Utilization of marine ulva lactuca seaweed and freshwater azolla filiculoides macroalgae feedstocks toward biodiesel production: Kinetics, thermodynamics, and optimization studies

Abstract

The current study compared between marine ulva lactuca seaweed and freshwater azolla filiculoides macroalgae to evaluate the productivity and characteristics of lipids and biodiesels. Using Soxhlet apparatus, 7.6 ± 0.10% and 12.52 ± 0.23% of lipids were extracted from ulva lactuca and azolla filiculoides, respectively. Besides, a second-order kinetic model and Eyring equations were used to conduct the study of kinetics and thermodynamics of oil extraction from both species. The activation energies were estimated to be 53.93 kJ/mol & 85.79 kJ/mol for ulva lactuca and azolla filiculoides, respectively. Analyses of thermodynamic properties of the Soxhlet extraction for both species revealed that the extraction was exothermal and temperature-dependent process. However, they are different in the Gibbs free energy of activation (ΔG) analysis where it revealed spontaneous extraction process for ulva lactuca and unspontaneous process for azolla filiculoides. The extracted oils from both species were subjected to optimized base-catalyzed transesterification in order to synthesize biodiesel fuel. Four independent varying variables were optimized using response surface methodology (RSM) which were oil to methanol molar ratio (1:3–1:12), NaOH catalyst ratio (1–2% w/w), temperature (55–70 °C), and time (1–2.5 h). The maximum biodiesel yield was from ulva lactuca lipids 88.77% obtained at 1:12, 1.5%, 63 °C, and 1.7 h. In contrast, 82.85% of biodiesel was yielded from azolla filiculoides lipids at optimal values 1:10, 1%, 60 °C, and 2 h for oil to methanol molar ratio, catalyst ratio, temperature, and time, respectively, for both species. Characteristics of both biodiesels revealed almost close compliance with ASTM D6751 and EN14214 standards. Apart from comparisons, it can be concluded that marine ulva lactuca seaweeds and freshwater azolla filiculoides macroalgae are obtainable, affordable, and potential feedstock toward sustainable biodiesel production.