Various kinds of clays occur naturally and are accompanied by particular cations in their interlayer domains. Here we report the reassembled membranes with nanofluidic channel arrays by using the natural clays montmorillonite, mica, and vermiculite, which were imparted with the natural selectivity for realizing precise recognition and directional regulation of the naturally occurring interlayer cations. A typical surface-governed ionic transport behavior was observed in the clay nanofluidic channels. Through asymmetric structural modification, cationic current rectification was realized in montmorillonite channels that performed as a nanofluidic diode. Interestingly, in the mica nanofluidic channel, the K+ that was naturally occurring in the interlayer domain of mica showed a reciprocating motion and resulted in a periodically fluctuating current. Electrodialysis demonstrated that such a fluctuating current reflects a directional selectivity for K+, achieving at least a 6000 times permeation rate difference with Li+ ions. The specific selectivity for Li+/Mg2+ on vermiculite reached up to 856 times with similar cations by the current technique. As-obtained clay membranes possess application prospects in energy conversion, brine resource development, etc. Such a strategy can achieve the designed selectivity through systematic screening of the building blocks, thus imparting them with the inherent characteristics of natural clays, which provides an alternative solution to the present manufacture of selective membranes.
Stacking two-dimensional (2D) nanosheets into lamellar membranes holds great promise in the selective separation of Li + and Mg 2+ from salt-lake brines, but revealing the intrinsic effect of nanosheet properties on the ion transport remains a great challenge. The primary reasons are inevitable emerging defects and changes in surface functional groups during nanosheet preparation. Here, we successfully demonstrated the intrinsic dependence of ion separation on the size and layer charge density of 2D building blocks using defect-free and inherently permanent charged clay nanosheets. The smaller-sized nanosheets readily assembled into lamellar membranes with narrower nanochannel dimension, which facilitated the steric hindrance effect to improve the Li + /Mg 2+ selectivity. Experiments and calculations demonstrated the layer charge density-dependent ion separation as well, for which a novel mechanism of intrinsic selective separation driven from the energy barrier difference of ions transport was proposed. Based on the “internal” regulation of the intrinsic nanosheet properties, MMT membranes realized stable and efficient Li + /Mg 2+ separation under extreme conditions, multi-cycle and long-term experiments, with an optimal S Li/Mg of 38.9, superior to most of the reported state-of-the-art membranes. This work reveals the intrinsic interplay of nanosheet properties tuning the ion transport and separation, which will inspire the design and development of advanced 2D lamellar membranes, particularly for sustainable and environmental energy exploitation.
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