Sustainable Recovery of Volatile Fatty Acids from Aqueous Solutions Using Terpenoids and Eutectic Solvents

Volatile fatty acids (VFAs) are produced in the anaerobic digestion of urban or food wastes and sewage sludge. The interest in these compounds has increased because of their use as platform chemicals. In this paper, the application of natural terpenoids and eutectic solvents to sustainably recover VFAs from aqueous solutions by extraction has been evaluated. First, a screening of solvents has been done using COSMO-RS, selecting three terpenoids: geraniol, citral, and eugenol, and two hydrophobic eutectic solvents formed by octanoic acid and menthol or thymol. These solvents have been tested in the extraction of acetic, propionic, butyric, and valeric acids. The highest yields were obtained using geraniol and the (octanoic acid + thymol) eutectic solvent, being comparable or higher than those for organic solvents from the literature. The back-extraction of the VFAs from both solvents has been studied using NaOH and Na2CO3 solutions, concluding that the eutectic solvent is not chemically stable to be reused, whereas the reusability of geraniol was confirmed in consecutive cycles of extraction. The technical viability of the proposal was confirmed in scale-up experiments using geraniol in a packed column, and VFA extraction yields higher than 98% were obtained in the simulation of a countercurrent extractor.