Two-dimensional horizontal and vertical array-based laminar membranes: An overview of synthesis, modification, and application in water treatment

• Two-dimensional membranes are reviewed for ion/molecule separation in water treatment. • Horizontal/vertical channeled lamellar membranes are focused according to transport modes. • The preparation, the modification and the application are discussed. • The transport mechanisms include size sieving, electrostatic interactions and chemical bonding effects. • As the prerequisite building blocks, the preparation of nanosheets is reviewed. Two-dimensional membranes composed of nanosheets can significantly reduce mass transport resistance and improve separation or rejection performance of molecules or ions given their regular sub-nano channels and robust chemical adaptability. This review focuses on the recent development in two-dimensional membrane for ion and/or molecule separation in water treatment, delving into the preparation of nanosheets, the recent advances of horizontal and vertical channeled lamellar membranes and their separation mechanisms. For the very thin horizontal channeled lamellar membranes (H-CLMs) that surpass the trade-off limitation, the assembly methods mainly include vacuum-assisted filtration, layer-by-layer assembly, and coating. Furthermore, modification methods during pre- and post-membrane fabrication are summarized in detail. Based on the ions/molecules transport through sub-nano channels, the filtration, the conductive membrane, and the pervaporation applications of H-CLMs are discussed. Specifically, the advanced vertical-channeled laminar membranes (V-CLMs) featuring short and straight transport paths, were comprehensively discussed for the first time in terms of the assembly, the modification, and the application. The different mass transport mechanisms of these two-dimensional membranes, i.e., size sieving, electrostatic interactions, and chemical bonding effects were elaborated given their importance in designing advanced membrane materials and enhancing separation performance. Finally, the challenges and outlooks of two-dimensional membranes were given in new membrane materials, assembly technology of V-CLMs, sub-nano channel tailoring, and long-term performance.