Biochar as an additive to improve the biomethanation of soybean straw: on-field implementation and cost benefit analysis aiding circular bioeconomy.

Soybean straw, an underutilized lignocellulosic residue generated at over 500 million tonnes annually, was converted into biochar through controlled carbonization at 250-550 °C, and the soybean straw-derived biochar produced at 450 °C (BC450) was applied as a same-feedstock additive to enhance the biochemical methane potential of soybean straw. Characterized by a mesopore-dominated surface (BET surface area 15.85 m2 g-1, peak pore diameter 3.82 nm, zero microporosity) and balanced elemental composition (C 52.32%, O/C 0.61) BC450 was dosed at 4.21, 4.57, and 5.09 g L-1 in 35-day mesophilic batch assays. The 4.21 g L-1 dose achieved the highest cumulative methane yield (1,019 NmL, net inoculum-corrected yield of 401.5 L CH4 kg-1 straw), nearly double that of conventional unamended agricultural-residue digesters. The Modified Gompertz model best described methane kinetics (R2 = 0.9994), revealing a markedly shortened lag phase (5.14 days) and rapid attainment of 90% methane potential (T90 = 24.8 days) relative to untreated lignocellulosic substrates. Process monitoring confirmed effective buffering, with pH recovering to 7.35-7.66 by Day 30 and volatile fatty acids declining by up to 77% between Days 15 and 30. FTIR, FESEM-EDS, and ICP-OES analyses revealed progressive biochar-substrate biotransformation and trace-metal redistribution consistent with active methanogenesis. A techno-economic assessment across the contexts of Germany, India, and Sub-Saharan Africa demonstrated simple payback periods of 3.2, 0.3, and 4.8 years, respectively, alongside GHG abatement of 1,786 t CO2-eq yr-1 at 5 tonnes per day scale. These findings establish soybean straw-derived biochar as an economically viable, circular-economy additive for accelerating and stabilizing biomethanation.