Abstract
This paper first reports on the selective separation of volatile fatty acids (VFAs) (acetic and hexanoic acids) using polymer inclusion membranes (PIMs) containing quaternary ammonium and phosphonium ionic liquids (ILs) as the carrier. The affecting parameters such as IL content, VFA concentration, and the initial pH of the feed solution as well as the type and concentration of the stripping solution were investigated. PIMs performed a much higher selective separation performance toward hexanoic acid. The optimal PIM composed of 60 wt% quaternary ammonium IL with the permeability coefficients for acetic and hexanoic acid of 0.72 and 4.38 µm s−1, respectively, was determined. The purity of hexanoic acid obtained in the stripping solution increased with an increase in the VFA concentration of the feed solution and decreasing HCl concentration of the stripping solution. The use of Na2CO3 as the stripping solution and the involvement of the electrodialysis process could dramatically enhance the transport efficiency of both VFAs, but the separation efficiency decreased sharply. Furthermore, a coordinating mechanism containing hydrogen bonding and ion exchange for VFA transport was demonstrated. The highest purity of hexanoic acid (89.3%) in the stripping solution demonstrated that this PIM technology has good prospects for the separation and recovery of VFAs from aqueous solutions.
Highlights
The conversion of organic residual waste into platform chemicals though anaerobic microbial fermentation is considered to be a promising alternative route to replace the petroleum based production of chemicals [1]
The value of the permeability coefficient (P) for hexanoic acids when using Aliquat 336 and Cyphos IL101 as the carrier can be calculated as 4.38 and 2.60 μm s−1, respectively (Equation (7)), which were much higher than that of acetic acid (0.72 and 0.60 μm s−1, respectively). These results demonstrate the potential of the Aliquat 336 based polymer inclusion membranes (PIMs) developed in this study for the selective separation of acetic and hexanoic acids from aqueous solutions
To investigate the effect of carrier content on the separation of acetic and hexanoic acids, membranes were prepared with different contents of Aliquat 336 (30, 40, 50, 60, and 70 wt%) while the total mass of the PIMs remained the same
Summary
The conversion of organic residual waste into platform chemicals though anaerobic microbial fermentation is considered to be a promising alternative route to replace the petroleum based production of chemicals [1]. The results obtained exhibited an obvious difference in transport flux for four VFAs and butyric acid was the easiest to transport across the membrane, followed by propionic, acetic, and formic acids These studies all demonstrated the feasibility of LMs for VFA selective separation. Yang et al [34], using Aliquat 336 (methyltri-n-octylammonium chloride) as an extractant to extract lactic, acetic, propionic and butyric acids, found that Aliquat 336 had more potential to extract VFAs than that of tri-n-octylamine (TOA) as an extractant These solvents were found to possess the superiority of high selectivity for VFAs, suitable affinity strength for VFAs, and high biocompatibility toward microbial systems [15]. Aliquat 336 and phosphonium-based ionic liquids like trihexyltetradecylphosphonium chloride (Cyphos IL 101) have recently attracted considerable attention in PIM research for the transport of metal ions and small molecular species [35,36,37,38]. It is believed that this work may offer a method for green and sustainable VFA separation processes
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