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

Abstract

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-C3N4, biochar, biochar/g-C3N4 composite, graphene oxide and COF in sulfate radical-based and hydroxyl radical-based AOPs was investigated, mainly focusing on g-C3N4 and modified g-C3N4 in peroxymonosulfate (PMS)-based AOPs. The g-C3N4 prepared with different precursors and modified g-C3N4 were selected to investigate their structural stability in PMS-based AOPs in detail. The results showed that both g-C3N4 and modified g-C3N4 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-C3N4 and experimental conditions such as PMS concentration and inorganic ions. Modified g-C3N4 presented higher concentration of dissolved carbon than pristine g-C3N4. Based on the results of experiments and characterization, the decomposition of g-C3N4 and modified g-C3N4 were mainly due to their interaction with PMS. Modified g-C3N4 had higher adsorption capacity for PMS than pristine g-C3N4 leading to the enhanced interaction with PMS. The dissolved carbon from modified g-C3N4 would not affect the removal efficiency, but the mineralization efficiency of targeted organic pollutants. In addition, the dissolved components of g-C3N4 and modified g-C3N4 showed acute toxicity. In addition to g-C3N4-based catalysts, other materials such as covalent organic compound and graphene oxide could be also decomposed partially. The decomposition of g-C3N4 was also found in the peroxydisulfate and hydrogen peroxide-based AOPs. This study provided insight into the structural stability of g-C3N4-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-C3N4-based catalysts.