where ΔG is the free energy of adsorption (kJ·mol−1), T is the temperature
in Kelvin, R is the universal gas constant (8.314 J·mol−1·K−1), ΔH
is the heat of adsorption (kJ·mol−1), and ΔS is the standard entropy
change (kJ·mol−1). The values of ΔH and ΔS can be obtained from the
slope and intercept of a plot of lnKc against 1/T [22].
The calculated thermodynamic parameters are shown in Table 3 and
Fig. 7. ΔG values are negative at all temperatures used, confirming the
spontaneous nature of sorption. The value ofΔG becomesmore negative
as temperature increases, indicatingmore efficient adsorption at higher
temperature. The positive values of ΔS suggest that Zn(CN)4
2 − is not
restricted in the resin. The positive value of ΔH confirms the endothermic
nature of the adsorption process. The lower values of ΔH suggest
that the adsorption of cyanide on the resins is governed by hydrogen
bonding.