This research surveys biodiesel synthesis from used edible oil (UEO) using biochar/MnO–NiO nanocomposite, highlighting its efficiency and sustainability as a heterogeneous catalyst. The physical and chemical features of the manganese and nickel oxide nanocatalysts immobilized on biochar were thoroughly characterized through FTIR, BET, XRD, EDX, TEM, and FESEM analyses. The findings confirmed the successful deposition of manganese and nickel oxide nanoparticles onto the biochar surface. Moreover, the specific surface area of the biochar/MnO–NiO nanocatalyst was determined to be 25.436 m2/g, while EDX analysis validated the elemental composition, confirming the presence of carbon, manganese, nickel, and oxygen in the synthesized catalyst. The impacts of critical procedure variables on the transesterification process were investigated. Response surface methodology was applied to analyze and optimize the process. Among the variables studied, the methanol/UEO molar ratio was identified as the most influential variable affecting transesterification efficiency. Optimal conditions for maximum biodiesel yield were determined to be a methanol/UEO molar ratio of 14, a reaction time of 150 min, and a catalyst concentration of 2.5 wt.%. Under these conditions, the biodiesel production efficiency achieved an impressive 95.12%. Besides, a preliminary cost analysis of the biochar/MnO–NiO catalyst synthesis was performed to assess economic viability, reinforcing its potential for scalable biodiesel production. The production price of biochar/MnO–NiO catalyst per kg of biodiesel was calculated to be $0.05485, which is very economical and promising. This study highlights the significant potential of the biochar/MnO–NiO nanocatalyst as a highly effective and cost-effective catalyst for the transesterification of UEO.
