From waste to base oil: Supercritical CO2 extraction for regeneration of multi-source and saponified lubricant waste streams.

Waste lubricant oils, classified as hazardous waste, can be regenerated into valuable base oils, however, cleaner and more efficient technologies are needed to overcome the limitations of conventional processes, including safety risks, high costs, and inefficiency. While prior regeneration studies have focused on relatively homogeneous, single-source engine-drain oils, they have not addressed the challenges posed by complex, multi-source WLO blends or KOH-induced coagulation phenomena that hinder conventional re-refining routes. Here, we demonstrate for the first time that a standalone regeneration method based on supercritical CO2 extraction can effectively recover base oils from both heterogeneous WLO mixtures and saponified (coagulated) waste streams. Process optimization using response surface methodology identified optimal conditions of 140 bar and CO2 flow rate of 14 mL/min at 40 °C, achieving extraction yields up to 73% for non-coagulating oils and approximately 60% for coagulated samples. Regenerated oils exhibited significant improvements in physicochemical properties, including reductions in total acid number, saponification number, water content, chlorine concentration, and oxidation-related functional groups, as confirmed by ASTM methods, FTIR spectroscopy, and elemental analysis. Except for color, regenerated oils complied with technical specifications for base oils obtained from regeneration operations. These results demonstrate the ability of supercritical CO2 extraction to overcome coagulation barriers and extend green regeneration technologies to complex WLO feedstocks previously considered unrecoverable, supporting circular economy strategies for hazardous oil waste valorization.