Preparation of polyethersulfone/carbon nanotube substrate for high-performance forward osmosis membrane

Highlights

• Forward osmosis (FO) membrane with MWCNT–PES nanocomposite substrate is fabricated.

• MWCNT can modify the structure characteristics of substrate.

• PES composite substrate containing MWCNT shows excellent FO performance.

• The mechanical property of substrate is enhanced by the MWCNT.

Abstract

Forward osmosis (FO) process has attracted increasing interest because of its potential applications for low-energy desalination. However, the internal concentration polarization (ICP) has been considered as one of the key issues that can significantly reduce the water flux across the FO membrane. In this paper, we report the preparation of polyethersulfone (PES)/multiwalled carbon nanotube (MWCNT) substrate for the formation of a high-performance FO membrane. Nanocomposite MWCNT/PES substrates were obtained by dispersing carboxylated MWCNTs within PES via solution blending, and subsequent phase inversion process; The FO membranes were then prepared by depositing a polyamide active layer in-situ on the MWCNT/PES substrate with a finger-like macrovoid structure. The influence of addition of MWCNTs on morphology and properties of substrates and final FO membranes was systematically investigated. The results show that the performance of the FO membranes with MWCNT/PES nanocomposite substrates is better than that of the commercial membrane. Furthermore, the tensile strength of the substrate with MWCNTs is also greater than that of the neat PES. This work indicates that the FO membranes prepared from MWCNT–PES substrates are promising for practical FO applications.

Introduction

Nowadays, unprecedented growth in demand for fresh water and energy is a huge challenge for sustainable development of both society and economy due to population bulge, rapid urbanization and climate change [1]. Forward osmosis (FO) membrane process featuring lower energy consumption, and lower fouling tendency [2] than reverse osmosis (RO) process is an emerging technology for desalination [3], [4], waste water treatment [4], [5], [6] and many other applications such as in pharmaceutical industry [7], [8], agriculture and power generation [9], [10]. FO may be a viable alternative to RO as a low-cost and more environmentally friendly desalination technology if it was very well integrated with various processes [11]. However, the lack of high-performance membrane is a bottleneck in the development of the FO process. Similar to RO membranes, asymmetric aromatic polyamide membranes consisting of an active layer, porous substrate layer and a fabric layer fabricated by Loeb–Sourirajan process have been widely investigated for FO. However, the porous substrate and fabric layer of the FO membrane usually give rise to a severe internal concentration polarization (ICP) and thus lower water flux in the FO process [12], [13], [14], [15], [16], [17], [18]. A good protocol has been proposed for the development of FO membranes: firstly, sub-layer-free membranes may be well suited for FO membranes [19]; secondly, the substrate layer with low structure parameter (S) may be designed to minimize ICP [20]. The S value may be calculated by

$$S=\frac{\mathit{t\tau }}{\epsilon }$$

where t, τ and ε are the membrane thickness, tortuosity and porosity, respectively. Seen from Eq. (1), a thinner, less tortuous and more porous substrate will have a lower permeation resistance, and a higher water permeability. Recently, some researchers have reported the fabrication of optimal microstructure substrate layer with high porosity and low tortuosity for FO membranes. Yip et al. [18] produced a substrate layer with a mix of finger-like and sponge-like microstructure, which could significantly improve water flux. Since ICP problem is mainly responsible for water flux decline, the fabrication of a good FO membrane requires the minimization of ICP in the support layer. A scaffold-like nanofiber substrate fabricated by electrospinning technique successfully overcame this obstacle, and this unique structure offers direct paths for salt and water diffusion [21].

Polyethersulfone (PES) has been widely used as membrane material, especially for the fabrication of FO membranes, due to its excellent thermal stability, mechanical properties, chemical resistance and wide pH tolerance [22], [23]. On the other hand, multi-walled carbon nanotubes (MWCNTs) have superior separation capability as well as excellent physical properties including high tensile moduli and strength, which can be used as potential fillers in the fabrication of nanocomposite membranes [24]. It was reported that a high loading of MWCNTs likely resulted in a pore network in the membrane, and the hollow nanochannels of MWCNTs and their interspaces could provide new transport channels for water [25]. Also, it was hypothesized that MWCNTs could improve water flow by disrupting polymer chain packing, and creating external nanoscale channels in the membrane and additional internal channels from opening nanotubes [25], [26]. Importantly, the rejection of contaminants using the membranes with MWCNT filler may be improved by forming a special porous substrate interface for polymerization of the active layer. In addition, the special structure of MWCNTs makes them particularly attractive for reinforcement of composite materials [27]. However, the uniform dispersion of CNTs in a polymer matrix has been identified as a critical issue that must be addressed in the fabrication of high-performance membranes [28].

Herein, we report our attempt for modifying the microstructure of PES substrate using MWCNTs as filler for the purpose for preparing FO membranes. The formation of such nanocomposites is expected to not only strengthen the substrate but also provide favorable microstructure for promoting the separation properties in the FO process. In particular PES or MWCNT/PES composite substrate membranes with different loadings of carboxylated MWCNTs were fabricated by solution blending and phase inversion method; m-phenylenediamine and 1,3,5-trimesoylchloride were used as the monomers for the in situ polycondensation reaction to form a thin aromatic polyamide selective layer on the substrate surface. The effects of MWCNTs on the microstructure and FO performance were systematically investigated.