Valorization of industrial coal fly ash into a magnetically recoverable adsorbent for PFOA removal from water

Per- and polyfluoroalkyl substances (PFAS) are persistent contaminants that are challenging to remove using conventional water-treatment processes. In this study, coal fly ash (FA) was functionalized with cetyltrimethylammonium bromide (CTAB) and subsequently loaded with Fe 3 O 4 to produce a waste-derived, magnetically responsive adsorbent for perfluorooctanoic acid (PFOA) removal. FTIR, XRD, SEM–EDS, XPS, and N 2 adsorption–desorption analyses confirmed CTAB functionalization, Fe 3 O 4 incorporation, and the development of a mesoporous composite structure. Mag–CTAB–FA exhibited a BET surface area of 82.35 m 2 g −1 , a total pore volume of 0.224 cm 3 g⁻ 1 , and an estimated mean pore diameter of 10.88 nm. A CTAB:FA ratio of 1:10 provided the highest performance, while the final Mag–CTAB–FA composite achieved 88.3% PFOA removal under the selected screening conditions. Adsorption increased markedly with adsorbent dosage, remained effective over a broad pH range, and was only slightly affected by temperature. Equilibrium uptake increased to a maximum experimentally measured capacity of 32.03 mg g⁻ 1 at an initial PFOA concentration of 100 mg L⁻ 1 . The Langmuir model provided a suitable representation of the equilibrium data and estimated a maximum adsorption capacity of 38.10 mg g⁻ 1 . Kinetic analysis showed rapid initial uptake, with the fractal-like Vermeulen model providing the best statistical description of the experimental data. The multilinear Weber–Morris plot revealed three uptake stages and indicated concurrent contributions from boundary-layer transport and intraparticle diffusion. Fe release remained below 28.39 μg L⁻ 1 and 0.00011% over 192 h, supporting the aqueous stability of the magnetic phase. The adsorbent retained 89.7% of its initial capacity after five adsorption–desorption cycles and achieved up to 99.9% PFOA removal from spiked municipal wastewater at sufficient dosage. Overall, Mag–CTAB–FA combines industrial-waste valorization, high PFOA removal, magnetic responsiveness, limited Fe release, and repeated-use capability, demonstrating its potential for further development in water-treatment applications.