Insight into the key factors for the tetracycline removal in biochar-mediated oxidative system

This study prepared pomelo peel biochar (PBC) under different pyrolysis temperatures (T) (300–800 °C) and investigated the removal efficiency of tetracycline (TC) under the oxidative systems established by PBC and oxidants (hydrogen peroxide (H2O2), peroxymonosulfate (PMS), and peroxydisulfate (PDS)). The correlation between the properties of PBC and the removal efficiencies of TC was studied by Pearson analysis to clarify the critical properties of biochar in catalytic processes. The inorganic carbon (IC), ash, pH, basic functional groups (BFG), specific surface area (SSA), microporous surface area (Smicro), pore volume (PV), and graphitization degree (ID/IG) of PBC were positively correlated with the increase of T (p < 0.05). Notably, the SSA and PV of PBC increased from 4.294 m2/g and 0.002 cm3/g to 496.864 m2/g and 0.261 cm3/g as the pyrolysis temperature increased from 300 to 800 °C. In contrast, the yield, organic carbon (OC), and acidic functional groups (AFG) of PBC displayed negatively correlated with the increase of T (p < 0.05). Rad, RPBC/H2O2, RPBC/PMS, and RPBC/PDS represented the removal efficiencies of TC by PBC adsorption, PBC/H2O2, PBC/PMS, and PBC/PDS system. RPBC/H2O2, RPBC/PMS, and RPBC/PDS showed a highly positive correlation to the Rad (r = 0.965, 0.952, 0.946, p < 0.01). Rad, RPBC/H2O2, RPBC/PMS, and RPBC/PDS were positively correlated with K, Mg, ash, pH, BFG, and ID/IG (p < 0.01). There was no statistically significant correlation between persistent free radicals (PFRs) and the removal efficiencies of TC. Furthermore, SSA and PV showed positive relationships with the removal efficiencies of TC in the PBC/PMS and PBC/PDS systems, but no significant correlation with the removal efficiencies of TC in the PBC/H2O2 systems. Moreover, the catalytic behaviors and mechanisms were studied in the PBC-800 based oxidative systems. The quenching experiments and EPR spectra indicated that singlet oxygen (1O2) was the main active species. Except for 1O2, superoxide radical (O2•−) also contributed to TC removal in the oxidative systems. From the changes in used PBC-800, the disordered sp2 hybrid carbon, sp3 C–C, and pyridinic-N were the main active sites in activation.