Agricultural Waste Derived Biochar: Production, Characterisation, Environmental Applications and Critical Perspectives—A Review

Worldwide generation of crop residues exceeds four billion tonnes per year, much of which is openly burnt or landfilled, with consequences for air quality, greenhouse-gas emissions, and soil organic matter. Thermochemical conversion of these residues to biochar offers a valorisation route with the potential to contribute simultaneously to water and soil remediation and to durable carbon storage. This structured narrative review consolidates progress on agricultural waste derived biochar (AWDB) reported between 2020 and 2026. Slow pyrolysis, fast pyrolysis, microwave-assisted pyrolysis, and hydrothermal carbonisation are compared in terms of operating conditions, yield, and product attributes. Activation routes (steam, CO2, KOH, ZnCl2, H3PO4) and post-synthetic modifications (heteroatom doping, metal impregnation, magnetic functionalisation) are evaluated against porosity, surface chemistry, and adsorbate selectivity. Applications span heavy-metal, dye, antibiotic, and CO2 removal, together with soil amendment and carbon sequestration. Persistent limitations are identified, including over-reliance on equilibrium capacities, inconsistent test protocols, sparse multicomponent data and incomplete life-cycle accounting. The distinction between biochar, activated biochar, and biochar-derived activated carbon is applied consistently, and the safety profile of the material—residual pyrolysis organics, potentially toxic elements, release of impregnated metals, and management of pollutant-loaded spent adsorbent—is treated as integral to the assessment. Priorities are proposed for standardised evaluation, mechanism-guided synthesis, data-driven design, and techno-economic assessment.