The thermodynamic parameters ΔH˚ and ΔS˚ can be calculated from the slope and the intercept of the plot: ln Keq vs. 1/T, according to the equation of Van’t Hoff Eq. (11), (Fig. 4). The numerical results are shown in Table 4.
The Gibbs free energy change is negative demonstrating that adsorption is spontaneous under the experimental conditions applied. The absolute values are increasing with temperature indicating a more favorable process at higher temperatures.
The positive values of standard enthalpy of adsorption indicates an endothermic process, a conclusion already found and justified earlier. Finally, the positive and low value of ΔS˚ suggests some minor structural changes in the adsorbent after adsorption.