Advancing microplastics remediation in bioretention systems using biochar/kaolin: Optimizing organics removal, plant health, and microbial community dynamics

Bioretention systems can efficiently eliminate microplastics (MPs) from stormwater and prevent their potential pollution in surface water. However, MPs dynamics in bioretention systems and their effects on microbes, plants, and organics removal are unknown. In this study, five lab-scale bioretention columns (i.e., control and four treatments) were established and filled with soil and fillers (zeolite and ceramsite). Various sorbents were utilized in columns, including biochar, kaolin and kaolin-biochar (KBC) composites for MPs adsorption. This study examines how biochar/kaolin amendment affects MPs and organics (COD and TOC) removal, plant health, and microbial community structure in bioretention systems. In the 60-day time-series column experiment, all amended columns removed over 90% of MPs compared to the control. The biochar, kaolin and their combined composite eliminated MPs by 90%, 94%, and 97%, respectively. Adding vegetation to the columns improved MPs removal. Moreover, bioretention systems were more effective in removing MPs ranging from 0.6 to 1 mm with a 71% removal rate than MPs ranging from 0.3 to 0.6 mm, resulting in a 54% removal. Organics were removed contrarily in the soil and filler layer of the bioretention system, with the soil layer removal higher due to increased microbial activity. The removal rate of total organic carbon was higher (90%) than that of chemical oxygen demand (80%). The most dominant phylum of the bacteria in the soil of treatment columns were Proteobacteria and Acidobacteriota, which constituted 16‐27% and 41‐58%, respectively. While the dominant phylum in that of fillers were Bacteroidota and Firmicutes, which constituted 18‐42% and 42‐65%, respectively. The maximum microbial enrichment was observed in the biochar and KBC vegetated columns. This work advances our understanding of the complex dynamics between microplastics and organic matter in stormwater and how, individually and in combination, vegetation, biochar, and kaolin vegetation, biochar, and kaolin, individually and in combination, enhance bioretention systems' effectiveness in managing multiple pollutants. • Microplastics removal in bioretention system was optimized with contrasting plants and sorbents. • Kaolin-biochar composite revealed comparatively higher removal rate of 97 % than pristine kaolin and biochar sorbents. • Microplastics retained more in vegetated columns compared to non-vegetated columns. • Sorbents improved organics removal and microbial growth, suggesting bioretention system sustainability for water treatment.