Two-dimensional fractal nanocrystals templating for substantial performance enhancement of polyamide nanofiltration membrane

Abstract

Significance Fractal structures and phenomena have existed in nature for hundreds of millions of years. Developing their practical applications in material design is of fundamental importance, but this goal has not yet been reached. In this work, NaCl crystals with a fractal structure are formed between the polyamide active layer and the support during an interfacial polymerization process. The branching of fractal NaCl nanocrystals creates numerous tiny interworking water channels that enable water transport, maximizing the effective permeating area of the polyamide nanofiltration (NF) membrane. The fractal NaCl nanocrystals–templated polyamide NF membrane exhibits an improved desalination performance with a three to four times increase in permeance. Applying fractal structure successfully to the design of artificial materials improves performance. In this study, we report the emergence of two-dimensional (2D) branching fractal structures (BFS) in the nanoconfinement between the active and the support layer of a thin-film-composite polyamide (TFC-PA) nanofiltration membrane. These BFS are crystal dendrites of NaCl formed when salts are either added to the piperazine solution during the interfacial polymerization process or introduced to the nascently formed TFC-PA membrane before drying. The NaCl dosing concentration and the curing temperature have an impact on the size of the BFS but not on the fractal dimension (∼1.76). The BFS can be removed from the TFC-PA membranes by simply dissolving the crystal dendrites in deionized water, and the resulting TFC-PA membranes have substantially higher water fluxes (three- to fourfold) without compromised solute rejection. The flux enhancement is believed to be attributable to the distributed reduction in physical binding between the PA active layer and the support layer, caused by the exertion of crystallization pressure when the BFS formed. This reduced physical binding leads to an increase in the effective area for water transport, which, in turn, results in higher water flux. The BFS-templating method, which includes the interesting characteristics of 2D crystal dendrites, represents a facile, low-cost, and highly practical method of enhancing the performance of the TFC-PA nanofiltration membrane without having to alter the existing infrastructure of membrane fabrication.