An ultrathin support layer based on carbon nanotubes/polyvinyl alcohol for forward osmosis membranes with outstanding water flux†

Typical forward osmosis (FO) membranes utilize non-woven fabrics or polymers as support layers, which cause severe internal concentration polarization (ICP) effects due to their unalterable thickness, thereby limiting the water flux. Herein, an ultrathin composite film based on carbon nanotubes (CNTs) and polyvinyl alcohol (PVA), with a thickness controlled down to 10.3 μm, is proposed as a support layer for the fabrication of FO membranes for application in polluted water treatments and desalination. Our observations reveal that incorporating PVA into the CNT film boosts mechanical strength and hydrophilicity, thereby efficiently alleviating the ICP effect during the FO process. Moreover, employing a uniform polydopamine (PDA) film as the interlayer reduces the pore size and roughness of the substrates, thus facilitating the formation of an ultrathin, continuous, and intact polyamide (PA) selective layer. With deionized water as the feed solution and a 1 M sodium chloride solution as the draw solution, the optimized membrane achieves an extremely high water flux of up to 90.86 L m⁻² h⁻¹ and a specific salt flux of 0.22 g L⁻¹. This work demonstrates an effective strategy to develop FO membranes with reduced membrane thickness while still exhibiting exceptional water flux and good mechanical properties, showcasing its potential in practical FO membrane applications.