Aging or degradation? Transformation mechanisms of microplastics in soil environments

Soil serves as a primary sink for microplastics. This review explicitly focuses on the mechanisms underlying microplastic aging and degradation in soil environments, and the ecological toxicity arising from microplastic transformation. The fragmentation processes of different microplastics depend on the characteristics of their mechanical dynamics and microstructures. Mechanical disruption can result in surface cracking, fragmentation, and subsequent formation of smaller microplastic particles. Exposure to ultraviolet and infrared radiation under natural sunlight can accelerate the embrittlement, stiffening, and eventual fragmentation of microplastics. Thermal degradation of microplastics primarily involves high temperature facilitating the dissociation of chemical bonds between polymer molecules. This process is similar to photodegradation that refers to oxidative reactions following the breakage of polymer chains. The humidity stability of microplastics is influenced by the hydrophilicity of their functional groups. Microplastics that enter the soil environment undergo aging and degradation due to the chewing action of soil fauna and enzymatic erosion within their digestive tracts. The causal relationship between microplastic aging and degradation is further explored, providing a comprehensive understanding of how microplastics transform under soil conditions. This work supplements targeted data on microplastic behavior in soil ecosystems and establishes a foundation for studies on microplastic pollution management in terrestrial environments.