Fe-N co-doped biochar: Enhanced removal efficiency and mechanistic insight into perfluorooctane sulfonate (PFOS)

Perfluorooctanesulfonic acid (PFOS) poses a potential threat to human health owing to its exceptional environmental persistence and bioaccumulation. Biochar exhibits considerable potential for PFOS removal, and metal/heteroatom doping or co-doping has been shown to effectively enhance its efficiency. In this study, Fe-N co-doped biochar was fabricated from agricultural rice straw waste, using potassium ferrate and melamine as precursors for Fe and N, respectively. The PFOS removal performance of the as-prepared biochar in solution was systematically evaluated by investigating the effects of initial solution pH, contact time, initial PFOS concentration, and co-existing substances (i.e., anions and humic acid). Results demonstrated that the Fe-N co-doped biochar pyrolyzed at 800 °C (Fe/N-BC8) possessed a significantly higher specific surface area of 290.76 m²/g, larger than that of the pristine biochar (BC8) with 12.95 m²/g. Moreover, the modified surface chemistry of Fe/N-BC8 endowed it with superior adsorption capacity and broader pH adaptability for PFOS. The PFOS adsorption behavior on Fe/N-BC8 was well fitted by the liquid film diffusion and the Langmuir-Freundlich model. Fe/N-BC8 exhibited a considerable PFOS adsorption capacity with a maximum value of 2.06 mg/g. Surface characterizations of Fe/N-BC8 revealed that the incorporated Fe-N active sites play a critical role in PFOS removal, which was governed by a synergism of electrostatic attraction, surface complexation (i.e., ligand exchange), and hydrophobic interactions. This work provides a high-performance and cost-effective adsorbent for PFOS remediation, and offers a novel design strategy and theoretical basis for developing functionalized biochars of persistent organic pollutants.