Electrocatalytic degradation of norfloxacin using biochar electrode derived from oily sludge: Synergistic effects of hydrothermal and pyrolytic treatment

The disposal of large quantities of oily sludge remains a significant global challenge in terms of resource utilization. In this study, oily sludge was transformed into biochar specifically via a sequential process involving hydrothermal carbonization (HTC) at 250 °C followed by pyrolysis at 800 °C (HTC-250-PY-800). This integrated biochar was comprehensively compared against those produced solely using HTC or pyrolysis. The results showed that HTC-250-PY-800 exhibited a higher density of structural defects and a more highly developed spatial structure. It demonstrated the highest oxygen evolution potential alongside the lowest charge transfer resistance. When employed as a biochar electrode, HTC-250-PY-800 achieved exceptional removal efficiency (98.5%) of norfloxacin within 2 h, significantly outperforming biochars produced from either individual process. Additionally, this integrated biochar maintained robust catalytic performance across a broad pH range (3−11) and retained high degradation efficiency and stability over five continuous cycles. Quenching experiments confirmed that •OH and 1 O₂ collaboratively drove norfloxacin degradation. Based on detailed DFT calculations and UPLC-TOF-MS analyses, potential degradation pathways for norfloxacin were proposed. This study not only provides an effective two-stage strategy for converting waste oily sludge into high-value functional electrode materials, but also highlights the potential of oily-sludge-derived biochar as a sustainable catalyst for degrading emerging pollutants.