Enhancing TFC membrane permeability by incorporating single-layer MSN into polyamide rejection layer

Highlights

• A new pathway to incorporate MSN into polyamide rejection layer was provided.

• Vinyltrimethoxysilane works as silane coupling agent to graft the MSN on substrate surface.

• The prepared MSN-TFC membrane has excellent permeability and desalination properties.

Abstract

A new kind of thin film composite membrane (TFC) was prepared through a simple layer-by-layer assembly method. Vinyltrimethoxysilane (VTMS), a kind of silane coupling agent, was grafted on PVDF membrane surface through free radical polymerization method firstly. Then, mesoporous silica nanoparticles (MSN) were uniformly grafted on surface of VTMS/PVDF membrane through reacting with the methoxide group (Si-OCH₃) of VTMS. Under the optimized grafting conditions, the MSN grafted density can reach up to 8.73 mg/dm². Eventually, the MSN were embedded into polyamide rejection layer through an interfacial polymerization process. The prepared MSN-TFC membrane has excellent permeability and desalination properties. Furthermore, this research provides a new pathway to incorporate MSN into polyamide rejection layer.

Introduction

Reverse osmosis (RO) process, which just needs about one-tenth energy consumption than distillation and evaporation processes in purifying equivalent amount of wastewater, is gradually becoming a currently mainstream technique for both brackish and sea water desalination [1], [2], [3]. However, the development of RO technique has encountered bottleneck, due to the contradictory between membrane selectivity and permeability [4]. Therefore, how to significantly enhance the membrane permeability without decreasing its separating property has become the key point to break the bottleneck.

Recent years, lots of efforts have been payed to improve RO membrane permeability and avoid damage to the membrane selectivity. There are some studies tried to improve the membrane permeability through enhancing the membrane hydrophilicity by incorporating large amounts of hydrophilic additives into polyamide rejection layer. Yi Li et al. successfully improved the permeability of TFC RO membrane by adding hydrophilic carbon dots into polyamide rejection layer [5]. Lin Zhao et al. incorporated o-aminobenzoic acid–triethylamine, a hydrophilic additive, into the rejection layer of TFC RO membrane and successfully increased the membrane water flux [6]. However, these types of improvement approaches were too limited to significantly enhance the membrane permeability. Meanwhile, several researchers mimicked the structure of biological membrane to incorporate varied “water channels” into polyamide rejection layer to improve membrane permeability. Aquaporins as a kind of water channel proteins have been widely used to fabricate high-flux RO membranes. Yang Zhao et al. successfully introduced Aquaporin Z-containing proteoliposomes into polyamide rejection layer improving the membrane permeability by about 40% [7]. Whereas, the protein activity is difficult to remain for long, due to the high salinity and high pressure in practical applications. Carbon nanotubes as a kind of 1 D inorganic water channel material have also been extensively applied in the fabrication of high permeability RO membrane. Xiao-Hua Ma et al. successfully manufactured a TFC membrane containing carbon nanotubes in rejection layer by a special interfacial polymerization technology with the assistance of a high voltage electric field [8]. Nonetheless, the arrangement of carbon nanotubes in rejection layer is out-of-order. As a result, it was unable to make full use of the 1 D water channels inside the nanotubes. Therefore, developing inorganic 3 D water channel materials which sprawling in all directions is still very promising to enhance the RO membrane permeability in practical applications.

Mesoporous silica nanoparticles (MSN), as a kind of porosity material inside which the mesopore size is easy to adjusted as required, have plentifully molecular-based 3 D nanochannels [9]. Given the special characters of abundant 3 D nanochannels sprawling in all directions, tunable mesopore size, and high porosity, MSN have been widely applied in many fields, such as nanomaterial carrier [10], desalination [11] and medical delivery [12], [13]. Especially in desalination process, MSN have been introduced into polyamide rejection layer to fabricate high-flux membranes. Hui-Qing Wu et al. incorporated mesoporous silica nanoparticles, which had been modified by amino groups, into polyamide rejection layer and successfully improved the membrane permeability and anti-fouling ability [14]. Masoumeh Zargar et al. fabricated a kind of hollow mesoporous silica nanoparticles (HMSN) and successfully introduced the HMSN into polyamide rejection layer improving water flux by about 40% [15]. However, the agglomeration of MSN seriously restricted the exertion of water channels within MSN and then limited the further increase of membrane permeability.

To solve the problem of MSN agglomeration, we developed a simple layer-by-layer assembly method based on free radical graft polymerization to manufacture MSN-TFC RO membranes. Vinyltrimethoxysilane (VTMS), as a kind of silane coupling agent, contains organic radical which can react with organic materials and silane radical which can react with inorganic materials [16], [17], [18], [19]. Considering the high reaction activity and low price, VTMS was chosen as the linking agent to immobilize MSN on PVDF membrane surface uniformly. Then, the MSN were embedded within polyamide rejection layer within the interfacial polymerization process. Therefore, 3D water channels within MSN would be able to significantly enhance the permeability of TFC membrane.