Plastic pollution is a critical environmental issue that requires innovative approaches to mitigate the level of plastic waste entering the environment. Now a review article has been published in the journal Materials Science in Semiconductor Processing which evaluates the potential use of solid state photocatalysis to combat plastic pollution.
Study: Solid State Photocatalysis for Plastics Reduction: A Review. Image Credit: Larina Marina/Shutterstock.com
Since the development of plastics in the mid-20e century, plastic waste has accumulated at a rate that has caused enormous environmental problems. While there have been increased efforts to tackle plastic pollution over the past few decades, the COVID-19 pandemic has thrown the proverbial wrench into the works. Disposable masks, PPE, tests and single-use medical equipment have exacerbated the problem.
Once polymeric materials enter the environment, they can degrade into toxic by-products, and microplastics from mechanical, thermal, and microbial degradation can accumulate in the food chain. Plastics are not biodegradable, with products and particles remaining in the environment for long periods of time. Plus, they can release harmful greenhouse gases, contributing to climate change.
In recent years, several methods have been developed to degrade plastics, including mechanical methods, physiochemical processes, and methods using microorganisms. These processes aim to reduce occurrences of polluting by-products and recover value-added products from waste streams, in line with circular economy objectives. Biodegradable polymers have also been developed.
Photocatalysis uses sunlight or artificial light to degrade materials. This oxidation process has been used to combat several pollutants, such as antibiotics and other pharmaceuticals, dyes, phenolics, disinfectants, and airborne contaminants such as carbon monoxide, NOXs, and toluene.
Photocatalysis has been explored in recent research on the degradation of polymeric materials. In this process, the degradation is accelerated by the formation of oxidizing species which favor the decomposition of the constituent organic components. Photocatalysis requires a lot of water, oxygen and photons with sufficient energy.
Much research has focused on the aqueous phase of photocatalysis. A major limitation of this phase is mass transfer, which impairs efficiency. Nevertheless, these studies have reported the successful photocatalytic conversion of polymeric materials into value-added products such as biofuels.
The aqueous phase of photocatalytic processes requires large amounts of water, which can have the undesirable side effect of contaminating water supplies due to the use of aggressive reagents. Due to the problems of aqueous-phase photocatalysis, research has turned to solid-state methods.
The review study examined solid-state photocatalysis methods, highlighting the benefits, processes, study results, and current perspectives on this important method for plastic waste mitigation. Common assessment methods (molecular weight determination, carbonyl index, and weight loss assessment) are discussed in depth. Sixty-eight studies were reviewed in the paper.
Complementary analytical methods used to assess the performance of photocatalytic plastic degradation methods were reviewed, including XRD, XPS, methods for determining mechanical properties, and microscopy techniques. Current methods to promote hydrophilicity in polymer materials and the polymer/semiconductor interface were discussed by the authors.
The study provided an in-depth analysis of the use of these processes in the degradation of PP, PE and PVC, which are the main components of plastic products. The review’s focus on these materials provides essential information for recycling products such as PPE, plastic bags and disposable masks and producing value-added products from waste streams.
Prospects and future challenges
Although still a relatively new area of research, solid state photocatalysis offers many advantages for plastic waste disposal. There are still some challenges to overcome before the process can be considered for industrial-scale recycling and the production of value-added products.
The review highlighted several challenges and opportunities for future research in this area. Embedding photosensitive materials into plastic materials can improve mechanical, physical, and chemical properties. Additional loads can also provide benefits such as improved transportation.
Another strategy to improve semiconductors in solid state photocatalysis is to improve the initial properties of molded plastics. Parameters such as film thickness and particle size can inhibit caking and mechanical properties.
Another area of research that requires special attention is the nanotoxicity of semiconductors, which could negatively affect living organisms. Additionally, research into value-added products is a potential key area of study that will improve processes.
According to the authors, solid-state photocatalysis is still an understudied area of research that promises immense potential to combat plastic pollution. However, in recent years there has been increasing interest in this method. Because the problem of plastic waste has increased since the start of the COVID-19 pandemic, innovative solutions are essential. The review was intended to provide guidelines for future researchers to achieve this.
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Castilla-Caballero, D et al. (2022) Solid State Photocatalysis for Plastic Reduction: A Review Materials Science in Semiconductor Processing 149 106890 [online] sciencedirect.com. Available at: https://www.sciencedirect.com/science/article/pii/S1369800122004279