Casting of a superhydrophobic membrane composed of polysulfone/Cera flava for improved desalination using a membrane distillation process

Abstract

Superhydrophobic membranes are necessary for effective membrane-based seawater desalination. This paper presents the successful fabrication of a novel electrospun nanofibrous membrane composed of polysulfone and Cera flava, which represents a novel class of enhanced performance membranes consisting of a superhydrophobic nanofibrous layer and hydrophobic polypropylene (PP). Cera flava, which helps lower the surface energy, was found to be the ideal additive for increasing the hydrophobicity of the polysulfone (PSF) polymeric solution because of its components such as long-chain hydrocarbons, free acids, esters, and internal chain methylene carbons. In the fabricated membrane, consisting of 10 v/v% Cera flava, the top PSF-CF nanofibrous layer is active and the lower PP layer is supportive. The hybrid membrane possesses superhydrophobicity, with an average contact angle of approximately 162°, and showed high performance in terms of rejection and water flux. This work also examined the surface area, pore size distribution, fiber diameter, surface roughness, mechanical strength, water flux, and rejection percentage of the membrane. The salt rejection was above 99.8%, and a high permeate flux of approximately 6.4 LMH was maintained for 16 h of operation.

Fig. 1. Surface modification of a polypropylene membrane with an electrospun nanofibrous polysulfone layer incorporating Cera flava for improved MD applications.

Fig. 2. Typical block diagram of the electrospinning technique.

Fig. 3. Schematic of a typical setup for lab DC-MD.

Fig. 4. Contact angle and contact diameter analysis of different membranes [note: error bars are based on standard errors from three replicate tests].

Fig. 5. FT-IR analysis of the PP membrane and surface-modified PSF–CF/PP membrane [note: FT-IR analysis was carried out in the range of 600–4000 cm⁻¹].

Fig. 6. SEM image analysis: (a) 16% PSF (b) 16% PSF–CF_(5%) (c) 16% PSF–CF_(7.5%); (d) 16% PSF–CF_(10%) (e) fiber diameter analysis for different concentrations of PSF incorporated with CF [note: error bars are based on standard errors by analyzing at least 10 measurements].

Fig. 7. SEM micrograph: (a) surface morphology of the PP membrane. (b) Cross-sectional view of the PSF–CF/PP membrane before heat-pressing post-treatment. (c) Cross-sectional view of the PSF–CF/PP membrane after heat-pressing post-treatment.

Fig. 8. Two-dimensional AFM micrographs of membrane surfaces before and after modification by PSF–CF (dimensions: 2 μm × 2 μm).

Fig. 9. Surface area and pore volume analysis of various modified membranes [note: error bars are based on standard errors from three replicate tests].

Fig. 10. BJH adsorption/desorption average pore diameter and average pore width for various modified membranes [note: error bars were based on the standard errors of three replicate tests].

Fig. 11. Effect of temperature difference on water flux in MD [note: feed solution = 30 g L⁻¹ NaCl solution, time period = 1 h]. Error bars are based on standard errors from three replicate tests.

Fig. 12. MD performance. (a) Effect of time interval on salt rejection. (b) Effect of time interval on permeate water flux of different fabricated membranes utilized in the MD system [note: feed solution = 30 g L⁻¹ NaCl solution, temperature difference = 60 °C]. Error bars are based on standard errors from three replicate tests.

Fig. 13. Long term performance analysis in terms of water flux decline: graphical representation of water flux and water flux after physical cleaning [note: membrane used: PSF–CF_(10%)/PP, time: 30 h, feed stream: 30 g L⁻¹, T_f: 70 °C, T_p: 20 °C].