Maximizing the wetting resistance of fluorine-free omniphobic membranes for hypersaline wastewater desalination.

Membrane distillation (MD) equipped with omniphobic (non-wetting) membranes has found a niche in water reclamation from hypersaline industrial wastewater. Here, we examined the efficacy of non-fluorinated materials as surface coating agents for omniphobic MD membrane fabrication, and identified necessary mechanisms to attain a maximized wetting resistance using fluorine-free materials. We first prepared MD membranes with different surface chemistries using a series of linear alkylsilanes and polydimethylsiloxane (PDMS) as representative fluorine-free, low surface energy materials. Membranes modified with a longer chain alkylsilane exhibited a lower surface energy and demonstrated a greater wetting resistance in direct contact MD experiments using feedwaters of various surface tensions. Despite the nearly identical surface energy measured for the longest alkylsilane and PDMS, PDMS-modified membrane exhibited an extended antiwetting performance as compared to the membrane treated with the longest alkylsilane. To elucidate the source of the distinctive wetting resistance, we examined the nucleation and condensation kinetics on the surfaces with the different surface chemistries via environmental scanning electron microscopy. Our analysis suggests that the membranes treated with long chain alkylsilanes contain surface defects (i.e., hydrophilic regions) whereas the high mobility of the PDMS effectively minimizes the defect exposure, slowing down the condensation and subsequent surface wetting.

Influence of photothermal nanomaterials localization within the electrospun membrane structure on purification of saline oily wastewater based on photothermal vacuum membrane distillation.

Today, synergistic combination of special nanomaterials (NMs) and electrospinning technique has emerged as a promising strategy to address both water scarcity and energy concerns through the development of photothermal membranes for wastewater purification and desalination. This work was organized to provide a new perspective on membrane design for photothermal vacuum membrane distillation (PVMD) through optimizing membrane performance by varying the localization of photothermal NMs. Poly(vinylidene fluoride) omniphobic photothermal membranes were prepared by localizing graphene oxide nanosheets (GO NSh) (1) on the surface (0.2 wt%), (2) within the nanofibers structure (10 wt%) or (3) in both positions. Considering the case 1, after 7 min exposure to the 1 sun intensity light, the highest temperature (∼93.5 °C) was recorded, which is assigned to the accessibility of GO NSh upon light exposure. The case 3 yielded to a small reduction in surface temperature (∼90.4 °C) compared to the case 1, indicating no need to localize NMs within the nanofibers structure when they are localized on the surface. The other extreme belonged to the case 2 with the lowest temperature of ∼71.3 °C, which is consistent with the less accessibility of GO NSh during irradiation. It was demonstrated that the accessibility of photothermal NMs plays more pronounced role in the membrane surface temperature compared to the light trapping. However, benefiting from higher surface temperature during PVMD due to enhanced accessibility of photothermal NMs is balanced out by decrease in the permeate flux (case 1: 1.51 kg/m2 h and case 2: 1.83 kg/m2 h) due to blocking some membrane surface pores by the binder. A trend similar to that for flux was also followed by the efficiency. Additionally, no change in rejection was observed for different GO NSh localizations.

Long-Chain PFASs-Free Omniphobic Membranes for Sustained Membrane Distillation.

Omniphobic membranes possessing high wetting resistance have been created for the treatment of challenging hypersaline feedwaters with low surface tension through membrane distillation (MD). However, virtually all such membranes are fabricated with long-chain per- and polyfluoroalkyl substances (PFASs, ≥8 fluorinated carbons). The environmental risks and high bioaccumulation potential of long-chain PFASs have raised increasing concerns. Developing highly wetting-resistant MD membranes while avoiding the use of long-chain PFASs is essential to improve the viability of MD for resilient and sustainable water purification. We demonstrate that MD membranes with exceptional wetting resistance can be designed through the combination of hierarchically structured membranes consisting of re-entrant texture at different length scales and (ultra)short-chain fluorocarbons, which have lower acute toxicity and bioaccumulation potentials than long-chain PFASs. Our hierarchically structured membrane with three-tier micro/nanostructure fabricated with short-chain fluorocarbon possesses superior wetting resistance, which is comparable to or higher than the long-chain PFASs-based omniphobic membranes reported in the literature. Furthermore, the hierarchically structured membranes fabricated with ultrashort-chain fluorocarbons display improved wetting resistance against feedwaters with low surface tension. Our findings indicate that long-chain PFASs are not required when designing wetting-resistant membranes and that the balance between sustainability and wetting resistance should be tailored to the wetting potential of the feedwater.

Fluorinated silica-modified anti-oil-fouling omniphobic F-SiO2@PES robust membrane for multiple foulants feed in membrane distillation.

Direct-contact membrane distillation (DCMD) can be eminent solution for oily wastewater treatment if the membrane provided is slippery and tolerant to low surface tension complex solutions. This study describes preparation of an anti-oil-fouling omniphobic polyethersulfone membrane using fluorinated silica nanoparticles (F-SiO2@PES) combined with perfluorodecyl triethoxysilane and polydimethylsiloxane for application against oil-In-water (o/w) emulsions. Feed solutions consist of different concentrations of oil (hexadecane), different charge surfactants (anionic sodium dodecyl benzenesulfonate, non-ionic Tween 20, and cationic hexadecyltrimethylammonium bromide, and salt (NaCl). The hierarchical re-entrant micro structured surface of the omniphobic F-SiO2@PES membrane and functional groups are confirmed by atomic force microscopy, scanning electron microscopy, and Fourier-transform infrared spectroscopy. The anti-oil-fouling and anti-wetting performance of omniphobic F-SiO2@PES membranes are investigated using contact-angle, sliding angles, DCMD tests with multiple foulants of surfactants. Omniphobic F-SiO2@PES membrane exhibited effective anti-oil-fouling and anti-wetting performance against emulsions as no severe fouling and a conductivity rises were evident regardless of surfactant charge and the concentration of components. Flux reduction and rejection rates for the omniphobic F-SiO2@PES membranes are in a range of 5-15% (only) and >99%, respectively, for various combinations of feed solution components.

Omniphobic Nanofibrous Membrane with Pine-Needle-Like Hierarchical Nanostructures: Toward Enhanced Performance for Membrane Distillation.

Wetting and fouling phenomena are the main concerns for membrane distillation (MD) in treating high-salinity industrial wastewater. This work developed an omniphobic membrane by growing titanium dioxide (TiO2) nanorods on polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) nanofibers using a hydrothermal technique. The TiO2 nanorods form a uniform pine-needle-like hierarchical nanostructure on PVDF-HFP fibers. A further fluorination treatment provides the membrane with a low-surface-energy omniphobic surface, displaying contact angles of 168° and 153° for water and mineral oil, respectively. Direct contact MD experiments demonstrated that the resulting membrane shows a high and stable salt rejection of >99.9%, while the pristine PVDF-HFP nanofibrous membrane suffers a rejection decline caused by intense pore wetting and oil fouling in the desalination process in the presence of surfactant and mineral oil. The superior antiwetting and antifouling behaviors were ascribed to a nonwetting Cassie-Baxter state established by the accumulation of a great deal of air in the hydrophobized hierarchical re-entrant structures. The development of omniphobic membranes with pine-needle-like hierarchical nanostructures provides an approach to mitigate membrane wetting and fouling in the MD process for the water reclamation from industrial wastewater.

Omniphobic re-entrant PVDF membrane with ZnO nanoparticles composite for desalination of low surface tension oily seawater.

In this study, an omniphobic membrane was fabricated by electrospraying fluorinated zinc oxide (ZnO) nanoparticles (NPs) mixed with polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) on the surface of an organosilane functionalized polyvinylidene difluoride (PVDF) membrane. Our results revealed that the functionalized ZnO NPs membrane exhibited a rough hierarchical re-entrant morphology with low surface energy which allowed it to achieve high omniphobic characteristics. It was observed that the addition of 30% ZnO (w/w of PVDF-HFP) was found to be optimal and imparted a high repulsive characteristic. The optimized PVDF/ZnO(30)/FAS/PVDF-HFP referred as cPFP-30Z membrane exhibited a high contact angle values of 159.0 ±â€¯3.1°, 129.6 ±â€¯2.2°, 130.4 ±â€¯4.1° and 126.1 ±â€¯1.2° for water, sodium dodecyl sulfate (SDS) saline solution (0.3 mM SDS in 3.5% NaCl), ethanol, and vegetable oil, respectively. The low surface energy and high surface roughness (Ra) of optimised membrane was assessed as 0.78 ±â€¯0.14 mN m-1 and 1.37 µm, respectively. Additionally, in contrast with the commercial PVDF membrane, the cPFP-30Z membrane exhibited superior anti-wetting/anti-fouling characteristics and high salt rejection performance (>99%) when operated with a saline oil solution (0.015 v/v) and SDS (0.4 mM) feed solutions.

Recent advances in membrane development for treating surfactant- and oil-containing feed streams via membrane distillation.

Membrane distillation (MD) has been touted as a promising technology for niche applications such as desalination of surfactant- and oil-containing feed streams. Hitherto, the deployment of conventional hydrophobic MD membranes for such applications is limited and unsatisfactory. This is because the presence of surfactants and oils in aqueous feed streams reduces the surface-tension of these media significantly and the attachment of these contaminants onto hydrophobic membrane surfaces often leads to membrane fouling and pore wetting, which compromises on the quantity and quality of water recovered. Endowing MD membranes with surfaces of special wettabilities has been proposed as a strategy to combat membrane fouling and pore wetting. This involves the design of local kinetic energy barriers such as multilevel re-entrant surface structures, surfaces with ultralow surface-energies, and interfacial hydration layers to impede transition to the fully-wetted Wenzel state. This review critiques the state-of-the-art fabrication and surface-modification methods as well as practices used in the development of omniphobic and Janus MD membranes with specific emphasis on the advances, challenges, and future improvements for application in challenging surfactant- and oil-containing feed streams.

Antiwetting and Antifouling Janus Membrane for Desalination of Saline Oily Wastewater by Membrane Distillation.

In this study, we develop Janus membranes comprising a hydrophilic zwitterionic polymer layer and an omniphobic (all-liquid-repelling) porous substrate that simultaneously possess fouling and wetting resistances. An omniphobic membrane was first fabricated by attaching silica nanoparticles (SiNPs) to the fibers of a quartz fiber mat, creating multilevel re-entrant structures, followed by surface fluorination to reduce the surface energy. The Janus membrane was then fabricated by grafting a zwitterionic polymer brush layer via surface-initiated atom-transfer radical-polymerization (ATRP) on the omniphobic substrate. Membrane characterizations, including Fourier-transform infrared spectroscopy, fluorescence microscopy, and contact angle measurements, confirm that the surface hydrophilicity can be finely tuned by adjusting the duration of the ATRP reaction. Also, the zwitterionic polymer brush layer was confined on the top surface of the Janus membrane, rendering the surface hydrophilic, while the remaining part of the Janus membrane remained omniphobic, resisting the wicking of low-surface-tension liquids including ethanol and hexane. A static oil-fouling test showed that crude oil droplets irreversibly fouled an omniphobic membrane (without a hydrophilic top layer) in water. In contrast, oil droplets placed on the Janus membrane in air were immediately desorbed upon its immersion in water. Finally, we performed direct-contact membrane distillation (MD) experiments using a crude-oil-in-saline (NaCl) water emulsion as a feed solution, simulating highly saline oily wastewater. The Janus membrane exhibited superior wetting and fouling resistances, with a stable water flux and nearly perfect salt rejection, while an omniphobic membrane failed in the desalination process. Our work highlights the great potential of antiwetting and antifouling Janus membranes for water reclamation from challenging industrial wastewaters through MD.