Almost all covalent organic framework (COF) materials conventionally fabricated by solvothermal method at high temperatures and pressures are insoluble and unprocessable powders, which severely hinder their widespread applications. This work develops an effective and facile strategy to construct flexible and free-standing pure COF membranes via the liquid-liquid interface-confined reaction at room temperature and atmospheric pressure. The aperture size and channel chemistry of COF membranes can be rationally designed by bridging various molecular building blocks via strong covalent bonds. Benefiting from the highly-ordered honeycomb lattice, high solvent permeances are successfully obtained and follow the trend of acetonitrile > acetone > methanol > ethanol > isopropanol. Interestingly, the imine-linked COF membrane shows higher nonpolar solvent permeances than b-ketoenamine-linked COF due to their difference in pore polarity. Both kinds of COF membranes exhibit high solvent permeances, precise molecular sieving, excellent shape selectivity, and sufficient flexibility for membrane-based separation science and technology.

A new type of biocompatible draw solute, Na(+)-functionalized carbon quantum dots (Na_CQDs) with ultra-small size and rich ionic species, in forward osmosis (FO) is developed for seawater desalination. The aqueous dispersion of Na_CQDs demonstrates a high osmotic pressure, which allows high FO water flux and negligible reverse solute permeation.

Thermoresponsive magnetic nanoparticles (MNPs) as a class of smart materials that respond to a change in temperature may by used as a draw solute to extract water from brackish or seawater by forward osmosis (FO). A distinct advantage is the efficient regeneration of the draw solute and the recovery of water via heat-facilitated magnetic separation. However, the osmotic pressure attained by this type of draw solution is too low to counteract that of seawater. In this work, we have designed a FO draw solution based on multifunctional Fe3O4 nanoparticles grafted with copolymer poly(sodium styrene-4-sulfonate)-co-poly(N-isopropylacrylamide) (PSSS-PNIPAM). The resulting regenerable draw solution shows high osmotic pressure for seawater desalination. This is enabled by three essential functional components integrated within the nanostructure: (i) a Fe3O4 core that allows magnetic separation of the nanoparticles from the solvent, (ii) a thermoresponsive polymer, PNIPAM, that enables reversible clustering of the particles for further improved magnetic capturing at a temperature above its low critical solution temperature (LCST), and (iii) a polyelectrolyte, PSSS, that provides an osmotic pressure that is well above that of seawater.

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A dendrimer-based forward osmosis (FO) draw solute, poly­(amidoamine) terminated with sodium carboxylate groups (PAMAM-COONa), was investigated for seawater desalination. Compared with existing FO draw solutes, PAMAM-COONa offers unique advantages: (1) Its aqueous solution can generate high osmotic pressure because of the large number of −COONa groups. (2) The low viscosity of PAMAM-COONa solution can reduce internal concentration polarization (ICP), which adversely affects FO water flux. (3) PAMAM-COONa has a relatively large molecular size, favoring reduced reverse solute flux. In our FO tests using 2.5-generation (2.5G) PAMAM-COONa draw solution (33.3 wt %) and seawater (Singapore coast) feed solution, a relatively high water flux of 9 L m–2 h–1 was achieved with commercial HTI FO membrane. In addition, a considerably reduced reverse solute flux of PAMAM-COONa compared to that of NaCl was attained. After FO testing, the diluted PAMAM-COONa solution was reconcentrated to its original osmotic pressure with membrane distillation to produce desalinated water and to regenerate the draw solution. In addition to seawater desalination, the dendrimer-based FO draw solute may find applications in wastewater treatment and protein enrichment.