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The adsorption thermodynamics equilibrium constant K is calculated according to intercepts of the fitting line of ln (qe /Ce) vs. qe and the formula (10). Thermodynamic parameters are calculated according to formula (11) and (12) and the results are listed in Table 4. The decreased K values and the negative enthalpy change with the increase of adsorption temperature indicated the adsorption was an exothermic process, which resulted in the close removal efficiency at different temperatures as seen in Figure 3. The values of standard Gibbs free energy change (ΔG) were negative, suggesting that the adsorption process was spontaneous. The negative entropy change ΔS demonstrated that the system degree of freedom was decreasing, which meant the system was in order when OTC was adsorbed on the surface of modified 13X.

Table 4

The calculated thermodynamic parameters

T/KKΔG (kJ·mol−1)ΔS (J·mol−1·K−1)ΔH (kJ·mol−1)
298 44.70 −9.41 − 47.69 − 23.64 
308 34.47 −9.07 
318 24.78 −8.49 
T/KKΔG (kJ·mol−1)ΔS (J·mol−1·K−1)ΔH (kJ·mol−1)
298 44.70 −9.41 − 47.69 − 23.64 
308 34.47 −9.07 
318 24.78 −8.49 

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