Adsorption of anionic dye onto homoionic montmorillonite: effect of the exchangeable cation on the adsorption

The objective of this study was to investigate the adsorption of anionic dye represented by Congo red (CR) using adsorbents prepared starting from Wyoming montmorillonite (Mt). The first group of adsorbents named homoionic montmorillonite (HMt) was obtained using Ca²⁺, Mg²⁺, Na⁺, and K⁺, while the second group called modified montmorillonite (Mt-CTA) was acquired after intercalation of cetyltrimethylammonium (CTA⁺) into HMt. The obtained XRD results analysis revealed that Mt-K-2CEC gave a maximal d₀₀₁ value of 19.07 Å due to a relatively high amount of CTA⁺ cations in the interlayer spaces of Mt-K. From BET measurements, total pore volume, average pore size, and the BET surface area decreased with the increase in CTA⁺ loading on montmorillonite from 1 to 2CEC, except for Mt-Ca. On the other hand, adsorption kinetics data showed accordance with the pseudo-second-order equation, and the adsorption isotherm was in good agreement with the Langmuir model. Up to 274.48 and 258.46 mg g⁻¹ of the dissolved CR were adsorbed by Mt-Mg and Mt-Na-2CEC, respectively. Finally, as shown in the FTIR spectra and SEM analyses, the adsorption of organo-clays was explained as a chemical adsorption process between the groups –N=N– and –SO₃ of CR and CTA⁺. Hence, the use of these adsorbent materials to remove CR dye is highly beneficial due to their abundance and low cost.