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The equilibrium partition coefficient (Kc) is calculated as:
formula
7
The thermodynamic parameters such as change in free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) are calculated from the following equations and are given in Table 4.
formula
8
formula
9
where R is the gas constant (8.314 J/mol K), Cs and Ce are the equilibrium concentrations of the arsenic in the adsorbent (mg/L) and solution (mg/L) respectively and T is the solution temperature (K). ΔH and ΔS are calculated from the slope and intercept of the plot of ln Kc versus 1/T as shown in Figure 9. The estimated values of thermodynamic parameters are shown in Table 3. The negative ΔG values confirm the feasibility of the adsorption process and the spontaneous nature of As(III) adsorption onto IOISB adsorbent. The more negative values of ΔG with the rise in temperature imply a greater driving force to the adsorption process and show an increase in feasibility of adsorption at higher temperatures (Ramesh et al. 2007). A positive value of ΔH indicates endothermic nature of As(III) adsorption on to the adsorbent, whereas the positive value of ΔS indicates that the randomness increased with temperature during the adsorption of arsenic onto IOISB adsorbent.
Table 3

Estimated thermodynamic parameters (ΔH, ΔG and ΔS) for As(III) adsorption onto IOISB adsorbent at different temperatures

Temperature (K)ΔG (kJ/mol)ΔH (kJ/mol)ΔS (J/mol K)R2
283 –763.305 8,030.49 30.71 0.963 
293 –928.983 
303 –1,375.37 
313 –1,649.89 
Temperature (K)ΔG (kJ/mol)ΔH (kJ/mol)ΔS (J/mol K)R2
283 –763.305 8,030.49 30.71 0.963 
293 –928.983 
303 –1,375.37 
313 –1,649.89 
Figure 9

Estimation of thermodynamic parameters (adsorbent dose 10 g/L, initial concentration 0.5 mg/L, contact time 90 min, mixing speed 50 rpm, pH 6).

Figure 9

Estimation of thermodynamic parameters (adsorbent dose 10 g/L, initial concentration 0.5 mg/L, contact time 90 min, mixing speed 50 rpm, pH 6).

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