[Effect of Free Water Surface Constructed Wetland on the Occurrence Characteristics of Microplastics in Water in the Upper Reaches of the Erhai Lake].

Abstract

Microplastics （MPs）, an emerging contaminant, have been widely detected in environmental media including water, sediment, soil, and atmosphere. Exploring the occurrence and transport characteristics of MPs in different environments is essential for accurately evaluating its ecological and environmental effects. However, the extent to which a free water surface （FWS） constructed wetland alters the occurrence characteristics of MPs in water has not been clearly elucidated. Therefore, five FWS constructed wetlands in the Luoshijiang Watershed located in the upper reaches of the Erhai Lake were selected as the research objects. The effects of these FWS constructed wetlands on MPs characteristics in water bodies were explored based on their abundance, shape, particle size, and polymer type in the wetland inlet and outlet. The results showed that： ① The quantity and mass abundance of MPs in the water of the FWS constructed wetland in the Luoshijiang Watershed were in the range of 10 375-21 000 n·m^-3 and 5.5-9.8mg·m^-3, and the corresponding average abundances were （14 115±2 790） n·m^-3 and （7.6±2.3）mg·m^-3, respectively. MPs with small particle size （0.2-1 mm, 73.8%±7.5%）, transparent color （64.0%±12.2%）, and fiber shape （98.9%±1.7%） were dominant in water bodies of the FWS constructed wetland, while cellophane （CP, 58.5%±13.9%）, polyethylene terephthalate （PET, 22.1%±11.4%）, and polyester （PES, 12.8%±16.8%） were the main polymer types. The MPs abundance in the water of FWS constructed wetland in this watershed was relatively high, whereas the pollution load index and ecological risk index were both in low levels （Level I）. ② After water flowing through the FWS constructed wetland, the film- and fragment-shaped MPs disappeared, and the proportions of dominant color （transparent, black, and white） and polymer type （PET） decreased by 1.4% and 4.1%, respectively. By contrast, the proportions of large size and minority-color （red, yellow, green, purple, etc.） MPs increased, with 1-2 mm, 2-5 mm, and minority-color increasing by 3.0%, 2.2%, and 5.4%, respectively, and polymer types of rayon and PES, which did not exist in the wetland inlet, were detected. ③ The quantity and mass abundance of MPs differences between the inlet and outlet of the FWS constructed wetland in the Luoshijiang Watershed were both insignificant （P>0.05）, and the retention of MPs in water bodies by the FWS constructed wetland was also insignificant. However, the FWS constructed wetland weakened the correlations among different MPs characteristics in water bodies. The results indicated that the FWS constructed wetland could alter the occurrence characteristics of MPs in water bodies, including particle size, shape, color, and polymer type, and exhibited the dual roles of source and sink of MPs.