Singlet oxygen-dominated peroxydisulfate activation by sludge-derived biochar for sulfamethoxazole degradation through a nonradical oxidation pathway: Performance and mechanism

In this study, sludge-derived biochar (SDBC) was prepared and applied in peroxydisulfate (PDS) activation for sulfamethoxazole (SMX) degradation. Compared to the slight adsorption (16.5%) by SDBC alone and low direct oxidation (10.1%) by PDS alone, the SMX degradation rate was drastically increased to 94.6% in the combined SDBC/PDS system, suggesting that SDBC can successfully and efficiently activate PDS. The observed rate constant of the combined SDBC/PDS system was 48.3 times those of both PDS alone and SDBC alone processes. Material characterization and comparative experiments showed nitrogen doping and iron loading into the carbon layer might be the important active sites of the graphene-like SDBC material in PDS activation for SMX degradation. More importantly, singlet oxygen (¹O₂), instead of traditional sulfate radicals or hydroxyl radicals, was the predominant reactive species of the SDBC/PDS system, which involved a new nonradical oxidation method for PDS activation by SDBC. The SMX degradation pathways by the nonradical ¹O₂ oxidation were first studied by combining density functional theory (DFT) calculations with experimental results. Different from the well-known pathways of SMX through the cleavage of the sulfanilamide bond by the attack of radicals, the ¹O₂ was likely to attack the aniline ring of SMX to initiate and accelerate the decomposition process. Finally, the energy cost analysis of the SDBC/PDS system further demonstrated the possible and economic application of the SDBC/PDS technique for SMX degradation. Thus, this study proposed a novel and economic method for PDS activation through a new nonradical oxidation pathway predominated by ¹O₂, which also promoted the safe and efficient transformation of antibiotics or other contaminants by PDS activation processes.