Isotherm, kinetic, and mass-transfer analysis of free fatty acid adsorption using wood twig-derived biochar and biochar-bentonite composite adsorbents

The increasing generation of used cooking oil (UCO) presents significant environmental and health challenges, necessitating efficient and sustainable purification methods. This study investigates the removal of free fatty acids (FFA) from UCO using low-cost, wood twig-derived adsorbents, including bentonite, biochar, biochar–bentonite composite, and NaOH-activated composite. Adsorption equilibrium was evaluated using Langmuir, Freundlich, Temkin, and Harkins-Jura isotherm models, while kinetics were analyzed through pseudo-first-order, pseudo-second-order (PSO), and intraparticle diffusion (ID) models. Mass-transfer mechanisms were further examined using the Generalized Fulazzaky (GF) equations to distinguish external, internal, and global mass-transfer contributions. The novelty of this work is the integration of conventional adsorption analyses with the GF equations to quantitatively distinguish mass-transfer mechanisms and identify the dominant rate-controlling step governing free fatty acid removal from used cooking oil. Equilibrium results showed good agreement with isotherm models (R² > 0.8116), with the Harkins–Jura model (R² = 0.7752–0.9990) indicating multilayer adsorption on heterogeneous surfaces. The PSO model best described adsorption kinetics (R² > 0.9203), while ID analysis confirmed the role of pore diffusion. GF analysis revealed that global mass transfer is governed primarily by intraparticle diffusion, identifying it as the dominant rate-controlling step. NaOH activation enhanced surface reactivity and improved adsorption performance. The combined isotherm, kinetic, and GF analyses demonstrate that FFA adsorption proceeds through heterogeneous multilayer adsorption and is predominantly controlled by ID, while NaOH activation enhances adsorbent reactivity and adsorption efficiency, providing important mechanistic insights for the development of sustainable UCO purification technologies. These findings provide mechanistic insight and a practical framework for designing efficient adsorbents for sustainable UCO purification.