This experiment has shown that the adsorption capabilitiesof LDH increases with an increase in temperature. This suggeststhat the interaction of adsorbate and adsorbent is endothermic innature. The temperature dependence of adsorption process is asso-ciated with changes in several thermodynamic parameters such asstandard Gibbs energy (G◦), enthalpy (H◦) and entropy (S◦) ofadsorption and were calculated by using the equation:
Go =
−RT ln (K) (13)where R is the universal gas constant, T is temperature, and Kis Langmuir constant. For calculating the other thermodynamicparameters (H◦) and (S◦) Van’t Hoff equation was used:
ln (K) =
−H
RT
+ S
R
(14)Constants in this equation (H◦and S◦) were found using 1/Tand ln(K) graphics. The slope of the line is (−H/R), and interceptis (S/R).The negative values of G◦at different temperatures(298–318 K) indicate the spontaneous nature of malic acid adsorp-tion. The value of H was calculated to be 15.18 J mol−1. This datashowed that the adsorption process of malic acid onto LDH is physi-cal adsorption, so because the value is below 80 kJ mol−1this showsthat the adsorption reaction is endothermic. The negative S◦val-ues pointed out that no significant change occurs in the internal