Sulfadiazine oxidation by peroxodisulfate and cobalt loaded graphitic biochar system: The dominant role of graphitic domain

A series of Co and biochar composite catalysts were fabricated by impregnation-pyrolyzation, acid-etching, and second annealing. The peroxodisulfate (PDS) activation capacity of these catalysts were evaluated among which the catalyst with largest surface area and graphitization degree achieved the best sulfadiazine (SDZ) degradation performance. The result of mechanism research excluded the essential contribution of Co species toward PDS activation, and pinpointed the paramount role of a single electron transfer pathway involved the formation of inner sphere complexes between catalyst and PDS (catalyst/PDS*), which could capture electron from SDZ, resulting in SDZ oxidation. The graphitic carbon with excellent electronic conductivity was discerned as the active site and especially the graphite microcrystals along c axis played more significant role in the catalytic degradation than the graphene sheet along a axis. The catalyst/PDS* originated from these catalysts with more graphitization degree possessed higher redox potentials and thereby stronger oxidation capacity. The selective degradation capacity of prepared catalyst and PDS system was demonstrated. Eight degradation intermediates from four proposed degradation pathways were identified and the toxicity of most transformation products was lower than that of SDZ.