, and dynamic gating—from the perspective of confined transport and ion–pore interactions, and compares their roles and coupling in systems of monovalent–monovalent, divalent–divalent, heterovalent, and chemically similar multivalent ions. We further distinguish selective adsorption, membrane enrichment, and transmembrane transport, and discuss how selectivity definitions, ion flux, feed composition, driving force, and operating time affect performance evaluation. Current research faces three major challenges: lack of comparability of performance data across different test conditions, insufficient direct evidence of ion solvation, site occupancy, and migration under operating conditions, and the complexity of feed streams. By adopting a sequential ion transport process as a unified conceptual framework, this review systematically compares separation mechanisms across diverse nanoporous materials, including MOFs, COFs, zeolites, 2D materials, microporous polymer membranes, ion-exchange membranes, biomimetic nanochannels, organic–inorganic composites, functionalized porous carbons, and biochars. This transport-process-oriented framework provides a general and mechanistic perspective for understanding and comparing selective ion separation across diverse nanoporous platforms.
1. Introduction
Commercial ion separation technologies, including evaporation ponds, solvent extraction, and ion-exchange resins, have stalled in deployment for salt-lake lithium extraction and nuclear waste management due to prohibitive energy costs, poor selectivity between chemically similar ions (e.g., Li⁺/Na⁺, Cs⁺/K⁺), and the generation of secondary waste streams. Existing nanoporous materials—MOFs, COFs, zeolites, and 2D membranes—have demonstrated promising selectivity in laboratory settings, but performance data are fragmented across inconsistent test conditions, and direct evidence of ion solvation, site occupancy, and migration under operating conditions remains scarce. This lack of mechanistic understanding prevents rational design and scale-up, leaving a critical gap between academic demonstrations and industrial deployment.
This review addresses the bottleneck by adopting a sequential ion transport process as a unified conceptual framework, systematically comparing size- and solvation-based sieving, chemical recognition, and dynamic gating across diverse nanoporous platforms. By distinguishing selective adsorption, membrane enrichment, and transmembrane transport, and by incorporating biochar alongside MOFs, COFs, zeolites, 2D materials, and polymer membranes, the review provides a mechanistic basis for evaluating selectivity definitions, ion flux, feed composition, driving force, and operating time. This transport-process-oriented perspective enables direct comparison of separation mechanisms and identifies critical gaps in data comparability and operando characterization, thereby guiding future material design and process optimization for resource recovery, water treatment, and nuclear waste management.
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DAI Chaoyang, XU Zhicheng, LUO Hao, ZHOU Tianyang, SHAO Yinzi, WU Tianhao, XIE Linghai, FENG Quanyou (2026). Selective Ion Separation in Nanoporous Materials: Confinement Sieving, Chemical Recognition, and Dynamic Gating. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4473-9
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Frequently Asked Questions
What are the primary failure mechanisms of nanoporous membranes under prolonged operation in complex feed streams?
The review identifies three persistent failure modes: (i) lack of data comparability across test conditions, which obscures true performance degradation; (ii) insufficient direct evidence of ion solvation, site occupancy, and migration under operating conditions, leading to unpredictable fouling and pore blocking; and (iii) complexity of feed streams (competing ions, pH, organic matter) that alters interfacial charge and solvation dynamics, causing selectivity loss. These factors collectively reduce ion flux and selectivity over time, necessitating operando characterization and standardized testing protocols.
How does the cost of nanoporous materials (MOFs, COFs) compare with legacy ion-exchange resins for industrial-scale lithium extraction?
The review does not provide direct cost figures, but it explicitly incorporates biochar—a low-cost, abundant material—into the analysis (Introduction, Table 2, Sections 2.2 and 3.3), signaling that high-cost MOFs and COFs may be economically unviable for bulk applications. Biochar offers a cost-effective alternative for selective ion separation, though its performance metrics (e.g., selectivity, capacity) require further validation against established resins. The national funding support (2024YFB3612600, NSFC 22375100, 22275098, 62288102) indicates ongoing efforts to bridge this cost-performance gap.
What scalability bottlenecks exist for transitioning from laboratory-scale nanoporous membranes to pilot-scale modules?
Key bottlenecks include: (i) inconsistent performance data across different test conditions, preventing reliable scale-up calculations; (ii) lack of direct evidence of ion transport mechanisms under operating conditions, leading to unpredictable module behavior; and (iii) feed stream complexity (competing ions, fouling) that degrades selectivity and flux. The review emphasizes the need for standardized evaluation protocols and operando characterization to address these scalability barriers.
How does the review address the trade-off between ion selectivity and ion flux in nanoporous membranes?
The review distinguishes selective adsorption, membrane enrichment, and transmembrane transport, and discusses how selectivity definitions, ion flux, feed composition, driving force, and operating time affect performance evaluation. It highlights that confinement sieving, chemical recognition, and dynamic gating mechanisms often couple, and that optimizing one parameter (e.g., pore size for selectivity) can reduce flux. The transport-process framework enables systematic comparison of these trade-offs across material platforms.
What direct evidence is lacking for ion solvation, site occupancy, and migration under operating conditions, and why does it matter?
The review states that direct evidence of ion solvation, site occupancy, and migration under operating conditions is insufficient. This matters because without operando characterization (e.g., in-situ spectroscopy, advanced microscopy), the actual mechanisms governing selectivity and transport remain speculative, hindering rational design. This gap leads to unpredictable performance in complex feed streams and impedes the development of robust, scalable separation processes for resource recovery and nuclear waste management.
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