Effects of biochar-dissolved organic matter on the photodegradation of sulfamethoxazole and chloramphenicol in biochar solutions as revealed by oxygen reduction performances and free radicals

Highlights: • BDOMs promoted the photodegradation rates and efficiencies of SMX and CAP. • The currents of BCs significantly increased during photodegradation process. • The DMPO-OH from BDOMs in BCs solutions improved the photodegradation efficiencies. Photodegradation plays an important role to directly and indirectly degrade residual antibiotics in environments. Biochar as an emerging material can be sustainably produced and used in various environmental remediation fields. However, very few efforts and mechanisms have been made to elucidate the photodegradation of sulfamethoxazole (SMX) and chloramphenicol (CAP) in the biochar (BCs), modified-biochar (MBCs), and biochar-derived dissolved organic matters (BDOMs) solutions under Xenon-lamp irradiation. In this study, different concentrations of BCs and MBCs (100, 250, 500, 1000 and 2000 mg L⁻¹), and BDOMs (2, 4, 6 and 10 mg L⁻¹) have been chosen to research the effects on the photodegradation of SMX and CAP. By studying the oxygen reduction performances and environmental persistent free radicals (EPFRs) of BCs or MBCs and DMPO-·OH adducts from the photodegradation process, the different photodegradation pathways were proposed. SMX and CAP can be directly photodegraded in the BCs or MBCs solutions by Xenon-lamp. In addition, in the low-concentration of BCs (100, 250, 500 mg L⁻¹) and MBCs solutions, the oxygen reduction performances and EFPRs of BCs and MBCs can indirectly photodegrade SMX and CAP by promoting the oxidation process and electron transfer on BCs surface under Xenon-lamp irradiation. While the high-concentrations of BCs (1000 and 2000 mg L⁻¹) and MBCs hindered the penetration of Xenon-lamp, resulting in decreasing the photodegradation efficiency. In particular, BDOMs released from BCs solutions significantly improved the photodegradation rates and efficiencies of SMX and CAP by generating ·OH radicals to photodegrade SMX and CAP indirectly. Therefore, this study provides a theoretical basis for the photodegradation of SMX and CAP with biochar and modified-biochar, and also gives a comprehensive evaluation of the effects and mechanisms of BDOMs on the photodegradation.