Tailored carbonaceous materials as biofilter amendments for PFAS removal in stormwater runoff

This study investigates the efficacy of metal-modified biochar (metal char) as biofilter amendments for the removal of six per- and polyfluoroalkyl substances (PFAS) in stormwater runoff. The metal identity (i.e., Zn, Al, and Fe) and production conditions substantially influenced the performance of metal char, with Zn Char exhibiting a greater surface area (586.28 m² g^-1), smaller pore size (3.44 nm), higher metal content (5.53 % w/w), and highest affinity toward PFAS compared to Al and Fe Char. Zn Char's enhanced PFAS removal was attributed to the synergistic effects of increased surface area, optimized porosity, and localized electrostatic interactions with metal oxide domains. Structural differences among the six PFAS, including carbon chain length, terminal head group, and degree of fluorination, strongly influenced their adsorption kinetics and capacity. Batch experiments revealed that Zn Char demonstrated substantially enhanced PFAS removal compared to unmodified char, achieving adsorption performance that closely tracked granular activated carbon (GAC) for long-chain PFAS. Under variable flow conditions, Zn Char delayed the PFAS breakthrough compared to unmodified char and tracked GAC performance despite being used at higher doses, with particularly enhanced retention for PFOA and 6:2 FtS. While PFOS showed no breakthrough during both storm events, short-chain PFAS exhibited rapid breakthrough but with improved retention by Zn Char relative to unmodified biochar. These findings highlight metal char as a promising alternative to conventional adsorbents for PFAS removal in dynamic flow conditions experienced during stormwater management.