Biodegradation of typical plastics and its mechanisms

Due to excellent properties such as ease of processing, transportation and storage, as well as outstanding chemical stability, plastics are widely used in daily production and life, which induce the possibility of plastics entering the natural environment and long-term staying in the environment. Plastics could release into different environmental compartments such as sea, lake, soil and sediment through various ways, and have toxicity to organisms through ingestion and bioaccumulation. In particular, the physical and chemical properties of plastics can be changed by biotic and abiotic factors in the environments. Biodegradation is one of the inevitable environmental processes of plastics, and it is also an eco-friendly treatment for plastic waste. Therefore, exploring the biodegradation of plastics has great practical and ecological significances. In this review, the biodegradation process of plastics by animals, plants, microorganisms and enzymes were summarized and analyzed, and the biodegradation mechanisms of plastics were revealed. Animals (e.g., insects) could physically crush plastics by gastric grinding and chewing, and also change the chemical properties of the ingested plastics under the complex biochemical reactions in their bodies. Current researches on the biodegradation of plastics by animals mainly focus on insects and their intestinal microorganisms. Limited investigations focused on plants, in which the colonization of plants (e.g., algae) on the plastics surface was observed. The biodegradation by plants could be attributed to the enzymes existing on the surface of stems and leaves or secreting by roots. Therefore, it can be concluded that the biodegradation of plastics by animals and plants is closely related to microorganisms and enzymes. It is noted that the microorganisms from animals, plants and external environments showed similar pathways on plastics biodegradation. The microorganisms could first colonize on the plastics surface to form a biofilm, and then secrete extracellular or intracellular enzymes to contact with the plastics and decompose the plastics into low molecular weight oligomers, dimers or monomers through catalysis of their binding sites. These generated oligomers, dimers and monomers can be absorbed by microorganisms as a carbon source through in vivo metabolic mechanism (e.g., the assimilation of organisms). Finally, plastics can be mineralized to CO2, CH4 and/or H2O. In addition, this review deeply analyzes the factors that affect the efficiency of plastic biodegradation, including plastic properties (e.g., plastics types, glass transition temperature, surface hydrophobicity, types of additives), organism species (e.g., insects, marine animals, plants, bacteria, fungi) and abiotic environmental factors (e.g., temperature, oxygen content, solar radiation, physical abrasion). In addition, the environmental behavior of the products after plastics biodegradation was discussed. The small-sized fragments, oligomers and plasticizers released after the biodegradation of plastics could further show different effects on organisms and ecosystems. In conclusion, challenges and future research perspectives for the plastic biodegradation were suggested at the end of this review. Further investigation on the biodegradation mechanisms of plastics and the degradation and fate of plastics additives are suggested. More efforts are recommended on the environmental behaviors and possible biological impacts of the degraded microplastics and nanoplastics. The unified measurement standards and characterization methods should be explored. This review will provide forceful scientific supports for better understanding the environmental fate of plastics.