Encapsulating MnFe LDH in Biochar Tunes Persulfate Activation from Radical to Nonradical Pathway: Significant Role of Electron Transfer

Nonradical peroxydisulfate (PDS) oxidation has attracted great interest due to its mild oxidant dosage and little environmental impact. In this study, biochar-supported flower-like MnFe layered double hydroxide (BC-LDH) was prepared, and the PDS activation mechanisms were probed with ciprofloxacin (CIP) as representative contaminant. Compared to biochar (BC), MnFe LDH, and physical mixed BC/LDH, PDS activation was tuned to an electron-transfer-dominated nonradical pathway with coprecipitated BC-LDH. Electrochemical techniques including electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), Tafel, and two-chamber experiments confirmed that the synergistic effect between BC and LDH remarkably facilitated electron transfer from CIP to PDS. Degradation efficiency ranging from 92 to 94% was achieved with a PDS dosage ranging from 0.2 to 4 mM, and degradation rate constant was inversely proportional to the electron transfer resistance of PDS activators. Three degradation pathways for CIP were proposed based on the intermediates analyzed by ultra-performance liquid chromatography-mass spectrometry/MS (UPLC-MS/MS), and the toxicity of CIP was significantly decreased. This study proposed a novel strategy for enhancing electron-transfer-dominated nonradical PDS activation pathway with biochar/transition-metal oxide composites for the remediation of contaminants.