Optimized In-Situ transesterification of spirogyra using KOH-activated wheat straw biochar catalyst

• KOH-activated wheat straw biochar developed as a low-cost heterogeneous catalyst. • In-situ transesterification of Spirogyra achieved 98.82% biodiesel yield at 50°C. • Response Surface Methodology optimized catalyst loading, methanol ratio, and time. • Characterization confirmed enhanced porosity, graphitic structure, and basic sites. • Demonstrates circular bioeconomy by converting agricultural waste into clean fuel. This study presents a pioneering circular bioeconomy approach by converting wheat straw agricultural waste into KOH-activated biochar catalyst for Spirogyra biodiesel production via in-situ transesterification, eliminating energy-intensive oil extraction steps. Wheat straw underwent slow pyrolysis at 500°C followed by KOH activation, yielding biochar with significantly enhanced fixed carbon content (65.03±2.46% versus 14.48±1.11% in raw material) and reduced volatile matter (14.94±2.71%). Response Surface Methodology with Box-Behnken Design optimized three critical parameters: catalyst concentration (0-2% w/w), methanol-to-Spirogyra ratio (50-100% v/v), and reaction time (60-180 minutes) at 50°C across 17 experimental runs. Statistical analysis demonstrated excellent model accuracy (R²=0.9823, F-value=43.62, p<0.0001), identifying optimal conditions of 1.42% catalyst loading, 97.56% methanol ratio, and 106.45 minutes reaction time, achieving 98.82% predicted biodiesel yield with only 1.882% deviation from experimental value (96.96%). The novelty encompasses first systematic optimization of wheat straw-derived activated biochar for Spirogyra transesterification, triple sustainability advantage (agricultural waste valorization, elimination of extraction, low-temperature operation), and scalable implementation potential for developing regions with abundant agricultural residues, offering simultaneous waste management and renewable fuel production solutions.