A study on the role of surface functional groups of metakaolin in the removal of methylene blue: Characterization, kinetics, modeling and RSM optimization.

In this study, thermally activated kaolinite clay is explored as a suitable material for dye removal applications, which gave rise to highly reactive silica species in a broad range of aluminosilicate clusters. Multinuclear NMR studies described it as a short-range network in which Al sites in IV, V, and VI are coordinated, and Si is present mainly as Si(Q4(1Al)). Critical parameters for methylene blue (MB) were determined by the Placket Burman Design (PBD) as initial dye concentration, contact time, adsorbent dosage, pH and size. The % of MB removal studied after optimizing the parameters by central composite design (CCD), based on Response Surface Methodology, was found to be 90%. The adsorption kinetics and thermodynamics were systematically studied and reported by fitting them into different models. The maximum removal of the dye reached 97.8 mg/g according to the Freundlich isotherm, accomplished through chemisorption, following a pseudo-second-order reaction and the process is thermodynamically spontaneous and endothermic. The line spectrum of X-ray photoelectron spectroscopy (XPS) shows the participation of Si, Al, O, Ca and Na of Metakaolin (AK) and nitrogen of MB in the adsorption process. The appropriate stabilization of the N atom of the chromophore on the Si and Al atom in AK resulting from the ionic interaction on the surface is established from an increase in the binding energy of Al and Si. A single bridging oxygen signal at 532.32eVcorresponding to AK after dye adsorption tends to form siloanol/aluminol, and their interaction is lowered to 531.58eV. Regeneration of adsorbent after thermal treatment without loss of efficiency proved.