Reduction of Free Fatty Acids in Waste Frying Oil Using Thermally Treated Mortar

Amid growing global efforts to mitigate climate change and decarbonize energy systems, biodiesel derived from waste cooking oils (WCOs) has emerged as a sustainable and economically viable alternative. However, the high free fatty acid (FFA) content in WCOs presents a significant barrier to efficient transesterification, requiring cost-intensive pretreatment stages. To address this challenge, thermally treated mortar waste from construction and demolition activities was evaluated as a low-cost adsorbent for reducing the acidity of WCOs collected from a university food service facility. Material characterization by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) and Fourier transform infrared spectroscopy (FTIR) revealed that thermal treatment at 850 °C enhances surface porosity and increases the presence of basic calcium oxide (CaO) phases, facilitating FFA adsorption. The process achieved a 78% reduction in acid value, reaching approximately 0.55 mg KOH.g^-1, which is below the typical threshold required for efficient transesterification. These results were obtained using a Central Composite Rotatable Design (CCRD) with a total of 17 experimental runs, including triplicates at the central point. The statistical model yielded a coefficient of determination (R ²) of 0.91. Under optimized conditions (20.29 °C, 153.53 rpm, 0.5 g of adsorbent), an acid value reduction of 78% and an adsorption capacity of 81.67 mg.g^-1 were achieved. Kinetic modeling showed the best fit with the pseudo-second-order (PSO) model (R ² = 0.97893), indicating a chemisorption-dominated process. Potential limitations include calcium leaching into the oil and the high energy demand associated with the calcination process.