Effect of pore size distribution on the desalination performance of the selective layer of nanoporous atomically-thin membranes

Abstract

Over the last decade, molecular simulations and experiments have shown that nanoporous atomically-thin membranes (NATMs) have the potential for high-permeance, high-selectivity separations, including seawater desalination. Realistic NATMs contain polydisperse pore sizes that could impede their performance, as large pores lead to salt leakage. This paper computationally examines the effect of pore size distributions (PSDs) on the desalination performance of the selective layer of NATMs by reverse osmosis (RO), considering size exclusion as the dominant selection mechanism. Analogous to thin-film composite RO membranes, the finite width of PSDs leads to a trade-off between water permeability and water/salt selectivity in NATMs. Tight PSDs with average pore sizes slightly below the size of the salt are needed to ensure high selectivity comparable to TFC membranes with high water permeance. Sealing of large, salt-permeable pores (e.g. by interfacial polymerization) limits salt leakage but substantially reduces water permeance. Introducing energy barriers that impede salt permeation by tuning nanopore structure and chemistry can make NATMs more robust to wider PSDs. In summary, some combination of control of PSDs, leakage mitigation, and pore functionalization is essential for NATMs to surpass the permeability/selectivity trade-off of polymeric RO membranes and achieve high water permeance with good salt rejection.