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 degrees 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 (S-micro), pore volume (PV), and graphitization degree (I-D/I-G) of PBC were positively correlated with the increase of T (p < 0.05). Notably, the SSA and PV of PBC increased from 4.294 m(2)/g and 0.002 cm(3)/g to 496.864 m(2)/g and 0.261 cm(3)/g as the pyrolysis temperature increased from 300 to 800 degrees 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). R-ad, R-PBC/H2O2, R-PBC/PMS, and R-PBC/PDS represented the removal efficiencies of TC by PBC adsorption, PBC/H2O2, PBC/PMS, and PBC/PDS system. R-PBC/H2O2, R-PBC/PMS, and R-PBC/PDS showed a highly positive correlation to the R-ad (r = 0.965, 0.952, 0.946, p < 0.01). R-ad, R-PBC/H2O2, R-PBC/PMS, and R-PBC/PDS were positively correlated with K, Mg, ash, pH, BFG, and I-D/I-G (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 (O-1(2)) was the main active species. Except for O-1(2), superoxide radical (O-2(center dot-)) also contributed to TC removal in the oxidative systems. From the changes in used PBC-800, the disordered sp(2) hybrid carbon, sp(3) C-C, and pyridinic-N were the main active sites in activation.