Publication: Superhydrophobic polyvinylidene fluoride-carbon membrane for membrane distillation with electrochemical cleaning evaluation
Loading...
Date
2022-09-01
Authors
Zakaria, Nur Ain
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Membrane distillation (MD) is driven by the vapor pressure differences between feed and permeate circulating on different sides of a porous membrane. Water can be recovered below boiling point at low pressure. However, water vapor permeation through the porous membrane is significantly affected by membrane wetting and fouling. In this work, carbon black (CB), poly(methyl methacrylate) (PMMA) and commercial conductive ink (CI) were added to the polyvinylidene fluoride (PVDF) dope solution for creating a superhydrophobic surface with minimum wetting and fouling. PVDF dope solution was varied with CB content from 2 to 5 wt% and different types of additives to study the effects of CB on membrane
properties and separation performance in membrane distillation. All the fabricated membranes were further tested in electrochemical cleaning and the WCA of the results obtained were compared. The PVDF/CB, PVDF/PMMA/CB, PVDF-CI and PVDF-PMMA-CI membranes were produced using phase inversion. The solidified membranes were peeled off from the woven support to gain a micro-roughness membrane surface. The addition of CB beyond 3 wt.% could result in a superhydrophobic surface with a water contact angle (WCA) higher than 150°. Meanwhile, the PMMA had caused small reduction of PVDF-Carbon (PVDF-C3) membrane WCA but still higher than 150 °. As PVDF membrane incorporated with
CI, the WCA was decreased up to 141.4 ± 0.8 °. Yet, incorporation of PMMA in PVDF-CI membrane improved the WCA value. PVDF membranes blended with 2 to 5 wt% of CB showed PVDF characteristic peaks and interactions with CB in Fourier transform infrared spectra. The presence of CB in the dope solution caused the length of finger-like voids to reduce but the membrane thickness to increase. The pore size increased by adding 2 or 3 wt% of CB. A higher amount of CB resulted in reduced pore size and porosity due to pore blockage by CB. Nevertheless, CB particles increased the surface roughness to form a superhydrophobic surface without using any hydrophobic agent. As for PVDF membrane incorporated with CI, the membrane thickness dropped and increased significantly as PVDF-CI membrane was mixed with PMMA. The addition of PMMA in PVDF-CI membrane dope solution increased the viscosity, thus increased the membrane thickness and membrane hydrophobicity after being casted and dried. However, the mean pore size of PVDF-PMMA-CI membrane became smaller due to the high dope solution viscosity. Although the superhydrophobic PVDF/CB membrane showed similar permeate flux to the neat PVDF membrane in MD, it could be electrochemically cleaned within 4 min to restore its permeate flux after wetting by the salt solution containing surfactant. The membrane could be cleaned by hypochlorite (OCl−) and hypochlorous acid (HOCl) or metal hydroxides produced in electrochemical cleaning. PVDF-PMMA-CB and PVDF-CI membranes showed similar membrane morphology to PVDF/CB. However, PVDF-PMMA-CB and PVDF-CI membranes with more conductive additives could be cleaned more effectively than PVDF-CB
membranes. In conclusion, incorporating conductive additives improved the membrane morphology with minimum impacts on membrane distillation but significant changes in electrochemical cleaning.