Regulating the interfacial polymerization process toward high-performance polyamide thin-film composite reverse osmosis and nanofiltration membranes: A review

Highlights

• Regulating the IP process could effectively improve the permselectivity of TFC RO and NF membranes.

• The representative methods of regulating the IP process were summarized.

• Insights into the IP process were discussed from the perspective of monomer diffusion and reaction.

• The respective mechanisms and advantages of the methods to regulate the IP process were discussed in detail.

• The challenge and direction for the future development of IP process to prepare high-performance TFC membranes were also proposed.

Abstract

Polyamide (PA) thin-film composite (TFC) reverse osmosis (RO) and nanofiltration (NF) membranes are the core elements for the membrane-based desalination technologies. Nowadays, the preparation of TFC membranes is dominated by the interfacial polymerization (IP) process, of which the reaction system is composed of the substrates and the two phases (aqueous and organic phases). For the TFC RO and NF membranes, the improved permselectivity, desirable for both industry and academia, can be realized by regulating the IP process, which is usually neither process-intensive nor time-consuming. In this review, the representative methods of regulating the IP process are outlined and divided into two categories: the reformation of substrates and the development of aqueous and organic phases. Additionally, this review analyzes the similarities of these methods and, based on our knowledge, discusses the possible reasons underlying the contradictory conclusions in the mutually independent studies. At the end of each category, the imperative discussion on the direction of further research is proposed. It is anticipated that this review can offer guidance for further regulating the IP process, thereby improving the permselectivity of TFC RO and NF membranes.

Introduction

Reverse osmosis (RO) and nanofiltration (NF), the representative membrane-based desalination technologies, are drawing worldwide consideration to alleviate the shortage of clean and potable water [[1], [2], [3], [4], [5], [6]]. Both RO membranes and NF membranes are pressure-driven and possess the relatively dense structure. Therefore, a certain similarity could be found in the selection of membrane materials for the fabrication of RO and NF membranes. Nowadays, the polyamide (PA) thin-film composite (TFC) membranes have become the most prevailing type of RO and NF membranes [[7], [8], [9], [10], [11]], of which the ultra-thin PA separation layer (tens to hundreds of nm in thickness) is assembled on the porous polymeric substrate (tens of micrometers in thickness) with the non-woven fibrous polyester backing below it [[12], [13], [14], [15], [16], [17]]. The porous substrates can compensate for the poor mechanical endurance of ultra-thin separation layer, which is necessary to withstand the high-pressure driving force during the RO and NF process [18]. The schematic diagram of multi-layered TFC RO and NF membranes is depicted in Fig. 1. As for the TFC membranes, the advantage that each layer can be tailored respectively puts them in a superior position to the asymmetric cellulose acetate (CA) desalination membranes [19]. Through decades of development, the performance of TFC membranes has been significantly improved in terms of permeability and selectivity, which are the two primary parameters to evaluate membrane performance [20]. The preparation of the TFC RO and NF membranes with high permselectivity can effectively improve the quality of produced water, reduce energy consumption, and further promote the application of membrane technology [3]. Therefore, the intention to develop the TFC membranes combining excellent permeability and selectivity has always been enthusiastic in RO and NF fields.

For the permselectivity of TFC RO and NF membranes, the separation layer is of paramount importance. In other words, an unqualified separation layer is even entitled to exercise the one-vote veto over the satisfied permselectivity of the composite membrane. Given this, to guarantee stable membrane performance and further improve membrane permselectivity, the methods by which the goal to precisely control the structure and properties of separation layer can be achieved should be developed. The separation layer is mainly prepared by the interfacial polymerization (IP) process [[21], [22], [23], [24]]. In the IP process, the porous substrates would be saturated with the aqueous solution containing the amine monomers for a certain amount of time. Then, the excess aqueous solution would be removed, followed by introducing the immiscible non-polar organic solvent containing the acyl chloride monomers onto the substrate surface. The encounter between the acyl chloride monomers and the amine monomers remaining in the substrates would trigger the nucleophilic substitution polymerization reaction. The schematic diagram of IP reaction is shown in Fig. 2. Given the immiscibility between the two solutions, a sharp organic-water interface would be formed. In view of the low solubility of acyl chloride monomers in the aqueous phase, during the reaction, the amine monomers would diffuse across the organic-water interface and enter the organic phase. The polymerization was restricted in a narrow region at the organic side of the organic-water interface [25,26]. At the beginning of IP reaction, the rapid diffusion and intense reaction would produce a relatively cavernous and immature PA barrier at the organic side of the organic-water interface. Due to the additional resistance caused by the PA barrier, the diffusion of amine monomers would be gradually inhibited [27,28]. As the inceptive PA layer became dense and integrated, the subsequent diffusion of amine monomers would be further slow-moving, eventually leading to the termination of the IP reaction [25,28]. Consequently, the TFC RO and NF membranes were successfully synthesized, of which the relatively thin separation layer was deposited on the substrate surface [26,29].

In order to tailor the structure and properties of IP-made separation layer, a lot of methods have been carried out. Taking the moment of the termination of the IP reaction as the basis for classification, these methods can be mainly categorized into two approaches. One approach is to modify the separation layer after the termination of the IP process, which can be further classified into post-treatment, surface modification, etc. However, these methods usually require additional operating units in the production process. The other approach is dedicated to making full use of the IP process to tailor the separation layer. A notable feature of the latter is that the formation and adjustment of separation layer are carried out simultaneously. By comparison, the latter demonstrated a favorable characteristic that is usually neither process-intensive nor time-consuming, making the adjustment of IP process highly compatible with the continuous membrane production process. Therefore, numerous researchers have carried out a lot of studies on regulating the IP process.

Despite some excellent review articles about TFC RO and NF membranes published in recent years, their interests were mainly focused on the functionalization of membrane surfaces [[30], [31], [32], [33], [34], [35], [36]], the introduction of new membrane materials [21,[37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51]], and the summary about improvement of membrane performance [52]. Some reviews considered the content about regulating the IP process as a minor part of the full text or only involved partial regulating methods of the IP process [[53], [54], [55], [56], [57], [58], [59], [60]]. Consequently, the relevant content in these reviews was often information-intensive but a bit brief. In addition, in these reviews, insights into the IP process from the perspective of monomer diffusion and reaction can be further discussed in-depth. Based on the above situation, this review intends to introduce and summarize the outstanding progress of the methods for regulating the IP process to improve the permselectivity of TFC RO and NF membranes. The reaction system of IP process mainly consists of two essential parts: the substrates and the two phases (aqueous and organic phases). Thus, in this paper, these methods are divided into two parts: the reformation of substrates and the development of aqueous and organic phases. It is well known that the structure and properties of substrates could largely determine the residual amount of the aqueous solution and the interaction with the amine monomers, thereby altering the IP process. For the development of aqueous and organic phases, several representative methods, such as adjusting the parameters of IP process, introducing the appropriate additives in aqueous or/and organic phases, and exploring the novel monomers, could effectively control the diffusion and reaction of monomers, thereby regulating the IP process.

By comparing the representative articles, the respective mechanisms and advantages of these methods to regulate the IP process are summarized. Further, the internal commonalities among these methods are analyzed in detail. According to our understandings, we also provide the underlying reasons for the seemingly contradictory conclusions of similar regulating methods in mutually independent studies. Additionally, the challenge and direction for the future development of IP process to prepare high-performance TFC RO and NF membranes are also discussed. It is believed that the content of this review provides significant insight into the full utilization of IP process and the exploitation of TFC RO and NF membranes with high permselectivity.