Decomposition of carbon-based catalysts in advanced oxidation processes: A neglected but noteworthy problem

Carbon-based catalysts have been widely investigated in the field of advanced oxidation processes (AOPs), however their structural stability during the AOPs has been neglected. In this study, the decomposition of different carbonaceous materials, including g-C 3 N 4 , biochar, biochar/g-C 3 N 4 composite, graphene oxide and COF in sulfate radical-based and hydroxyl radical-based AOPs was investigated, mainly focusing on g-C 3 N 4 and modified g-C 3 N 4 in peroxymonosulfate (PMS)-based AOPs. The g-C 3 N 4 prepared with different precursors and modified g-C 3 N 4 were selected to investigate their structural stability in PMS-based AOPs in detail. The results showed that both g-C 3 N 4 and modified g-C 3 N 4 were partially decomposed in the PMS-based AOPs leading to the carbon dissolution in solution. The concentration of dissolved carbon varied with the precursor of g-C 3 N 4 and experimental conditions such as PMS concentration and inorganic ions. Modified g-C 3 N 4 presented higher concentration of dissolved carbon than pristine g-C 3 N 4 . Based on the results of experiments and characterization, the decomposition of g-C 3 N 4 and modified g-C 3 N 4 were mainly due to their interaction with PMS. Modified g-C 3 N 4 had higher adsorption capacity for PMS than pristine g-C 3 N 4 leading to the enhanced interaction with PMS. The dissolved carbon from modified g-C 3 N 4 would not affect the removal efficiency, but the mineralization efficiency of targeted organic pollutants. In addition, the dissolved components of g-C 3 N 4 and modified g-C 3 N 4 showed acute toxicity. In addition to g-C 3 N 4 -based catalysts, other materials such as covalent organic compound and graphene oxide could be also decomposed partially. The decomposition of g-C 3 N 4 was also found in the peroxydisulfate and hydrogen peroxide-based AOPs. This study provided insight into the structural stability of g-C 3 N 4 -based catalysts in the PMS-based AOPs, which deserved much attention in the future studies from the point view of the design and environmental application of g-C 3 N 4 -based catalysts.