3.1 The results of synthesis of the geopolymer foams
The whole preparation was simple, and the overall cost was lower than that of traditional commercial inorganic adsorbents. The geo-polymerization mechanism for the zeolite-based geopolymer is that a polycondensation of [AlO4] and [SiO4] groups occurred to form polymer gels with three basic structural units in the hydration layer of the hydrated sodium ions during the condensation polymerization process; this layer was located at the interface between the solid and the liquid in the initial gel [27]. The geopolymer foams were prepared and evaluated using various initial conditions. Given the results in Table 2, examining the effect of raw material composition on the fabricated geopolymer foams showed that increasing sodium silicate more than the calculated stoichiometric value reduces the strength and the dye adsorption. On the other hand, thermal processing (activation) of the zeolite has a positive effect on the strength and adsorption capacity of the adsorbent foam. The results showed that the chemical foaming method resulted in greater strength of the final structure compared to the emulsion templating method with a positive effect on the dye adsorption capacity. By comparing the values related to strength and the ability of the adsorbent foam to absorb the dye, finally, sample 3 was selected as the optimal sample for further adsorption investigations.
Table 2
Examining the effect of raw material composition on geopolymer structure
Samples
|
Synthesis method
|
Sample density (g/mL)
|
Durability in water (day)
|
Dye adsorption (mg/g)
|
S1
|
Chemical foaming
|
0.0087
|
4
|
1.95
|
S2
|
Chemical foaming
|
0.0020
|
3
|
1.72
|
S3
|
Templating Emulsion/Chemical foaming
|
0.0126
|
4
|
2.69
|
S4
|
Templating Emulsion/Chemical foaming
|
0.0075
|
3
|
2.62
|
S5
|
Templating Emulsion/Chemical foaming
|
0.0080
|
2
|
2.04
|
S6
|
Templating Emulsion/Chemical foaming
|
Not produced
|
-
|
-
|
3.2 Analysis of the prepared geopolymer adsorbent
3.2.1 FESEM analysis results
The photograph and FESEM results of the surface structure and cross-sectional view of the prepared geopolymer adsorbent foam are shown in Fig. 2 for morphologic investigation. As shown in Fig. 2, the geo-polymerization reaction causes the formation of a series of macropore structures due to the bursting of bubbles formed by the hydrogen peroxide reaction in the geopolymer substrate:
$$2{\text{H}}_{2}{\text{O}}_{2}\left(\text{l}\right)\to 2{\text{H}}_{2}\text{O}\left(\text{l}\right)+{\text{O}}_{2}\left(\text{g}\right)\uparrow$$
4
Figures illustrate that geopolymer foam has high porosity and many pores with relatively uniform size distribution.
3.2.2 XRD analysis results
Figure 3 represented the XRD results and phase structure of the ZSM-5 zeolite and prepared geopolymer foam. By comparing the X-ray diffraction pattern of the foam created concerning the raw zeolite, it is found that the primary zeolite has a crystalline structure with sharp peaks, whereas the foam produced has an amorphous polymeric structure without the initial sharp peaks which is a representation of the geo-polymerization process to form the desired foam. The main feature was obtained by XRD measurements from the very broad reflection between 2θ = 15 and 35° assigned to an amorphous phase with the presence of a small number of trace minerals. The differences in the 2θ position of the broad reflection in the XRD patterns for zeolite and geopolymer foam provided evidence for the dissolution of SiO4 and AlO4− species from zeolite in the alkaline environment during the geo-polymerization reaction [28]. The sharp reflections due to the residual crystalline phases such as quartz were also observed alongside the broad reflection.
3.2.3 XRF analysis results
The chemical composition of the samples was determined by XRF and is shown in Table 1. Given the results, the presence of sodium and potassium components participating in the geo-polymerization reaction in the resulting foam can be confirmed.
Table 3
Chemical composition of ZSM-5 and zeolite-based geopolymer
Sample
|
Chemical composition
|
SiO2
|
Na2O
|
K2O
|
Cl
|
Al2O3
|
ZSM-5 zeolite
|
97.03
|
-
|
-
|
-
|
1.72
|
Geopolymer foam
|
60.87
|
11.77
|
10.52
|
2.45
|
0.88
|
3.2.4 TGA analysis results
TGA analysis was done to evaluate the thermal stability of the foam. Fig. 4 shows that there are three distinct steps in weight loss. Firstly, about 6 to 8% weight loss at 100°C took place because of water evaporation. As the processes of adsorption of most aqueous pollutants are done at temperatures below 100°C and ambient temperature, this weight loss shows the appropriate thermal stability of the adsorbent for these applications. Then the significant weight loss of 15% occurs between 100 to 400°C that may be because of the decomposition of the geopolymer adsorbent. Stabilization in the temperature range of 400 to 800°C occurred that showed an overall weight reduction of about 20% from 30°C to 800°C, showing geopolymer foam is stable at high temperatures.
3.2.5 BET analysis results
Figure 5 shows the BET analysis diagrams for the geopolymer foam. Based on the diagrams, the isotherm is an IV-type isotherm. This type of isotherm is used for porous materials. If the P/P0 ratio is low, it is similar to the type II isotherm, but when this ratio is very large, the material has very narrow, capillary pores, where the adsorption rate increases significantly and the adsorbent material condenses on the surface. This type of isotherm is usually seen for industrial catalysts and the corresponding curve is used to determine the pore size distribution. Furthermore, hysteria is seen in this type of isotherm. Hysteresis shows the presence of meso cavities in the material and can be used to obtain information about the geometry of the cavities.
BET analysis was used to investigate the behavior of nitrogen molecules in the adsorption phenomenon on the adsorbent surface and calculate the specific surface area, total pore volume, and average pore diameter of the adsorbent. According to the BET results, the specific surface area, total pore volume, and average pore diameter of the prepared adsorbent foam were 4.41 m2.g-1, 0.01 cm3.g-1, and 9.62 nm, respectively.
3.2.6 Determination of zero-point charge results
For the Determination of zero-point charge, the graph of (pHf-pHi) vs pHi was plotted as shown in Fig. 6. The intersection of the curve with the horizontal line is known as the endpoints of the pHZPC and this value is about pH =10 for the adsorbent. At high pH values (pH> pHZPC), the adsorbent surface is negatively charged, and the adsorption of dye molecules increases because of the electrostatic adsorption between the surface and the cationic dye molecules. At low pH values (pH <pHZPC), as the adsorbent surface, is positively charged, the expected tendency is to reduce the dye adsorption because of the electrostatic repulsion between the cationic dye molecules and the adsorbent surface [29].
3.3 The results of the methylene blue adsorption by the geopolymer adsorbent
3.3.1 Batch adsorption kinetics
Batch adsorption tests were performed to examine the adsorption kinetics and reach equilibrium time. Fig. 7 indicates the concentration decay of methylene blue over time. The concentration of methylene blue decreases over time, and the adsorption rate decreases as the curve slope. Over time, the value of adsorbed dye increases, and the active sites on the adsorbent surface occupy with dye molecules until approaching the adsorption equilibrium. Adsorption capacity in the equilibrium state was calculated to equal 9.82 mg/g for 10 mg/L initial concentration. The results of the study of kinetic models of pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion are given in Table 4. The proximity of the correlation coefficient (R2) to 1 shows less deviation and more accuracy of the model. According to R2 values, the models were well-fitting to the experimental data but, the pseudo-second-order model has the best adjustment. Given the obtained R2 and the proximity of the obtained adsorption capacity with its experimental value as shown in the figure, one can state that the adsorption process was consistent with the pseudo-second-order kinetic model. In the intraparticle diffusion model Ki,1> Ki,2> Ki,3 examines the adsorption steps on the outer surface, inner section, and equilibrium. In the first stage, the molecules of the adsorptive must be transferred from the solution bulk to the adsorbent surface. The adsorbed molecules must pass through the solution boundary layer that surrounds the adsorbent particle, called film diffusion. In the second stage, the adsorbed molecules must penetrate the adsorbent pores. In the last stage, the particle must be attached to the adsorbent surface, which eventually reaches equilibrium.
Table 4. The Linear form of different kinetics models, and the parameters related to different kinetics models for methylene blue adsorption
3.3.2 Continuous adsorption kinetics
The results of the investigation of the effluent solution from the continuous adsorption column (Fig. 8) indicated that over time the dye concentration inside the column increased, which shows that dye adsorption decreased because the adsorbent active sites were saturated by the dye molecules over time. Adsorption capacity in equilibrium state and column saturation was calculated using equation 3 equal to 5.38 and 8.17 mg/g for 5 and 10 mg/L initial concentration, respectively. The adsorption process of methylene blue was investigated using the Thomas, Bohart-Adams, and Yoon-Nelson models, and the adsorption conformity to each model is represented in Table 4. Investigation of the correlation coefficients led to the conclusion that the Thomas and Yoon-Nelson models have the best fitting which states that adsorption of the adsorbate on the adsorbent occurs directly, meaning that the adsorption rate is controlled by the surface reaction between the adsorbate and the unoccupied capacity of the adsorbent.
3.3.3 Investigation of methylene blue adsorption isotherm by geopolymer adsorbent
Adsorption isotherm is one of the most important factors in designing adsorption systems. Indeed, the adsorption isotherm explains how the adsorbent and the adsorbate interact. Thus, it is always considered as a basic factor in determining the adsorption capacity and optimize the adsorbent consumption. The equilibrium of the adsorption process is usually shown by matching experimental data with different isotherm models such as Langmuir, Freundlich, and Temkin models. The models were plotted (Fig. 9) and the results were represented in Table 5. Investigation of the R2 led to the conclusion that the adjusted models were consistent with the experimental data and the Temkin model has the best fitting.
Table 5. Different isotherm models and their parameters
3.3.4 Examining of thermodynamics of methylene blue adsorption by geopolymer adsorbent
Thermodynamics is carried out to see if the process is endothermic and exothermic as well as spontaneous. In this study, Gibbs free energy (∆G), enthalpy (∆H), entropy (∆S), and energy activation (Ea) were examined at temperatures of 298.15 and 313.15 K. The parameters intended were calculated by the Van't Hoff equation and plot (Fig. 10).
$$\text{l}\text{o}\text{g}\left(\frac{{\text{q}}_{\text{e}}}{{C}_{e}}\right)=-\frac{\varDelta H}{2.303 R} \frac{1}{T}+\frac{\varDelta S}{2.303 R}$$ 4
ΔG = ΔH - TΔS (5)
Eα = ΔH + RT (6)
The results of the calculations of the desired parameters are shown in Table 6. Negative values of enthalpy, entropy and Gibbs free energy show the exothermicity, a reduction in irregularity, and spontaneity of the process, respectively.
Table 6
Thermodynamic parameters for methylene blue adsorption process
C0
(mg/L)
|
T
(K)
|
Ea
(kJ/mol)
|
∆G
(kJ/mol)
|
∆H
(kJ/mol)
|
∆S
(J/mol.K)
|
2
|
298.15
|
45.622
|
-1.103
|
-48.121
|
-157.2
|
313.15
|
45.51
|
1.262
|
|
|
4
|
298.15
|
29.713
|
0.781
|
-32.192
|
-104.3
|
313.15
|
29.589
|
-0.799
|
|
|
6
|
298.15
|
27.445
|
0.025 -
|
29.924 -
|
-100.3
|
313.15
|
27.321
|
1.479
|
|
|
8
|
298.15
|
21.319
|
0.291
|
23.798-
|
-80.8
|
313.15
|
21.194
|
1.503
|
|
|
10
|
298.15
|
15.254
|
1.212
|
17.733-
|
-63.5
|
313.15
|
15.130
|
2.165
|
|
|
3.3.5 Examining the effect of various parameters on the adsorption of methylene blue dye
The effect of temperature
Given the adsorption data at various temperatures in Fig. 11a, the adsorption capacity has decreased with an increase in temperature. With the increase in temperature, the adsorption interaction between methylene blue molecules and active sites of the adsorbent decreases, which reduces the dye adsorption.
The effect of initial concentration
The initial concentration of the solution affects the adsorption. According to the results in Fig. 11a, the adsorption increases with an increase in concentration. This process can be explained by the improvement of the driving force that reduces the mass transfer resistance between the adsorptive and adsorbent. At higher concentrations, it provides a fundamental driving force to reduce the mass transfer resistance between the liquid and the solid phases.
The effect of pH
As a key element in adsorption, pH affects the chemical properties of the adsorbent and solution. Methylene blue is a cationic dye with a positive charge in the solution phase. Hence, methylene blue is an ionic species and its adsorption on the adsorbent surface is primarily affected by the adsorbent surface charge, affected by the pH of the solution. The effect of pH was examined in the range of 5 to 9 (Fig. 11b). As can be seen in the Figure, an increase in the percentage of removal dye was observed by both increasing and decreasing pH. At high pH values, the increase of the dye adsorption can be explained as the negative charging of the surface depending on the pHzpc value of the adsorbent. However, at low pH values, the reason for the dye adsorption value not decreasing can be explained as the replacement of the hydrogen ions on the adsorbent surface with the cationic methylene blue dye molecules in the solution. In an acidic medium, the initial pH value of adsorption firstly increases and then reaches an initial value by decreasing can be accepted as an indication of the previous situation.
3.3.6 Regeneration and reusing the adsorbent
Regeneration and reusing are very important in the adsorbent application. Two methods were used to reduce the adsorbent dye. As Fig. 11c shows, the adsorbents are still capable of adsorbing the dye after four cycles of regeneration and reusing, and in the acidic method, the adsorbent has shown better efficiency than the thermal method, which can be because of the adsorbent degradation in the furnace and thus decreasing its weight and as well as decreasing adsorption level.