3D-Printed Hierarchical Porous MgAl-Layered Double Hydroxide/Sodium Alginate Monoliths for Efficient Recovery of Volatile Fatty Acids from Anaerobic Fermentation Broth

Recovering volatile fatty acids (VFAs) from anaerobic fermentation broth as preferred carbon sources is often hindered by the poor recoverability of powdered adsorbents and the slow mass transfer within conventional hydrogel beads. Herein, we report a MgAl-layered double hydroxide (LDH) and sodium alginate (SA) composite ink tailored for extrusion-based 3D printing, followed by Ca2+ crosslinking and freeze-drying to fabricate monolithic adsorbents. Rheological tests show that the ink exhibits shear-thinning behavior and elastic-dominated viscoelasticity (G′ > G″), ensuring smooth extrusion and high shape fidelity during printing. Under optimized conditions (25 °C, 3.0 g/L, 120 min, pH 7.0), the 3D-printed monolith removed 55.74% of total VFAs. Kinetic analysis reveals that the adsorption follows the pseudo-second-order model (R2 = 0.915), consistent with a chemically controlled adsorption contribution and yielding a calculated equilibrium capacity of 286.5 mg/g. Notably, the adsorbent exhibits a pronounced selectivity toward acetic acid, which accounts for 62% of the total adsorbed VFAs. These findings demonstrate that 3D printing offers a viable strategy to overcome the intrinsic limitations of LDH-based adsorbents, namely, difficult separation and limited internal diffusion, thereby enabling efficient and selective VFA recovery from complex fermentation streams.