Applications in Modern Water Purification

Water is vital for the survival of life on Earth.¹ Rapid population growth, fast industrialization, and accelerated climate change have endangered water security and aggravated global water scarcity.² Failure to ensure water sustainability can lead to drought-desertification, poor quality water, decreased forestation and agriculture, or increased chronic diseases.¹

Image Credit: Angel Soler Gollonet/Shutterstock.com

The limited freshwater sources and the ease of water pollution necessitate efficient treatment systems. Membranes are commonly employed as filters and quality control materials in water treatment systems. Among different types, graphene-based membranes have emerged as important functional materials in water treatment applications.¹

This article explores the properties of graphene membranes and their applicability in water purification systems.

What are Graphene Membranes?

Graphene has a distinct two-dimensional structure with rapid mass transmission channels, making it ideal for membrane-based applications such as water purification.²

Graphene-based membranes are highly functional due to graphene’s lattice structure. They exhibit enhanced anti-fouling and antimicrobial activities, as well as contamination resistance.¹

Nano-porous graphene membranes offer several advantages over conventional polyamide membranes in water treatment systems, including two to three times higher water flux and almost 100 % salt rejection. The above-average hydrophilicity of graphene oxide (GO) membranes also helps alleviate fouling and inhibits the adhesion of impurities. This significantly reduces energy consumption and costs.¹

Graphene membranes are explored for sustainable water treatment due to modest energy demand, pollutant adsorption, antibacterial and anti-fouling properties, rapid water transfer, and filtering potential.¹ Graphene-based materials also have high thermal conductivity, essential for solar-driven photothermal desalination processes.²

Various methods are employed to synthesize graphene membranes, such as vacuum filtration, phase inversion, pumping, hot press, rod-coating, dip-coating, and gravure printing.¹ High-quality membranes are also fabricated using chemical vapor deposition (CVD) and electrochemical exfoliation.²

Graphene can also be combined with other materials to enhance membrane performance. For example, graphene, carbon nanotubes (CNTs), and metal oxide composites exhibit high photocatalytic behavior when removing dyes and metals from water. These membranes are suitable for ultrafiltration and fouling recognition and can enhance surface pore structures, roughness, and hydrophilicity.¹

Graphene can be combined with a nano/micro-porous polymer substrate such as polyvinylidene fluoride (PVDF), nylon, or polycarbonate (PC) to enhance the membrane’s structural durability. Additionally, doping graphene membranes with nanoparticles improves their permeability, stability at high temperatures, separation capabilities, and selectivity.¹

Using Graphene Membranes in Water Purification

Ultra/micro-filtration and forward or reverse osmosis are prevalent methods in existing membrane-based water treatments to remove agrochemicals, pharmaceutical compounds, heavy metals, and other toxins. These methods generally operate under great pressure and utilize ultraviolet light. Graphene membranes can be employed in all such applications.¹

Compared to traditional materials, graphene membranes are more economical for water treatment, including desalination, filtration, purification, dye or metal removal, ultrafiltration, pollutant recognition, or separation.¹ Graphene nanocomposites can eliminate organic (dyes or phenol derivatives) and inorganic (salts or heavy metals) pollutants from water through adsorption.¹

Acoustic filtration systems comprise light-sensitive graphene membranes, CNTs, and a metal oxide photocatalyst. Such membranes prevent fouling and allow the use of solar energy instead of intense ultraviolet light and catalysts, significantly reducing costs. Membrane-based water treatment systems utilizing sound waves also exhibit improved water flow, contamination prevention, and pollutant collection.¹

Desalination can resolve the global water crisis by expanding water availability outside the hydrological cycle.

Until now, reverse osmosis (RO) using polyamide (PA) membranes has been preferred for large-scale desalination. However, it suffers from low stability due to less tolerance to chemicals like chlorine and high temperature and pressure conditions. As a solution, the incorporation of GO with PA membrane enhances the antimicrobial and antifouling characteristics of RO.²

Another important function of water treatment systems is to remove infectious pathogens carried by water. Traditionally, polymeric membranes are used to separate viruses from water depending on their pore size. However, an ultrafiltration membrane is usually required to eliminate a virus.

Graphene and graphene-derived materials have great potential for such applications due to graphene’s antibacterial activity, which can be elevated by doping with nanoparticles of materials such as Ag, TiO₂, and ZnO.³

Challenges and Limitations

The utilization of graphene membranes is mainly restricted to laboratories with rare instances of successful commercialization.² The primary hurdle is the synthesis of high-quality and large-area graphene membranes, which are still under development. Achieving high performance using prevalent fabrication methods such as chemical exfoliation is difficult.¹

Alternatively, CVD is promising for large-area graphene membrane synthesis. However, it requires high energy and harmful gas chemistries and mainly produces multilayer films.^1,2

Generating regular nanopores on graphene sheets is another challenge. Suitable techniques such as ion bombardment and plasma etching are costly and not yet developed. They can create non-uniform nanopores, increasing stress and decreasing the mechanical strength of graphene membranes.²

Tackling global water quality challenges needs sustainable, economical, highly functional, and durable water treatment methods. These methods should exhibit high fouling resistance, antimicrobial behavior, and membrane service life.¹ A complete techno-economic analysis is also important to estimate the total cost of a water treatment system.²

Overall, the design and manufacturing process need improvement for the commercial synthesis of graphene-based water treatment systems with cost-effective and efficient performance.²

Future Outlook

Novel methods are being explored to enhance the water treatment performance of graphene membranes.

For instance, a recent study in the Journal of Membrane Science proposed intercalation of GO laminates with Prussian blue (PB) nano-cubes to produce a photo-Fenton self-cleaning membrane. The GO/PB membrane, fabricated by a straightforward vacuum filtration method, exhibited outstanding antifouling performance and separation efficiency.⁴

Additionally, the GO/PB membrane was reusable as it retained over 95 % dye elimination efficiency and effective permeability after three separation cycles of photo-Fenton treatment. This exceptional performance of GO laminates was attributed to PB functioning as a nano-spacer and photo-Fenton catalytic nano-filler.

The proposed approach can be used to develop hybrid nano-laminated membranes for water filtration and textile wastewater treatment.⁴

Producing graphene membranes with a uniform multilayer structure and controlled pore size is challenging. A recent study in Membranes demonstrated the synthesis of ultrathin GO-based nanocomposite membranes via an easy and efficient method.

The researchers obtained MoO₂@GO and WO₃@GO membranes with regulated structure and interlayer distances. These membranes effectively separated inorganic contaminants due to enhanced stacking and electrostatic behavior, making them suitable as advanced desalination and water treatment membranes.⁵

In conclusion, developing innovative membrane structures with cost-efficient fabrication methods is essential to establish graphene membranes as a viable solution to clean water shortage.

More from AZoNano: Recent Advances in Twisted Bilayer Graphene

References and Further Reading

1. Memisoglu, G., Murugesan, RC., Zubia, J., Rozhin, AG. (2023). Graphene Nanocomposite Membranes: Fabrication and Water Treatment Applications. Membranes. doi.org/10.3390/membranes13020145
2. Dai, Y., et al. (2022). Graphene-Based Membranes for Water Desalination: A Literature Review and Content Analysis. Polymers. doi.org/10.3390/polym14194246
3. Lavorato, C., Fontananova, E. (2023). An Overview on Exploitation of Graphene-Based Membranes: From Water Treatment to Medical Industry, Including Recent Fighting against COVID-19. Microorganisms, 11(2), 310. doi.org/10.3390/microorganisms11020310
4. Pan, Y., et al. (2023). Graphene oxide laminates intercalated with Prussian blue nanocube as a photo-Fenton self-cleaning membrane for enhanced water purification. Journal of Membrane Science. doi.org/10.1016/j.memsci.2023.121465
5. Soomro, F., et al. (2023). Ultrathin Graphene Oxide-Based Nanocomposite Membranes for Water Purification. Membranes. doi.org/10.3390/membranes13010064

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