Plasma-Induced Superhydrophobicity as a Green Technology for Enhanced Air Gap Membrane Distillation

Abstract

Superhydrophobicity has only recently become a requirement in membrane fabrication and modification. Superhydrophobic membranes have shown improved flux performance and scaling resistance in long-term membrane distillation (MD) operations compared to simply hydrophobic membranes. Here, we introduce plasma micro-and nanotexturing followed by plasma deposition as a novel, dry, and green method for superhydrophobic membrane fabrication. Using plasma micro-and nanotexturing, commercial membranes, both hydrophobic and hydrophilic, are transformed to superhydrophobic featuring water static contact angles (WSCA) greater than 150°and contact angle hysteresis lower than 10°. To this direction, hydrophobic polytetrafluoroethylene (PTFE) and hydrophilic cellulose acetate (CA) membranes are transformed to superhydrophobic. The superhydrophobic PTFE membranes showed enhanced water flux in standard air gap membrane distillation and more stable performance compared to the commercial ones for at least 48 h continuous operation, with salt rejection >99.99%. Additionally, their performance and high salt rejection remained stable, when low surface tension solutions containing sodium dodecyl sulfate (SDS) and NaCl (down to 35 mN/m) were used, showcasing their antiwetting properties. The improved performance is attributed to superhydrophobicity and increased pore size after plasma micro-and nanotexturing. More importantly, CA membranes, which are initially unsuitable for MD due to their hydrophilic nature (WSCA ≈ 40°), showed excellent performance with stable flux and salt rejection >99.2% again for at least 48 h, demonstrating the effectiveness of the proposed method for wetting control in membranes regardless of their initial wetting properties.

Antifouling Plasma-Treated Membranes with Stable Superhydrophobic Properties for Membrane Distillation

“…In the case of PTFE membranes, porosity increased by 3% after plasma treatment, from 59 to 62%, while for the CA membranes, the initial value is much higher at 77% (untreated) and reaches 81% after plasma treatment, which can explain the fact that these membranes yield higher flux in MD. The surface pore widening effect of plasma treatment on membranes has been investigated in our previous publication using SEM image analysis . Additionally, using the ethanol permeability test, the effective pore size was determined and calculated using the Guerout–Elford–Ferry equation.…”

“…This increase was in the range of 27% for the CA membranes, with average pore sizes being 176 ± 5 nm for the untreated and 225 ± 10 nm after plasma treatment. More information on porosity, tortuosity, and average pore size of these membranes can be found in our previous publication …”

“…We have already reported the performance of plasma-treated membranes in AGMD using a 60 °C temperature difference (75–15 °C) during a 48 h experiment duration . Plasma-treated membranes not only featured a 10% higher flux compared to pristine samples but also demonstrated a stable performance.…”

“…We have extensively worked on transforming rigid polymeric substrates to superhydrophobic, − and we have reported that superhydrophobic surfaces fabricated using our methodology, which involves plasma treatment, can repel bacteria containing solutions in static mode for more than 72 h, while sustaining superhydrophobicity underwater for several days . Recently, we have demonstrated that it is possible to render commercial membranes superhydrophobic, regardless of their initial wetting properties, featuring enhanced performance in MD and exhibiting superior antiwetting properties, against a saline feed solution featuring the surfactant sodium dodecyl sulfate (SDS) …”

“…49 Recently, we have demonstrated that it is possible to render commercial membranes superhydrophobic, regardless of their initial wetting properties, featuring enhanced performance in MD and exhibiting superior antiwetting properties, against a saline feed solution featuring the surfactant sodium dodecyl sulfate (SDS). 50 Herein, we demonstrate the long-lasting, antifouling properties of superhydrophobic and plasma-treated PTFE and CA membranes in continuous flow conditions, using a high concentration (up to 3 g/L) of bovine serum albumin (BSA) in the saline feed solution. Using our plasma technology, we alter membranes' surface morphology and chemistry at will through a combination of plasma etching and plasma deposition that results in superhydrophobicity (Figure 1).…”