A Critical Review on the Biochar-mediated Formation of Reactive Species: Detection Methods, Transformation Mechanisms and Environmental Implications.

In recent years, studies on the degradation of environmental contaminants by sulfate radicals (SO₄^•-) and hydroxyl radicals (^•OH) based advanced oxidation processes (AOPs) have garnered increasing attention. Notably, biochar, which can activate chemical oxidants to produce reactive species, has been increasingly employed to enhance the removal of the contaminants in AOPs. Nevertheless, a systematic understanding of the biochar-mediated reactive species is still lacking in contaminants remediation during environmental applications. This review outlines the identification and determination methods of reactive species induced by biochar, the underlying reaction mechanisms, and their contaminant removal efficacy in AOPs systems. Multiple factors, such as the characteristics of biochar/chemical oxidants, the nature of the target contaminants, and some soil active constituents can affect the generation, diffusion, and transformation of a wide range of reactive species. This review systematically compares two reaction pathways, free radical mechanisms and non-free radical mechanisms. Environmental applications are emerging from the controlled generation of reactive species through biochar-mediated processes, as comprehensively discussed.