Effect of Biochar–Soil Interactions on Stormwater-Derived Trace Organic Contaminant Fate in Amended Bioretention Systems

Soil amendments such as biochar are increasingly used in bioretention systems for enhanced removal of trace organic contaminants (TrOCs). However, the effects of interactions between biochar and soil in amended bioretention systems are not well understood. Herein, we investigated the sorption and degradation of 8 representative stormwater TrOCs in bench-scale columns and batch experiments designed to replicate field bioretention systems. We exposed columns containing unamended soil, biochar-amended soil, and biochar-amended sand to TrOC-spiked synthetic stormwater over nine months. We then conducted batch experiments with fresh media, column-aged media, and media aged under field conditions. TrOC concentrations in the effluent of the biochar-amended columns were consistently below detection limits. Batch experiments with fresh substrates showed that sorption of TrOCs to biochar-amended soil substantially exceeded that of unamended soil and, for most TrOCs, amended sand. Aging reduced the sorption capacity of biochar-amended substrates but enhanced microbial degradation, particularly in experiments containing column-aged soil–biochar. Overall, this study demonstrates that TrOC removal in biochar-amended systems is governed by a dynamic balance between sorption and biodegradation processes that evolve with substrate aging, and that interactions between soil and biochar may improve the TrOC removal efficiency and support the biodegradation of degradable TrOCs.