Development of composite membranes with irregular rod-like structure via atom transfer radical polymerization for efficient oil-water emulsion separation

Abstract

Development of superhydrophilic, stable and cost-effective composite membranes for efficient oil-water emulsion separation is highly desirable. Herein, an irregular rod-like composite membrane was prepared through 3-aminopropyltriethoxysilane (APTES) modification, followed by acrylamide polymerization with atomic transfer radical polymerization (ATRP). The as-prepared membrane exhibits superhydrophilicity/underwater superoleophobicity due to its irregular rod-like structure and pores-induced capillary actions. The composite membrane has demonstrated sufficient stability in acidic, alkaline and salty environments due to the polymerization of acrylamide. Moreover, the as-prepared composite membrane has effectively separated various oil-water emulsions and demonstrated high permeation and superior flux recovery. The present work demonstrates that the ATRP-assisted composite membrane is a promising material in a wide range of applications, such as industrial wastewater recovery and drinking water treatment.

Introduction

In recent decades, water pollution has become a worldwide problem due to ever-increasing oil spill accidents, urban sewage and industrial wastewater, which seriously threatens the ecosystem and human health [1], [2], [3], [4]. The conventional technologies, such as oil skimming, in-situ burning and air flotation, are not capable of efficiently separating oil-water emulsion because of quite a small size (<20 µm) of oil droplet [5], [6]. The membrane separation technology (MST) has garnered significant academic and industrial attention as an attractive alternative due to its distinct advantages, such as easy operation process, space saving and low energy consumption [7], [8], [9]. The poly(vinylidene fluoride) (PVDF) membranes, polysulphone (PES) membranes and cellulose membranes are being widely used for wastewater treatment [10], [11], [12]. Among them, PVDF is considered as an ideal material for complex separation systems due to its outstanding chemical stability, thermal stability and inoxidizability. For instance, Zhang et al. have prepared superhydrophobic and superoleophilic PVDF membranes by using ammonia solution induced a phase-inversion process and utilized for oil-water emulsion separation [13]. However, the as-prepared superhydrophobic PVDF membranes were easily fouled by oil droplets, suffered from membrane pore blockage and reduced permeation, which dramatically limit the long-term application of filtration membranes. Therefore, hydrophilic modifications of polymer membranes are carried out to enhance the antifouling performance and obtain desirable chemical strength for oil-water separation applications.

Graphene oxide (GO) is a potential filler material to form hybrid membranes with polymer materials for liquid separation processes due to its unique 2D structure and high specific surface area [14], [15], [16]. Some studies have demonstrated that the presence of reactive functional groups, such as carboxyl, hydroxyl and epoxide groups, on GO surface provide secondary reaction platform for further membrane modification and improve the antifouling performance of hybrid membranes. For instance, Mahlangu et al. have reported a PES/GO hybrid membrane and demonstrated enhanced hydrophilicity and higher water flux compared to pure PES membrane [17]. Sun et al. have prepared GO/SiO₂ membrane for highly efficient separation of oil-in-water emulsion [18]. In 2009, superhydrophilic/underwater superoleophobic materials were first reported by Jiang et al. [19] and had garnered significant research attention for oil-water separation [20], [21], [22], [23].

Superhydrophilic/underwater superoleophobic materials are mostly mixtures of polymers and nanoparticles with micro nano-scale structure. For instance, Shi et al. have prepared PVDF/TiO₂ nanocomposite membranes and demonstrated high oil-water separation efficiency [24]. Yang et al. have reported superhydrophilic composite membrane by decorating multiwall carbon nanotubes (MWCNTs) on PVDF surface for the separation of oil-water emulsions [25]. Despite the excellent separation performance of these materials, the weakly attached surface nanoparticles tend to fall off from the membrane surface during operation. Atomic transfer radical polymerization (ATRP) is considered as a stable and industry-viable membrane modification technology [26], [27], [28], [29], which offers excellent control over chemical composition, chain length and graft density. Therefore, ATRP is an effective approach for the surface modification of membranes to achieve superhydrophilic/underwater superoleophobic membrane materials.

Herein, we aimed to enhance the oil-water emulsion separation performance by using an irregular rod-like composite membrane. The GO offers enhanced hydrophilicity and feasibility of secondary reaction platform, whereas the PVDF provides excellent support due to its chemical and thermal stability. The composite membrane was fabricated by grafting polyacrylamide onto the APTES-modified PVDF membranes. The grafted polymers mitigated the problem of weak interactions between membrane and surface modifiers. Most importantly, the superhydrophilicity/underwater superoleophobicity of as-prepared composite membrane originated from the –NH₂ groups of polyacrylamide and the formation of irregular rod-like structure. Thus, a stable composite membrane with enhanced antifouling properties has been demonstrated, which presents a great potential for long-term application in oil-water emulsion separation.