Optimized one-step pyrolysis synthesis of zinc-modified sludge biochar for enhanced perfluorooctane sulfonate removal from water

Perfluoroocatane sulfonate (PFOS) has drawn increasing global attention due to its environmental omnipresence and adverse health effects. This study presents a synergistic strategy to address the dual challenge of legacy PFOS contamination and sustainable sewage sludge disposal by developing a novel, zinc-modified sludge-based biochar (Zn/SBC) for highly efficient PFOS removal. The innovation lies in the one-step pyrolysis using ZnCl 2 as an activator, which dramatically enhanced the specific surface area by 50-fold (to 261.33 m 2 /g) by creating a well-developed micro- and mesoporous structure. Under optimized conditions, 26%Zn/SBC exhibited a superior PFOS adsorption capacity of 60.14 mg/g, 5.83 times than that of pristine biochar, and achieved >99% removal across a wide pH range (3–11). The material exhibited remarkably stable performance, sustaining a removal efficiency over 93.6% in the presence of high concentration of competing ions and humic acid. After 5 cycles of repeated uses, 26%Zn/SBC remained highly reactive in adsorption for PFOS. In addition, the superior adsorption of PFOS by optimized 26%Zn/SBC is attributed to a unique concurrent mechanism, including electrostatic attraction facilitated by the elevated point of zero charge (pH PZC  = 8.1), surface complexation with Zn/ZnO sites, π-anion interaction, and hydrophobic effects. This study not only presents a high-performance and low-cost adsorbent for PFOS remediation, which holds the potential for more cost-effective PFAS removal, but also establishes a sustainable "waste-to-wealth" strategy, paving the way for practical solutions to concentration persistent organic pollutants in aquatic environments.