《Persistent free radicals in carbon-based materials on transformation of refractory organic contaminants (ROCs) in water: A critical review》

Abstract:With the increased concentrations and kinds of refractory organic contaminants (ROCs) in aquatic environments, many previous reviews systematically summarized the applications ofcarbon-based materials in the adsorption and catalytic degradation of ROCs for theireconomically viable and environmentally friendly behavior. Interestingly, recent studiesindicated that carbon-based materials in natural environment can also mediate thetransformation of ROCs directly or indirectly due to their abundant persistent free radicals(PFRs). Understanding the formation mechanisms of PFRs in carbo-based materials and theirinteractions with ROCs is essential to develop their further applications in environmentremediation. However, there is no comprehensive review so far about the direct and indirectremoval of ROCs mediated by PFRs in amorphous, porous and crystalline carbon-basedmaterials. The review aims to evaluate the formation mechanisms of PFRs in carbon-basedmaterials synthesized through pyrolysis and hydrothermal carbonization processes. Theinfluence of synthesis conditions (temperature and time) and carbon sources on the types aswell as the concentrations of PFRs in carbon-based materials are also discussed. In particular,the effects of metals on the concentrations and types of PFRs in carbon-based materials arehighlighted because they are considered as the catalysts for the formation of PFRs. Theformation mechanisms of reactive species and the further transformation mechanisms ofROCs are briefly summarized, and the surface properties of carbon-based materials includingsurface area, types and number of functional groups, etc. are found to be the key parameterscontrolling their activities. However, due to diversity and complexity of carbon-basedmaterials, the exact relationships between the activities of carbon-based materials and PFRs are still uncertain. Finally, the existing problems and current challenges for the ROCstransformation with carbon-based materials are also pointed out.

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