Biodiesel Production by Transesterification of Recycled Oil Catalyzed with Zinc Oxide Prepared Starting from Used Batteries

The consumption of batteries and cooking oil have been increasing. Most used batteries are disposed of incorrectly, leading to health and environmental problems because of their composition. In a similar form, cooking oil, once used, is often released by the discharge reaching the wastewater, polluting soil, and water, which affects its treatment. In Ecuador, these environmental passives are recollected and stored without further treatment, which is a temporary and unsustainable solution. To address this issue, the circular economy concept has gained increasing attention. In this study, zinc oxide was prepared from discarded batteries using the hydrometallurgical method to use as a catalyst; it achieved 98.49% purity and 56.20% yield and 20.92% of particles presented a particle size of 1–10 nm. Furthermore, the catalyst morphology was investigated in an SEM, which showed that particle size ranged from 155.69 up to 490.15 nm and spherical shapes. Due to its characteristics, the obtained catalyst can be used in the industry instead of the zinc oxide obtained by mining processes. These processes are known to produce heavy contamination in the ecosystems and human health. Additionally, a zinc oxide lifecycle in the environment was analyzed through a material flow analysis (MFA), taking into consideration two paths, one assuming the disposal of used batteries and the other assuming the recycling of zinc. Biodiesel was produced with a heterogeneous catalyst. This took place with a transesterification reaction with used cooking oil, ethanol, and zinc oxide (ZnO) as catalysts. The biodiesel obtained had the following characteristics: 37.55 (Formula presented.) of heating power, 0.892 (Formula presented.) of density, 4.189 mm²/s of viscosity, 0.001% of water content, and a 70.91% yield. Furthermore, the energy consumption in biodiesel production was quantified, giving a total of 37.15 kWh. This kind of initiative prevents that waste from becoming environmental pollutants and potential health risks by giving them a second use as a resource. Moreover, turning waste into a valuable product makes the processes self-sustaining and attractive to be implemented.