Superoxide-derived singlet oxygen generation via electron transfer-mediated activation of persulfate by heterogeneous biochar catalysts for naproxen degradation: Effects of aromatization degree on electron exchange capacity.

In this study, heterogeneous biochar catalysts derived from spent coffee grounds (HCBCs) and walnut shells (HWBCs) were synthesized at three pyrolysis temperatures (500 °C (HCBC500, HWBC500), 650 °C (HCBC650, HWBC650), and 800 °C (HCBC800, HWBC800)) to elucidate effects of changes in the aromatization degree determining electron exchange capacity (EEC) of heterogeneous biochar catalysts on the degradation of naproxen (NPX) via the electron transfer-mediated activation of peroxydisulfate (PDS). The greater EEC values of highly aromatic HCBCs and HWBCs produced at higher pyrolysis temperatures led to increased degradation efficiencies of NPX by the HCBCs/PDS and HWBCs/PDS systems. The HCBC800/PDS system achieved the highest degradation efficiency of NPX, at 80.9%, compared to 16.4-48.1% for other systems. These observations highlight that the EEC relying on the aromatization degree of heterogeneous biochar catalysts is a key factor governing the degradation of NPX via the electron transfer-mediated activation of PDS. In the HCBC800/PDS system, electrophilic decarboxylation induced by superoxide-derived singlet oxygen was mainly responsible for the degradation of NPX rather than hydroxyl radical-driven electrophilic hydroxylation. Moreover, the HCBC800/PDS system exhibited excellent reuse efficiency (≥73.2%) for the degradation of NPX over four consecutive cycles. Although increases in bioaccumulation potential and mutagenicity were detected for some degradation intermediates of NPX produced via the HCBC800/PDS system, most of them were less harmful to aquatic ecosystems. Therefore, HCBC800 could be a promising option as a carbonaceous material-based heterogeneous catalyst to activate PDS via the electron transfer for eliminating NPX.