Mechanisms and influences on the formation of electron-transfer complexes in the activation of N,B co-doped graphitic biochar for peroxydisulfate degradation of oxytetracycline

The activation mechanism and degradation mechanism of peroxydisulfate (PDS)-based advanced oxidation technologies in applications have received increasing attention. In this paper, N,B co-doped biochar graphene (N, B-rGO-900) was synthesised from discarded coconut shells, which showed good adsorption and catalytic properties in the removal of oxytetracycline (OTC), there is no energy barrier to impede the electron transfer process. The adsorptive removal rate was 53 % in 30 min, and the degradation rate was 94.14 % in 15 min, with a reaction rate constant of 0.1363 min- 1 (k0- 15min), which was higher than the degradation rate(47 %) in 15 min for N-rGO with an energy barrier of 0.215 eV. More importantly, this study clarifies the structure-activity relationship of the catalyst, in the radical pathway, the number of electrons transferred to the PDS from the lone pairs of pyrrolic N and pyridinic N on N,B-rGO-900 was 0.045 e, which was higher than that of N-rGO (0.032 e) and pristine GO (0.019 e), contributing to the decomposition of PDS to produce radicals; in contrast, the N,B-rGO-900 in the nonradical pathway acts as an electron channel to promote electron migration from OTC to surfaceconfined N,B-rGO/PDS* complexes, the electron migration is driven by the potential energy difference between the highest occupied molecular orbital (HOMO, -4.932 eV) and the lowest vacant molecular orbital (LUMO, -1.923 eV), a process in which PDS extracts 2 electrons to the OTC, while ketones or quinones in the catalyst can act as Lewis acid sites to activate PDS to produce 1O2, in addition the introduction of N and B atoms reduces its activation energy in the reaction process (Ea = 45.45 kJ mol- 1). The highest contributing active substance in the degradation process was O2 center dot -(97 %) in the free radical pathway, and it was capable of degrading OTC into small molecules that are non-toxic to fish, daphnid, and algae, toxic concentrations much greater than 100 mg/L, and the catalyst was able to maintain an excellent performance after four reuse experiments and antiinterference experiments. This study systematically elucidates the process mechanism of antibiotic degradation by persulfate and provides new insights into the design of economic, efficient and stable targeted carbonaceous catalysts.