Optimization of waste cooking oil biodiesel production via NaOH-catalyzed transesterification and dry washing using olive pit–derived activated carbon

Biodiesel production from waste cooking oil (WCO) represents both an economic opportunity and an environmental strategy to address global challenges related to renewable energy. Among available routes, alkaline transesterification remains the main process for converting vegetable oils into biodiesel. However, conventional purification by wet washing requires large amounts of water, encouraging development of alternatives. Adsorption and ion-exchange technologies, known as dry washing methods, have emerged as promising approaches for crude biodiesel purification, using adsorbents to selectively remove impurities from the biodiesel liquid phase through surface interactions. This work presents the optimization of biodiesel production from waste cooking oil by transesterification using sodium hydroxide as catalyst. The process was optimized by response surface methodology using a Box–Behnken design with three variables: temperature, catalyst loading, and alcohol-to-oil molar ratio. The optimized reaction conditions were 30 °C, with a catalyst loading of 1% by weight, and an alcohol-to-oil molar ratio of 7.5:1. The crude biodiesel was then purified by adsorption using activated carbons produced from olive pits. These activated carbons showed surface areas up to 936.94 m2/g and strong affinity for free glycerol adsorption, allowing purification without pre-treatment. Purified biodiesel reached free glycerol limits required by ASTM D6751 and EN 14214, with values below 0.012% by weight.