Critical field evaluations of biochar-amended stormwater biofilters for PFAS and other organic micropollutant removals

Biochar is often promoted as an ideal amendment for stormwater biofilters; however, its effectiveness has rarely been tested under field conditions. This study evaluates the impact of biochar addition on the removal of organic micropollutants (OMPs) in field-scale biofilters operating under real-world conditions for the first time. The research comprised four vegetated biofilter facilities (3 - 5 years old), two without and two with 2.1 wt. % (10 vol. %) biochar amendment. Stormwater and filter material samples from various locations after four years of operation were analyzed for a wide range of common and emerging OMPs found in urban runoff. Unlike hydrophobic OMPs (hydrocarbons, polychlorinated biphenyls, and di(2-ethylhexyl) phthalate), the investigated biofilters demonstrated low, or inconsistent, removal of hydrophilic and slow-adsorbing OMPs like bisphenol A, monobutyltin, and per-fluoroalkyl substances (PFASs). Although the physiochemical properties of biochar were well-adapted to pollutant removal, biochar amendment did not significantly improve OMP removal when compared with the status quo. This can be attributed to several field conditions and suboptimal design interfering with the biochar's sorption capacity, namely, the large particle size (D₅₀ ∼4 mm) and low quantity of biochar, high levels of competing agents (i.e., dissolved oxygen carbon (DOC) and cations), co-contaminants in stormwater, limited contact time, biochar pore blockage (e.g., by DOC molecules and sediments/minerals), diminished biochar surface porosity, and sometimes increased removal uncertainty due to low influent concentrations. Our findings demonstrated the complexities associated with applying biochar for stormwater treatment. Further research on biochar-specific biofilter designs is needed to optimize the sorption potential of this material under field conditions.