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

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.