Productivity evaluation, multi-scale characterisation, and safety assessment of biochar derived from green and woody agricultural biomass in Qatar

Qatar produces approximately 2 million tons of agricultural biomass waste annually. Less than 20% is currently recycled or valorized, mainly through limited composting, landfilling, and, in some cases, open burning. This study presents a systematic production and multi-scale characterization of biochar derived from local mixed green agricultural waste (GAGW) and woody agricultural waste (WAGW), providing a characterization-based foundation to inform their potential integration into the energy-water-food (EWF) nexus of arid regions. Each feedstock was analyzed for moisture content (MC), volatile matter (VM), ash content, fixed carbon (FC), elemental composition (C, H, N, S), molar ratios (H/C, O/C), structural, elemental, morphological, thermal combustion and specific surface area properties. The experimental design follows a double-split arrangement. GAGW and WAGW were considered as the main plots, pyrolysis temperature (300, 400, 500, and 600 degrees C) as the sub-main plots, and residence time (20, 60, 90, and 120 min) as the sub-sub plots. Results demonstrate a fundamental divergence in carbonization pathways. WAGW achieved a peak solid yield of 71.28% at 300 degrees C in 20 min, significantly outperforming GAGW (60%). As thermal severity increased to 600 degrees C, WAGW underwent superior structural development, reaching a specific surface area of 684 m(2)/g, about 3 times higher than GAGW. Van Krevelen analysis validated the transition to highly recalcitrant aromatic frameworks, with WAGW reaching a superior H/C ratio of 0.30 compared to 0.35 for GAGW. While WAGW produced a carbon-dense matrix (82.6 atom% C), GAGW evolved into a nutrient-active medium, concentrating potassium (88,780 mg/kg) and calcium (63,932 mg/kg) in stable sylvite and calcite phases, as affirmed via X-ray Diffraction (XRD). Crucially, all produced biochar samples were free from hazardous heavy metals (As, Cd, Cr, Pb, Hg < 0.1 mg/kg), confirming their safety profile and suitability for further evaluation in potential soil and water applications.