Elsevier

Journal of Membrane Science

Volume 505, 1 May 2016, Pages 61-69
Journal of Membrane Science

Amphiphobic PVDF composite membranes for anti-fouling direct contact membrane distillation

https://doi.org/10.1016/j.memsci.2015.12.042Get rights and content

Highlights

  • The amphiphobic PVDF composite membrane has been developed via a surface modification.

  • The membrane has superhydrophilicity/oleophilicity.

  • The membrane has good permeation flux rates and high tolerance to various organic foulants.

Abstract

This study aimed to develop an effective method to fabricate the amphiphobic polyvinylidene fluoride (PVDF) composite membranes for membrane distillation (MD) with excellent tolerance to various organic foulants. A facial surface modification method was explored to obtain amphiphobic membranes with mechanical and thermal robustness by dynamically forming perfluorooctyl trichlorosilane (PFTS) and coating SiO2 nanoparticles onto the membrane surface. A variety of techniques such as environmental scanning electron microscopy (ESEM), Fourier transform infrared spectrometry (FTIR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), liquid entry pressure (LEP) measurement and contact angle goniometry were applied to examine the effects of surface modification on surface chemistry, morphology and wettability of the derived membranes. The surface modification conferred the modified membrane superhydrophobicity and oleophobicity, stemmed from micro-fluorinated SiO2 particles covering the membrane surface. The anti-fouling property of the pristine and modified membranes were examined in a direct contact membrane distillation (DCMD) process using sodium chloride solution containing three model foulants (hydrophobic, hydrophilic or amphiprotic). The dynamically formed SiO2-PFTS/PVDF-2 membranes exhibited good thermal and mechanical resistance for DCMD operation. DCMD test showed that the surface modification did not sacrifice the permeate flux and the salt rejection. By adding the organic foulants, the pristine membrane displayed severe permeate flux decay and salt penetration. In contrast, the modified membrane presented a stable permeate flux and high salt rejection with the presence of three foulants respectively. The anti-fouling and anti-wetting properties of the modified membrane could be attributed to the enhanced amphiphobicity of membrane surface.

Introduction

Membrane distillation (MD) is a promising thermally driven desalination technology still in its embryonic stage for future industrial and commercial development [1], [2]. In this process, water is purified using a hydrophobic membrane, which is permeable to water vapor but repels liquid water. In desalination via MD, as hot saline feed flows over the membrane, water vaporizes and crosses through the membrane pores by the water vapor pressure difference derived from temperature difference of feed/permeate sides [3]. Despite many attractive advantages, MD has not been widely applied due to the challenges still waiting for us to overcome. Now the research enthusiasm or efforts have been placed in two areas. One is the process design and modeling to optimize the operating conditions (mass/heat transfer rates, temperature gradient, flow distribution) with the parameters from the membranes (thickness, length, or module configurations) to maximize the mass transfer and energy efficiency [4], [5], [6]. The other area is the membrane material development. A basic requirement for MD is that the membrane material should be intrinsically hydrophobic to prevent pore wetting. Therefore, the popular hydrophobic membrane materials such as polytetrafluoroethylene (PTFE), polyvinylidenefluoride (PVDF) and polypropylene (PP) are widely used in MD [7].

In practice, sustaining a stable flux and controlling the heat and mass transfer throughout the membrane module is the main obstacle to commercial implementation of the MD process. Sustained operations are compromised by pore wetting, various membrane fouling (scaling, biofilm, particulate and colloidal fouling), temperature and concentration polarization at the membrane interface, and heat loss via conduction through the membrane [8]. In particular, membrane fouling reduces the permeate flux and process efficiency by depositing a foulant layer on the membrane surface and plugging the pores. Under extreme cases, foulant can block feed channels, causing high feed pressure drops, reflected by the increase of liquid entry pressure (LEP) exceeded in parts of the module [8]. In general, most of wastewater contained substances causing membrane pore wetting or fouling such as humic acid, biomass, oily substances and surfactants. Humic acid [9] and oily substances [10] can easily adsorb on hydrophobic membrane due to their high affinity to hydrophobic surface, while surfactants strongly wet membranes and facilitate membrane fouling. Therefore, membrane fouling is a big concern for a highly hydrophobic membrane surface when in contact with an aqueous media containing hydrophobic species [11].

The most common method to improve membrane anti-fouling properties is membrane surface modification. Shifting the hydrophobicity toward super hydrophobicity helps to introduce an air gap between liquid droplet and the surface [12]. This air gap provides an opportunity to increase the allowable pore sizes prior to the pore wetting occurrence, consequently ensuring higher mass flux [13]. Recent studies mainly focus on development of superhydrophobic membranes with improved anti-fouling resistance [14], [15], [16], [17], [18]. For example, after CF4 plasma treatment, hydrophilic asymmetric PES membranes turns to hydrophobic and the water contact angle increases to 120°. The long-term stability test shows a stable water flux and 100% sodium chloride rejection in DCMD [14]. Alternatively, by depositing fluorosilanized TiO2 nanoparticles on the surface of microporous PVDF membranes, superhydrophobic PVDF membrane displays a hierarchical structure with multilevel roughness. The derived membrane displays superhydrophobicity and desirable anti-fouling properties against sodium chloride and humic acid solution [15]. The anti-biofouling property of superhydrophobic coating for marine application is also studied by Zhang et al. [16]. Interestingly, no microorganism traces are observed in the first week after the immersion of modified substrate. However, the superhydrophobic coating shows limited term of antifouling performance in the complicated real sea water environment. Privett et al. synthesizes a superhydrophobic xerogel coated with a mixture of nanostructured fluorinated silica colloids, fluoroalkoxysilane, and a backbone silane. The derived membranes exhibit significant effect in reducing the bacterial adhesion [17]. Meng et al. also investigates the fouling behavior of superhydrophobic PVDF-based composite membranes [18]. Fouling experiments using humic acid and salt, indicate that less salt deposition would occur inside the pores; therefore the extent of partial wetting is significantly reduced [18].

On the other hand, shifting the hydrophobicity toward superhydrophilicity also helps to increase the anti-fouling resistance. A highly hydrophilic membrane surface can be developed via plasma induced grafting of polyethylene glycol (PEG) and subsequent TiO2 particles deposition onto the PVDF membrane surface [10]. The hydrophilic-modified membrane is tolerant to salty feed solution containing mineral oil. A similar composite membrane has been synthesized by attaching a thin layer of agarose hydrogel on the surface of a hydrophobic Teflon membrane, achieving good anti-wetting properties against surfactant [19].

For the feed containing hydrophobic foulants such as oils, amphiphobic modification can be more effective to prevent the contact between oil droplet and membrane surface. Recently, superamphiphobic (both superhydrophobic and near-superoleophobic) surfaces, displaying contact angles larger than 150° and contact angle hysteresis less than 5° with water and oils, have attracted extensive applications including self-cleaning, anticorrosion, and improved blood compatibility [20], [21], [22], [23]. A variety of techniques is used to fabricate superamphiphobic surfaces, such as plasma treatment, lithography, solgel technology, nanoparticle deposition on substrates, fluoroalkylsilane coating and phase separation of a multi-component mixture. The superamphiphobic surfaces can be tailored by reducing the surface free energy with the functionalization of low surface energy materials (e.g. fluorosilanes) and by increasing the surface roughness [24]. Alternatively, attempts have been made to generate a hierarchical nanostructure surface morphology with multilevel surface roughness in order to modulate surface wettability toward extremes [25], [26]. However, the amphiphobic membrane for DCMD process is rarely reported in literature. The fouling behavior and mechanism of different organic foulants such as oily substances and surfactants in modified amphiphobic membrane have not been explored. In general, the instability of the surface coating layer on the membrane surface is caused by its relatively weak interaction. It is still a great challenge to fabricate a stable and amphiphobic membrane with a straightforward and cost-effective method.

In this study, we explore an effective method to fabricate amphiphobic membranes with anti-fouling and anti-wetting properties for membrane distillation. For this purpose, porous PVDF microfiltration membrane is surface-modified by dynamically forming of 1H,1H,2H,2H-perfluorooctyl trichlorosilane (PFTS) and fluorinated SiO2 nanoparticle suspensions (NPs) via different methods (Scheme 1). The surface chemistry, morphology and integrity of the modified membranes are investigated. The impacts of surface modification on membrane performance in DCMD process is examined using sodium chloride, kerosene, surfactant and humic acid as the model foulants. It is expected that this study not only provides an effective method to fabricate anti-wetting and anti-fouling amphiphobic membranes for the MD application but also reveals the anti-fouling mechanism for different foulants in MD process.

Section snippets

Materials

1H,1H,2H,2H-perfluorooctyl trichlorosilane [CF3(CF2)5(CH2)5SiCl3, PFTS] was obtained from Bailingwei Co. Ltd, and used as received. SiO2 NPs (~30 nm) were obtained from Maikun Chemical Co. Ltd. PVDF flat sheet microporous membrane (0.45 μm) was obtained from Ande Membrane Separation Technology & Engineering Co. Ltd. Cyclohexane and ethanol were purchased from Sinopharm Chemical Reagents and used without further purification. Sodium chloride (NaCl, synthesis grade) was supplied by Beijing Yili

Assessment of membrane amphiphobicity

The surface wettability is characterized by the static contact angle using water and diiodomethane droplets. Fig. S3 shows the typical contact angle profiles of water (a–d), diiodomethane (e–h) for the prepared different membrane samples. The value was calculated by averaging the five measurements at different locations and summarized in Table S1. The water contact angle (WCA) and diiodomethane contact angle (DCA) for the pristine PVDF membrane is 124.2° and 87.9°, respectively (Fig. 1a and b).

Conclusions

An amphiphobic PVDF composite membrane was successfully prepared by dynamically functionalizing PFTS and coating the modified SiO2 nanoparticles onto the membrane surface. The modified membrane displays large water and diiodomethane contact angles of 167.3° and 140.9°, respectively, stemmed from micro-fluorinated SiO2 particles covering on the membrane surface. The dynamically formed SiO2-PFTS/PVDF-2 membrane also exhibits good thermal and mechanical resistance for DCMD operation. The LEPw

Acknowledgments

Authors thank the National Natural Science Foundation of China for financial support (21176008). Profs. Zhu and Liu also acknowledge the support from the Australian Research Council Future Fellowship.

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