Unveiling the mechanism of biochar-activated hydrogen peroxide on the degradation of ciprofloxacin

Biochar (BC) is usually applied in the remediation of contaminated water and soil as an excellent adsorbent, but recently, the BC-induced catalytic degradation of organic contaminants by persistent free radicals (PFRs) is attracting considerable attention. In this study, BC was prepared from the pyrolysis of sewage sludge and used to activate hydrogen peroxide (H2O2) for the degradation of ciprofloxacin (CIP) in water. Results showed that the HNO3 modified-BC exhibited better catalytic ability to activate H2O2 than the raw BC. Under similar conditions: pH 7 and H2O2 1.0 mmol/L, the removal efficiency of CIP by HNO3 modified-BC/H2O2 (93.00%) was obviously superior to that of BC/H2O2 (70.60%). The calculated contributions of adsorption and degradation to CIP removal demonstrated that BC exhibited certain CIP adsorption ability, but the contributions of adsorption led by raw BC and HNO3 modified-BC were only 38.31% and 29.81%, respectively. The proportion of catalytic oxidation induced by PFRs in BC accounted for 61.69% and 70.19%, suggesting the important role of PFRs in CIP removal. HNO3 modification not only altered the surface characteristics of BC, but also improved the quantities and types of PFRs in BC, which promoted the production of OH. In addition, the possible mechanism for H2O2 activation by PFRs in BC is that the single-electron transferring from sp2 CC, CO, pyridonic-N and pyridinic-N of PFRs in BC to H2O2, thus leading to the generation of OH.