Thermally Activated Construction Waste as an Efficient Adsorbent for Free Fatty Acid Removal from Waste Cooking Oil

Abstract This study aimed to evaluate the reduction of free fatty acids (FFAs) in waste cooking oil (WCO) using calcined concrete, brick, and ceramic tile residues as adsorbents. Concrete, brick, and ceramic tile residues were collected, ground (590–297 μm), and calcined at 850 °C. Characterization using SEM-EDS, FTIR, XRD, and TGA revealed that the calcination process increased porosity and surface alkalinity, enhancing the presence of calcium oxide (CaO) and silicate (SiO2) phases, both active in acid neutralization. Among the tested materials, concrete waste showed the highest efficiency, achieving a 62% reduction in free fatty acids, compared to 23% for ceramic and 17.5% for brick residues. The kinetic data fit best to the Elovich model (R2 = 0.9865), indicating a chemisorption-controlled mechanism on heterogeneous active sites dominated by interactions between Ca2+ and carboxyl groups of fatty acids. The adsorption capacity reached 75 mg g–1, a value comparable to other alkaline adsorbents such as quicklime or calcium silicate, confirming the strong reactivity of concrete-derived materials. The FTIR spectra confirmed the formation of calcium carboxylates after adsorption, while XRD patterns showed crystalline transformations consistent with CaO and C–S–H phase reactivation. The pH increase from 7.5 to 10.5 during the adsorption process further confirmed the alkalinity of the adsorbent and the neutralization of FFAs. Comparative clustering analysis with literature data demonstrated that thermally treated concrete achieved high FFA reduction under mild operational conditions (25 °C, 150 rpm, 360 min), offering an energy-efficient alternative to conventional adsorbents. The results validate calcined concrete waste as a technically viable and environmentally sustainable adsorbent for the pretreatment of waste cooking oils.