Enhanced desalination using a three-layer OTMS based superhydrophobic membrane for a membrane distillation process

Abstract

Superhydrophobic membranes are essential for improved seawater desalination. This study presents the successful casting of a three-layered membrane composed of a top superhydrophobic coating onto a polypropylene (PP) mat through simple sol-gel processing of octadecyltrimethoxysilane (OTMS), and the bottom layer was casted with hydrophilic poly(vinyl alcohol) (PVA) by using a knife casting technique; this membrane represents a novel class of improved-performance membranes consisting of a top superhydrophobic coating onto a hydrophobic PP mat and a hydrophilic layer (PVA) at the bottom. OTMSs are well known low-surface-energy materials that enhance superhydrophobicity, and they were observed to be the ideal chemical group for increasing the hydrophobicity of the PP mat. The PVA layer acted as base layer absorbing the condensed vapor and thus enhancing the vapor flux across the membrane. The hybrid three-layered membrane exhibited superhydrophobicity, with an average contact angle of more than 160°, and demonstrated high performance in terms of rejection and water flux. This study also examined the pore size distribution, surface roughness, surface area, tensile strength, water flux, and salt rejection of the fabricated membrane. The salt rejection level was calculated to be 99.7%, and a high permeate flux of approximately 6.7 LMH was maintained for 16 h.

Fig. 1. Schematic illustration of three layered membrane for MD application.

Fig. 2. Schematic of lab-scale direct contact membrane distillation system.

Fig. 3. Analysis of average contact angle and contact diameter of various membranes [note: error bars are based on standard errors from three replicate tests].

Fig. 4. SEM micrographs analysis: (a) surface morphology of polypropylene (PP) mat (middle layer); (b) morphology of PP mat modified with OTMS chemical group (top layer); (c) morphology of PP mat modified with PVA layer (bottom layer); (d) cross sectional view of three layered membrane OTMS-PP/PVA.

Fig. 5. AFM 3-D micrographs of membrane surfaces before and after modification by OTMS chemical group (dimensions: 20 μm × 20 μm).

Fig. 6. Surface area and pore volume analysis: (a) BET and Langmuir surface area with pore volume analysis; (b) surface area to pore volume ratio of different fabricated membranes [note: error bars are based on standard errors from three replicate tests].

Fig. 7. Dynamic mechanical analysis (DMA) of different fabricated membrane in terms of loss modulus and storage modulus.

Fig. 8. Barrett–Joyner–Halenda (BJH) analysis of adsorption/desorption average pore diameter and pore width for different modified membranes.

Fig. 9. Effect of temperature difference on permeate water flux in membrane distillation [note: feed solution = 30 g L⁻¹ NaCl solution, time period = 1 h]. [Note: error bars are based on standard errors from three replicate tests].

Fig. 10. MD performance of various membranes (a) effect of time interval on water flux; (b) effect of time interval on salt rejection% of fabricated membranes used in the MD process [note: feed solution = 30 g L⁻¹ aqueous NaCl solution, temperature difference = 50 °C]. Error bars are based on standard errors from three replicate tests.

Fig. 11. Analysis of reusability of membrane in terms of water flux decline: graphical representation of water flux and water flux after physical cleaning [note: membrane used: OTMS-PP/PVA, time: 16 h, feed stream: 30 g L⁻¹, T_f: 70 °C, T_p: 20 °C].