Enhanced desalination using a three-layer OTMS based superhydrophobic membrane for a membrane distillation process Saikat Sinha Ray, a Shiao-Shing Chen, * a Hau-Ming Chang, a Cao Ngoc Dan Thanh, a Huy Quang Le a and Nguyen Cong Nguyen b 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. 1. Introduction The demand for fresh water has increased gradually in the past 20 years. Membrane distillation (MD) is one of the most effec- tive technologies for seawater desalination. MD is a thermally driven separation process in which only vapor molecules can pass through a porous hydrophobic membrane. 1,2 Typically, MD possesses many unique features such as requiring lower oper- ating temperatures compared with those encountered in conventional processes, and placing lower demands on membrane mechanical strength. In addition, the hydrostatic pressure encountered in MD is much lower than that in pressure-driven membrane processes such as reverse osmosis. Hence, MD is a cost-effective process that places lower demands on membrane characteristics. 2–4 Hydrophobic materials such as polyvinylidene uoride (PVDF), polypropylene (PP), and polytetrauoroethylene (PTFE) are generally utilized in the MD process, and they are fabricated through processes such as stretching, electrospinning, ther- mally induced phase separation, and phase inversion. 5,6 Direct contact membrane distillation (DCMD) conguration is a type of MD in which an aqueous solution at a lower temperature is in direct contact with the permeate stream of the membrane. DCMD conguration has been widely studied because of its convenience and simplicity. 7 To prevent membrane wetting and the subsequent forma- tion of liquid-lled pores with higher mass transfer resistance, hydrophobic membranes composed of PVDF and PTFE are commonly utilized in MD. 8 Commercial membranes such as PP, PTFE, and PVDF demonstrate higher hydrophobicity and chemical and thermal resistance, compared with other membranes. Recently, membrane researchers have further modied these MD membranes into superhydrophobic membranes to avoid membrane fouling by reducing the direct contact between membrane foulants and membrane surface at the Cassie–Baxter state. 9 Superhydrophobic-specialized surfaces with versatile features have gained increased atten- tion in the eld of desalination and ltration. In general, MD processes must generate pure water with lower conductivity continuously. However, liquid penetration and pore wetting during long-term operation lead to lower salt rejection. According to some researchers, an increase in the hydropho- bicity of MD membranes can reduce membrane pore wetting effectively. 10,11 Therefore, researchers are attempting to develop superhydrophobic membranes by utilizing hydrophobic addi- tives. 12 Superhydrophobicity introduces an air gap between water molecules as well as on the membrane surface, which can a Institute of Environmental Engineering and Management, National Taipei University of Technology, No. 1, Sec. 3, Zhongxiao E. Rd, Taipei-10608, Taiwan. E-mail: f10919@ ntut.edu.tw b Faculty of Environment and Natural Resources, Dalat University, Vietnam Cite this: RSC Adv. , 2018, 8, 9640 Received 2nd February 2018 Accepted 28th February 2018 DOI: 10.1039/c8ra01043a rsc.li/rsc-advances 9640 | RSC Adv. , 2018, 8, 9640–9650 This journal is © The Royal Society of Chemistry 2018 RSC Advances PAPER Open Access Article. Published on 06 March 2018. Downloaded on 5/30/2019 8:27:38 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online View Journal | View Issue