Comparison of Remazol Brilliant Blue Reactive Adsorption on Pristine and Calcined ZnAl, MgAl, ZnMgAl Layered Double Hydroxides

Layered double hydroxides (LDHs) are promising adsorbents for the removal of various contaminants from water. However, a comprehensive understanding of how ternary LDHs differ from the divalent ones in terms of dye adsorption capacity and the underlying mechanisms still remain unknown. Remazol brilliant blue reactive (RBBR) adsorption on pristine and calcined (C) ZnAl, MgAl, and ZnMgAl LDHs were comprehensively investigated for the first time in this study. The characteristics of the pristine and calcined samples were established with X-ray diffractometer, Fourier transform infrared spectrophotometer, zeta potential analyzer, and Brunauer-Emmett-Teller method. A considerably larger adsorption capacity was obtained with pristine MgAl (220 mg/g) and ZnAl (191 mg/g) compared to that of ZnMgAl (164 mg/g) at all studied initial dye concentrations (10-250 mg/L) and solution pH (3-12). However, when the LDH samples were calcined, the largest RBBR mass adsorbed was achieved with ZnMgAl-C (263 mg/g) followed by MgAl-C (247 mg/g) and ZnAl-C (236 mg/g). In comparison to the pristine samples, the faster adsorption rate and higher adsorption capacity of the calcined samples were attributed to the large specific surface area and enhanced electrostatic interactions due to the higher positive charge obtained after calcination. ZnMgAl-C had the largest surface area and the charge, explaining its superior adsorption capacity over the divalent LDHs. Pseudo-first-order, pseudo-second-order, and Elovich models described the kinetics data well while Freundlich and Redlich-Peterson isotherms suitably fit to the equilibrium data for the pristine and calcined LDHs. Surface adsorption via electrostatic interactions was found to be the effective mechanism for RBBR adsorption on all pristine and calcined LDHs while intercalation was not.