CHAPTER 6: A COMPARATIVE STUDY ON CESIUM IONS REMOVAL
6.5 Discussions
6.5.1 Factors affecting the cesium adsorption
The adsorption of cesium onto slag was affected by the several factors as demonstrated in the results section. The contact time is one of the important parameters for adsorption process. In the experimental results, the fast adsorption of cesium by the modified slag was observed. The equilibrium contact time of the different adsorbents varies depending on their physical (porous structure) and chemical characteristics (surface charge, functional groups, cation exchange capacity, etc.) as these properties affect the adsorption performance [23]. The fast uptake rate of cesium occurs as a result of direct electrostatic attraction in the ion exchange process and complexion interaction with modified slag [34]. Moreover, the well-developed porous structure of the modified slag would promote the rapid adsorption of cesium.
The pH of the adsorbate solution is one of the dominating factors for adsorption process. The adsorption reduction in the extreme acidic condition might be due to the competition between hydronium ions (H3O+) and Cs+ during the sorption process. The higher concentration of H3O+ in the adsorbate solution could generate a repulsive force closer to the adsorbent surface that obstructs the cesium adsorption [38][39]. However, the surface of the adsorbents was deprotonated at a higher pH, and this could have enhanced the electrostatic interactions and led to a higher cesium removal capacity [40][20].
In this study, the cesium removal performance dropped when the concentration of cesium in solution increased. This is might be the cause of the higher energy sites are being occupied with excess Cs+ and they would interact with lower energy sites of adsorbents [41][42].
The removal of radioactive cesium from the real environment water is always challenging because the sea water or groundwater contains different cations (Na+, K+, NH4+
, etc.). Therefore, the presence of the foreign ions and the target adsorbing metal ions creates a form of competition for the adsorption sites on the adsorbents. In the competitive ions study, it was observed that the presence of the Na+ and K+ at their lower concentration (Cs:Na+/K+=1:10) did not hamper the cesium adsorption. However, the
131 opposite was true for the presence of their higher concentration. For higher concentrations of Na+ and K+ ions, the adsorption sites were surrounded by the counter ions, resulting in the partial loss of charges and the binding force of electrostatic interaction. Therefore, the ion exchange mechanism between the metal ions and adsorbents is interrupted [23][43].
Moreover, particularly the cesium adsorption was highly disturbed for the presence of K+ when compared with Na+. This may be due to the fact that in this experiment, the concentration of K+ was much higher than the adsorption capacity of Cs+ for the modified slag. In general, K+ is considered highly competitive during the cesium adsorption, as the ionic radii of Cs (1.69 Å) is closer to K (1.33 Å) than Na (0.95 Å) [44].
6.5.2 Adsorption kinetic modeling
The kinetic modeling of adsorption process helps to determine the adsorption rates and suitable rate expression characteristics of the possible reaction mechanism.
From several kinetic models, the experimental data attained from the effect of contact time (0–120 min) for the raw and the modified slag were used to investigate two typical adsorption kinetic models, namely the pseudo-first order and the pseudo-second order rate equations. These kinetic equations are described in Chapter 2 (Equations 2.5 and 2.6).
The adsorption of cesium does not follow the pseudo-first order kinetic due to the large variation between the experimental and calculated adsorption capacity (qe) (data are not demonstrated).
The kinetic parameters for cesium adsorption by raw and modified slag were calculated using a linear plot based on the pseudo-second order kinetic equation. The kinetic parameters are listed in Table 6.1. The correlation coefficients (R2) for raw and modified slag were 0.9993 and 1.00, respectively. Moreover, the experimental values of were extremely close to their corresponding theoretical values. Therefore, it can be concluded that the cesium adsorption process for both the raw and the modified slag follows the pseudo-second order kinetic model and the adsorption reaction is mostly dominated by the chemisorption process rather than physisorption [4].
132 Table 6.1: Pseudo-second order kinetic parameters for the adsorption of cesium by raw and modified slag.
6.5.3 Adsorption isotherm studies
The adsorption isotherm indicates that the distribution of the adsorbed molecules between the liquid and solid phase when the adsorption process reaches an equilibrium state [45]. Adsorption isotherms were studied to determine the maximum cesium adsorption capacity of the raw and the modified slag for a wide range of initial concentrations of cesium (20 mg/L to 800 mg/L). The obtained equilibrium data were evaluated by using the Langmuir and the Freundlich isotherm models described in Chapter 2, Equations 2.1 and 2.4.
The adsorption isotherm parameters obtained from these models are presented in Table 6.2. As evidenced by the regression coefficient (R2), the Langmuir model (R2=0.287) does not fit for cesium adsorption by the raw slag, while the Freundlich model (R2=0.964) was a better fit. On the other hand, for the modified slag, both Langmuir (R2=0.989) and Freundlich isotherms (R2=0.988) could be applied perfectly to fit the experimental adsorption parameters for the removal of cesium. The maximum values of the adsorption capacities (qe) were 52.36 mg/g and 14.50 mg/g for the modified and the raw slag, respectively.
Moreover, the non-linear regression fitting curve of Langmuir isotherm for the experimental data of cesium adsorption by modified slag is shown in Figure 6.8 for better understnding the Cs concentration in solid phase and the liquied pahse at equilibrium conditions.
Parameters Raw slag Modified slag
Experimental value, 3.18 10.30
K2 (g/mg.min) 2.32 1.37
Calculated value, (mg/g) 3.21 10.30
0.999 1.0
133 The favourability of the cesium adsorption process by modified slag can be explained by the separation factor (RL) according to the Langmuir isotherm model. The values of RL were calculated according to the Equation 2.2 in Chapter 2 and plotted against the different initial cesium concentrations (Figure 6.9 (A)). In this study, the values of RL were in the range of 0 to 1 (0< RL<1), which indicated that the adsorption process of cesium by the modified slag was favourable. The adsorption process of cesium by the modified slag was further investigated by surface coverage (θ), which is also related to the Langmuir isotherm. In accordance with the Equation 2.3 in Chapter 2, the surface coverage was plotted against the different initial concentration, as shown in Figure 6.9 (B). It was observed that the surface coverage increased rapidly at lower concentrations of cesium, while it stabilized for a concentration of 400 mg/L with θ being close to 1. This observation reveals that the surface of the adsorbent was closely covered with a monomolecular layer.
It can be concluded that the adsorption process of cesium by the modified slag could be successfully explained by the Langmuir and the Freundlich isotherm. Therefore, the modified slag could be used more efficiently for the cesium adsorption.
Table 6.2: Langmuir and Freundlich isotherm parameters for the adsorption of cesium on raw and modified slag.
Isotherm models Parameters Adsorbents
Raw slag Modified slag
(mg/g) 14.50 52.36
Langmuir isotherm (L/mg) 0.0008 0.1496
0.287 0.989
(L/mg) 0.03 12.16
Freundlich isotherm 0.798 0.269
0.964 0.988
134 Figure 6.8: Langmuir isotherm for Cs adsorption by raw and modified slag (non- linear form).
Figure 6.9: (A) Plot of separation factor (RL) against initial concentration and (B) surface coverage against initial concentration on cesium removal efficiency by modified slag.
6.5.4 Adsorption thermodynamic studies
The thermodynamic parameters ( as listed in Table 6.3) were also determined to understand the effect of temperature on the adsorption mechanism. The
135 standard enthalpy change ( and entropy change ( were calculated from the slope and intercept from the plot based on the Equations 2.7 and 2.8 in Chapter 2 as shown in Figure 6.10. The Gibbs free energy was calculated from the Equation 2.9 in Chapter 2. In the present investigation, the negative value of at different temperatures and negative amounts of indicate that the cesium adsorption process was spontaneous and exothermic in nature. The adsorption process is exothermic, suggesting that the adsorption capacity should decrease with the increase in temperature. The negative value of represents a random reaction that leads to the transfer of the adsorbate ions from a disorderly state in the solution to a more ordered state.
Table 6.3: Thermodynamic parameters of Cs adsorption by modified slag.
Figure 6.10: The linear plot of ln Kd versus 1/T for the variation of temperature.
T (K) R% Kd (ml/g) G
(kJ/mol)
H
(kJ/mol) S (kJ/mol)
288 90.11 913.78 -16339.08
298 87.83 721.66 -16272.34 -18260.04 -6.67
308 84.77 556.66 -16205.68
136 6.5.5 The mechanism of cesium adsorption by modified slag
The high cesium adsorption performance by the modified slag when compared to the original one was likely due to the zeolitization that occurred during the alkali hydrothermal process of the slag. The surface of the modified slag comprised of two basic functional oxide groups, namely SiO2 and Al2O3. The central ions of silicate (Si4+) and aluminate (Al3+) have a strong affinity for electrons. Therefore, oxygen atoms are bonded to silicon and aluminum ions to subsequently form a silica tetrahedral (SiO4
-) and an alumina tetrahedral (AlO4
-) units. These are surrounded by four oxygen atoms at the corners, which are shared with the neighboring tetrahedral units to form a three-dimensional network structure. Consequently, the net charge of alumina tetrahedral is -1, which is balanced by Na+ in their structure. During the cesium adsorption process, Na+ ions are replaced by Cs+ ions according to the cation exchange mechanism. This adsorption mechanism can be easily understood from Figure 6.11.
Moreover, the higher cesium adsorption capacity of modified slag could also be explained by the CEC value and the cesium saturation capacity. The CEC value of the modified slag was almost two times higher than the raw slag. Therefore, the modified slag can exchange a higher amount of cations with Cs+ in aqueous solution compared to the raw slag. This can be clearly understood by comparing the cesium saturation capacity of the raw and the modified slag. The calculated cesium saturation capacity from the CEC value was 0.248 and 0.422 g of Cs+/g for the raw and the modified slag, respectively. Moreover, the surface of the modified slag was enriched with micropores and had a well-developed porous structure. This surface condition can enhance a physisorption mechanism along with a cation exchange process, where the cesium ions in solution are adsorbed on a porous solid surface by electrostatic forces [46].
Figure 6.11: Cesium adsorption mechanism by modified slag.
Cs Na O
Si Al
O O
O
O O O O
+
Si
+
137 6.5.6 Evaluation of the adsorption capacity
The cesium adsorption performance of the raw and the modified slag was compared with other low-cost clay and zeolite-based natural adsorbents to evaluate its potential for the application (Table 6.4). The comparison shows that the adsorption capacity of cesium by modified slag is comparable with other adsorbents in the same group.
Table 6.4: Comparison of maximum adsorption capacities of cesium with some low-cost clay minerals.
