O2-Involved Electro-activation of Persulfate on Oxidized Carbon Black for Effective Sulfamethoxazole Degradation: Key Role of Side-Reactive Oxygen Reduction

The oxygen reduction reaction (ORR) was an unavoidable side reaction in the electro-activating persulfate (PS) process, especially on the ORR-active carbon materials, which has usually been ignored in previous reports. In this work, the O2/N2-involved electro-activation of the PS system was constructed to confirm the role of side-reactive ORR on oxidized carbon black (OCB) for sulfamethoxazole (SMX) degradation. With O2 involvement, 820.8 μM •OH was produced by PDS electro-activation, over 7 times more than 115.8 μM under N2 aeration, leading to a high SMX removal of 90.6% (only 67.8% under N2 aeration). Such a huge difference in •OH yield and SMX removal performance was mainly attributed to side-reactive 2e– ORR, in which as-generated H2O2 was electro-activated to produce abundant •OH and form an advanced homogeneous •OH/SO4•– oxidation system via mutual transformation with gradual acidification. Meanwhile, the introduction of oxygen functional groups and defects on the surface of carbon black calcined at 600 °C (CB600) also contributed to the improvement of SMX removal performance, attributing to their enhancing effects on SMX electro-adsorption, PDS electro-activation, and O2 electro-reduction. Thus, side-reactive oxygen reduction played the spontaneous and synergistic oxidation roles in the O2-involved electro-activating PS process, helping to design highly effective advanced oxidation systems in the future.