Raw Clay-Assisted Fenton Removal of Dark Blue Reactive Dye: Experimental Assessment Supported by DFT Insights

This study examines the removal of the azo dye dark blue (DB) using natural raw clay as an iron-bearing material in a clay-based Fenton system. Characterization of the natural mineral via XRD, FTIR, and SEM/EDX confirmed a quartz-rich structure containing native iron. The main objectives were to evaluate the effect of pH on color removal efficiency over time, to assess the contribution of adsorption on the clay surface, and to clarify the molecular interactions between DB and the clay. Optimization of the initial pH showed that the highest color and COD removal efficiencies were achieved at a pH of 4. Comparative experiments involving conventional Fenton reactions at pH 3 and pH 4, together with ultrasound-assisted clay-based Fenton oxidation under identical molar ratios, showed that the presence of clay improved dye removal at pH 4, whereas adsorption accounted for only 8% of the overall removal under the investigated conditions. To gain deeper insights into the adsorption step, density functional theory (DFT) calculations based on the cluster approximation were performed to model the molecular and electronic interactions between DB and the clay surface. The calculations, carried out at the B3LYP-D3-(BJ)/6-311G-(d,p) level with CPCM water solvation, indicate that adsorption is mainly governed by weak noncovalent interactions and suggest two distinct adsorption modes: DB is stabilized on the CLAY-(Si-O) surface primarily through localized hydrogen bonds, whereas its stabilization on the CLAY-(Al-O) surface results from the cumulative contribution of weaker and more diffuse noncovalent interactions.