Directional modification and design of mineral-based two-dimensional layered kaolinite/montmorillonite membranes with high water flux and ions rejection rate

The development of energy-efficient desalination membranes is of great significance. Achieving high water flux while maintaining considerable ion rejection remains challenging. In this work, two-dimensional (2D) kaolinite/ montmorillonite membranes have been prepared and the modification routes have been investigated with four typical modified molecules: polyacrylic acid (PAA, molecular chains, enriched with -COOH functional groups), polyvinyl alcohol (PVA, molecular chains, enriched with -OH functional groups), gallic acid (GA, enriched with phenolic -OH and -COOH mixed functional groups) and tannic acid (TA, enriched with phenolic -OH functional groups) to explore the functional regulation of 2D membranes in terms of mass ratio (mineral to modified molecule), active sites, nanosheet size, and shape, etc. The results demonstrated that when the mass ratio of kaolinite to GA was 1:1, the dead-end filtration water flux of the membranes could reach up to 7000 L m- 2 center dot h- 1 center dot bar- 1, which was 4.5 times higher than graphene oxide membranes (1529 L m- 2 center dot h- 1 center dot bar- 1) and 17 times higher than COF membranes that have been reported under comparable ion rejection rates so far. Meanwhile, at the feed concentration of 3.5 wt% Na+, Ca2+, Mg2+ solutions and temperature of 25 degrees C, either the kaolinite or montmorillonite membranes can maintain the ions rejection rates of more than 95 %. Surprisingly, the membranes modified by -OH and -COOH have higher ions rejection rate and water flux, respectively. When -OH and -COOH are co-modified, the membranes can have high water flux and ions rejection rate at the same time. This study provides a constructive strategy for the design of mineral-based membranes from low-cost resources with high water flux and rejection performance, which obviously broadens the fundamental theory for the development and application of 2D membranes desalination.