Raw Clay–Based Fenton Catalysis for Dark Blue Reactive Dye Removal: Experimental Assessment Supported by DFT Insights

The present research focuses on the degradation of azo dye Dark Blue (DB) using natural clay as a catalyst source for heterogeneous Fenton reaction. The main objectives of this study were to evaluate the influence of pH on the degradation efficiency over time, to assess the contribution of adsorption on the clay surface, and to investigate the molecular interactions between DB and clay. The optimization of the pH value showed that maximum removal was observed at pH 4. Comparative analyses with classical Fenton reactions at pH = 3 and pH = 4 and heterogenous Fenton reaction assisted with ultrasound confirmed the dominant catalytic role of the clay, while adsorption accounted for only 8% of the overall dye removal. To gain further insight into the adsorption mechanisms, Density Functional Theory (DFT) calculations using the cluster approximation were performed to model the molecular and electronic interactions between DB and clay, predominantly composed of kaolinite. The DFT results confirmed that adsorption is driven by physical interactions and highlight a clear distinction in adsorption mechanisms: while DB anchors to the KLNT(Si-O) surface through localized hydrogen bonds, its stabilization on the KLNT(Al-O) facet arises from a collective contribution of weaker non-covalent interactions.