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Microplastics are tiny plastic particles, typically less than 5 mm in size, that originate from a variety of sources, such as the fragmentation of larger plastic objects, microbeads in personal care products, and synthetic fibers in garments. These microscopic particles pose a significant environmental risk because they can readily infiltrate the ecosystem via waterways and accumulate in the food chain[1]. Microplastics can be harmful to aquatic life and have the potential to negatively impact human health if they are ingested through the food. Through enhanced waste management, product limits, and public awareness-raising, efforts are being made to reduce microplastic pollution[2].

The photocatalytic degradation of various environmental pollutants, including microplastics, is essential. Utilizing photocatalysts, which are substances that can accelerate chemical reactions when exposed to light, specifically ultraviolet (UV) radiation, is involved. i.Photocatalytic degradation is a "green" technology that produces no deleterious byproducts. Upon exposure to light, photocatalysts produce reactive oxygen species (ROS) such as hydroxyl radicals (•OH), superoxide anion radicals (•O2-), and hydrogen peroxide (H2O2), which can degrade various pollutants into less hazardous substances such as water, carbon dioxide, and inorganic ions [25]. ii.Targets for photocatalytic degradation include organic molecules, heavy metals, and microplastics, among other types of contaminants. It has been effectively used to rid water of impurities including phenol, colors, and medications [26]. iii.Photocatalytic degradation is an energy-efficient technique for eliminating pollutants that may be driven by direct sunlight or low-energy artificial light sources. iv.Photocatalysts are economical for large-scale applications because they may be reused several times without losing their potency. v.Compared to other processes like photodegradation, biodegradation, or chemical oxidation, photocatalytic degradation is more efficient in breaking down microplastics. Research has demonstrated that the removal of microplastics from the environment may be improved even further by combining photocatalysis with other degrading techniques [27].

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Primary microplastics and secondary microplastics are the two categories of main sources of microplastics. Primary microplastics are microplastics which are manufactured at micro size (1 µm – 5 mm), such as plastic particles in personal care products and synthetic fibers in textiles [3]. On the other hand, secondary microplastics result from the fragmentation and degradation of larger plastic items, such as plastic bags, bottles, and fishing gear, due to physical, chemical, and biological processes to which plastics are exposed in the environment [4]. Microplastics pollution has become widespread in all types of terrestrial ecosystems as these particles are found in water, soil, and air. Marine ecosystems are among the most extensively investigated in terms of microplastic contamination. Microplastics have been found in a wide range of marine habitats, including surface waters, deep-sea sediments, and polar ice [5-7], where they can be ingested by a large variety of marine organisms, becoming part of marine food chains and resulting in negative effects, such as decreased nutrition, altered growth rates, and reproductive problems [8]. Freshwater systems are not exempt from microplastic pollution. These particles have been identified in rivers, lakes, and wetlands [9]. Urban waterways and wastewater treatment plants are significant entry points for microplastics in freshwater systems, as they receive and process effluent from both residential and industrial sources [10]. Some studies have reported microplastic concentrations in freshwater ecosystems comparable to or even surpassing those in marine environments [11]. The recognition of terrestrial habitats as potential sources of microplastic pollution is also growing. Agricultural soils have been shown to contain microplastics, with the use of compost and sewage sludge as fertilizers being recognized as a major source [12]. The microplastic load in terrestrial systems is further influenced by the fragmentation of plastic trash in landfills and the abrasion of synthetic fibers from garments during washing [13]. Recent studies additionally highlighted the presence of microplastics in atmospheric deposition [14]. Through wind-driven processes, such as the resuspension of soil or the release of fibers during the drying of textiles, both primdary and secondary microplastics can become airborne [15]. The presence of atmospheric microplastics in urban, remote, and even polar regions indicates long-distance transport and global distribution [16]. In conclusion, microplastics are ubiquitous in diverse environmental matrices and pose significant hazards to ecosystems and human health. The pervasiveness of microplastics in marine, freshwater, terrestrial, and atmospheric environments underscore the urgency of instituting effective waste management strategies, product bans, and public awareness campaigns to combat this global problem.

While photocatalytic degradation can degrade microplastics into non-toxic chemicals (CO2 and H2O as the main degradation products) and thus reduces environmental risks, photocatalytic reforming, as an opportunity to not only remove MP pollution but also reduce CO2 emissions and produce useful chemicals and/or fuels has been attracting increasing attention recently [19, 24]. This is a challenging but rapidly developing research area, which has a great potential to make a significant difference in tackling microplastics pollution, as it has a potential to provide economic incentive for processing MP waste.

Microplastic degradation can be accomplished by a variety of physical, chemical, and biological techniques. The following techniques have been investigated by different research groups for microplastics degradation. Each of these approaches has advantages and disadvantages, and a combination of strategies may be required to effectively combat microplastic pollution. I.Biodegradation: Microorganisms such as bacteria and fungi can be used in biodegradation to degrade microplastics through enzymatic processes. Researchers are investigating the capability of these microorganisms to degrade various plastic polymers [17]. II.Enzymatic Degradation: Enzymatic degradation is the process of breaking down microplastics using enzymes that are either created through recombinant technology or extracted from microorganisms. For example, a newly discovered deep-sea enzyme that breaks down PET plastic, has been reported according to recent studies [18]. III.Chemical Degradation: Microplastics can be chemically degraded by oxidation, hydrolysis, and the use of certain reagents or catalysts. By using these advanced oxidation or thermochemical processes, the plastic polymers are either broken down into smaller molecules or changed into less dangerous materials [19, 20]. a.Photocatalytic Degradation: This process speeds up the degradation of microplastics in the presence of ultraviolet (UV) light by using photocatalysts, such as titanium dioxide (TiO2). Microplastics may be effectively broken down into smaller molecules by photocatalytic degradation, which will eventually. b.Ozonation: Ozonation is a chemical oxidation process that uses ozone to break down microplastics. Strong reactivity of ozone can contribute to oxidation and cleavage of plastic polymers, resulting in fragmentation and degradation [21]. c.Thermochemical processes (pyrolysis, hydrogenolysis, hydrothermal and solvothermal treatment): Microplastics are heated in a reducing atmosphere (inert gas or hydrogen-containing atmosphere) or in solution at high pressure, commonly in the presence of a catalyst, to transform them into valuable substances and fuel [19, 22-24].

1.“Photocatalytic degradation of different types of microplastics by TiOx/ZnO tetrapod photocatalysts” Yanling He, Atta Ur Rehman, Muxian Xu, Christelle A. Not, Alan M. C. Ng, Aleksandra B. Djurišić, Heliyon 9 (2023) e22562 2.“MXene photocatalysts for hydrogen evolution and microplastics degradation under simulated solar illumination” Atta Ur Rehman, Sin Yi Pang, Kang Ding Han, Chunyang Dong, Yingchuan Zhang, Yanling He, Christelle A. Not, Alan Man Ching Ng, Jianhua Hao, Zheng Xiao Guo, Aleksandra B. Djurišić, submitted. 3.Niobium oxide for microplastics degradation – effect of crystal structure and morphology, Atta Ur Rehman, Kang Ding Han, Muhammad Umair Ali, Chunyang Dong, Yanling He, Christelle A. Not, Alan Man Ching Ng, Zheng Xiao Guo, Jasminka Popović, Aleksandra B. Djurišić, in preparation.