Mn-Doped Biochar Derived from Sewage Sludge for Ciprofloxacin Degradation

Three different MnO2 polymorphs, α-MnO2, β-MnO2, and δ-MnO2, with K+-tuned tunnel structures were loaded on sludge-based biochar (BC). Then MnO2-loaded BCs served as the catalyst for ciprofloxacin (CIP) removal. Results showed that as-prepared materials exhibited better catalytic activity than raw BC, among which BC-α-MnO2 achieved superior CIP removal than BC-β-MnO2 and BC-δ-MnO2. CIP removal increased when pH increased from 3 to 5, then slightly decreased with further increase of pH. Desorption experiments confirmed that both adsorption and degradation contributed to the CIP removal by MnO2-loaded BCs. Compared with the raw BC, the contribution of degradation increased 27.87%, 19.87%, and 14.19%, respectively, for BC-α-MnO2, BC-β-MnO2, and BC-δ-MnO2. The radical quenching experiments and electron paramagnetic resonance tests demonstrated the existence of reactive oxidation species (ROS) including O2−•, •OH, and O21. The O2−• and •OH were the main ROS during CIP degradation by BC, while O21 was the main ROS for MnO2-loaded BCs. BC-α-MnO2 featured the most abundant ROS, followed by BC-β-MnO2 and BC-δ-MnO2. The persistent free radicals and Mn in MnO2-loaded BCs may serve as the active site for electron transfer during the degradation process, thus leading to the generation of O2−•, •OH, and O21.