Turing structured nanofiltration membrane fabricated by sulfonated cellulose nanocrystals mediated interfacial polymerization for enhanced permeability separation performance

Preparing Turing structure nanofiltration (NF) membrane is a representative strategy to obtain high permeability and selectivity. However, getting a tailored Turing structure by adjusting the interfacial polymerization (IP) process remains a significant challenge. Herein, sulfonated cellulose nanocrystals (S-CNC) with excellent hydrophilicity and negative surface charges were selected as aqueous phase additives to explore their influence on the morphology and composition of polyamide (PA) layer as well as its NF performance. It was found the surface morphology of the PA layer was effectively changed by adjusting the content of S-CNC in the aqueous phase. As the content of S-CNC increased, the Turing structure underwent a series of evolutions, from an "octopus leg structure" to a densely packed ridge-valley structure to a large ridge-valley structure filled with dense nodular particles. Finally, it changed back to the typical PA morphology of densely distributed nodular particles. By exploring the interaction between S-CNC and piperazine (PIP), it was demonstrated that the hydrogen bond and electrostatic interaction between S-CNC and PIP synergistically inhibited the diffusion of PIP, thus resulting in various Turing structures. Benefiting from the unique Turing structure, the prepared SCNF-10 NF membranes showed excellent permeability and separation properties with pure water permeability (PWP) of 16.9 LMH bar(-1) and Na2SO4 rejection rate of 98.0 %. SCNF-10 NF membranes also exhibited good selectivity with a separation factor of 34.6 for mono-bivalent salts and long-term permeation stability. It is anticipated that this regulation strategy for various Turing structures preparation can provide a deeper understanding of the application of cellulose nanocrystals to NF membranes in the later stage, which will also promote the development of the high performance of NF membranes.