Xrd Diagrams Of Samples A) Balla-Tx And B) Balfe-Tx Prepared At Temperature

3.4.2. Evaluation of physical and chemical properties of BAlLa and BAlFe materials

3.4.2.1. Chemical composition and surface area

The mass percentage, surface area and total pore volume of BAlLa and BAlFe are shown in Table 3.22. The BAlLa sample has higher mass percentages of Al2O3 and La than the B90 sample (Table 3.2). But the Al2O3 mass percentage of BAlLa is lower than that of the BAl sample (Table 3.15) and the La percentage is also lower than that of the B90-La sample (Table 3.9). The Fe2O3 mass percentage in BAlFe is much higher than that of the B90 sample (Table 3.2). Besides, the mass percentages of Al2O3, La and Fe2O3 increased significantly in the BAlLa and BAlFe samples, the mass percentages of Na2O and K2O decreased significantly. The mass percentages of MgO and CaO also decreased slightly, while the other components did not change significantly. This also confirms that Binh Thuan bentonite belongs to the alkaline bentonite type with interlayer hydrated cations mainly being Na+, K+. The surface area and total pore volume also increased significantly in the BAlLa and BAlFe samples compared to the raw bentonite sample. The surface area increased from 60.72 m2/g (B90) to 181.51 m2/g (BAlLa) and 131.48 m2/g (BAlFe). Thus, it can be confirmed that the interlayer hydrated cation exchange process of bentonite clay and the polyoxocation agents Al/La and polyoxocation Al/Fe occurred [101, 121, 132].

Table 3.22. Chemical composition and surface area of ​​BAlLa and BAlFe

% mass

3.4.2.2. Surface properties

(MKN: loss on heating)

The results of surface morphology analysis of BAlLa, BAlFe samples by SEM method are shown in Figure 3.22. Samples B90 and BAlLa, BAlFe have different surface morphology. Sample B90 has a thin leaf structure, sharp edges and exists

face-face structure (Figure 3.1), the difference in surface morphology of BAlLa and BAlFe is due to the interlayer hydrate cations of bentonite clay being exchanged with the polyoxocation modifier during the preparation process. The structure of BAlLa exists in a flocculated form and some small bubbles appear as observed in Figure 3.22a. When the Al/Fe polyoxocation agent is inserted into the bentonite layer structure, the face-face structure is destroyed and the layered structure (edge-to-face orientation and edge-to-edge orientation) is formed (Figure 3.22b), increasing the degree of disorder of the Al/Fe modified bentonite material. As a result, the sharp leaf structure of B90 is replaced by a chaotic layered structure (with possible layering), leading to an increase in surface area (Table 3.22). These findings are consistent with the XRD analysis results of BAlLa and BAlFe samples with a broad spectral peak compared to the B90 sample [80, 122].

a)

b)

Figure 3.22. SEM images of materials a) BAlLa and b) BAlFe.

3.4.2.3. FTIR spectrum

The FTIR spectrum analysis results of BAlLa, BAlFe and B90 samples are shown in Figure 3.23. The characteristic spectral signals shown in Table 3.23 have demonstrated that the modification of aluminosilicate layers by polyoxocation Al/La and polyoxocation Al/Fe was successful. In Figure 3.23, it can be seen that the intensity of the spectral peak at 470 cm-1 of BAlLa, BAlFe is much higher than that of the spectral peak at 471 cm-1 (B90), the change in spectral peak intensity is due to the binding of Fe3+, La3+ ions into the structure of polyoxocation Al13 [70, 131, 140].

1039

3432

1034

3625

1035

918

470

3699

1636

529

693

655

783

c)

3436

3624

918

534

471

3698

1638

782

9 16

b)

3626

532

3445

1431

3698

1638

471

780 639

a)

704

1....000

0....95

0....90

0....85

0....80

0....75

0....70

0....65

0....60

0....55

0....50

0....45

0....40

0....35

0....30

0....25

0....20

0....15

0....10

0....05

0....000

4000....0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400....0

cm - 1

Figure 3.23 . FTIR spectra of samples a) B90, b) BAlLa and c) BAlFe .

Table 3.23. Frequency and FTIR spectral signals of BAlLa and BAlFe samples

Spectrum signal

3.4.3. Conclusion on the preparation process of BAlLa, BAlFe materials

The suitable conditions for the preparation of BAlLa and BAlFe materials with the Al:La, Al:Fe ratios of 5:1 and 2:1 were determined. The ratio of (Al3++La3+)/bentonite was 15 mmol/g and (Al3++Fe3+)/bentonite was 20 mmol/g; the aging time of the Al/La and Al/Fe polyoxocation solutions was 14 days, the denaturation reaction temperature was 50oC (for BAlLa) and 70oC (BAlFe) with the molar ratio of OH-

/(Al3++La3+), OH-/(Al3++Fe3+) is 2.0.

Under these conditions, the obtained BAlLa, BAlFe materials have a layer spacing greater than 18.2Å, corresponding to an interlayer spacing greater than 8.6Å and compared with the average size of Keggin ions of 8.6Å. This shows that the Al/La polyoxocation and Al/Fe polyoxocation have a Keggin ion-type structure. The surface area and total pore volume also increased compared to the raw bentonite. This also confirms that the interlayer hydrate cations of bentonite have been exchanged and replaced by the Al/La polyoxocation and Al/Fe polyoxocation.

3.5. SURVEY OF FACTORS AFFECTING THE PHOSPHORUS ADSORPTION PROCESS OF MODIFIED BENTONITE MATERIALS

In this study, the modified bentonite material used was synthesized from Binh Thuan bentonite and the agents La, Al, Fe, Al/La and Al/Fe. The hydrated cations in the middle layer were exchanged by the modifying agents, they are located on the surface of the clay layer to form active centers located in the interlayer between the modified clay minerals. These active centers have the ability to adsorb phosphorus and lock them into the structure of the modified bentonite. The adsorption process of phosphate anions in solution on the surface of solid materials depends on many factors such as: the nature of the adsorbent, the mass of the adsorbent, the concentration of the blocking ion, the diffusion rate, the concentration, the reaction time, the pH of the solution, the reaction temperature, etc.

3.5.1. Determination of factors affecting the phosphorus adsorption capacity of modified bentonite

3.5.1.1. Effect of stirring speed

In this study, the adsorption capacity of phosphorus in synthetic aqueous solutions of modified bentonite samples at different stirring speeds was compared. The adsorption conversion (or adsorption efficiency) Hx% was calculated according to formula (2.2). The x index below indicates the stirring speeds of 150, 170, 200 and 240 rpm, respectively. The mass of adsorbent used was 1 g/L, the phosphate concentration (calculated as P) was 20 mgP/l (Appendix P11). The results of the study are presented in Table 3.24.

From the data shown in Table 3.24, it can be seen that the Hx adsorption efficiency (%) of stirring speed 170; 200 and 240 rpm is not much different and is larger than that of stirring speed at 150 rpm. Under this condition, the speed