Eco-friendly adsorption of dye pollutants by palygorskite in aqueous effluents: Experimental and computational studies

Palygorskite clay mineral (Pal) was employed in the removal of Congo red (CR) and methylene blue (MB) dyes pollutants in aqueous effluents by the adsorption process. The materials, Pal raw and acid Pal (Apal), were characterized by SEM, EDX, XRD, XFR, FTIR, XPS and Raman spectroscopy techniques that evidenced the main active sites of clay mineral. Characterization data indicated that acid treatment caused a leaching process of metallic cations on the Pal surface. As result, the maximum adsorption capacity was increased from 11.3 to 120.5 mg center dot g(-1) and from 2.7 to 238.1 mg center dot g(-1) for MB and CR dyes, respectively. The regeneration result after five cycles was of 75% recovered to MB adsorption into Apal. Semi-empirical quantum mechanical (SQM) calculations were performed to identify the mechanism of interaction between the Pal surface and dyes. High correlation (R-2>0.99) was observed for the experimental data using the pseudo-second-order kinetic model, that were confirmed by computed enthalpy values (-298.7 to -84.5 kJ center dot mol(-1)), suggesting a chemisorption process as the determining step. Furthermore, the experimental and computational results indicated that the Pal also could work removing efficiently two dyes simultaneously with an adsorption capacity of 37.2 and 40.4 mg center dot g(-1) for MB and CR, respectively.