Conventional and microwave-assisted pyrolysis biochars: comparative mechanistic insights, structural evolution, and environmental remediation applications

Abstract The dual challenges of waste biomass disposal and escalating contamination by inorganic, organic, and emerging microplastic pollutants demand sustainable, circular remediation strategies. Biochar, a carbon-rich, porous material produced through the thermal decomposition of biomass, has emerged as a multifunctional sorbent capable of adsorbing pollutants, enhancing soil quality, and sequestering carbon. This review critically compares biochars synthesized via conventional pyrolysis (CP) and microwave-assisted pyrolysis (MAP), highlighting their physicochemical differences, sorption mechanisms, and contaminant removal efficiencies. As a key novelty, this review presents the first mechanistically integrated comparison of CP and MAP biochars in relation to their structural characteristics and performance relationships. Factors such as temperature, reactor design, and pyrolysis method significantly influence the properties of biochar, including surface area, porosity, pH, carbon content, and adsorption capacity. Comparative analysis indicates that MAP offers advantages over CP, including faster volumetric heating, higher product yield, enhanced specific surface area, greater selectivity for targeted contaminants, and potentially improved energy efficiency under optimized conditions. The review discusses biochar functionalization, adsorption-reduction methods, and optimization for contaminants such as heavy metals, dyes, medicines, and microplastics. Beyond remediation, biochar is used for soil improvement, composting, catalysis, and electrode materials. Key information gaps persist, including a mechanistic understanding of contaminant interactions, long-term stability, and MAP scalability. Addressing these gaps is critical for optimizing biochar for targeted remediation, incorporating MAP-derived biochar into circular bioeconomy and carbon-neutral methods, and directing the development of next-generation biochars for sustainable pollution management and resource recovery. Graphical Abstract