CHAPTER 3: PREPARATION, MODIFICATION AND
4.4 Discussions
85
86 The pHpzc of BC, BC-AO and BC-AC were 10.20, 9.80 and 2.10 respectively. The surface charge of the adsorbents was positive when the pH of the adsorbate solution was lower than the pHpzc value and the pH was above the pHpzc when the surface of the adsorbents became negative. Therefore, the solution pH at above the respective pHpzc valueled to increase the cesium adsorption rate.
The adsorbent dose is also one of the important parameters for adsorption. The adsorption of cesium by BC and BC-AO increases proportionally with an increase of the adsorbent doses. In adsorption solution, the number of active sites increases for higher doses that cause the enhance of the rate of adsorption. However, for BC-AC, the higher amount of adsorbents in the solution led to increasing larger amounts of surface acidic functional groups in turn to increase the cesium adsorption through ion exchange mechanism. Similar phenomenon for the effect of adsorbent dose on adsorption was also reported by the other researchers [20][24]. Nevertheless, the cesium removal percentage did not increase with the same trend with increasing adsorbent dose. This is maybe one of the reasons of unsaturation of adsorption sites in the adsorption process.
The concentration of cesium in solution affected the adsorption ability of the adsorbents. When the concentration of the cesium in solution increased then the adsorption rate steadily declined. In the adsorption process, the higher energy sites are occupied with excess cesium ions and they obstructed the lower energy sites of the adsorbents [25][26].
During the Fukushima nuclear power plant accident, the plant was urgently cooled down by using sea water. The sea water generally contains several types of cations (Na+, K+) etc. These competing ions and adsorbing ions are available for the adsorption sites also affect the target metal ions adsorption by the selected adsorbent [27].
Therefore, adsorption of cesium ions in the presence of these cations needs to investigate to understand the practical applicability. The cesium adsorption by using BC-AO did not significantly affect for the presence of the Na+ and K+. It might be the cause of the lower molar concentration of Na+ and K+ (Cs: Na+/K+=1:80) during cesium adsorption. In the optimum adsorption conditions, the cesium removal performance by BC-AO in the presence of competitive ions is much higher than the other hybrid and mesoporous adsorbents [15] [27]. On the other hand, the cesium adsorption by using BC-AC seriously
87 affected by the higher molar concentration of Na+ and K+ (Cs: Na+/K+ =1:1000). This may be due to the fact that in this experiment, the concentration of Na+ and K+ was much higher than the adsorption capacity of Cs+ by BC-AC. However, the presence of these competitive ions at their lower concentration (Cs: Na+/K+ =1:100) did not significantly affect the cesium adsorption.
4.4.2 Adsorption isotherm studies
The adsorption isotherm basically explains the diffusion status of the adsorbate molecules in liquid and solid phase during the equilibrium state of the adsorption process.
It also establishes a correlation between the adsorption capacity of adsorbent and liquid phase concentration of adsorbate ions [28]. Therefore, two well-known adsorption isotherm models were studied to fit the experimental data obtained from the effect of Cs ions concentration (20 mg/L to 1000 mg/L) on adsorption (Figure 4.4).
The cesium adsorption experimental data were fitted by the Langmuir and Freundlich isotherm model (Chapter 2, Equations (2.1) and (2.4)), and the isotherm parameters were presented in Table 4.1. It is clearly seen that the Freundlich isotherm ( =0.981) is more appropriated than the Langmuir isotherm ( =0.901) for adsorption of cesium by BC although the both isotherms can be applied to explain the adsorption mechanism. On the other hand, for BC-AC, the higher value of regression coefficient suggests that the Langmuir isotherm ( ＝0.993) can be applied instead of Freundlich isotherm ( =0.797) to fit the experimental adsorption parameters for the uptake of cesium. The maximum Langmuir isotherm adsorption capacity was 48.54 mg/g for BC-AC. On the other hand, for BC-AO, the Langmuir isotherm model ( ＝0.991) presents a better fit in terms of higher R2 value as compared to the Freundlich isotherm model (
＝0.966). The justification of Langmuir isotherm model by the adsorption experiments data represents the homogenous nature of the oxidized-BC surface and adsorption sites contain the uniform adsorption energy. This observation also suggests that the mostly monolayer sorption takes place on the adsorbent surface and the maximum experimental monolayer adsorption capacity was 55.25 mg/g according to the Langmuir isotherm model. Moreover, according to the Langmuir isotherm, the non-linear regression fitting
88 curve for the experimental data of cesium adsorption by BC-AO and BC-AC is shown in Figure 4.7.
Furthermore, the suitability of the adsorption process of BC-AO and BC-AC can be explained by the separation factor according to the Langmuir isotherm equation. The different values of separation factor ) were calculated according to the Equation (2.2) (Chapter 2) and plotted against the various initial cesium concentrations (Figure 4.8).
The values of ranging from 0 to 1 confirm that the BC-AO and BC-AC are favorable for the adsorption of cesium under the conditions of a wide range of initial concentrations.
It can be summarized that considering the higher correlation coefficient, the suitable range of separation factors the cesium adsorption by BC-AO mostly follow the Langmuir isotherm model and the adsorption mechanism also can be explained by the Freundlich isotherm model. On the other hand, for BC-AC adsorbent, the adsorption mechanism mainly dominated by the Langmuir isotherm model.
Table 4.1: Langmuir and Freundlich isotherms parameters for the adsorption of cesium onto BC BC-AO and BC- AC.
Isotherm models Parameters Adsorbents
BC BC-AO BC-AC
Langmuir Isotherm (mg/g) 0.172 55.25 48.54
(L/mg) 1.652 0.021 0.176
0.901 0.991 0.993
Freundlich Isotherm (mg/g) 0.233 1.474 10.56
0.6397 0.535 0.2628
0.981 0.966 0.797
89 Figure 4.7: Langmuir adsorption isotherm for Cs adsorption by BC-AC and BC-AO (non-linear form).
Figure 4.8: The plot of separation factor against initial concentration on cesium adsorption by (A) BC-AO and (B) BC-AC.
4.4.3 Adsorption kinetics model studies
In kinetics studies, the experimental data obtained from the effect of contact time (Section 4.3.1) on cesium adsorption were investigated by the pseudo-first order kinetic model and pseudo-second order kinetic model in order to analyze the adsorption kinetics
90 by BC, BC-AO and BC-AC. The experimental data were fitted by the pseudo-first order (Chapter 2, Equation 2.5) and pseudo-second order (Chapter 2, Equation 2.6).
Among the two applied kinetic models, pseudo-first order model is not appropriate for the adsorption of cesium by these adsorbents (data are not shown).
According to these models, the theoretical value of equilibrium adsorption exhibits a significant difference with the experimental values . The kinetic parameters of the pseudo-second order model is listed in Table 4.2 and the data were plotted in Figure 4.9.
The pseudo-second order model shows the higher correlation coefficient value for BC ( , BC-AO and BC-AC for cesium adsorption.
Besides, the adsorption capacity obtained from this model is close to the experimental values. Therefore, this result suggesting that the cesium adsorption by these adsorbents could be well-explained by the pseudo-second order sorption mechanism.
Table 4.2: Pseudo-second order kinetic model parameters for the adsorption of cesium by BC, BC-AO and BC-AC.
Adsorbent Experimental value
K2(mg/g. min) Calculated value (mg/g)
BC 1.927 0.017 1.972 0.963
BC-AO 3.16 0.016 3.44 0.998
BC-AC 9.021 0.314 9.090 0.999
91 Figure 4.9: The plots of pseudo-second order kinetic model for adsorption of cesium by (A) BC, (B) BC-AO and (C) BC-AC.
4.4.4 Thermodynamics studies
A thermodynamic study was performed in order to determine the free energy change , entropy , and enthalpy during the adsorption reaction with BC-AO and BC-AC. A previous experimental study of the effect of temperature on cesium adsorption (Section 4.3.6) by these adsorbents allowed to determine the above mentioned thermodynamic parameters at 288-308K. The thermodynamic parameters were calculated from the equation mentioned in Chapter 2 (Section 2.4.6, Equations 2.7, 2.8 and 2.9) and listed in Table 4.3.
92 For BC-AO, the negative value of revealed that the adsorption process was favorable and spontaneous. The gradually increase of value with increase of temperature indicated that the cesium adsorption process was less effective at higher temperature. The positive value of was indicative of endothermic nature of the adsorption process. Moreover, the positive value of suggested that the adsorbent had good affinity towards Cs ions that promoted the randomness at the solid-solution interference during adsorption process.
On the other hand, for BC-AC, the positive value of ΔG indicated that the adsorption reaction is non-spontaneous in nature and the increase of with increase of studied temperature revealed that the sorption process is less effective at higher temperature. The reason of this behavior could promote the thermal destabilization of the adsorbent and inaugurate the desorption state. Moreover, the positive value of confirmed that sorption process was endothermic. However, the negative value of is considered the results of random reaction. In such a situation, adsorbate ions are transferring from the more disorder state to the order state.
Table 4.3: Thermodynamic parameters of cesium adsorption by BC-AO and BC-AC.
Temp (K) RE%* (kJ/mol) (kJ/mol) (kJ/mol)
BC-AO BC-AC BC-AO BC-AC BC-AO BC-AC BC-AO BC-AC
288 85.00 97.86 -8521.47 16188.62
298 85.11 96.35 -9073.61 16648.30 7380.16 83140 55.21 -279.67 308 85.22 95.50 -9624.52 16927.97
*RE is the cesium removal efficiency.
93 4.4.5 Cesium adsorption mechanism of modified BC
In this study, the prepared bamboo charcoal enriched with the higher surface area.
The larger surface area of the activated carbon is always welcome in order to obtain higher adsorption efficiency. However, oxidative modification (gas or liquid) can increase a certain degree of surface chemical heterogeneity with acidic functional groups.
The modification of BC with air is mainly enriched the surface porosity and a small amount of acidic functional groups (-OH and C=O) while the nitric acid modified BC boosted mainly acidic functional groups (-OH, C=O, and -COOH) (details in Chapter 3, Sections 3.3.3, 3.3.4 and 3.4.3). These surface properties of the raw and modified BC are strongly associated with the adsorption mechanism. In adsorption isotherm studies, it is observed that the raw BC is mainly followed the Freundlich isotherm model. The air oxidized BC (BC-AO) is most preferable for Langmuir isotherm model although Freundlich isotherm model can be applied for describing the adsorption mechanism.
However, the cesium adsorption mechanism of nitric acid modified BC (BC-AC) can be explained by only Langmuir isotherm. It is well known that the Freundlich isotherm is applicable for the physisorption process (BC and BC-AO) while Langmuir isotherm is appropriate for chemisorption process (BC-AO and BC-AC). It needs to clear that the BC-AO is suitable for both isotherm model because of owing the higher surface area (physisorption) and also the small amount of acidic functional groups (chemisorption). In chemisorption process, the proton (H+) of the carboxylic/carbonyl groups exchange with the cesium ions (Cs+) in the aqueous state. The chemisorption mechanism of the modified BC is shown in Figure 4.10.
Figure 4.10: The cesium adsorption mechanism of modified BC (chemisorption process).
94 4.4.6 Comparison of cesium adsorption capacity
The cesium adsorption capacity of the modified BC (BC-AO and BC-AC) found in this study is compared with the different activated carbon based adsorbents in Table 4.4. It can be seen that the cesium adsorption capacity by these adsorbents are comparable with other studied adsorbents.
Table 4.4: The comparison of cesium adsorption capacity by different charcoal or activated carbon.
