Sewage-sludge-based activated carbon as a sustainable adsorbent for near-complete removal of 6PPD-quinone at environmentally relevant levels

Abstract

The tyre-derived transformation product 6PPD-quinone (6PPD-q) poses a growing threat to aquatic ecosystems, particularly sensitive salmonid species. However, practical and affordable treatment options remain limited. This study investigated the potential of sewage-sludge-based activated carbon (SBAC) produced via ZnCl₂ activation and pyrolysis as a sustainable adsorbent for removing 6PPD-q from contaminated water and advancing circular-economy approaches for sludge valorisation. Fourier-transform infrared spectroscopy revealed the presence of oxygen-containing surface groups on SBAC that can facilitate hydrogen bonding and p–p interactions with 6PPD-q. Batch adsorption experiments were performed to evaluate equilibrium behaviour and thermodynamic properties under controlled conditions. Results showed rapid uptake, achieving >99% removal from an initial concentration of 200 microg/L within 0.5 h at pH 3.5. The Langmuir model best fit the equilibrium data, with R² value of 0.95, yielding a maximum adsorption capacity of 583.3 microg/g. Thermodynamic analysis indicated a spontaneous and endothermic process, suggesting chemisorption as the dominant mechanism. The adsorption efficiency remained stable within the temperature range of 7°C–35°C, and sequential treatments maintained high removal performance. The experimental results demonstrate that SBAC is an efficient and low-cost adsorbent for mitigating 6PPD-q contamination in stormwater, offering a sustainable solution for valorising sewage sludge within circular economy frameworks.