Valorization of soybean plant wastes in preparation of N-doped biochar for catalytic ozonation of organic contaminants: Atrazine degradation performance and mechanistic considerations

Selecting the biomass source is critical because different plants or tissues may exhibit varying textures and chemical properties. However, few researchers have compared the catalytic activity of different tissues from the same plant after carbonization. Soybean is widely cultivated worldwide and always suffers from pests and dis-eases. The inappropriate storage of harvested soybeans leads to mildew and its wastage. Herein, soybean biochar (SB), soybean shell biochar (SBS), and soy straw biochar (SS) catalysts were prepared using waste soybean plant tissues as precursors, which drastically improved the degradation efficiency of atrazine in catalytic ozonation. The specific reaction rate constant normalized by the surface area (kobs/SBET) of SB during catalytic ozonation indicated the catalytic performance of biochar was not affected just by SBET. The characterization results showed that the delocalized pi-electrons, nitrogen-, and oxygen-containing functional groups played key roles in catalytic ozonation, and contributions of these active sites to the generation of both radical and nonradical reactive ox-ygen species (ROS) including hydroxyl radicals (center dot OH), superoxide radicals (O2 center dot  ), and singlet oxygen (1O2) contributed to atrazine degradation. Compared with SBS and SS, SB contained the highest pyridine N content, the highest carbonyl group (-C = O) content, the strongest delocalized electron density, and electron donor ability, which was responsible for its superior catalytic performance. Such active sites on biochar enhanced the ozone adsorption and decomposition, accelerated the electron transfer process to produce more ROS, and achieved efficient pollutant elimination. Overall, guided by the "waste treating waste" concept, this study provided a valuable alternative in the recycling and reuse of agricultural waste soybean plants, proving that the derived biochar could be potentially used as a catalyst for advanced oxidation processes-based applications.