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

Abstract

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 Na 2 CO 3 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.

Green Extraction of Volatile Fatty Acids from Fermented Wastewater Using Hydrophobic Deep Eutectic Solvents

“…Later in 2019, two studies appeared in the literature, one by van den Bruinhorst et al [14], in which they showed that mixtures of dihexylthiourea and trioctylphosphine oxide (TOPO) are good extractors for VFAs. Another study by Rodrigues-Llorente et al [13] showed that an HDES composed of octanoic acid and thymol gave a comparable or higher extraction efficiency than organic solvents used in the literature. It should be pointed out that the three studies [13,14,22] concluded that the longer the chain of a VFA, and consequently the higher hydrophobicity, the higher the extraction efficiency.…”

“…Another study by Rodrigues-Llorente et al [13] showed that an HDES composed of octanoic acid and thymol gave a comparable or higher extraction efficiency than organic solvents used in the literature. It should be pointed out that the three studies [13,14,22] concluded that the longer the chain of a VFA, and consequently the higher hydrophobicity, the higher the extraction efficiency. Nonetheless, the studies available in the literature only discussed the single-stage extraction of the VFAs using HDESs, without investigating the influences of key operational parameters, multistage extraction, repetitive usage of solvent, or HDES regeneration.…”

“…However, over the past decades, the incentives for designing environmentally friendly, energy-efficient, and cost-effective processes have steadily grown. Therefore, affinity separations such as liquid-liquid extraction [9][10][11][12][13][14], adsorption [15], and membrane filtration [16] are becoming attractive alternatives when technically feasible.…”

“…An efficient separating agent for the extraction of the VFAs from the aqueous fermentation medium must primarily exhibit high hydrophobicity, high capacity, high solute distribution ratio, high selectivity, easy recoverability, environmental friendliness, and low cost. Different organic solvents such as medium-chain fatty acids (MCFAs) [12], organophosphorus [11], terpenes and terpenoids [13], and aliphatic amines [19,20] have been studied. However, several drawbacks were reported such as low selectivity, solvent miscibility, solvent losses via evaporation, and difficult regeneration.…”

“…However, several drawbacks were reported such as low selectivity, solvent miscibility, solvent losses via evaporation, and difficult regeneration. To address these limitations, designer solvents, particularly, deep eutectic solvents (DESs) [21] have been proposed for the extraction of VFAs [13,14,22].…”