Adsorptive removal of both cationic and anionic dyes by using sepiolite clay mineral as adsorbent: Experimental and molecular dynamic simulation studies

In the present work, sepiolite clay was assessed as an adsorbent for the removal of cationic (Methylene Blue: MB) and anionic (Direct Red-23: DR-23) dyes from aqueous media. Firstly, the sepiolite clay mineral was characterized using different instrumental techniques viz., XRD, SEM, EDX, FIR, X-ray Fluorescence, and BET analysis. Thereafter, the adsorption study was examined as a function of the adsorbent amount, the contact time, the aqueous phase pH, the ionic strength, and the initial dye amount. The data indicate that the adsorption was a function of the pH with high adsorption amounts of MB and DR-23, in basic and acidic, respectively, pH ranges. By comparing experimental data to the Langmuir predictions, the calculated maximum dye adsorbed amounts were 124.9 and 649.37 mg g(-1), for MB and DR-23, respectively. Further, kinetic adsorption studies have shown that adsorption of MB and DR-23 from aqueous media onto the sepiolite follow, respectively, the pseudo-second order and the pseudo first-order kinetic models. On the other hand, Molecular Dynamic (MD) simulations were performed to shed light on the pH-effect on the adsorptive properties of the sepiolite clay surface and the adsorption behavior of both hazardous MB and DR-23 dyes into the adsorbent surface. The better affinity of the MB molecules toward the sepiolite in the basic media could be related to the dispersion forces and electrostatic interactions. On the other hand, the DR-23 molecules show more favorable interaction in the acidic media as a result of the hydrogen bonds formed between the dye molecule and the sepiolite surface oxygen atoms. In addition, the negative values of interaction energies calculated show that the adsorption processes of both hazardous MB and DR-23 molecules on the sepiolite surface are spontaneous, which is in good agreement with the results observed experimentally. (C) 2020 Elsevier B.V. All rights reserved.