Biochar derived from different crop straws as persulfate activator for the degradation of sulfadiazine: Influence of biomass types and systemic cause analysis

In order to study the effect of biomass types on the catalytic activity of biochar, six kinds of biochar prepared from six common crop straws were used for persulfate activation to degrade sulfadiazine (SDZ). The SDZ removal rates of biochar samples from sorghum, reed, cotton, rape, sesame, and soybean stalks were 94.4%, 85.1%, 68.4%, 46.2%, 43.6%, 36.6%, respectively, their catalytic performances were significantly different. The six biochar samples were systematically characterized, and differences in their persistent free radicals, dissolved organic matter, oxygenated functional groups, as well as carbon configuration were found. The degradation mechanism was studied by identifying active species and electrochemical experiments. It was concluded that sulfadiazine was decomposed through nonradical pathways in all biochar + persulfate systems. The graphitic carbon in biochar acted as an electron shuttle for direct electrons transfer from sulfadiazine to persulfate, which mainly contributed to the degradation of sulfadiazine. The difference in graphitization of biochar from different crop straws led to a huge gap in their catalytic capacities. Finally, the degradation pathways of sulfadiazine in the system dominated by biochar-mediated electron shuttling were explored. It was found that the transfer pathways of sulfadiazine were similar to those of the radical pathway. This work found for the first time that the type of crop straw can affect the graphitization degree of biochar, which can provide a theoretical basis for the large-scale application of biochar catalysts.