Coal gasification slag as a low-cost alternative to synthetic adsorbents for C7-C9 perfluorocarboxylic acid removal

Per- and polyfluoroalkyl substances (PFAS) pose serious environmental risks due to their persistence and bioaccumulation potential. In this study, unmodified coal gasification fine slag (CGFS), an industrial solid residue, was directly used as an adsorbent for the efficient removal of C7-C9 perfluorocarboxylic acids (PFCAs) from aqueous solution, namely perfluoroheptanoic acid (PFHpA), perfluorooctanoic acid (PFOA) and perfluorononanoic acid (PFNA). Comprehensive characterization revealed that CGFS is a mesoporous adsorbent rich in carbon-based functional groups. Batch adsorption experiments demonstrated that the adsorption process was pH-dependent and exothermic, with optimal performance achieved at pH 3. The adsorption kinetics were well described by the pseudo-second-order model, whereas the equilibrium data were satisfactorily fitted by the Langmuir isotherm. The maximum adsorption capacities reached 181.5, 240.0 and 297.3 mg/g for PFHpA, PFOA and PFNA, comparable to those of widely reported carbon-based and amine-functionalized adsorbents. The adsorption process exhibited a clear chain-length dependence, favoring longer-chain PFCAs, and was governed by electrostatic attraction and hydrophobic interactions, as corroborated by density functional theory calculations. After four regeneration cycles involving desorption with 1 M NaOH solution for 3 h, CGFS retained over 80% of its initial adsorption capacity, indicating good stability and reusability. Moreover, CGFS showed promising performance in real water matrices, effectively removing organic substances and enhancing biodegradability. This work shows that low-cost CGFS can achieve high-efficiency removal of PFCAs, providing a promising strategy for the remediation of emerging PFCAs and the resource utilization of industrial solid waste.