N and S co-doped pine needle biochar activated peroxydisulfate for antibiotic degradation

A low-cost nitrogen and sulfur doped biochar (CNS) was prepared by oxygen limiting method using pine needles. In the CNS/PDS system, the degradation rate of tetracycline hydrochloride (TCH) reached 85.6% in 20 min when the pyrolysis temperature was 800 °C. X-ray photoelectron spectroscopy and Raman spectroscopy showed that high-temperature pyrolysis produced more carbonyl and defect, which enhanced the activation efficacy of CNS for PDS. Electron paramagnetic resonance and radical quenching have demonstrated that O2•- and 1O2 played key roles in radical pathway. A novel mechanism for degradation of TCH by electron transfer pathway was proposed by linear sweep voltammetry and electrochemical impedance spectroscopy. CNS served as a transport mediator and the electrons of TCH were continuous transferred to PDS via CNS, which accelerated its oxidation. Moreover, density functional theory demonstrated that pyrrolic N combined with the H atom of PDS to form an N–H structure and generated a PDS-catalyst complex. The projected density of state results showed that pyrrolic N and thiophene S further speeded up the electron transfer process due to their closer proximity to 0 Fermi level. This study provides insight into the reaction mechanism of heteroatom doped biochar for treatment of organic pollutants in aquatic environments.