Impact of Surfactant-Modified Biochar–Clay Soil Amendments on Per/Polyfluoroalkyl Substance (PFAS) Accumulation in Lettuce ( Lactuca sativa )

Abstract

Bioaccumulation of per/polyfluoroalkyl substances (PFASs) in agricultural plants is an emerging concern for human exposure. Immobilizing PFASs in soil using amendments is an effective strategy for reducing the levels of some PFASs in plants but is typically less impactful for PFASs with shorter perfluorinated chains. We evaluated low-cost, locally available clay-based amendments for reducing the uptake of short- and long-chain perfluoroalkyl carboxylates and sulfonamides in lettuce (Lactuca sativa) grown in PFAS-contaminated soil. We compared the performance of montmorillonite clay modified with cationic surfactant, biochar–clay composite, and surfactant-modified biochar–clay composite applied to soil at 0.5%, 1.0%, and 4.0% (w/w). PFAS concentrations in soil, porewater, and lettuce shoots and roots were measured after an 80-day growing period to track how amendments impacted PFAS partitioning, bioavailability, and ultimately accumulation in lettuce shoots. Amendment addition significantly decreased porewater ∑6PFAS, with long-chain PFASs removed more effectively than short-chain PFASs. Organic carbon–water partitioning coefficients demonstrated increased sorptive capacity in soils with surfactant-modified amendments, including for some short-chain PFASs like perfluorohexanoate (PFHxA). Total PFAS concentrations in tissues from lettuce grown in amended soils were significantly reduced, with surfactant-modified amendments appearing to be most effective. This suggests that surfactant addition enhanced both hydrophobic and electrostatic interactions promoting long- and short-chain PFAS sorption. Longer-chain PFASs were reduced in lettuce tissue to the greatest extent. Despite this, due to their high toxicity, long-chain PFAS in these lettuce tissues were still estimated to pose risks from regular human consumption.