To investigate the nature of adsorption, thermodynamic parameters viz. enthalpy (ΔH°), entropy (ΔS°) and Gibbs free energy (ΔG°) were estimated. The following Van’t Hoff equation was used to evaluate thermodynamic parameters:12 logqe/ce=ΔH/2.303RT+ΔS/2.303RThe negative value of ΔH indicates an exothermic reaction; a positive value denotes an endothermic reaction. If the value of ΔS is less than zero, it indicates the process is highly reversible and if more than or equal to zero, it indicates the irreversibility of the process. The negative value of ΔG indicates the spontaneity of adsorption whereas a positive value of ΔG indicates the non spontaneity of adsorption. Van’t Hoff plot is represented in Fig. 4 and the equation obtained was log (q e/c e) = 0.3105(1/T) + 1.3481. The positive experimental ΔH° value indicates that the adsorption process was endothermic in nature and there was a possible strong bonding between the metal ion and the adsorbent. As ΔS° was more than zero, the adsorption process appears to be irreversible (Desorption studies were hence not attempted). The Gibbs free energy is negative, suggesting the spontaneous nature of the adsorption process. The free energy change (ΔG°) increased with increase in temperature (10–50 °C), presumably due to activation of more sites on the surface of the adsorbent (Sengil and Ozacar 2008). The thermodynamic parameters at different temperatures are given in Table 3.
Fig. 4 Van’t Hoff plot for adsorption of copper using DNES–CH composite
Table 3 Thermodynamic parameters calculated by Van’t Hoff equation
Temperature (°K) ∆G° (kJ/mol) ∆H° (kJ/mol) ∆S° (kJ/mol K)
283 −7298.9075 5.9451 25.8122
293 −7557.0295
303 −7815.1515
313 −8073.2735
323 −8331.3955