Photocatalytic reactive oxygen species generation and their mechanisms of action in pollutant removal with biochar supported photocatalysts: A review

Wastewater treatment and energy production need low cost, efficient, sustainable and novel photocatalyst. Biochar produced by the thermochemical conversion of biomass not only helps to reduce atmospheric carbon emissions but is also an eco-friendly alternative for activated carbon. Owing to high surface porosity, crystallinity, stability, and functional groups, they find extensive applications in environmental pollution remediation. This review focuses mainly on biochar coupled photocatalysts synthesis routes and their mechanistic approaches. Modifying biochar by loading conventional photocatalytic materials produces biochar-supported photocatalysts (BSPs) which adds advantages of biochars with catalytic nanomaterials. Engineered biochars are for aqueous pollution remediation, air pollution control and microbial decontamination. While most existing reviews focus on the supportive, adsorptive and catalytic role of BSPs, much of the knowledge about the nature and types of oxidising species produced by BSPs are not well reported. Hence, the recent observations in photocatalytic mineralisation of pollutants with BSPs are first reviewed and later the nature and types of reactive oxygen species (•OH, •O2−, 1O2) produced by biochar in aqueous suspensions are detailed. Lastly, the mechanisms for improved photocatalytic activity upon forming heterojunctions with biochar are presented along with identifying recent research trends and outlook in implementing BSPs for guiding future research in this field. In the future solar energy intensified photocatalysis process needs to be experimented with and integrated into the wastewater treatment process.