Equilibrium, kinetics and thermodynamic studies for separation of malic acid on layered double hydroxide (LDH)

• LDH synthesized in this study has a highly crystalline and nanostructure.
• We examine the separation of malic acid from aqueous solution by LDH.
• The adsorption of malic acid by LDH, follows the Langmuir isotherm.
• The adsorption processes are physical and spontaneous.
• LDH synthesized in this study might be used as an alternative adsorbent.

The adsorption equilibria of malic acid onto layered double hydroxide (LDH) was studied. In the experimental context of this study, firstly LDH with a highly crystalline structure was synthesized with the co-precipitation method and characterized. In the adsorption experiment some important effects such as, adsorption equilibrium, influence of quantities of LDH as adsorbent, effect of adsorption temperature, and influence of starting concentration of malic acid were investigated. The removal of malic acid was 96.73% the highest percentage for 1 g LDH at 298 K. The results obtained from the experiment were used to plot Langmuir, Freundlich, and Temkin adsorption isotherms. It has been found that experimental data about solid–liquid equilibria of malic acid with LDH and isotherm equations were appropriate. The equilibrium data show that the adsorption isotherm is compatible with Langmuir isotherm (R2 is 0.9992 at 298 K). Adsorption depended on the starting malic acid concentration at various temperatures (298, 308, 318 K), respectively. The temperature dependence of adsorption process is associated with changes in several thermodynamic parameters such as standard Gibbs energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) of adsorption and were calculated. Elovich and other kinetic model equations were applied to the obtained data from the experiment.