Omniphobic Polymer/Graphene Oxide-Coated PVDF Membrane for Stable Membrane Distillation in Surfactant-Containing Seawater

Background

Membrane distillation passes only water vapor across a hydrophobic membrane from a hot saline feed to a cool permeate. Low-surface-tension contaminants such as surfactants lower the liquid entry pressure, so liquid fills the pores and both flux and salt rejection decline. GO lamellae provide narrow interlayer channels for vapor transport, but GO is intrinsically hydrophilic and swells in humid conditions. Reduction or fluoroalkyl silane functionalization requires multistep reactions that are difficult to apply uniformly over large areas, and solution-cast hydrophobic polymer films thicker than about a micrometer add vapor-diffusion resistance.

Design and approach

GO dispersed at 10 mg/mL in a water–ethanol mixture is slot-die coated onto a hydrophobic PVDF membrane, where shear between the die head and the moving stage aligns the nanosheets. The GO membrane is then dip-coated twice in a PFPE solution in a highly volatile solvent, which evaporates rapidly and leaves a conformal low-surface-energy film. The resulting omniphobic polymer–graphene oxide (OGO) membrane consisted of a GO layer of about 340 nm and a PFPE layer of 350 nm. The C–F-rich PFPE surface is intended to delay adsorption and penetration of surfactant, while the underlying GO lamellae restrict further permeation once liquid reaches them. Membranes up to 15 × 13 cm were produced, with the size limited by the coater stage.

How it was evaluated

The authors characterized the membranes by SEM, EDS, FTIR, contact angles with water, diiodomethane, ethanol, hexane, SDS and NaCl solutions, surface free energy, and XRD after water vapor exposure for up to 7 days at 60 °C. Desalination was tested by direct contact membrane distillation (DCMD) with a 3.5 g L−1 NaCl feed at 60 °C and DI water permeate at 20 °C over a 21 cm2 membrane area, with and without 0.4 mM SDS, for 24 h. Comparators were bare PVDF and uncoated GO on PVDF. Additional membranes on hydrophilic nylon supports, including thermally treated nanoporous multilayer graphene, were used to separate the effects of the GO structure, the substrate, and the number of PFPE coatings.

Key results

PFPE coating raised the water contact angle of bare PVDF from 93° to 132°, and the OGO membrane showed higher contact angles than GO for all tested liquids. The OGO contact angle remained above 95° in 0.4 mM SDS, and the calculated total surface free energy, including both dispersive and polar components, was lower than that of GO.

GO and OGO showed the same initial d-spacing of 7.3 Å. After 24 h of water vapor exposure, the d-spacing of GO increased to 8.5 Å, whereas that of OGO increased only to 7.7 Å, corresponding to an effective channel of 4.3 Å, smaller than the 7–8 Å hydrated diameter of SDS cited by the authors. Neither layer delaminated from PVDF after 7 days of vapor exposure at 60 °C.

With the 3.5 g L−1 NaCl feed, the OGO membrane maintained a flux above 25 LMH and salt rejection above 99.9% after 24 h. Its flux was lower than that of bare PVDF, which the authors attributed to the transport resistance of the PFPE layer. On nylon supports, a single PFPE coating gave very low flux and declining rejection, while two coatings gave an initial flux above 20 LMH, which the authors related to sealing of pinholes left after the first coating.

When 0.4 mM SDS was added, bare PVDF dropped to 89.8% rejection at 12.2 LMH after 24 h, and the uncoated GO membrane wetted rapidly and showed a negative flux as permeate moved back to the feed. The OGO membrane kept salt rejection above 99.7% with a flux of 15.4 LMH after 24 h, and neither coating layer delaminated during operation.

Open questions

The authors reported that the OGO flux in SDS-containing feed decreased during operation, which they attributed to partial pore wetting and fouling, and that the d-spacing increased slightly after operation owing to adsorption of salt and SDS. The surfactant tests were limited to 24 h of operation with 0.4 mM SDS.

How it connects to other research

The lab's earlier nonfluorinated superomniphobic surfaces repelled low-surface-tension liquids through mushroom-like re-entrant micropillar geometry, whereas this membrane relies on a thin fluorinated polymer film on GO lamellae to resist wetting while transmitting water vapor.