Confined highly dispersed trace cobalt species in shell porous carbon for activating peroxymonosulfate: Role of superoxide radical and singlet oxygen

The high cobalt leaching of cobalt-based catalysts during the activation of PMS is one of the bottlenecks in SR-AOPs. Herein, a series of trace cobalt confined in carbon shell catalysts (HDCo@C-X) with well-defined structures were designed using NH2-SiO2 nanospheres as a template via the surface-grafting−anchoring method, which was the innovation of the catalysts. AO7 removal rates were in an order of HDCo@C-700 (100%) ≈ HDCo@C-800 (100%) > HDCo@C-600 (80.4%) > Co/C-700 (43.8%). The apparent rate constant of AO7 in HDCo@C-700/PMS (0.0691 min−1) was 8.03 times higher than that of Co/C-700/PMS (0.0086 min−1). The synergistic index (SI) value was introduced to characterize the synergistic relationship between the active sites and confined structures of the catalyst. The SI value of HDCo@C-700/PMS system was 5.2, indicative of a strong synergistic effect between active sites and confined structures. The activation mechanism of HDCo@C-700 for PMS coexisted with free-radical (SO4•−, •OH, and O2•-) and non-radical (1O2 and electron transfer) pathways, and both O2•-and 1O2 played a major role in AO7 degradation. The catalytic activity of HDCo@C-700 gradually decreased to 39.6% and 27.0% for the 2nd and 3rd reuse, respectively, mainly ascribed to the surface oxidation of the carbon shell and the ultra-trace leaching of Co ions. Finally, the intermediates of AO7 in the HDCo@C-700/PMS system were detected by LC-MS, and a possible degradation path was proposed. The in silico toxicity predictions suggested the non-overlooked transformation-derived risks from the degradation products of AO7. This study provides a new insight into the design of PMS activators.