Volatile fatty acid adsorption: From equilibrium characterization to breakthrough dynamics

Abstract

This work presents a comprehensive experimental and modelling study of the adsorption and desorption behavior of volatile fatty acids (VFA) under well-controlled conditions, with the aim of developing an integrated modelling framework with experimental data (adsorption equilibrium and dynamic behavior) for better predictions of process performance. Batch experiments using single-component solutions were first performed to determine adsorption equilibrium and to identify a suitable adsorbent for VFA separation. The nonfunctionalized adsorbent (Amberlite XAD-4) was selected and used in the fixed-bed experiments, to determine single- and multicomponent breakthrough curves. In a multicomponent system, adsorption capacities ranged from 0.125 mmol·g_ads⁻¹ for acetic acid to 1.564 mmol·g_ads⁻¹ for hexanoic acid, highlighting significant differences in affinity among VFA. During these assays, some swelling of the adsorbent was observed with increase adsorbed quantity in the solid phase. A mathematical model was developed combining the dual-site Langmuir model and the Linear Driving Force model to describe adsorption equilibrium and intraparticle diffusion, respectively. Adsorbent swelling was incorporated by defining the mass transfer coefficient dependent on the average adsorbed quantity in the solid phase, enabling the model to capture swelling-induced variations in transport properties. The proposed model successfully described VFA adsorption and desorption behavior with single- and multicomponent solutions, and in both batch and fixed-bed systems. This work provides a consistent experimental-modelling methodology for VFA adsorption and represents a first step toward the development of intensified separation processes such as simulated moving bed systems and the future application to real effluents.