Leaching of PFOS and PFBS to Groundwater at US Air Force Bases: Could Biochar Offer an Effective Mitigation Strategy?

The use of aqueous film-forming foams at military bases potentially leads to the contamination of soils and groundwater by per- and polyfluoroalkyl substances (PFAS). Herein, an analysis of data from US Air Force bases was conducted to demonstrate that high levels of soil and groundwater PFAS contamination at such sites are common, particularly in testing and training areas, suggesting that prevention of PFAS leaching from such soils is a critical priority. Nonsaturated soil column experiments were conducted to optimize the use of soil-applied biochars as a low-cost strategy for adsorbing C8 perfluorooctanesulfonic acid (PFOS) and C4 perfluorobutanesulfonic acid (PFBS) and preventing their downward leaching. A wood-based biochar produced at >900 °C and added to the soil at 1% (m/m) was found to exhibit physio-chemical properties that facilitated excellent (>99%) retention of PFOS in a sandy loam soil. Moreover, postproduction thermal treatment (400 °C in air) of the biochar led to excellent retention of short-chain PFBS, essentially yielding nondetectable PFBS levels in the column leachate. The application of carefully selected biochars to areas with high levels of PFAS contamination at military facilities is likely a useful and low-cost approach for protecting groundwater from PFOS and PFBS (and perhaps other PFAS) contamination.