Recycling the powder of spent alkaline batteries as a sustainable and reusable catalyst in producing biodiesel from waste cooking oil.

Alkaline batteries are widely used in contemporary society. However, their black powder, which contains metals, categorizes them as hazardous waste, posing environmental risks if not disposed of correctly. Similarly, waste cooking oil (WCO), frequently produced in homes and restaurants, is often discarded into the environment as waste. In this study, the spent black powder from alkaline batteries was effectively utilized as a catalyst for biodiesel generation from WCO. This catalyst, containing carbonaceous materials, MnO, ZnO, and K, facilitated both esterification and transesterification processes. It featured a Brunauer-Emmett-Teller (BET) value of 31.87 m/g. Response Surface Methodology with Central Composite Design (RSM-CCD) was used to evaluate the influence of key variables on production efficiency. The highest biodiesel yield (99.23%) was attained with a methanol-to-oil ratio of 16:1, a temperature of 70 °C, a catalyst mass of 3 wt%, and a production time of 160 min. The regeneration process revealed that n-hexane effectively removes glycerol and biodiesel residues from the catalyst. Additionally, the catalyst demonstrated strong reusability for up to five cycles, with a significant decline in catalytic activity observed after the fifth cycle. The process demonstrated an activation energy of 22.046 kJ/mol and a pre-exponential factor of 62.878 min⁻. It was characterized as endothermic (ΔH: 19.274 kJ/mol) and non-spontaneous (ΔG: 94.666 kJ/mol). The economic assessment in this study showed that the production cost of 1 kg of biodiesel using a catalyst derived from waste batteries is $0.579, demonstrating its cost-effectiveness compared to alternative methods for large-scale applications. Fourier Transform Infrared Spectroscopy (FTIR), C-NMR, and H-NMR analyses validated the catalyst's effectiveness in converting WCO to biodiesel. Therefore, it is suggested that this catalyst be tested at an industrial scale.