Mn-Doped Biochar Derived from Sewage Sludge for Ciprofloxacin Degradation

Three different ${\mathrm{MnO}}_2$ polymorphs, $\alpha {\text{-}\mathrm{MnO}}_2$, $\beta {\text{-}\mathrm{MnO}}_2$, and $\delta {\text{-}\mathrm{MnO}}_2$, with ${\mathrm{K}}^{+}$-tuned tunnel structures were loaded on sludge-based biochar (BC). Then ${\mathrm{MnO}}_2$-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 $\mathrm{BC}\text{-}\alpha {\text{-}\mathrm{MnO}}_2$ achieved superior CIP removal than $\mathrm{BC}\text{-}\beta {\text{-}\mathrm{MnO}}_2$ and $\mathrm{BC}\text{-}\delta {\text{-}\mathrm{MnO}}_2$. 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 ${\mathrm{MnO}}_2$-loaded BCs. Compared with the raw BC, the contribution of degradation increased 27.87%, 19.87%, and 14.19%, respectively, for $\mathrm{BC}\text{-}\alpha {\text{-}\mathrm{MnO}}_2$, $\mathrm{BC}\text{-}\beta {\text{-}\mathrm{MnO}}_2$, and $\mathrm{BC}\text{-}\delta {\text{-}\mathrm{MnO}}_2$. The radical quenching experiments and electron paramagnetic resonance tests demonstrated the existence of reactive oxidation species (ROS) including ${\mathrm{O}}_2^{-}•$, •OH, and ${}^1{\mathrm{O}}_2$. The ${\mathrm{O}}_2^{-}•$ and •OH were the main ROS during CIP degradation by BC, while ${}^1{\mathrm{O}}_2$ was the main ROS for ${\mathrm{MnO}}_2$-loaded BCs. $\mathrm{BC}\text{-}\alpha {\text{-}\mathrm{MnO}}_2$ featured the most abundant ROS, followed by $\mathrm{BC}\text{-}\beta {\text{-}\mathrm{MnO}}_2$ and $\mathrm{BC}\text{-}\delta {\text{-}\mathrm{MnO}}_2$. The persistent free radicals and Mn in ${\mathrm{MnO}}_2$-loaded BCs may serve as the active site for electron transfer during the degradation process, thus leading to the generation of ${\mathrm{O}}_2^{-}•$, •OH, and ${}^1{\mathrm{O}}_2$.