Garlic Peel Waste as an Eco-Friendly Biosorbent for Used Cooking Oil Regeneration: Adsorption Efficiency and Mechanistic Evaluation

Abstract

This study explores the valorization of garlic peel waste as a sustainable biosorbent for the purification of used cooking oil through removal of free fatty acids (FFA) and other oxidation-related undesirable byproducts. Garlic peel adsorbent was prepared in both unactivated and chemically activated forms using hydrochloric acid (HCl) to enhance its adsorption performance. The effects of adsorbent mass and contact time on the adsorption process was systematically evaluated, while adsorption behavior and mechanisms were analyzed using isotherm, kinetic, and surface morphology approaches. The activated garlic peel exhibited superior adsorption performance, achieving a 76.74% reduction in FFA content under optimum conditions, using 24 g adsorbent/100 mL sample within 45 min. Adsorption equilibrium was best represented by the Langmuir isotherm model, suggesting monolayer adsorption on homogeneous active sites, whereas pseudo-second-order kinetics indicates that chemisorption dominates the adsorption process. Scanning Electron Microscopy (SEM) analysis confirmed that chemical activation enhances surface roughness, pore formation, and the availability of active adsorption sites, thereby improving adsorption efficiency. In addition to reducing FFA levels, the adsorbent contributes to lowering peroxide value and acid number, indicating a broader improvement in oil quality. These findings demonstrate that garlic peel waste possesses strong potential as a low-cost, environmentally benign, and renewable biosorbent for cooking oil regeneration. The study provides an alternative strategy for agricultural waste valorization while supporting sustainable and household-scale oil purification technologies.