Effects of microplastics on atrazine removal in constructed wetlands: Insight into the response characteristics of microorganisms, enzyme activity, and functional genes

Constructed wetlands (CWs) technology has been widely used to treat agricultural non-point source pollution. However, knowledge about the impact mechanism and distribution characteristics of microplastics (MPs) on pesticide treatment in CWs is limited. This study employed atrazine (ATZ), a representative pesticide, as a model contaminant, to systematically investigate the impacts of polyethylene microplastics (PE MPs) on the removal of ATZ and nutrients, as well as the enzyme activity and the distribution of functional genes in vertical subsurface-flow CW microcosm. The results showed that compared to the control group (CK), CWs treated with different concentrations of MPs had no significant difference in the removal of ATZ. Moreover, in the second stage (ATZ=400 μg/L), the average removal efficiency of ATZ by CWs containing MPs was slightly higher than that of the CK group. PE MPs reduced the nitrogen removal efficiency of CWs by 1.57 %-3.03 %, but had no significant effect on TP removal. The concentration distribution of PE MPs in the substrate layer exhibited a decreasing trend from top to bottom, and the interception capacity of CWs gradually decreased with time (from 100 % to 97.4 %); When exposed to PE MPs, the activities of enzymes in substrate related to nitrogen metabolism were inhibited; Moreover, the addition of PE MPs in CWs promoted the removal of ATZ by increasing the abundance of ATZ metabolizing bacteria (Hydrogenophaga, Zoogloea, Rhizobium, etc.) and ATZ degradation key genes (atzA and trzN). These results not only provide theoretical support for the practical application of CWs in the treatment of pesticide wastewater, but also provide a theoretical basis for the environmental risk control of pesticide non-point source pollution ecological treatment technology in the presence of MPs.