Preparation of polymer-carbon nanotubes composite hydrogel and its application as forward osmosis draw agent

Highlights

• Synthesis MWCNT incorporated polymer hydrogel.

• A new nanocomposite polymer hydrogel is proposed as draw agent.

• Increasing the swelling ratio of polymer hydrogels by MWCNT incorporation.

• Nanocomposite polymer hydrogel shows higher FO water flux than pure polymer hydrogel.

Abstract

In this study, we functionalized multiwall carbon nanotube with polar groups including COOH and OH groups (F.MWCNT) and synthesized composite polymer hydrogel via dispersion in monomer solution and by using a simple poly (acrylic acid-co-maleic anhydride) (PAA-co-PMA) crosslinking reaction. Incorporation of the F.MWCNT phase into the PAA-co-PMA hydrogel network was verified using Fourier transformation infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) analysis. The physico-chemical properties of the composite polymer hydrogels were investigated and compered to pure hydrogel. In addition, the effect of F.MWCNT incorporation on polymer hydrogels performance as forward osmosis draw agent was studied. The results indicated that the addition of F.MWCNT (1.2 wt%) within the polymer hydrogel enhanced the water adsorbing capacity of the composite hydrogel as expected. Furthermore, the composite hydrogel (1.2 wt% of incorporated F.MWCNT) displayed a much higher water flux compared with the pure PAA-co-PMA polymer hydrogel during the FO test. The facile and efficient preparation of the F.MWCNT incorporated PAA-co-PMA composite hydrogel means it will have potential applications as a novel draw agent.

Introduction

In recent years, fresh water scarcity has become one of the major global issues due to the growing population [1], [2]. Which has in part been addressed by the seawater desalination, as well as, recycling wastewater. To date, various technologies have been developed for sea and saline water desalination, in which membrane-based reverse osmosis (RO) and thermal-based multistage flash (MSF) evaporation processes are mostly dominated [3]. But this current technology is energy-intensive processes due to the high solute concentration of seawater and limited energy efficiency of conventional thermal separation [4], [5]. Thus, developing environmentally, low cost and high-efficiency technologies or methods for water purification and seawater desalination are of strategic importance for meeting the all current and foreseeable demands of clean water [6].

Forward osmosis (FO) as a novel membrane based water treatment process has attracted much attention in recent years owing to its lower energy consumption, easy operation, higher rejection of contaminants, and much lower fouling tendency compared to reverse osmosis desalination [3], [7], [8]. In FO process the natural osmotic pressure gradient between two solutions with different solute concentration act as driving force. Spontaneously, water molecules of the feed solution with lower osmotic pressure can be moved to the draw solution with higher osmotic pressure [9], [10], [11]. One of the obstacles to the development of FO desalination is the limited choice of draw agent, of which, type and concentration influence the efficiency of the FO process [12]. Thus the selection of a suitable draw agent is highly important for a high-performance FO system [13]. In general, an ideal draw or osmotic agent should generate high enough osmotic pressure relative to the feed solution and should be easily and efficiently regenerated and recovered from diluted feed solution with minimized energy consumption. In addition, other criteria like low reverse solute flux, non-toxicity and capability for large-scale production should also be taken into consideration [14].

Hydrogels as three-dimensional hydrophilic polymer networks when brought into contact with water are capable of swelling and retaining significant amounts of water within its structure [15], [16]. Of various hydrogels, chemically cross-linked polymer hydrogels are the most widely used in applications. In particular, presence of large amount of ionic functional groups in polymer backbone of hydrogels, cause to an osmotic pressure difference between the interior of the hydrogel and the surrounding solution, thus water molecules can be drawn into the polymer chains of hydrogel network [17], [18]. In synthesis of polymer hydrogels, polymer with ionic functional groups, also hydrophilic and flexible characteristic is required, but this polymer generally lacks sufficient rigidity, strength, and mechanical properties. Whereas, carbon nanomaterials such as carbon nanotube CNT and graphene oxide (GO) because of their exceptional electrical conductivity are stiffer and stronger. So by incorporation such nanomaterials within the polymer hydrogels, nanocomposite hydrogels with improved characteristic were obtained.

Carbon nanotubes (CNTs) as graphene sheets rolled into tubes, based on the number of concentric walls are classified into two different categories: single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) which usually don’t have polar functional groups in their pristine form. Non-functionalized or unmodified CNTs need to be functionalized with polar functional groups (i.e., carboxyl (COOH) or hydroxyl (OH) groups) via different methods to make high stable dispersed solution or make them soluble in aqueous medium [19]. CNT based nanocomposite polymer hydrogels, have applications in a wide variety of fields, including high-performance composites, drug delivery, sensors, actuators and water treatment [20].

In this study, composite poly (acrylic acid-co-maleic anhydride) (PAA-co-PMA_MW) hydrogels were developed by incorporation COOH functionalized MWCNT as the reinforcing phase within the PAA-co-PMA polymer hydrogels. The concentration of F.MWCNT within the PAA-co-PMA polymer hydrogel matrix was varied to investigate the effects on resultant mechanical and other physicochemical properties. Results will help establish composite PAA-co-PMA_MW polymer hydrogels as a new draw agent for forward osmosis process. To our knowledge, this is the first time functionalized MWCNT-based PAA-co-PMA polymer hydrogels have been fabricated and tested for FO draw agent.