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 k₁ = 1–7 h^–1 and k₂ = 0.001–0.016 h^–1. Electrostatic interactions dominated the desorption process, as both Na⁺ and Ca²⁺ reduced the fraction of fast-release sites (F₁), 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.