Degradation of Micro- and Nano-Plastics by Photocatalytic Methods

Abstract

In the last couple of years, an increasing number of dead marine animals that are full of plastic waste which ends up in the seas and oceans has been evident. Actually, micro-plastics (5-1000 μm) have been detected in the stomachs of commercially important fish from the Mediterranean, as well as some drinking water in plastic bottles. Today, humanity is struggling with the presence of micro- and nano-plastics in enormous quantities in the oceans, seas, rivers and lakes. World plastic production almost reached 350 million tonnes in 2017, where China is the largest producer of plastics, 29.4% followed by Europe with 18.5%. There is experimental evidence that three weeks of UV irradiation generates plastics particles in the nanometer size range. Nanoparticles (1-100 nm) usually exhibit different chemical and physical properties than macroscopic objects based on the same material. At the micro- and nanoscales, polyvinyl chloride (PVC), polyethylene terephthalate (PET), polystyrene (PS) and polyethylene (PE) were observed. Recent study has shown that carbon-based molecules could break down marine microplastics without harming surrounding microorganisms. Successful degradation of low-density polyethylene film (LDPE) in water by using visible light excited heterogeneous zinc oxide photocatalysts was achieved, as well as polyethylene (PE) plastic with polypyrrole/TiO2 (PPy/TiO2) nanocomposite as photocatalyst. Our current knowledge about nanoplastics in the environment is still very limited and only a few scientific publications are available. For this reason, micro- and nanoplastics are considered as an emerging issue of great concern. Photocatalytic degradation of plastics leads to the formation of lower molecular weight intermediates that can be further used as a raw materials for the chemical industry for the production of new petrochemical products or plastics or in organic synthesis. This review provides an overview of the latest catalytic and photocatalytic methods of degradation of microplastics, as well the quantities of global plastic produced in the world and their impact on human health.