Novel Thin-Film nanocomposite hollow fiber membranes in modules with reduced reverse solute flux for pressure retarded osmosis

Osmotic energy is released when mixing solutions with different salinities. Pressure retarded osmosis (PRO) is a promising membrane technology to harvest this osmotic energy. However, the current polymeric PRO membranes suffer from the high reverse solute fluxes during PRO processes, which would dramatically diminish the membrane performance, especially in big membrane modules. In order to control the reverse solute flux, two cup-like calix[n]arenes, sulfocalix[4]arene (SCA4) and sulfothiacalix[4]arene (STCAss) were incorporated into the polyamide network to enhance the PRO performance of thin-film nanocomposite (TFN) hollow fiber membranes in 1-inch modules. Due to the unique structures and the enhanced molecular-sieving abilities of both macrocyclic molecules, they were able to effectively improve the selectivity of the resultant TFN hollow fiber membranes. After optimizing the concentration of each nano-filler, it was found that the TFN membrane with 0.20 wt% of STCAss produced a PRO power density of 15.0 W/m2 and a reverse salt flux of 28.3 gMH, while the other containing 0.10 wt% of SCA4 produced a PRO power density of 14.2 W/m2 and a reverse salt flux of 31.6 gMH. In long-term PRO tests at 20 bar, the SCA4-incorporated membrane was more efficient in controlling the concentration polarization (CP) and maintaining the power density comparing to the STCAss membrane because the former has a smaller cavity opening than the latter. This research work may provide useful insights to further design PRO membranes and membrane modules for osmotic power generation.