Impact of microplastic physicochemical properties on denitrification in sediment: Emphasis on particle size, polymer type, and additives

Microplastics (MPs) are emerging contaminants that accumulate continuously in river sediments, posing potential effects on denitrification. However, the mechanisms by which the physicochemical properties of MPs affect denitrification remain unclear. In this study, four types of MPs (PS, PE, PET and PLA) with particle sizes of 1000, 300, and 1 μm, along with phthalate esters (PAEs) and bisphenol A (BPA), were used to investigate the effects of MPs on denitrification. Compared with the control without MPs, MPs of 1000 μm and 300 μm elevated denitrification rates by 2.08% – 13.75% and 10.91% – 46.81%, respectively. PLA enhanced denitrification to a greater extent than PS, PE, and PET. Conversely, 1 μm MPs, PAEs, and BPA suppressed the denitrification rate by 15.97% – 68.71%. Specifically, PS exhibited a greater inhibitory effect on denitrification than PE, PET, and PLA. MPs and additives reshaped the composition of keystone bacteria (Noviherbaspirillum, Steroidobacter, Massilia, Telluria) and fungi (Scedosporium, Aspergillus, Trichoderma, Fusarium) in sediments. Mediated by these keystone microbes, 1000 μm and 300 μm MPs promoted denitrification by enhancing enzyme activity and electron transfer, whereas 1 μm MPs and additives inhibited denitrification by reducing the abundance of denitrification-related functional genes. The study revealed the impact of MPs physicochemical properties on denitrification, aiding the ecological risk management of MPs.