Recovery of dilute (bio-based) volatile fatty acids by adsorption with magnetic hyperthermal swing desorption

Abstract

Adsorption followed by stepwise desorption to concentrate volatile fatty acids (VFAs) from very dilute aqueous streams is challenging and only a limited amount of VFAs can be collected at high concentration using N2-stripping. Here, we describe the preparation and use of superparamagnetic porous adsorbents to recover much larger fractions of VFAs in highly concentrated form from dilute aqueous streams than with the state-of-the-art N2-stripping technique. Our system is based on poly(divinylbenzene) (PDVB) impregnated with superparamagnetic magnetite nanoparticles (MNPs) synthesized by coprecipitation and functionalized with oleic acid (OA). The OA grafted MNPs (OA-MNPs) were embedded in the matrix of the polymer during suspension polymerization. The porous particles had an average size of 222 ± 40 µm with a surface area of 496 ± 10 m2/g and contained 11 ± 1 wt% MNPs with an average core size of 10 nm. VFAs adsorption from a dilute aqueous solution (containing 0.25 wt% of each acid) reached a saturation capacity of 43 g carboxylic acid per kg adsorbent,. The two-stage desorption was started with alternating magnetic field (AMF) heating at 25 mT and 52 kHz, followed by a hot N2 stripping stage removed 90 ± 9% of the water that had physically filled the pores during adsorption, and only 11 ± 2% of the loaded VFAs. Subsequently, 89 ± 3% of the VFAs were recovered almost water free using hot N2 stripping. Compared to sorbent regeneration and vapor fractionation by nitrogen stripping with a temperature gradient only, this new approach offers a much sharper acid fractionation from water, facilitating large energy savings in downstream operations.