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 effect on denitrification. However, the mechanism of physicochemical properties of MPs affecting denitrification remains unclear. In this study, four MPs types (PS, PE, PET and PLA) with particle sizes of 1000, 300, and 1 um, along with phthalate esters (PAEs) and bisphenol A (BPA), were used to investigate MPs effects on denitrification. Compared with the control without MPs, MPs of 1000 um and 300 um 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, respectively. Conversely,1um 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 um and 300 um MPs promoted denitrification by enhancing enzyme activity and electron transfer, whereas 1um 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.