Chapter 3: Results and Discussions
3.4 Adsorption Isotherm
The experimental data obtained from the adsorption studies were fitted to different adsorption isotherms using the model equations discussed in Section 1.6.1 As an example, fits of the adsorption processes of toluidine blue O and methyl violet to different isotherm are shown in Figures 20 to 25.
T (K) KL ΔG(Kj/mol) ΔH(Kj\mol) ΔSO
(J/mol/K)
R(square)
283 0.534 -13.039 14.428 46.127 0.9651
288 0.659 -13.270
301 0.794 -13.869
308 0.914 -14.192
Figure 20: Linear fitting of the adsorption of Toluidine blue O and Methyl violet on ACCG to the Freundlich isotherm
Figure 21: Linear fitting of the adsorption of Methyl violet and Toluidine blue O on ACCG to the Temkin isotherm
Figure 22: Linear fitting of the adsorption of Methyl violet and Toluidine blue O on ACCG to the Harkin-Jura isotherm
Figure 23: Linear fitting of the adsorption of Methyl violet and Toluidine blue O on ACCG to the Halsey isotherm
Figure 24: Linear fitting of the adsorption of Methyl violet and Toluidine blue O on ACCG to the Elovich isotherm
Figure 25: Linear fitting of the adsorption of Methyl violet and Toluidine blue O on ACCG to the Flory – Huggins isotherm
Table 5: Fitting the adsorption of methyl violet and toluidine blue to different isotherm models
Adsorption isotherm models
Parameters Linear fittings Methyl violet Toluidine blue o
Qe experiment(mg/g) 4.95 4.33
Langmuir isotherm Xm (mg/g) 4.198 4.424
B (L/mg) 88.225 1.876
RL 2.266 X 10-4 0.01
R2 0.908 0.9624
Freundlich isotherm N 4.25 3.375
KF (mg/L/g) 6.077 2.690
R2 0.9804 0.9591
Temkin isotherm BT (J/mol) 0.7635 0.9044
KT (L/mg) 1228.99 21.597
R2 0.9619 0.9747
Flory–Huggins isotherm NFH - 0.311 -0.5646
KFH (L/g) 4.518 X 10-3 5.202 X 10-3
R2 0.9683 0.9741
Halsey isotherm KH 2154.80 28.2151
NH 4.2535 3.3749
R2 0.9804 0.9591
Harkin- Jura isotherm AHJ (mg/g) 8.2919 5.9809
BHJ (mg2/L) -0.2098 -1.0
R2 0.920 0.8716
Qmax (mg/g) 2.5824 2.3996
KJ (L/g) 1.7415 0.1107
R2 0.7163 0.6993
QE 1.010 1.334
KE 8.006 X10-4 0.101
R2 0.9478 0.9447
Using the Langmuir model to fit the adsorption data for methyl violet and toluidine blue on ACCG, the plots give correlation coefficients of 0.908 and 0.9624, a sorption capacity of 4.198 and 4.428 mg/g, and RL values of 2.266 X 10-4 and 0.01,
respectively. Since 0<RL<1, the Langmuir isotherm fits well the data, and the adsorption behavior is favorable. This describes monolayer adsorption of methyl violet and toluidine blue on distinct localized ACCG sorption sites without transmigration of the dyes in the plane of the surfaces resulting in uniform energies of monolayer sorption onto the surface of the adsorbent (Balouch et al., 2013). The good fits of the adsorption data to the Langmuir isotherm also suggest a physical adsorption mechanism (Physisorption). The Langmuir isotherm was also discovered to be a good fit for adsorption of methyl violet on Casuarina equisetifolia needles (Dahri et al., 2013). Additionally, the adsorption isotherm of toluidine blue on natural zeolite was also fitted to the Langmuir isotherm model (Alpat et al., 2008).
Using the Freundlich model to fit the adsorption of methyl violet and toluidine blue on ACCG, the plots give a correlation coefficient of 0.9804 and 0.9591, and a determinant value (nF) of 4.25 and 3.37 respectively. The nF value also falls within the range of favorable adsorption behavior of nF within 1-10. The value of nF as given in Table 4 indicated that Freundlich isotherm is a good fit for the adsorption data. This result is in agreement with the adsorption isotherm of toluidine blue on natural zeolite fitted to the Freundlich isotherm model (Alpat et al., 2008).
Using the Temkin isotherm to stimulate the adsorption data, the correlation coefficients for methyl violet and toluidine blue are 0.9619 and 0.9747 respectively.
This high R2 indicates that the Temkin model is a good fit for the adsorption data.
Additionally, from Table 4, the values can be seen of B (L/mg) 0.763 and 0.9044, and KT (KJ/mol) 1228.99 and 21.597 for methyl violet and toluidine blue respectively. The low value of B (L/mg) suggests a weak interaction between the adsorbent and adsorbate, indicating weak ion-exchange interactions which support the physisorption
adsorption mechanism. The adsorption is also considered to be exothermic as a result of the Bbeing positive for both methyl violet and toluidine blue. Since the value of b is related to heat of adsorption i.e., b= ∆Q= -∆H. A positive value of b means adsorption process is exothermic.
Using the Elovich isotherm to fit the experimental data for the adsorption of methyl violet and toluidine blue on ACCG, the correlation coefficients are 0.9478 and 0.9447. The value of qE which is the maximum sorbent concentration on the sorbent surface (mg/g) is 1.010 and 1.334 for methyl violet and toluidine blue respectively which is very low when compared to the experimental sorption capacity. The assumption of exponential adsorption site coverage, which implies multilayer adsorption, does not match the concentration range examined. As a result, the Elovich isotherm is not a good fit for methyl violet and toluidine blue adsorption on ACCG.
Using Flory - Huggins isotherm to fit the experimental data of the adsorption of methyl violet and toluidine blue on ACCG, the high value of the correlation coefficients of 0.9619 and 0.9747 for methyl violet and toluidine blue suggest that the Flory- Huggins model is a good fit for the data. This suggests that the adsorption is spontaneous and the adsorption mechanism is feasible. A positive value of K gives a negative value of ∆G and in turn prove the spontaneity of the adsorption mechanism as a result of high adsorbate occupancy of the adsorbent surface.
Using the Halsey model to fit the experimental data, the high correlation coefficients of 0.9804 and 0.9591, and the nH values of 4.2535 and 3.3749 were found for methyl violet and toluidine blue, respectively, suggest that the experimental data is well fitted to the Halsey model. This indicates the heterogeneous nature of the ACCG.
Using the Javonovich model to fit the experimental data, the qmax values of 2.58 mg/g and 2.39 mg/g which is low when compared with the experimental sorption capacity of 4.95 and 4.33 mg/g are obtained, for methyl violet and toluidine blue respectively. In addition, there is the R2 value of 0.7163 and 0.6993, respectively.
Therefore, the Javonovich model does not seem to be a good fit for the experimental data.
Using the Harkin - Jura model to fit the experimental data, the R2 value are 0.920 and 0.8716 for methyl violet and toluidine blue, respectively. This suggests that the Harkin –Jura is not a great fit for the adsorption of methyl violet and toluidine blue on ACCG.