Does the aging behavior of microplastics affect the process of denitrification by the difference of copper ion adsorption?

Riparian sediment is a hot zone for denitrification that can withhold copper and microplastics (MPs) from outside. It has been proven that MPs affect denitrification and the existing forms of copper in the environment. However, the impact of copper on sediment denitrification under exposure to MPs remains unclear. This study revealed the response of sediment denitrification to copper availability under the adsorption of MPs and the complexation of MP-derived dissolved organic matter (DOM). These results showed that MP accumulation inhibited denitrification. However, aged MPs increased the activity of nitrite reductase (12.64%), nitrogen dioxide reductase (37.68%), and electron transport (28.93%) compared with pristine MPs. The aging behavior of MPs alleviated 28.18% nitrite accumulation and 16.41-118.35% nitrous oxide emissions. Thus, the aging behavior of MPs alleviated the inhibition of denitrification. Notably, we resolved the copper ion adsorption and complexation by MPs, MP-derived DOM contributed to the denitrification process, and we found that the key nitrogen removal factors were affected by KL, KM, and K2. These results fill a gap in our understanding of biochemical synthesis of MPs during denitrification. Furthermore, it can be used to build a predictive understanding of the long-term effects of MPs on the sediment nitrogen cycle.