Unraveling the Effect of Continuously Accumulating Microplastics on the Humification of Dissolved Organic Matter in the Composting System

Microplastics (MPs) accumulation in composting systems has received considerable attention in recent years. However, the ecological risk of MPs on composting environments remains scarcely explored. This study investigated the impact of polyethylene and polyethylene terephthalate MPs on the structural evolution of dissolved organic matter (DOM) during composting. The ultraviolet, excitation–emission matrix, and Fourier transform ion cyclotron resonance mass spectroscopy collectively suggested that MPs caused DOM of compost to have lower aromaticity and fewer humic acid-like substances, particularly at the early stage of composting. This inhibition effect of MPs on the aromatization of DOM of compost was proved to result from not only the altered compost characteristics (e.g., lower pH and temperature relative to the control) but also the preferential sorption of specific DOM compositions by MPs. The sorption of DOM by MPs before and after composting showed that the protein-like components and microbial by-product analogues, which were significant substrates for humification, were found to be preferentially sorbed by two MPs and thus were prevented from being directly humified by some microorganisms. The finding would contribute to an in-depth explanation of the mechanisms by which MPs affect compost humification and provide insight into the ecological risks of MPs for composting environments.