Thermodynamical and structural insights of orange II adsorption by MgRAlNO3 layered double hydroxides

[Mg1−x Alx(OH)2][(NO3)x, nH2O] Layered Double Hydroxide (LDH) sorbents with variable Mg/Al molar (R=(1−x)/x) ratios were investigated for adsorption of azo dye, orange II (OII) at various pH and temperature conditions. Mg2AlNO3 displays the highest adsorption capacity with 3.611 mmol of OII per gram of Mg2AlNO3 at 40 °C. Adsorption isotherms have been fitted using the Langmuir model and free energy of adsorption (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) were calculated. The experimental values for ΔG° in temperature range between 10 and 40 °C were found to be negative indicating that a spontaneous process occurred. Positive calculated enthalpy values, characteristic of an endothermic process were found. Characterization of solids (PXRD, FTIR, UV–vis, TGA/DTA, adsorption isotherm BET analysis, SEM and Zetametry) before and after adsorption showed that adsorption proceeds in two steps. First, adsorption occurs at the LDH surface, followed by intercalation via anion exchange.

Graphical AbstractStructural and thermodynamical insight of adsorption/Intercalation of OII in MgRAl LDHFigure optionsDownload as PowerPoint slideHighlights
► The nitrate containing hydrotalcite-like compounds (MgRAlNO3 LDH) were prepared by the coprecipitation method.
► Adsorption of anionic orange dye(OII) is studied on LDHs at different temperatures.
► The adsorption process is well described by the Langmuir isotherm model.
► Mg2AlNO3 displays the highest adsorption capacity with 3.611 mmol of OII per gram of Mg2AlNO3 at 40 °C.
► Adsorption process does not occur on the surface of the LDH only but an intercalation process is also occurring concomitantly according to the thermodynamical values.