Sustainable transformation of agricultural residues into high-value carbon materials via microwave-assisted pyrolysis: A review of energy efficiency, circular economy, and environmental applications

Agricultural waste accumulation presents significant environmental challenges, including greenhouse gas emissions and ecological degradation. Microwave-assisted pyrolysis (MAP) has emerged as a superior thermochemical strategy to valorise these residues into high-value biochar-based materials. Compared to conventional pyrolysis (CP), MAP provides unique advantages such as volumetric heating and precise structural control, enabling the synthesis of advanced carbon forms including biochar-derived graphene oxide (GO). This review systematically evaluates the fundamental mechanisms of MAP, emphasizing the influence of biomass composition and process parameters on the physicochemical properties of the resulting carbon. We highlight how MAP facilitates the fine-tuning of porosity, graphitization, and surface functionality, which are critical for high-end applications. Specifically, the review discusses the integration of these materials into additive manufacturing and their role as high-performance electrode modifiers in electrochemical sensors. A significant focus is placed on the emerging application of biochar-based coatings in providing anti-biofouling properties, which enhance the durability and efficiency of surfaces in marine and biomedical environments. Furthermore, we address technical bottlenecks in upscaling MAP and propose future research directions, such as optimizing process atmospheres to support a circular bioeconomy. This review provides a comprehensive roadmap for transforming agricultural waste into functional carbon materials with diverse industrial applications. ● MAP offers fast, low-energy routes to value-added carbon materials. ● MAP conditions tune biochar porosity, graphitization, and surface functionality. ● Biochar and derived carbons enable composites, coatings, and sensing applications. ● Upscaling challenges and process limitations of MAP are critically assessed. ● Future directions include atmosphere control and underexplored MAP parameters.