In previous studies of the desalination technology membrane distillation (MD), superhydrophobicity of the membrane has been shown to dramatically decrease fouling in adverse conditions, but the mechanism for this is not well understood. Additionally, air layers present on submerged solid superhydrophobic surfaces have been shown to dramatically reduce biofouling, and air-bubbling has been used to reducing fouling in MD. The present work studies the effect of maintaining air layers on the membrane surface and superhydrophobicity as a new method for preventing fouling of MD membranes by salts, particulates, and organic particles. Superhydrophobic MD membranes were prepared using initiated chemical vapor deposition (iCVD) of perfluorodecyl acrylate (PFDA) on poly(vinyldene fluoride) PVDF membranes and used to study the effects of hydrophobicity on fouling. A static MD setup with evaporation through an MD membrane but no condensing of permeate was used to examine the effect of air exposure on fouling, by measuring the increase in weight of the membrane caused by scale deposition. Theory was derived for the reduction of fouling on superhydrophobic surfaces. Air layers may displace fouling gels, reduce the area of feed in contact with the membrane, reduce foulant adhesion, and enhance superhydrophobicity in a Cassie-Baxter state. The study shows that the presence of air on the membrane surface significantly reduces biological fouling, but in some cases had mildly exacerbating effects on fouling of salts, especially when the air was not saturated with water vapor. Air recharging combined with superhydrophobicity reduced fouling in several cases where hydrophobic membranes alone did little.Keywords: membrane distillation; superhydrophobic surface; air layer; nucleation; anti-fouling
Electrospun
nanofiber membranes (ENMs) have garnered increasing
interest due to their controllable nanofiber structure and high void
volume fraction properties in membrane distillation (MD). However,
MD technology still faces limitations mainly due to low permeate flux
and membrane wetting for feeds containing low surface tension compounds.
Perfluorinated superhydrophobic membranes could be an alternative,
but it has negative environmental impacts. Therefore, other low surface
energy materials such as silica aerogel and polydimethylsiloxane (PDMS)
have great relevancy in ENMs fabrication. Herein, we have reported
the high flux and nonwettability of ENMs fabricated by electrospraying
aerogel/polydimethylsiloxane (PDMS)/polyvinylidene fluoride (PVDF)
over electrospinning polyvinylidene fluoride-co-hexafluoropropylene
(PVDF–HFP) membrane (E-PH). Among various concentrations of
aerogel, the 30% aerogel (E-M3-A30) dual layer membrane achieved highest
superhydrophobicity (∼170° water contact angle), liquid
entry pressure (LEP) of 129.5 ± 3.4 kPa, short water droplet
bouncing performance (11.6 ms), low surface energy (4.18 ± 0.27
mN m–1) and high surface roughness (R
a: 5.04 μm) with re-entrant structure. It also demonstrated
nonwetting MD performance over a continuous 7 days operation of saline
water (3.5% of NaCl), high antiwetting with harsh saline water containing
0.5 mM sodium dodecyl sulfate (SDS, 28.9 mN m–1),
synthetic algal organic matter (AOM).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.