Long-term efficiency of biochar amendment in PFAS-contaminated soil under unsaturated field conditions

The present study investigates the long-term immobilization efficiency of biochar on target per- and polyfluoroalkyl substances (PFAS) and precursors in well-drained soils contaminated by aqueous film-forming foam (AFFF) (Ʃ₂₇PFAS = 1624 ± 276 µg/kg) over 2 years. The total oxidizable precursor (TOP) assay revealed a large precursor reservoir in the soil. Fifteen outdoor field-scale columns were packed with contaminated soil (48 kg) without (control columns, triplicates) and with biochar amendments: Three sewage sludge-based biochars were homogeneously mixed into the soil at a 1% (w/w) dose in triplicate columns. One of the biochars was additionally applied as a barrier at the column base (1% w/w) in a separate set of columns. The best-performing biochar immobilized long-chain PFAS by 91.0 ± 35.0% and short-chain PFAS by 96.7 ± 32.9%, possibly due to a well-developed porosity. Compared to the control columns, the fluctuating PFAS leaching were negligible in columns amended with the best-performing biochar, but the immobilization efficiency of short-chain PFAS decreased after one year (from 97.8% to 74.2%). Applying biochar as a barrier was two times more effective than homogenous mixing, and the effect was most pronounced for long-chain PFAS. Our findings suggest that biochar may immobilize precursors, notably CF₃-CF₅ precursors, to the same extent or better than their typical target perfluoroalkyl acids transformation products. More research is, however, needed to confirm these trends. Going beyond simple lab experiments, this study suggests that biochar is a promising solution for PFAS remediation and brings the technology closer to field application.