Sorption of per- and polyfluoroalkyl substances across soil horizons

This study evaluated the sorption of seven per- and polyfluoroalkyl substances (PFAS) across four synthetic soil horizons representative of biosolids-impacted soils in Maine. The experimental soil profiles comprised four distinct layers (Depth 1–4) with varying compositions, where Depth 1 represented organic-rich surface soils and Depth 4 was the mineral layer characterized by higher sand and clay contents. This design enabled controlled investigation of depth-dependent PFAS sorption. Sorption kinetics for PFAS were fit by the pseudo-second-order model (R² = 0.96–0.99), where PFOS exhibiting slower sorption than PFOA. In terms of sorption capacity, q e decreased from 2.08-1.54 µg g⁻¹ for PFOS and from 0.67-0.52 µg g⁻¹ for PFOA from surface to deeper soils, respectively. PFOS exhibited the highest sorption capacity at Depth 1 (qₘₐₓ=28.17 µg/g) while 90% less capacity of 2.10 µg/g at Depth 4. Across all depths, long-chain PFAS exhibited higher sorption capacities (q e =0.52-2.5 µg/g) than short-chain compounds (q e =0.001-0.74 µg/g). Overall, PFAS sorption increased with OM, PFAS chain length and hydrophobicity, indicating organic-rich surface soils acting as primary sinks, while mineral-dominated subsoils exhibited limited sorption capacity. These findings can enable vertical transport models where the mobility of short-chain PFAS and the depth-sensitive sorption behavior of long-chain analogues are crucial.