Emerging graphitic carbon nitride-based membranes for water purification

Highlights

• Recent advances of g-C₃N₄-based membranes for water purification are reviewed.

• Regulation strategies of g-C₃N₄ molecules for membrane preparation are discussed.

• Fabrication and applications of the state-of-the-art membranes are summarized.

• g-C₃N₄ with multifunctional properties is extremely potential in membrane development.

• Challenges and prospects of g-C₃N₄-based membranes for water treatment are proposed.

Abstract

Membrane separation is a promising technology that can effectively remove various existing contaminants from water with low energy consumption and small carbon footprint. The critical issue of membrane technology development is to obtain a low-cost, stable, tunable and multifunctional material for membrane fabrication. Graphitic carbon nitride (g-C₃N₄) has emerged as a promising membrane material, owing to the unique structure characteristics and outstanding catalytic activity. This review paper outlined the advanced material strategies used to regulate the molecule structure of g-C₃N₄ for membrane separation. The presentative progresses on the applications of g-C₃N₄-based membranes for water purification have been elaborated. Essentially, we highlighted the innovation integration of physical separation, catalysis and energy conversion during water purification, which was of great importance for the sustainability of water treatment techniques. Finally, the continuing challenges of g-C₃N₄-based membranes and the possible breakthrough directions in the future research was prospected.

Introduction

With the population explosion and fast development of industrial production, water crisis has emerged as one of the biggest global challenges facing us the next decades (Castro-Muñoz, 2020). The discharge of industrial wastewater, brings about large quantities of heavy metals and synthetic pollutants, leading to the continuous deterioration of water quality. In order to deter the environmental risk of contaminated water, various conventional and advanced treatment methods have been applied for water purification (Hayat et al., 2017; Jiao et al., 2017; Pintor et al., 2016; Zheng et al., 2015; Zhu et al., 2018). Among these approaches, membrane-based technologies have been considered as one of the most promising ways, owing to the advantages of versatile, efficient, economical, low energy intensive, and easy for operation (Goh and Ismail, 2015; Koros and Zhang, 2017). Most importantly, besides water treatment, membrane processes provide a sustainable way to augment the water supply by combining with the technologies of new energy generation. So far, membrane filtration systems, such as pressure driven seawater and brackish water reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), microfiltration (MF), forward osmosis (FO) and pressure retarded osmosis (PRO), have been widely employed in drinking water production, desalination, water reuse, and wastewater treatment (Ali et al., 2020; Anand et al., 2018; Meng et al., 2020; Qasim et al., 2019; Song et al., 2019; Zhao et al., 2019). Emerging electrically driven membrane processes represented by electrodialysis and electrophoresis further promote the application of membrane technology in combating water issues (Zhu and Jassby, 2019).

Essentially, the performance of water treatment membranes is strongly related to the physicochemical characteristics of membrane materials. Therefore, the core task of membrane technology is exploring appropriate materials for membrane fabrication (Wei et al., 2018). By far, various of polymeric and ceramic materials have been used to fabricate water treatment membranes. Because of the rich resources, low price and tailorable structures, macromolecule polymers predominate raw materials for membrane construction. After all, polymer membranes are currently the most mature commercial system in the forefront of desalination and wastewater treatment (Hendrik, 2012; Samaei, Mohsen et al., 2018; Zakrzewska-Trznadel, 2013). Compared to organic polymers, inorganic ceramic materials possess obvious advantages, such as good chemical stability, mechanical strength and fouling resistance. Thus, ceramic membranes are more suitable for challenging water purification processes such as oil-water separation, pharmaceutical wastewater and liquid radioactive waste treatment (Chen et al., 2020b). Despite great progresses have been achieved, either polymeric or ceramic materials are confronted with inherent shortages for industrial applications. For example, organic polymeric materials have deficient resistance to heat and long-term ageing, while inorganic materials suffer from poor corrosion resistance and high production cost. This has brought about technical dilemmas of membranes for practical water purification, such as the unavoidable membrane fouling and the trade-off between separation efficiency and water permeability (Goh and Ismail, 2017). Currently, many efforts are being pursued in alleviating the above difficulties, such as the construction of thin film nanocomposite (TFN) membranes and mixed matrix membranes (MMMs) by combining the unique properties of polymer substrates and inorganic fillers to enhance the sustainable application of polymer membranes (Kumar et al., 2020; Zhao et al., 2020), the preparation of low-cost ceramic membranes with cheap clay and organic pore-making agents to promote the prevalence of ceramic membranes (Hamdi et al., 2016; Hubadillah et al., 2017). Nevertheless, it is still a huge challenge to obtain a kind of membrane with satisfactory cost, high selectivity, high permeability, superior stability and antifouling ability.

Advances in nanotechnology brings about new opportunities for the development of membrane science and technology. In recent years, numerous nanostructured materials have been explored for membrane synthesis, such as gold nanocrystal (He et al., 2015), carbon nanotubes (CNTs) (Li et al., 2019b), graphene derivatives (Anand et al., 2018; (Wei et al., 2018)), MXenes (Al-Hamadani et al., 2020), metal organic frameworks (MOFs) (Wang et al., 2019a) and zeolites (Dong et al., 2016). The multifunction of nanomaterials endows as-developed membranes modified by these nanomaterials with improved surface hydrophilicity, unique surface charge as well as anti-scaling and antibacterial properties (Liu et al., 2019a). With earth-abundant and environmental-friendly components, carbon-based materials, shows remarkable advantages over other types of nanomaterials for high separation efficiency via different permeation mechanisms (Anand et al., 2018). Taken the mostly focused graphene as an example, it has been reported that water molecules could flow through the two dimensional (2D) channels with nearly zero friction (Joshi et al., 2014; Nair et al., 2012). However, the high-cost of materials, the technological challenge in large-scale membranes manufacturing, as well as the limited structure stability have become the main significant barriers to slow down the adoption of graphene-based membranes (Cheng et al., 2020; Cohen-Tanugi and Grossman, 2014; Goh et al., 2016). Therefore, new type of carbonaceous nanomaterials are highly desirable for the development of high-performance water treatment membranes.

Due to the facile and inexpensive synthesis procedures, outstanding physicochemical stability and unique catalytic properties, polymeric g-C₃N₄ with a graphene-like structure is attracting more and more attention in environmental remediation and energy conversion (Kessler et al., 2017; Liu et al., 2019b). g-C₃N₄ is composed of 2D layered structure and the neighboring layers are held together by weak van der Waals interactions. Triazine (C₃N₃) and tri-s-triazine (C₆N₇) are the two fundamental tectonic units for establishing g-C₃N₄ (Fig. 1) (Ong et al., 2016; (Wang et al., 2012b)), in which tri-s-triazine-based g-C₃N₄ is deemed as the most stable g-C₃N₄ form with regularly distributed triangular nanopores (3.11 Å) throughout the entire laminar structure. Both the laminar structure and triangular nanopores contribute to the fast permeation of small molecules (H₂O, H₂, and so on) and the effective retention of larger molecules (Tian et al., 2016; Zou et al., 2019). Furthermore, g-C₃N₄ generally possesses a defect-rich structure that is generated by thermal condensation. These defects with the size of 3.1~3.4 Å can shorten the transmission path and endow g-C₃N₄ to be more water permeable (Lotsch et al., 2007). In contrast to the conjugated double bonds in the tri-s-triazine units, the weaker single bond of tertiary amino (N-(C)₃) makes it easy to eliminate the tri-s-triazine units in g-C₃N₄, so that the nanostructure of g-C₃N₄ and transport path of different molecules can be further optimized (Wang et al., 2017c). Moreover, with the combination of inorganic and organic characteristics, g-C₃N₄ is wildly considered as an ideal material for photocatalytic membrane preparation. The relatively narrow bandgap (2.7 eV) and maximum absorption wavelength at about 460 nm fully improve the utilization of g-C₃N₄ for visible light. Its capability of light energy conversion makes it possible to in-situ degrade retained contaminants or generate new energy. Undoubtedly, g-C₃N₄ can provide an opportunity for the construction of next-generation water treatment membranes with high separation efficiency and self-cleaning ability.

Although the applications of g-C₃N₄ for environment catalysis have been well summarized by several groups, comprehensive assess of the latest progress of g-C₃N₄-based water purification membranes is rare. In this critical review, we begin with the fabrication and modification methods of g-C₃N₄-based self-supporting film and composite membranes. We emphasized the material strategies used to regulate the structure of bulk g-C₃N₄ for membrane fabrication. The applications of g-C₃N₄-based membranes in typical water purification processes have been elaborated. Finally, some concluding remarks and perspective on the future development of g-C₃N₄-based membranes are discussed.