Subcritical Water Hydrolysis-Derived Hydrochar from Grape Pomace (Vitis vinifera L.): A Novel Adsorbent for Removal of Fermentation Inhibitors

Abstract

This study explores the production of adsorbents from grape pomace (Vitis vinifera L.) via subcritical water hydrolysis (SWH) and their application as sustainable materials for removing fermentation inhibitors. Grape pomace, an abundant lignocellulosic byproduct of the wine industry, underwent SWH to yield a hydrolysis residue, which was subsequently chemically and thermally modified to generate various adsorbents (C2–C4). Among these, the C4 material, activated with phosphoric acid and treated under autoclave conditions, exhibited the most favorable properties, including a high surface area (777.2 m² g^–1), microporosity, surface acidity, and negative surface charge. Batch and fixed-bed adsorption experiments confirmed C4’s superior capacity to selectively adsorb inhibitory compounds such as acetic acid, 5-hydroxymethylfurfural (5-HMF), and furfural, while preserving fermentable sugars. The adsorption mechanisms were comprehensively analyzed using kinetic, equilibrium, and thermodynamic models. Adsorption was determined to be spontaneous and endothermic, primarily governed by physisorption, as evidenced by low activation energies and favorable thermodynamic parameters. Regeneration studies demonstrated that C4 could be effectively reused for over five cycles with minimal loss of adsorption capacity, particularly for furfural and 5-HMF, using alkaline water as the eluent. The practical utility of C4 was validated through fermentability tests, which demonstrated significantly improved performance in detoxified hydrolysates, including a 3.2-fold increase in ethanol production compared to nondetoxified samples, as well as xylitol production. Overall, this study highlights the promise of subcritical hydrochar produced from water as an affordable, effective, and eco-friendly adsorbent suitable for biorefinery processes.