Fabrication of thin film composite polyamide membrane for water purification via inkjet printing of aqueous and solvent inks

Highlights

• Thin film PA membrane was synthesized via inkjet printing assisted IP process.

• Printable TMC ink was prepared with the combination of toluene and dodecane solvents.

• Multiple printing cycles of monomer inks enabled the effective synthesis of PA layer.

• A defect-free PA layer formed using minimal volume of MPD and TMC solutions.

• The optimal TFC-M2T3-15 membrane performed a high NaCl rejection of 95.33 %.

Abstract

Thin film composite (TFC) polyamide (PA) membranes are typically prepared using the interfacial polymerization (IP) method, which synthesizes the PA layer by soaking in aqueous diamine and organic acyl chloride solutions and allowing the occurrence of amidation. The conventional IP process has been used over several decades in the industrial production of TFC membranes for water purification and excess amounts of monomer solutions are used for PA thin film layer synthesis, generating a lot of chemical wastes. In this work, we first demonstrated the TFC PA membrane fabrication using simple inkjet printing of aqueous and organic inks to effectively prepare a PA layer with no chemical wastes. The organic acyl chloride ink was newly formulated by the combination of dodecane and toluene solvents to allow printability. Precise and uniform depositions of monomer inks via the inkjet printing approach were able to coat a dense and defect-free PA film onto the support membrane using only small volumes of chemicals. After multiple printing numbers of inks, the PA layer was optimized with two printing cycles (30.17 mL m⁻²) and three printing cycles (8.36 mL m⁻²) of diamine and triacyl chloride inks, respectively, followed by heat treatment. The inkjet-printed TFC-M2T3-15 membrane exhibited a high salt rejection (95.33 %) which showed higher performance compared to that of the conventional IP-prepared TFC membrane (89.64 %).

Introduction

Interfacial polymerization (IP) method has been considered as the most useful and efficient approach to prepare the polyamide (PA) selective layer of thin film composite (TFC) membranes for nanofiltration (NF) and reverse osmosis (RO) applications, thus it has been adapted for industrial application over several decades [1]. These commercial TFC membranes have been extensively used for water treatment, including seawater desalination. The IP process typically utilizes two different monomers of aromatic diamine, such as phenylenediamine (MPD) and piperazine (PIP), and aromatic acid chloride, such as trimesoyl chloride (TMC), dissolved in deionized water (DI) and non-polar organic solvent (i.e., n-hexane), respectively [2], [3], [4], [5]. The immiscible interface between the aqueous and organic phases subsequently enables the formation of PA with thin film layer on the surface of membrane supports [6], [7], [8].

Due to the critical role of PA selective layer of TFC membranes for desalinating saline waters (i.e., brine and seawater) and the influence of membrane performance on overall efficiency of the membrane process, extensive efforts have been exerted by researchers in developing the ideal structure of PA layer for high membrane performance. For instance, Livingston et al. first introduced the synthesis of ultra-thin with defect-free PA layer in a thickness of about 10 nm using porous cadmium hydroxide nano-strands as an interlayer between the PA layer and the support layer, which enabled to achieve superior desalination performance with high water permeance [9]. Inspired by the concept of interlayer construction for ultra-thin PA layer synthesis, some studies have also been achieved to fabricate high performance membranes using various nanomaterials or polymers, such as carbon nanotubes (CNTs) [10], [11], [12], graphene oxide (GO) [13], metal organic frameworks (MOF) [14], [15], polyelectrolyte [16], and polyvinyl alcohol (PVA) [6], for constructing interlayers. Although these recent research activities have been putting a lot of effort into developing high performance membrane materials; however, oftentimes these membranes still cannot be exploited for commercial opportunities. Most of the PA fabrication techniques often involve the use of expensive nanomaterials and chemicals, producing a large volume of toxic chemical wastes from complicated synthesis processes. Thus, the current membrane production technologies are not only costly and limited in terms of commercial viability, but also not environmentally friendly.

In recent years, interest in 2D and 3D printing technologies has grown for membrane fabrication and enhancement of solute/solvent separations [10], [17], [18], [19], [20]. Chowdhury et al. firstly demonstrated a successful fabrication of 3D printed PA membrane for desalination using an electrospray technique with direct depositions of MPD and TMC monomers to the membrane support with precisely controlled conditions (i.e. spray solutions droplet size and monomer concentrations) which enabled to form thinner and smoother PA layer compared to the PA layer formed via conventional IP process [21]. Likewise, 2D inkjet printing technology also has proven to be a versatile tool in precise and uniform depositions of various types of inks composed of different nanomaterials and polymers [22], [23], [24], [25], [26], [27], [28]. As such, 2D and 3D printing approaches may have great benefits to efficiently deposit or coat diverse materials with minimal volume usages and less production of chemical wastes, making these processes more eco-friendly and less costly [17], [29]. While both printing technologies have shown huge potential for a larger scale membrane preparation, the inkjet printing approach may provide more specialized precision with the right location and accurate volume with a high-speed deposition of ink droplets on a substrate, in comparison to the 3D electrospray which results in random deposition of solution droplets [21], [30]. Considering these major advantages of inkjet printing for material deposition, Badalov and Arnusch first demonstrated the fabrication of TFC PA membranes with printing of MPD aqueous ink on a polyethersulfone (PES) ultrafiltration (UF) support [29]. Multiple printing cycles of MPD ink were sufficiently able to coat the membrane support surface with MPD monomer, prior to the introduction of organic TMC solution similar to the conventional IP method. The resultant TFC membrane exhibited comparable separation performances with the TFC membrane fabricated via the conventional IP process. This study exposed one of the biggest limitations of the inkjet printing technology at the time, which was finding a suitable inkjet ink with high printability. In general, the printability of inkjet ink is determined by several properties, such as solution viscosity, surface tension, density, among others [22], [30], [31]. For the inkjet printing of IP monomers, especially, the use of n-hexane as the organic solvent for TMC was found to be difficult to perform using commercial drop-on-demand piezoelectric inkjet printers, which are designed for water-based inks [29], [30], [31]. This could be the main reason why the previous study only performed inkjet printing of a water-based MPD ink to the PES support membrane [29].

In this study, we report the first attempt in the formation of the PA layer via inkjet printing of both MPD and TMC inks for TFC membrane fabrication. A commercial home-office Deskjet Hewlett-Packard (HP) printer was used for both water-based and organic solvent inks. To formulate the printable TMC monomer ink, a solvent system containing two different solvents (dodecane and toluene) was mixed at a suitable ratio prior to printing of TMC. Since typical TFC membrane supports, such as PES or polysulfone (PSf), can easily be dissolved in toluene, toluene- and dodecane-insoluble polyvinylidene fluoride (PVDF) was used as the membrane support. After performing multiple printing cycles of MPD and TMC inks, an optimal printing condition was found in which the membrane performance such as salt selectivity and water flux is comparable with the TFC membrane fabricated by the conventional IP process. Changes in PA layer properties were further characterized and the experimental results proved that a uniform TFC PA membrane could efficiently be synthesized with minimal use of MPD and TMC monomers under a controlled environment of solution coatings by inkjet printing technology. From this investigation, we believe that simple inkjet printing technology could play a key role in improving the TFC membrane manufacturing process by saving production costs with a simpler process, minimal use of materials, and less waste production.