Noncondensed aromatic carbon of sludge-derived biochar predominated peroxydisulfate activation mechanism for tetracycline degradation via an electron transfer pathway

Discrimination of the catalytic ability of heterogeneous biochar components is often challenging. Herein, a sewage sludge-derived biochar (SDBC) was prepared to activate peroxydisulfate (PDS) for tetracycline (TC) degradation. To verify the contribution of different carbon components, SDBC was bleached with NaClO2 and CH3COOH to remove noncondensed aromatic carbon (NAC) contained in biochar, which was confirmed by 13C Nuclear Magnetic Resonance. The batch degradation experiment revealed that NAC removal decreased TC degradation by SDBC from 84.1% to 33.2% within 2 h, indicating its significant role in PDS activation. The quenching and electron paramagnetic resonance experiments suggested a very minor contribution of radical pathway in TC degradation. Instead, the electron transfer pathway predominated TC degradation mechanism as inferred by electrochemical tests. This is likely ascribed to formation of a biochar-PDS metastable complex, facilitating electron transfer from tetracycline-like compounds. An X-ray photoelectron spectroscopy confirmed that the percent of graphitic N in SDBC decreased after the degradation reaction, which suggested graphitic N is an important active site in biochar. Besides, acid-washed SDBC did not change TC degradation behavior excluding significant contribution of minerals in SDBC to PDS activation. Thus, the roles of biochar components in catalyzing PDS were quantified for the first time, proving insight for selection and manipulation of biochar in catalyzing PDS in environmental application.