Occurrence, environmental behavior, and co-pollution of microplastics with HMs in urban channelized rivers

Abstract

Complex and frequent anthropogenic activities have released tons of numerous exogenous pollutants into aquatic ecosystems, notably heavy metals (HMs) and microplastics (MPs) in urban channelized rivers. A stereomicroscope, Raman spectroscopy and ICP-MS were employed to systematically investigate the occurrence characteristics, environmental behaviors, and combined pollution effects of microplastics and HMs in urban channelized rivers. The average abundance of microplastics in the urban channelized rivers was (10.1 ± 0.7) items per L, predominantly in fibrous form (54.79%) and with particle sizes <0.5 mm (61.96%), mainly composed of polypropylene (37%) and polyethylene (24%). Compared to Chinese Environmental Quality Standards for Surface Water (GB 3838-2002), the exceedance rates of HMs Mn and As in the river water relative to the standard limits were 22.22% and 61.11%, respectively. The average concentrations of HMs in river water were as follows: W-Mn (580.43 μg L⁻¹) > W-As (184.22 μg L⁻¹) > W-Cr (168.88 μg L⁻¹) > W-Cu (157.76 μg L⁻¹) > W-Ni (153.76 μg L⁻¹) > W-V (78.03 μg L⁻¹) > W-Pb (28.54 μg L⁻¹) > W-Co (15.90 μg L⁻¹) > W-Cd (9.42 μg L⁻¹). The average concentrations of HMs adsorbed by microplastics in the river followed the order: MP-V (42.37 μg g⁻¹) > MP-As (22.33 μg g⁻¹) > MP-Cr (18.97 μg g⁻¹) > MP-Mn (18.82 μg g⁻¹) > MP-Pb (13.88 μg g⁻¹) > MP-Cu (3.41 μg g⁻¹) > MP-Ni (3.23 μg g⁻¹) > MP-Co (1.85 μg g⁻¹) > MP-Cd (1.60 μg g⁻¹). The concentration of microplastics adsorbing HMs in river water at the discharge outlets of wastewater treatment plants was significantly higher than at non-discharge sampling sites. The concentration of HMs adsorbed on microplastics is associated with the color, shape, and size of the microplastics. MP-Cr shows a significant negative correlation with W-Cr, while MP-As demonstrates a significant positive correlation with W-As (p < 0.05). Approximately 72.22% of MP-Cd, 16.67% of MP-Pb, and 5.56% of MP-As were identified as posing moderate ecological risks, while the remaining six HMs were classified as low ecological risk. This study recommends enhancing source control, optimizing wastewater treatment processes, and promoting basin-wide joint prevention and control to provide a scientific basis for managing microplastic–heavy metal composite pollution in urban channelized rivers.