Valorization of spent coffee grounds: a multi-solvent extraction approach and blending with waste cooking oil for sustainable biodiesel production

A worldwide waste stream, spent coffee grounds (SCG), presents serious environmental challenges because of landfill disposal and methane emissions. In this work, the valorization of SCG through oil extraction and biodiesel synthesis has been investigated. Hexane, a non-polar solvent, achieved the highest spent coffee grounds oil (SCGO) yield of 18.73 ± 2.31% (dry weight basis). Non-polar solvents usually produce lower free fatty acid (FFA) content than polar solvents. For instance, hexane extracted SCGO with an acid value of 7.85 mg KOH/g, much below ethanol (16.20 mg KOH/g). The direct alkaline transesterification via blending SCGO with low FFA WCO is a creative way to lower the FFA of SCGO. This reduced production costs, energy requirement and catalyst consumption as it eliminates the need for the 2-step esterification-transesterification process. A biodiesel yield of 99% was achieved with 90%WCO:10%SCGO blend. The biodiesel quality was validated via GC-MS, FT-IR, NMR and TGA analyses. Results indicated that SCGO is a promising biodiesel feedstock when blending with WCO. This helps make the economy more sustainable and circular by reducing reliance on fossil fuels and waste generation. This study falls well within the UN Sustainable Development Goals (SDGs) such as Affordable and Clean Energy (SDG7) and Climate Action (SDG13).