Performance improvement of polyethersulfone ultrafiltration membrane containing variform inorganic nano-additives

Highlights

• The lamellar GO and hollow HNTs were successfully embedded into PES membranes.

• Both HNTs and GO could improve the wettability of the membrane.

• The HNTs endowed the membrane with better filtration performance than GO.

Abstract

Lamellar graphene oxide (GO) and hollow halloysite nanotubes (HNTs) have aroused increasing interest with their unique structure and performance, deriving a new research direction for materials science. In this work, polyethersulfone (PES) ultrafiltration (UF) composite membrane incorporated with lamellar GO and hollow HNTs were prepared via non-solvent induced phase separation (NIPS) process, respectively. The prepared membranes were characterized by using various techniques, including Fourier Transform Infrared (FTIR), Raman spectra, Scanning Electron Microscopy (SEM), Confocal Scanning Microscopy (CSM), etc. Remarkably, the pure water flux of composite membranes with 0.05 wt% GO and 0.01 wt% HNTs loading reached maximum values of 327 and 346 L m⁻² h⁻¹, respectively. Moreover, the permeability of PES/HNTs composite membranes were more stable than PES/GO composite membranes. The rejection of the prepared composite membranes for bovine serum albumin (BSA) almost reached 98.5%, while it exhibits quite different flux recovery ratio in the anti-fouling test, which indicates that the variform inorganic nano-additives could produce widely divergent influences on the performance of membrane.

Introduction

It is well known that shortage of fresh water resources has seriously threatened the surviving of human being and development of society on account of the growth of human population and the industrialization [1]. To alleviate this predicament, efficient and sustainable water treatment technologies are needed (such as decontamination, sterilization and desalination), using unconventional sources of water (such as, sea water, brackish groundwater and waste water) to keep up a sufficient water supply. However, these technologies are usually costly with high energy consumption, and require large amount of chemicals that may produce toxic byproduct. These issues need to be solved from the beginning instead of replaced by another set of modes.

Among various membrane separation techniques, ultrafiltration (UF) has the advantages of high efficiency and environmental protection with relatively simple operation process, which is widely used in water purification [2]. In order to obtain asymmetric UF membranes with high permeability, favorable selectivity and excellent anti-fouling performance, it's quite important to choose a suitable polymer as membrane material. Polyethersulfone (PES) could be regard as the most common UF membranes' materials, due to its excellent chemical resistant, high thermal stability and good mechanical behavior [[3], [4], [5]]. However, the poor anti-fouling property of the pure PES membrane limits its practical application in water treatment on account of its inferior hydrophilicity, which bring about low water flux, high energy consumption, etc [6]. Therefore, it is necessary to make some modification for the UF membrane via chemical modification or blending with additives for the purpose of improving its hydrophilicity and anti-fouling ability. Up to now, the preparation of UF composite membrane by adding inorganic material into PES matrix has attracted broad attention due to the simple operation and mild condition [[7], [8], [9]]. Many kinds of nano-materials have been brought into PES membrane to prepare high-performance composite membrane, such as carbon nanotube [10,11], SiO₂ [12,13], TiO₂ [6,14], Al₂O₃ [15], and so on. Generally, the performance of PES membrane could be improved by the addition of these nano-materials.

Especially, graphene oxide (GO) is regarded as an important derivative of graphene with a monatomic lamellar structure, which has been widely studied by researchers [[16], [17], [18]]. Compared with graphene, the surface and edge of GO are inlaid with a large number of hydroxyls, epoxy, carboxyl and other oxygen-containing functional groups, showing excellent hydrophilicity and providing a large number of active sites for further modification of polymeric membranes [19]. Zinadini et al. [3] prepared PES/GO mixed matrix membranes which exhibited wider finger-like pores with the highest mean pore radius. Moreover, the roughness and hydrophilicity of composite membrane exhibited an influence on its anti-fouling properties with GO addition [20]. Besides, the mechanical strength of PES/GO membrane could be enhanced [21].

Similarly, halloysite nanotubes (HNTs) are natural alumino-silicates clay with the molecular of Al₂Si₂O₅(OH)₄·2H₂O, which have been widely used in many industries such as paper, rubber, polymer, etc [22]. The HNTs own a special combination hollow nanotubular structure with an internal diameter of 20–30 nm and outer diameter of 30–70 nm, while the length varies between 0.2 and 2 μm [23]. HNTs could be regarded as physical additives to enhance the permeability and anti-fouling ability because of hydroxyl groups surface, unique crystal structure, and large surface area [[24], [25], [26]]. KeziaBuruga et al. [27] reported that polystyrene-halloysite nanotube (PS-HNT) membranes exhibited good thermal and mechanical properties along with great water permeation ability and rejection. Compared with other nano-additives, such as carbon nanotubes (CNTs), HNTs not only have nanotube structure but also possess of good hydrophilicity, and they are easier to be obtained in nature and much cheaper than CNTs. Consequently, HNTs have good potential in membrane separation fields as hydrophilic nano-additives.

Nevertheless, the research of composite membranes comprising uniformly dispersed inorganic nano-additives in polymeric matrix has been a hotspot for a long time. Among various nano-additives, GO and HNTs have been at the center of attention due to their unique structure and performance [28,29]. However, for these two nano-additives, the published articles were reported that only one type of nano-additives or modified nano-additives was incorporated into polymeric matrix to prepare membrane with almost the same characterization. And few previous works paid attention to the effects about different shape of nano-additives in the performance of PES UF composite membrane.

In this work, we prepared the PES UF composite membranes with introducing lamellar GO and hollow HNTs, respectively. The objective of this paper is to investigate the effect of nano-additives with different shapes on the membrane. The effects of nano-additive content on the structure, permeation ability, mechanical property of the membranes were discussed in detail. Finally, the anti-fouling properties of the prepared membranes were also evaluated.