Water pollution, caused by the large disposal of chemicals, dyes, wastes, plastics, and other organic pollutants, has severely affected the natural water resources, causing a significant threat to clean drinking water and raising chronic effects to human health.
Image Credit: JonShore/Shutterstock.com
The WHO’s record shows that contaminated water is responsible for approximately 485,000 diarrhoeal deaths per year (World Health Organization, 2019). The lack of proper water facilities is expected to force half of the world’s population to live in water-stressed areas by 2025.
The cause of water pollution is mainly attributed to insufficient water treatment facilities of wastewater from households, hospitals, and industries. Besides, environmental pollution due to transportation and agriculture has promoted organic pollutants that remain in the atmosphere and consequently affect surrounding lakes and streams.
Current technology uses physical, chemical, and biological processes to remove pollutants or contaminants from wastewater. However, it still requires continuous investigation to make the purest form available to humans and the environment.
Water Treatment Solutions
Based on the application, different solutions are currently available in treating wastewater. For example, sedimentation is a physical process that allows particles to settle out of water due to gravity. As a result, the concentration of solids in suspension is reduced, minimizing the chemicals needed for the treatment (Bache, et al., 2007). Another common chemical treatment, coagulation, removes solids from water by manipulating electrostatic charges of particles (Bradley, 2019).
For drinking water, the adsorption process is one of the widely used technologies for removing organic substances. Despite this, according to (Mohd Kaus, 2021), the technology has low efficiency in processing medical waste and generates secondary waste.
As industrialization has advanced, exploration of different chemicals and composites has generated unsorted species that cause toxicity in wastewater. For this purpose, the Advanced Oxidation Process (AOP) has been the most appealing and ideal choice since the introduction of the technology in the 1980s (Glaze, 1987).
Advanced Oxidation Process
AOP uses highly reactive species to destruct pollutants by creating hydroxyl radicals in the water. As a result, most organic compounds are oxidized until they are fully mineralized as carbon dioxide and water (Suez Water Technologies & Solutions, 2021).
A popular process for treating drinking water, the efficacy of the treatment depends on the choice of AOP, the properties of the contaminants, and the environmental parameters. However, the short lifetime of radicals means the process is less suitable for pathogen disinfection, making it only applicable in a small-scale application.
Other frequently used AOPs, such as photolysis, photo-Fenton, and photocatalysis, create radicals using semiconductors and light energy (Mohd Kaus, 2021). The formation of electron-hole pairs and radical production occurs when the light intensity is higher than the semiconductor bandgap energy.
It is a challenge for some semiconductors to meet this demand. As a result, the process requires enhancement by integrating with other materials such as polymers, graphene, or other metals.
Earlier this year, scientists from Malaysia and Thailand developed a photocatalyst for AOP water treatment, based on reduced graphene oxide (rGO) that can minimize environmental pollution (Mohd Kaus, 2021). The research, published in the journal Catalysts on February 25th 2021, shows that this process enables unique characteristics of graphene, such as the extended range of light absorption, the separation of charges, and the high capacity of adsorption of pollutants. These properties enormously improve photocatalytic efficiency.
Semiconductor Nanomaterials as a Photocatalyst
A photocatalyst is a material that decomposes and detoxicates harmful substances by exposing the light. Although various materials are available that show photocatalytic capability (Palccoat, 2021), many of them are expensive or suffer from agglomeration that reduces the material reactivity. Consequently, the efficiency of photocatalytic operation is reduced.
The growing popularity of the nanotechnology industry has meant that semiconductor nanomaterials (NMs) have been considered in various applications that utilize their unique size-dependent material properties.
Impressive modifications to their electronic and optical properties occur as particle size changes because of the quantum confinement effect. Due to their wide bandgaps and ability to bandgap tuning to acquire desired properties, applications are being found in photocatalysis, photo-optics, and electronic devices (Tiwari, et al., 2019). Their ability to use light energy and produce charge carriers vital for various applications has promoted significant research efforts over the past few years to enhance their photocatalytic efficiencies.
According to the research conducted by scientists in Malaysia and Thailand, the most commonly used semiconductor NMs in photocatalysis are TiO2, ZnO, BiFeO3, BiVO4, SnO2, and CdO. Still, the introduction of graphene into the prospect becomes highly desirable for the tuning surface.
ANU scientists develop new technique to purify wastewater with sunlight
Video Credit: ANU TV/youtube.com
Graphene as a Photocatalyst
The semiconductor NMs hybridization with graphene and its derivative, particularly rGO, has attracted considerable attention. Graphene has three sp2 hybrid orbitals bonded with carbon atoms, leaving the fourth bond to dangle perpendicular to the hexagonal array of carbon atoms. This dangling bond is attractive to any functional groups in the solution for the attachment.
Once graphene is deposited in a liquid solvent, functional groups in the solvent cover the sheets, oxidizing graphene into graphene oxide (GO) and then into rGO once reducing agents are added. The use of rGO gives the advantage of remarkable properties of high charge carrier mobility, good mechanical properties, high thermal conductivity, and high surface area.
Besides, due to the reduction process, rGO does not have some absorption peaks that seem to appear in GO, particularly the functional group peaks containing oxygen. The research team used the composites-based rGO that presented excellent photocatalytic ability and powerful photodegradation of a wide variety of organic pollutants.
This recent development of using rGO as a photocatalyst is considered a groundbreaking and significant innovation to remove toxic contaminants from wastewater. Interestingly, the theory also responds to broader issues of environmental pollution that require urgent attention. With this technology in progress, scientists have enough supporting theory to upscale and provide a cleaner environment and safe drinking water to human populations.
References and Further Reading
Bache, D. H. and Gregory, R. (2007) Sedimentation Processes. [Online] IWA Publishing. Available at: https://www.iwapublishing.com/news/sedimentation-processes (Accessed August 2021).
Bradley, E. (2009) Wastewater Coagulation. [Online] Dober. Available at: https://www.dober.com/water-treatment/resources/wastewater-coagulation (Accessed August 2021).
Glaze, W. H. (1987) Drinking-water treatment with ozone. Environ. Sci. Technol. ACS Publications. Available at: https://doi.org/10.1021/es00157a001 (Accessed August 2021).
Mohd Kaus, N. H., et al. (2021) Effective Strategies, Mechanisms, and Photocatalytic Efficiency of Semiconductor Nanomaterials Incorporating rGO for Environmental Contaminant Degradation. Catalysts. Available at: https://doi.org/10.3390/catal11030302 (Accessed August 2021).
Palccoat (2021) What is a Photocatalyst? [Online] Palccoat. Available at: https://www.palccoat.com/en/about/ (Accessed August 2021).
Suez Water Technologies & Solutions (2021) AOP Advanced Oxidation Process Systems. [Online] Suez Water Technologies & Solutions. Available at: https://www.suezwatertechnologies.com/products/disinfection-oxidation/aop-systems (Accessed August 2021).
Tiwari, A. P. and Rohiwal, S. S. (2019) Chapter 2 - Synthesis and Bioconjugation of Hybrid Nanostructures for Biomedical Applications. Hybrid Nanostructures for Cancer Theranostics. book auth. Bohara, R. A. and Thorat, N. Available at: https://doi.org/10.1016/B978-0-12-813906-6.00002-0 (Accessed August 2021).
World Health Organization (2019) Drinking-water. [Online]. Available at: https://www.who.int/news-room/fact-sheets/detail/drinking-water (Accessed on August 2021).