Although mechanical recycling has become the primary recycling method and is applied for reusing thermoplastic wastes, the properties of most recycled materials are significantly compromised after a number of processing cycles, and the resulting commercial values are thus limited. Incineration of plastic wastes can reduce the demand of landfills and recover heat energy, but we also need to reduce the environmental effects of secondary pollutants generated from the incinerating process, such as dioxins, carbon monoxide, nitrogen oxides, and so on. However, the accumulated plastic wastes have occupied a great amount of land. In most countries, especially the developing countries, landfilling is the major method for plastic wastes disposal due to its operability and low cost. The current methods for disposing of plastic wastes mainly include landfilling, incineration, and mechanical and chemical recycling ( Peng et al., 2018). As a result, plastic wastes have become a malevolent symbol of our wasteful society.
It is predicted that up to 26 billion tons of plastic wastes will be produced by 2050, and more than half will be thrown away into landfills and finally enter ecospheres, such as oceans and lakes, leading to serious environmental pollution ( Jambeck et al., 2015 Lönnstedt and Eklöv, 2016 Geyer et al., 2017). Concomitant with the growing consumption of plastics, the generation of plastic wastes increases rapidly around the world. China and the European Union account for 29.4 and 18.5%, ranking first and second in the world, of all the world’s plastic use, respectively ( China Plastics Industry, 2017 Plastics Europe, 2018). According to the latest statistics of Plastics-Europe, the global yield of plastics reached 348 million tons in 2018 ( Plastics Europe, 2018). Synthetic plastics, including polyethylene (PE), polystyrene (PS), polypropylene (PP), polyvinyl chloride (PVC), polyurethane (PUR), and polyethylene terephthalate (PET) ( Table 1), have become fundamental to almost every aspect of our lives. Last, but not least, we have discussed the challenges toward microbial degradation and valorization of plastic wastes. Taken together, these findings will contribute to building a conception of bio-upcycling plastic wastes by connecting the biodegradation of plastic wastes to the biosynthesis of valuable chemicals in microorganisms. In addition, we have highlighted the microbial metabolic pathways for plastic depolymerization products and the current attempts toward utilization of such products as feedstocks for microbial production of chemicals with high value. In this review, we have comprehensively summarized the microorganisms and enzymes that are able to degrade a variety of generally used synthetic plastics, such as polyethylene (PE), polystyrene (PS), polypropylene (PP), polyvinyl chloride (PVC), polyurethane (PUR), and polyethylene terephthalate (PET). In recent years, reports on biodegradation of synthetic plastics by microorganisms or enzymes have sprung up, and these offer a possibility to develop biological treatment technology for plastic wastes. It is urgent to develop innovative approaches for the disposal of plastic wastes. 2Key Laboratory of Molecular Medicine and Biotherapy, Beijing Institute of Technology, Beijing, ChinaĪ growing accumulation of plastic wastes has become a severe environmental and social issue.1Department of Biology, School of Life Science, Beijing Institute of Technology, Beijing, China.Jiakang Ru 1, Yixin Huo 1,2 and Yu Yang 1*
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