Kinetics and energetics of biodiesel oxidation stability: The impact of Uapaca kirkiana-derived natural antioxidants

Abstract

Despite considerable progress in understanding biodiesel autoxidation inhibition, the kinetics and energetics of the inhibition reactions involving natural antioxidants remain underexplored. Most existing research on natural antioxidants has focused on enhancing oxidation stability and other fuel properties. This study aimed to investigate the oxidative stability of croton biodiesel (CBD) and assess the kinetics and energetics of natural antioxidants derived from the roots, pulp, and fruit peels of the Uapaca kirkiana plant. The oxidation stability of biodiesel samples was assessed using the OXITEST method at temperatures of 90, 100, 110, and 120 °C. These tests enabled the calculation of kinetic parameters such as reaction rates and activation energies, crucial for understanding the inhibition role of antioxidants during oxidative degradation. Activation energy for antioxidant consumption, determined using the Arrhenius equation, was found to be 81.39 kJ mol⁻¹ for fruit peel extracts, 77.73 kJ mol⁻¹ for pulp extracts, and 63.85 kJ mol⁻¹ for root bark extracts. The higher activation energy for fruit peel extracts suggests that they are more effective at preventing oxidation, especially under high-temperature conditions. Enthalpy, entropy, and Gibbs free energy parameters were calculated using the Eyring equation, indicating a nonspontaneous endothermic process for the antioxidant samples. The study found an inverse relationship between antioxidant concentration and rate constants, demonstrating the antioxidants' effectiveness in slowing down the oxidation process. These kinetics and energetics analyses provide detailed insights into how antioxidants function, facilitating the optimization, selection, and validation of their efficiency in stabilizing biodiesel.