(568b) Janus Hollow Fiber Membranes with Bio-Inspired Polydopamine Coating for Desalination

Membrane distillation (MD) is a thermally driven water desalination process. Traditionally, hydrophobic membranes are applied to separate the hot feed and cold permeate. The hydrophobic membranes prevent the saline feed permeation and only allow the transport of water vapor. In addition, they also prevent the membrane pore wetting. For the direct contact membrane distillation (DCMD) process, the mass and heat transfer are coupled transport processes, and high mass transfer accompanied with low heat transfer are preferred to maintain the desalination driving force and subsequently permeation flux. However, the reduction of membrane thickness would benefit the mass transfer, but it also reduces the heat resistance simultaneously. Such a phenomenon leads to a dilemma in the DCMD membrane fabrication.

During the DCMD process, the water vapor transport through the membrane pores while the heat is mainly conducted via the membrane matrix due to its higher heat transfer coefficient. As a result, if we can strategically introduce a single-sided hydrophilic layer onto the hydrophobic membrane, we may reduce the mass transfer distance while remain the heat transfer resistance relatively unchanged. Therefore, the Janus membrane with different wettability could be a promising candidate to fulfill the requirement of DCMD process. Conventional approaches to constructing a Janus membrane require complicated fabrication processes or highly specialized equipment, and a facile, scalable method to fabricate a Janus membrane has not been available to date, especially for hollow fiber membranes.

In this work, we applied dopamine (DA) / polyethyleneimine (PEI) co-deposition method to coat the lumen side of a hydrophobic polypropylene hollow fibre membrane (PPHFM, the inner diameter is 1800±150 µm, the wall thickness is 450±50 µm, and the pore size is ~200 nm). The PPHFM was firstly assembled into a membrane module and then the mixture (DA/PEI) were recirculated within the membrane module to allow its gradual deposition on the membrane lumen surface. In order to improve the membrane surface wetting by the deposition, the PPHFM was firstly pre-wetted by ethanol and the residual ethanol in the pores was absorbed by a filter paper.

Due to the capillary effect, single-faced modification of the membrane is challenging through a wet modification process. However, the bio-inspired deposition process enables us a facile process to modify the hydrophobic membranes by an aqueous solution. Initially, only the lumen side was wetted by the deposition solution. Then the solid-liquid-air triple phase contact line moved to the membrane outer surface with the increase of deposition time. As a result, the hydrophilic layer deposition thickness can be regulated by applying different deposition times. After the deposition, the original membrane porous structure was still well preserved.

One of the most remarkable characters for the Janus membrane is the asymmetric wettability. We further investigated the water contact angle of the Janus membrane, and results suggest the successful single-sided deposition of the DA/PEI. The inner side gradually became hydrophilic with the increase of deposition time (contact angle 30 ° for 6 h deposition), while the water contactor angle for the hydrophobic outer side was relatively unchanged. Furthermore, the liquid entry pressure (LEP) test reveals the introduction of a hydrophilic layer only marginally reduced the LEP value, suggesting the Janus membrane could be applied for DCMD process.

We further evaluated the Janus membrane performance for DCMD. With the increase of deposition time, the pure water flux gradually increased for the Janus membrane, and the flux increment was more significant at elevated temperature. The most significant flux improvement of 120 % was observed at 80 °C using 6 h deposited membrane. In terms of the salt rejection test, the Janus membrane exhibited good operational stability. For example, over 99 % of the rejection was obtained over a 72 h testing period with 4 h deposited membrane.

Conclusively, we developed a facile approach to prepare Janus hollow fibre membranes via the DA/PEI deposition approach. The Janus membrane exhibited asymmetric surface components and wettability. During the DCMD testing process, the Janus membrane had high salt rejection and improved permeate flux. This work introduced a novel approach to developing the Janus membrane for various applications.