Perfluoroalkyl and Polyfluoroalkyl Substances Release from Biosolid-Derived Compost

This study investigated the release potential and controlling mechanisms of representative per- and polyfluoroalkyl substances (PFAS) of varying carbon-chain lengths from a commercial biosolid-derived compost using sequential leaching with water and saline solutions (10 mM NaCl and 5 mM CaCl₂). The compost contained >40% organic matter, PFAS concentrations up to 140 ng·g^-1, and precursor levels below 5 ng·g^-1. Eluate analyses revealed dissolved organic matter (DOM) concentrations up to 1400 mg·L^-1 and major ions (Ca, Mg, Na, K, Cl, P, and S) reaching 600 mg·L^-1, accompanied by PFAS concentrations up to 2600 ng·L^-1. PFAS desorption followed a biphasic patternrapid release within the first hour followed by a slower, sustained phase over 48 hwell described by a first-order two-compartment model with rate constants k ₁ = 1-7 h^-1 and k ₂ = 0.001-0.016 h^-1. Electrostatic interactions dominated the desorption process, as both Na⁺ and Ca²⁺ reduced the fraction of fast-release sites (F ₁), with Ca²⁺ showing a stronger suppression due to cation bridging and charge screening. Perfluorohexanoic acid (PFHxA) and perfluorohexanesulfonic acid (PFHxS) were almost completely released within 1 h, whereas perfluorobutanoic acid (PFBA), perfluorobutanesulfonic acid (PFBS), perfluorooctanoic acid (PFOA), and perfluorooctanesulfonate (PFOS) exhibited prolonged desorption associated with interactions with solid organic matter and DOM. DOM-facilitated desorption was evident, as PFAS-DOM complexation enhanced release under certain ionic conditions. Overall, the results reveal the intricate coupling among compost matrix properties, PFAS molecular structure, and eluent chemistry governing PFAS mobility.