Synergistic efficiency of COD removal and CO₂ emissions in biochar-modified bioretention systems

The efficiency of biochar-modified bioretention system (BRS) in runoff pollutants removal and carbon reduction has attracted increasing attention. In this study, BRS modified with coconut shell biochar and corn stalk biochar at different ratios (2 %, 4 % and 6 %) were constructed to investigate their synergistic efficiency. The influence of different initial chemical oxygen demand (COD) concentrations, ambient temperatures, rainfall intensities and drying periods were investigated. The results showed that coconut shell biochar could improve the COD removal efficiency while reducing carbon dioxide (CO₂) emission fluxes of BRS together, while the effect was significantly better than that of corn stalk biochar. Meanwhile, coconut shell biochar increased the abundance of Proteobacteria and Actinobacteriota by up to 31.43 % and 24.76 %, thereby enhancing pollutant degradation and carbon sequestration efficiency in BRS. With the addition of 4 % coconut shell biochar in BRS, the synergistic efficiency was best that COD removal rate was up to 98.15 % and CO₂ emission flux was only 246.38 mg/(m²∙d). Furthermore, the study found that lower initial COD concentration, moderate ambient temperature and heavier rainfall intensity could all improve the synergistic efficiency of runoff pollutants removal and carbon reduction in BRS. Prediction equations could directly calculate the COD removal rate and CO₂ emission fluxes of BRS under different independent variables. This study provided a theoretical basis for the synergistic efficiency of pollutant removal and carbon reduction in biochar-modified BRS.