Omniphobic membranes for direct contact membrane distillation: Effective deposition of zinc oxide nanoparticles

Highlights

• ZnO nanoparticles were effectively deposited on a hydrophilic glass fiber membrane using a CBD method.

• The omniphobic membranes had a particulate morphology and an extremely high surface fluorine concentration.

• The contact angles of the omniphobic membrane for water and ethanol were as high as 152.8° and 110.3°, respectively.

• In the DCMD tests, the initial water flux was maintained after the addition of 0.3 mM SDS to a 60 °C 1 M NaCl feed.

Abstract

An omniphobic membrane was fabricated for membrane distillation (MD) by effectively depositing ZnO nanoparticles on a hydrophilic glass fiber (GF) membrane using a chemical bath deposition method to create hierarchical re-entrant structures, followed by surface fluorination and the addition of a polymer coating to lower the surface energy of the membrane. The omniphobic membranes possessed a particulate membrane morphology and an extremely high fluorine concentration on the surface. The omniphobicity of the fabricated membrane was indicated by the contact angles for water and ethanol, which were as high as 152.8 ± 1.1° and 110.3 ± 1.9°, respectively. Compared to hydrophobic GF membranes without deposited ZnO nanoparticles, the omniphobic membrane revealed a higher wetting resistance to low surface tension feed solutions in direct membrane distillation (DCMD) experiments. The water flux was stably maintained in a 0.3 mM sodium dodecyl sulfate (SDS) solution (60 °C 1 M NaCl) with salt rejection rate higher than 99.99%. The results suggest that the omniphobic membrane exhibited not only superior wetting resistance to low surface tension liquids but also promise for desalinating low surface tension wastewaters.

Introduction

Membrane distillation (MD) is a thermally driven process that has been considered a promising alternative to conventional separation technologies, such as reverse osmosis (RO) and distillation [1], [2]. In a MD process in which a hydrophobic porous membrane acts as a barrier to create a liquid-vapor interface at the entrance of the membrane pores. The temperature difference across the membrane induces a vapor pressure difference that drives water molecules from the feed to the permeate in the form of vapor [3]. MD can be not only operated at mild pressure and temperature, but also partially powered by alternative energy sources, such as solar energy [4], waste heat [5] and geothermal energy [6].

MD has been widely applied in the desalination of high salinity waters to address the issues of water scarcity [7], [8], [9], [10], [11] and is a potential technology toward an ultimate goal of zero-liquid discharge [12], [13], [14], [15], [16], [17]. However, conventional hydrophobic membranes are limited to the treatment of relatively clean water sources that have minimal surface-active agents [18]. The low surface tension contaminants that exist in wastewater streams lead to potential wetting of the membrane pores. The wetted membrane pores undermine the function of the membrane as an effective barrier for creating a liquid-vapor interface for fluid streams, and the practical MD applications are thus restricted [19]. The failure to treat low surface tension wastewaters results in the loss of valuable clean water sources. Therefore, the development of robust membranes is imperative for extending membrane applications to the desalination of challenging wastewaters, especially high salinity wastewaters from major industries, such as the textile [20], chemical [21], shale gas [22] and petroleum industries [23].

Robust omniphobic surfaces that repel both water (hydrophobic) and oil (oleophobic) have attracted significant attention in a wide range of technological areas. Tuteja et al. demonstrated that re-entrant geometries and specific chemical compositions are critical factors in the design of omniphobic surfaces that display excellent wetting resistance to a number of liquids with low surface tension [24]. For a liquid drop that is placed on the re-entrant structures, the net traction on the liquid-vapor interface is directed upward, which supports the metastable Cassie state for the composite solid-liquid-air interface at thermodynamic equilibrium [24], [25]. In the past few decades, a variety of methods have been employed to create re-entrant structures on several substrate materials to impart surface omniphobicity with low surface tension [24], [26], [27], [28], [29]. Recently, re-entrant geometries have been successfully created on a variety of membrane substrates by the deposition of silica nanoparticles. The modified membranes exhibited omniphobic properties and were applicable to MD with a low surface tension feed [18], [30], [31], [32].

Chemical bath deposition (CBD), which is based on a controlled chemical reaction in solution, is particularly attractive, because it provides a simple, cost-effective and scalable chemical approach for depositing ceramic films [33]. Zinc oxide (ZnO) is well-known for its useful properties [34] and richest growth morphologies [35], [36], [37] for a wide variety of practical applications. It is one of the most-studied materials produced by CBD deposition from readily available raw materials. In most studies, a ZnO seed layer is deposited on the substrate prior to the CBD of the ZnO films [38], [39]. Kokotov and Hodes presented a simple reproducible CBD method for depositing uniform ZnO films with controllable morphologies in the absence of a ZnO seed layer. Instead, treatment with a potassium permanganate solution was used to form a Mn-(hydroxyl)oxide deposit that acted as an efficient seed layer on a variety of substrates [40]. This CBD method is not specific to particular substrates and can be easily applied to large surface area [41], [42]. Perry et al. and Dufour et al. further applied this CBD method to the deposition of ZnO nanoparticles on silicon substrates for the preparation of omniphobic surfaces [28], [29]. However, to the best of our knowledge, studies have yet to be reported that use the CBD method to prepare omniphobic microporous membranes.

In this study, we fabricated an omniphobic membrane for MD by depositing ZnO nanoparticles on a hydrophilic glass fiber (GF) membrane to create hierarchical re-entrant structures, followed by depositing surface fluorination and a polymer coating to lower the surface energy of the membrane. The fabricated omniphobic membrane was then compared to GF membranes without deposited nanoparticles in terms of their liquid repellency using contact angle tests with water and ethanol. Direct contact membrane distillation (DCMD) experiments were also conducted to compare the desalination performances of the membranes, and a commonly used surfactant, sodium dodecyl sulfate (SDS), was added to lower the surface tension of the feed solutions.