Green solvent-based lipid extraction from guava seeds and spent coffee grounds to produce biodiesel: Biomass valorization and esterification/transesterification route

Abstract

Spent coffee grounds and guava seeds were used as representative biomass residues to analyze the replacement of hexane with alternative green solvents for the extraction of lipids and their application to obtain biodiesel with the aim of contributing to the global zero-waste strategy. Lipid extraction studies were performed with these biomass wastes under different operating conditions. The results showed that the green solvents (i.e., ethyl acetate, ethyl butyrate, and ethyl propionate) outperformed the lipid extraction using hexane, achieving a maximum oil recovery of 24% for guava seed and 16.4% for spent coffee grounds, with ethyl propionate. The impact of solvent-to-biomass ratio, extraction temperature, and biomass particle size on lipid recovery from tested biomass residues was analyzed and discussed. The optimal extraction conditions for both biomasses were ethyl propionate, 0.3 mm of particle size and 40 °C; while the best solvent to biomass ratio were 10 and 8 mL/g for guava seeds and spent coffee grounds, respectively. The lipids extracted with green solvents showed similar physicochemical properties and compositions to those lipids extracted with hexane. Lipid extraction kinetics exhibited an endothermic performance. The kinetic data were suitably described with the first-order model (R2 = 0.92 – 1) with kinetic rate parameters in the range of 0.38–0.59 and 0.28–0.72 min−1, for guava seeds and spent coffee grounds, respectively. The lipids extracted from tested biomass were analyzed as raw source for biodiesel production via transesterification. The best reaction conditions to produce biodiesel (90%) were found at 60 °C, 6 h, 15% of catalyst-to-oil, and molar methanol-to-oil ratio of 26:1 using a one-step process. FAME formation up to 96% was obtained using a two-step esterification/transesterification methodology. A detailed characterization of the biodiesel obtained from the lipid fraction from these biomass wastes was also performed, indicating that transesterification reduced the acid value and viscosity to meet the ASTM standard criteria for biodiesel. The study contributes to the valorization of biomass wastes to produce renewable energy and to minimize the waste management problem.