Altered interactions and joint toxicity between microplastics and zinc induced by activated sludge composting process

Sludge-derived organic compost is widely recognized as a key carrier of microplastic (MP) input to agricultural soils; however, the accompanied environmental modification, such as aging, and the subsequent environmental impact of MPs are largely unknown. In this study, we investigated the aging of polyethylene terephthalate (PET), polypropylene (PP), and polyethylene (PE) in organic compost of sludge, its effect on the environmental behavior of co-occurring heavy metal zinc (Zn), and their joint toxicity on model organism Caenorhabditis elegans (C. elegans). Cracks and grooves were observed to have formed on the surface of aged MPs (A-MPs), and the abundance of oxygen-containing functional groups increased significantly following composting treatment. The adsorption capacity of A-MPs for Zn^2 + was generally enhanced, with A-PET showing the most profound increase. The dissolved organic matter (DOM) released during PET aging primarily consisted of three components, and Zn²⁺ was preferentially complexed with the protein-like C1 fraction (48.50 %). We found that the aging process enhanced the joint reproductive toxicity of PET and Zn²⁺ to C. elegans. This may result from accelerated Zn²⁺ desorption from PET in the nematode's lower pH gut and promoted Zn²⁺ bioaccumulation. Our results elucidated the relevant mechanism through which oxidative aging in the composting environment remodels MPs' surface properties and enhances heavy metal adsorption and bioavailability. These findings provided a key scientific basis for evaluating the environmental risks of MP-pollutant composites.