Development of natural deep eutectic solvent-assisted liquid-liquid extraction method for soap removal from biodiesel: optimization and kinetics

The soap content in biodiesel is an important challenge during the production and purification processing of biodiesel. Natural deep eutectic solvents (NADES) have recently attracted considerable interest as an environmentally suitable substitute for traditional solvents in the biodiesel industry. This work investigates the soap removal from the contaminated biodiesel using NADES. Eight choline chloride-based deep eutectic solvents (DESs) were screened using the conductor-like screening model for real solvents (COSMO-RS) to identify the most suitable solvent for soap removal and were validated experimentally. The effect of NADES molar ratio, NADES:biodiesel ratio, mixing speed and extraction time on the extraction efficiency were investigated. COSMO-RS screening revealed that the malonic acid-based NADES possess higher soap elimination, and this is compatible with the experimental screening. The higher extraction efficiency of 99.18% was achieved under the optimum conditions of 1:3 of NADES molar ratio, 1:1 DES:biodiesel, 150 rpm and 15 min of extraction time. The soap removal followed the first-order kinetic equation with a rate constant of 0.183 min⁻¹. This technique offers innovative and environmentally friendly routes for downstream processing of contaminated biodiesel.