The frequency factors and the activation energies can be calculated from the slope and the intercept on the plot of ln k vs. 1/T, according to the linear equation of Arrhenius as shown in Fig. 3.
The numerical results are presented in Table 3.
Adsorption activation energy is quite low. Desorption activation energy is higher, although in the order of magnitude of physisorption. At low temperature, the cyanide is hydrated and it is adsorbed loosely in the diffuse layer of the electrical double layer. Adsorption energy is low since adsorption (electrostatic) interactions between carbon positive charges and cyanide is “not strong” due to the water molecules shield between carbon surface and cyanide ion. By increasing temperature, energy is supplied to the system, enough to remove and exchange the solvation sheath around the cyanide ion. This is the most probable explanation given also by other researchers. The amount of energy is quite low, 0.29 kcal/mol, equal to the activation energy calculated for adsorption (Table 3). Generally, increase in adsorption with temperature has been encountered in a number of works. The usual explanation is that although adsorption is exothermic, the dehydration process that precedes adsorption is endothermic. The enthalpy of the later is higher than that of the former and the overall process results endothermic. In parallel, the overall equilibrium constant is affected by the kinetics of dehydration. Since K of adsorption is expected to decrease with temperature, the kinetic constant of dehydration increases sufficiently to overcome the decrease of K. In order to further elucidate the adsorption mechanism of cyanide on carbon surface, the adsorbent–adsorbate bond must be investigated experimentally using techniques such as SEM, FTIR, and XPS. Experiments are under way and results would be published in an upcoming paper