Over 2 billion people are facing a water crisis, and water-related hygiene and sanitation problems. Clean water is the base for economic development of any society. Water treatment produces clean water. Water treatment includes sedimentation, filtration, aeration, solar treatment, chlorination, and sterilization by boiling. A wide range of treatment processes have evolved to suit the different local conditions. Water treatment must produce ‘clean water’, which is having all contaminants safely below the maximum permissible limits (MPL). With oft revised MPL, new materials are explored to address the presence of contaminants such as microbes, heavy metal ions, oils, pesticides, disinfection byproduct precursors, and innumerable chemicals.
Carbon has been in use for water treatments with evidence found dating back to the Harappan civilization. The latest technologies involve ultrafiltration (UF), reverse osmosis (RO), and desalination methods. Carbon is also an essential component in all the water treatments available today in the form of graphene.
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With graphene, a single atomic layer of carbon, a clean water solution has emerged. Cutting edge water filtration systems are developed using graphene to remove highly hazardous contaminants that are otherwise not efficiently removed by previous technologies. The properties that make graphene unique to water treatments are large surface area, little or no cytotoxicity, large delocalized ∏-electrons and tunable chemical properties. Importantly, graphene can be easily reused for the purpose with minimal chemical alterations.
Graphene is well-known for its adsorption capabilities and photo-catalysis of water pollutants. By etching pores and creating nanoscale apertures in the graphene, it is also used as a sieve for water purification. The atomically defined nanopores in graphene are comparable to the hydrated ions. This is ideal for ultrafast and selective water transport. Graphene is impermeable to standard gases; selective defects make it permeable to water.
Image Credits: nature.com
Graphene-based materials comprise of graphene, graphene oxide and reduced graphene oxide - all of which constitute the graphene familyis similar in structure but different in sp2 region predominantly and the surface groups. The adsorption capacity of these materials is higher than traditional materials such as activated carbon and resins, and also carbon nanotubes. Graphene possesses “ultrathin thickness for maintaining high flux efficiency, high mechanical strength to withstand high pressures, high chemical and thermal stability, allowing for membrane design and fabrication” - potential factors in the water treatment process.
Use of graphene in water desalination through membrane distillation has been demonstrated with high salt rejection rates, with the capacity to remove various contaminants and with anti-fouling capability. Here, the water is permeated by “nanochannels of multilayer, mismatched, partially overlapping graphene grains.” Recent simulation studies show that these nanochannels between the mismatched grains serve as an effective water permeation path.
Compared to commercial membranes, graphene membranes exhibit superior water vapor retention flux, high salt rejection rates, and anti-fouling capability, which are essential for water treatment problems in the desalination process.
Graphene is the promising material for use in water treatments across the world to meet the water demands. A bottleneck challenge is in the perfection of fabricating large graphene sheets for large scale production considering the cost-efficiency and the need for environmental-friendly procedures.
Sources and Further Reading
- Maliyekkal, S. M., Sreeprasad, T. S., Krishnan, D. , Kouser, S. , Mishra, A. K., Waghmare, U. V. and Pradeep, T. (2013), Graphene: A Reusable Substrate for Unprecedented Adsorption of Pesticides. Small, 9: 273-283. doi:10.1002/smll.201201125
- Kaijie Yang,ab Jun Wang,c Xiaoxiao Chen,d Qiang Zhao,ab Abdul Ghaffarab and Baoliang Chen, Application of graphene-based materials in water purification: from the nanoscale to specific devices, Environ. Sci.: Nano, 2018,5, 1264-1297. DOI:10.1039/C8EN00194D
- Yang, K., Wang, J., Chen, X., Zhao, Q., Ghaffar, A., & Chen, B. (2018). Application of graphene-based materials in water purification: from the nanoscale to specific devices. Environmental Science: Nano, 5(6), 1264–1297.doi:10.1039/c8en00194d
- Seo, D. H., Pineda, S., Woo, Y. C., Xie, M., Murdock, A. T., Ang, E. Y. M., … Ostrikov, K. (2018). Anti-fouling graphene-based membranes for effective water desalination. Nature Communications, 9(1).doi:10.1038/s41467-018-02871-3
- Shahin Homaeigohar and Mady Elbahri, Graphene membranes for water desalination, NPG Asia Materials (2017) 9, e427; doi:10.1038/am.2017.135