Review
Two-dimensional graphitic carbon nitride-based membranes for filtration process: Progresses and challenges

https://doi.org/10.1016/j.cej.2021.130955Get rights and content

Highlights

  • The progresses on 2D g-C3N4 based membrane filtration are summarized.

  • G-C3N4 membrane fabrication, composition and applications are reviewed.

  • The challenges and perspectives of g-C3N4-based membrane are highlighted.

Abstract

Two-dimensional (2D) graphitic carbon nitride (g-C3N4) membranes with lamellar structure have gained extensive attention in the past decade due to their good separation performance. Studies carried out on the filtration performance in the fields of Microfiltration, Ultrafiltration, Nanofiltration, Reverse and Forward osmosis reveal that 2D g-C3N4-based membranes have a high selectivity towards all kinds of contaminants present in water such as organic components as well as divalent and monovalent ions while maintaining high permeability and that by their photocatalytic properties they are able to remedy the membrane fouling thus allowing long-term use over several cycles of filtration process. Therefore, the limitations encountered by conventional polymeric membranes have thus been corrected by the use of 2D nanomaterials-based membranes such as g-C3N4. This review mainly focuses on summarizing recent trends in research on g-C3N4 nanosheets-based membrane filtration, including g-C3N4 nanosheets and membrane fabrication, composition and application. We also attempt to provide future directions in which the field is likely to be realized. The key challenges and issues related to the synthesis and application of g-C3N4-based membranes in the filtration field are highlighted.

Introduction

In the context of resource management and sustainable development, the efficient, rapid and inexpensive separation of the various molecules and/or pollutants from aqueous, organic solutions and gas mixtures is essential.[1] Therefore, energy efficient and environmentally friendly separation methods are required to significantly reduce the cost of processing liquid and gaseous mixtures for purification, recovery and recycling. Membrane separation technologies are recognized as efficient and economically profitable separation techniques, they present a simple operating process with a high separation rate, they have a wide range of applications in the industrial environment such as the water purification by desalination of sea water.[2], [3], [4], [5], [6] Compared to traditional separation methods for water treatment, pressure-driven membrane separation processes offer multiple benefits, including energy efficiency, easy integration and scalability. Membrane filtration has particularly attracted a lot of attention due to its unique advantages in the purification of wastewater and the separation of organic molecules, multivalent ions and industrial residues. Depending on the size of the pores, the purification process can be classified as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO) and forward osmosis (FO). [5], [7], [8], [9], [10], [11] The filtration processes was commonly based on polymeric membranes such as Polyvinyl Alcohol (PVA), Polyether sulfone (PES), Polyvinylidene fluoride (PVDF), Polyvinyl chloride (PVC), Polypropylene (PP), Polyacrylonitrile (PAN), Polyimide (PI), Polyethylene (PE), Polyvinyl Alcohol (PVA) Cellulose Acetate (CA), etc. However, polymeric membranes have usually exhibited a tradeoff between permeability and selectivity and had a problem of fouling. Since the use of conventional polymer membranes for water treatment has had certain limitations, these limitations have led researchers to introduce nanotechnology with nanoporous membranes to increase the performance of the membrane separation. [11], [12] Among nanoporous membranes, 2D nanomaterials-based membranes have demonstrated exceptional properties in the field of membrane separation.[13], [14], [15], [16], [17] Graphene and its derivatives paved the way in the using of 2D nanomaterials-based membrane for separation. [18], [19], [20], [21], [22], [23], [24] Other 2D structures, such as MXenes, among others, have been object of recent investigations in the field of membrane separation. [25], [26], [27], [28], [29], [30] Similar to graphite, graphitic carbon nitride (g-C3N4) a sp2 hybridized material, have aroused much interest in recent years owing to is layered structure involving weak Van der Waals interactions between adjacent C-N layers. [31] g-C3N4 is a semiconductor photocatalyst with visible-light response. The band gap energy of g-C3N4 is 2.7 eV and its maximum adsorption wavelength is around 460 nm. The g-C3N4 also possesses many advantages, such as strong mechanical property, large surface area, non-toxic nature, and facile synthesis readily available precursor. [15], [31], [32], [33] Taking these advantages into account, g-C3N4 has been a revolutionary material in the construction of membranes with high separation performance and having photocatalytic properties allowing the decrease of membrane fouling. Since its synthesis in the 19th century by the work carried out by Berzelius and Liebig as well as the discovery reported by Wang et al. in 2009 as a metal-free conjugated semiconductor photocatalyst for hydrogen, various nanostructures of g-C3N4 were developed, namely 0D quantum dots, 1D nanotubes-nanofibers-nanowires-nanorods, 2D nanosheets and 3D microspheres. This has given researchers a wide range of choice of g-C3N4 based nanomaterials for the design and construction of composite membranes with superior separation performances. [32], [34], [35] The possibility of modulating the physico-chemical properties of g-C3N4 allows the adjustment of the functionalities of membranes based on the latter. Thus, g-C3N4 has become one of the main materials in membrane separation. More than two hundred articles on g-C3N4-based membranes have already been published to date, and it is from the year 2016 that the research aroused particular interest on the part of researchers because the publications have been multiplied due to the great fascination offered by this material. Fig. 1 traces the publications relating to g-C3N4-based membranes from 2011 to 2020. For the last 5 years, our research group has focused on 2D membranes [118], [121], [141] as well as 2D photocalytic membranes [139], [140], [142] for water filtration. On the basis of our own experiments, we therefore set out to study the filtration performance of 2D g-C3N4 membranes by clarifying the synthesis of the nanosheets as well as the preparation of membranes and the application in filtration processes. Thus, this review summarizes the trends on 2D g-C3N4 synthesis as well as that on g-C3N4-based photocatalytic membranes while highlighting the influence of membrane microstructure on separation performance and attempts to provide future directions in which the field is likely to come true.

Section snippets

Synthesis and properties of g-C3N4

Before talking about 2D g-C3N4-based membranes, the intrinsic synthesis and properties of g-C3N4-based materials will be briefly presented, as they strongly determine the quality of the membrane, the microstructures and the separation performance.

Progress on 2D g-C3N4-based membrane

The recent development of the 2D-enabled membrane architecture shows a distinctive pattern that differs from the development of the conventional polymeric membrane designs.[16] The rapid growth in interest and the large number of studies in recent years suggests that 2D g-C3N4-based membranes are becoming a family of high separation performance membranes. The low nanometric thickness, the high specific surface area as well as the high hydrophily of the nanosheets have stimulated the continuous

Challenges in 2D g-C3N4-based membranes for filtration

Though significant progresses have been achieved, several challenges are still to be faced, and some of the issues must be solved before g-C3N4-based photocatalytic membranes can be used in practical applications. Based on the studies investigated, we propose the following:

  • (1)

    Since polymeric substrates such as CA, PVDF, PTFE, etc and ceramic substrates are different in term of their structures and surface properties, their interaction with g-C3N4 to form lamellar membranes, mixed matrix membranes,

Conclusions and perspectives

The development of the transport and communication industry, the proliferation of chemical fertilizer factories, etc. have resulted in increased water pollution. Until then, filtration was an alternative to water pollutants, but the development of industries has made commercial polymeric membranes obsolete. However, membrane filtration is remaining the effective method for water treatment. Membranes based on 2D nanomaterials have made it possible to revitalize the field of membrane filtration

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the Hunan Provincial Science and Technology Plan Project, China (No. 2016TP1007) and Hunan Provincial Natural Science Foundation of China (No. 2020JJ4107). Kai Han acknowledges the support from Innovation-Driven Project of Central South University (No. 2020CX037).

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