Spirulina-Derived Biochar as a Bifunctional Green Catalyst for Conversion of Waste Cooking Oil to Biodiesel: Synthesis, Characterization, Optimization, and Green Performance Metrics

Biochar-based catalysts have emerged as sustainable alternatives for biodiesel production, achieving high yields (up to 99%) from various feedstocks. This study aimed to utilize Spirulina-derived biochar as a bifunctional green catalyst for biodiesel synthesis from waste cooking oil (WCO) through transesterification and assess its green performance metrics. Biochar synthesized by carbonization (324 °C) was modified with calcium and sulfuric acid, featuring dual acid-base sites. Energy dispersive spectra revealed impregnation of calcium (11.11%) compared to the raw biomass (2.34%), followed by peaks of methoxy group and methylene group, and with methylene and β-carbonyl protons shown by nuclear magnetic spectroscopy. Thus, the biochar catalyst tested on WCO achieved a 93.27% yield under optimized conditions (65 °C, 1:15 methanol-to-oil ratio, 3% catalyst, 3.5 h) via central composite design. Catalyst reusability was maintained over four cycles with an average biodiesel yield (90%). Further, green metrics validate their eco-friendliness with a single-cycle reaction mass efficiency (RME) of 60.8%. When the initial catalyst mass is amortized over four cycles, the cumulative biodiesel yield per initial catalyst input reaches the equivalent of 243% of a single-batch theoretical yield (catalyst productivity = 3.12 g FAME/g catalyst). E-Factor at 0.67 (reduced to 0.17) and mass intensity at 1.68 (down to 0.42), contrasting with business-as-usual scenarios such as sulfuric acid catalysis (RME 70.0%, E-Factor 0.25) using 8.85 g H2SO4 vs. ~5 g H2SO4/kg biochar. Our results demonstrate that bio-based catalysts minimize non-benign inputs, supporting a circular economy from algal waste.