A comparative study of the amaranth azo dye adsorption/desorption from aqueous solutions by layered double hydroxides

Magnesium aluminum layered double hydroxides (LDH) were synthesized by the co-precipitation method with nitrate and carbonate as interlayer anion (MgAlNO3 and MgAlCO3) followed by the calcination of a carbonate containing LDH sample to obtain the mixed oxide MgAl500. The resulting materials were characterized by several physicochemical techniques. Calcined and uncalcined LDHs were used as adsorbents to remove the amaranth azo dye in an aqueous solution. Adsorption results indicated that MgAl500 had a greater adsorption capacity (1.6 mmol·g− 1) than non-calcined LDHs (0.8 and 0.2 mmol·g− 1for MgAlNO3 and MgAlCO3, respectively). Isotherms showed that the adsorption of the dye was consistent with the Langmuir model. The kinetic experiments of dye adsorption showed that the system reached the adsorption equilibrium in 3 h for the three adsorbents. The kinetic data fitted well with the pseudo-second order model. Powder X ray diffraction patterns of the adsorption products suggest that adsorbed amaranth anion was intercalated in the MgAlNO3 interlayers by anion exchange and in the MgAl500 by the rehydration and reconstruction process, according to the value of the basal space d003 = 17.7 Å. Whereas in the case of MgAlCO3, .it was probably adsorbed on the external particle surface. The influence of common inorganic anions present in the solution was also studied and a decrease in the dye removal was observed in the following order HPO42 −> CO32- > SO42− > Cl-. The dye desorption was tested with carbonate and phosphate solution and it was partial in all cases. The recyclability of the used adsorbents was performed by calcinations, and the percentage of removal decreases after the first cycle for MgAlNO3 and MgAl500. Dye removal improved after the first cycle then decreased after the second cycle for MgAlCO3.