Dopamine incorporating forward osmosis membranes with enhanced selectivity and antifouling properties

Abstract

A new type of polyamide thin-film composite forward osmosis (FO) membranes were prepared by controlling dopamine self-polymerization in the aqueous phase during interfacial polymerization. The as-prepared membranes were investigated by attenuated total reflection Fourier transform infrared, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, atomic force microscopy and water contact angle measurements. The influence of the dopamine self-polymerization degree with different polydopamine particle sizes on membrane morphologies and chemical properties was studied by regulating dopamine concentrations in the aqueous phase. FO performance of the membrane was evaluated under two different modes, i.e. active layer facing draw solution (AL-DS) and active layer facing feed solution (AL-FS). The optimized FO membranes achieved a doubly enhanced water flux (22.08 L m^-2 h^-1) compared with the control membrane without dopamine incorporation, and a half-reduced reverse salt flux (32.77 mmol m^-2 h^-1) with deionized water as the feed and 1 M NaCl as the draw in the AL-FS mode. The optimized FO membrane showed a significantly reduced structural parameter (176 μm) compared with the control membrane (635 μm), indicating the minimised internal concentration polarization. Moreover, the new FO membranes had less flux decline than the control membrane, suggesting the improved antifouling performance of the membrane. Incorporation of dopamine during interfacial polymerization can be an effective strategy to fabricate high-performance FO membranes with excellent antifouling properties.

Fig. 1. Flow diagram of TFC membrane synthesis.

Fig. 2. Schematic diagram of FO performance testing system.

Fig. 3. Polydopamine particle size as a function of the dopamine concentration. The testing was performed in 30 ± 1 min and 3.0 wt% MPD monomers were dissolved in PBS solutions to simulate the environment in interfacial polymerization.

Fig. 4. SEM and AFM images of (a) TFC-0, (b) TFC-1, (c) TFC-2, (d) TFC-3, and (e) TFC-4, in which l means PA layer thickness.

Fig. 5. ATR-FTIR images of membranes PSf substrate, TFC-0, TFC-1, TFC-2, TFC-3 and TFC-4.

Fig. 6. Performance of the TFC membranes in FO performance tests, (a) water flux and (b) reverse solute flux in both AL-FS and AL-DS mode with 2 M NaCl as draw solution and DI water as the feed solution.

Fig. 7. (a) Water flux and (b) reverse solute flux for TFC-0 and TFC-1 membranes at AL-FS modes using DI water feed solutions and different concentrations of NaCl draw solutions.

Fig. 8. (a) FO fouling curves for the benchmark membrane and the modified membranes (TFC-1 and TFC-4). The feed solution was supplemented with seawater collected from Briton Beach in Melbourne as model foulant. The initial permeate water flux of around 10 L m⁻² h⁻¹. The cross flow velocities of the FS and DS were 4.9 cm s⁻¹ (T = 20 ± 0.5 °C, DS = NaCl 0.5–4 M), (b) comparison between the FO water flux of foaled membrane and recovered after the physical cleaning step. Crossflow velocity during cleaning step was 14.9 cm s⁻¹.