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Time Dependence of Phosphorus Adsorption Capacity of Modified Bentonite on Time.


The stirring speed or external diffusion speed did not affect the adsorption capacity. Based on the indications in Table 3.24, the stirring speed of 170 rpm was chosen to study the adsorption laws.

Table 3.24. Hx adsorption efficiency (%) of phosphate anion on modified bentonite after 120 minutes of stirring


Sample

H150(%)

H170(%)

H200(%)

H240(%)

B40-La

39.77

41.74

41.84

41.94

B90-La

45.12

48.37

48.44

48.27

BAl

59.73

61.42

61.40

61.54

BFe

65.57

68.79

69.93

69.33

BALa

53.24

56.79

56.98

57.40

BAlFe

67.17

70.07

70.63

72.74

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3.5.1.2. Determination of time to reach adsorption equilibrium

The effect of time on the adsorption capacity of phosphorus in the form of phosphate onto modified bentonite was studied under the following conditions: phosphate concentration was 10 mgP/l (for B40-La, B90-La) and 15 mgP/l (for BAl, BFe, BAlLa and BAlFe), adsorbent amount was 1 g/L, pH = 6, temperature 25oC, stirring speed 170 rpm. Samples were taken after intervals of 8 hours and the remaining phosphorus concentration in the solution was analyzed. The results of the study on the effect of time on the adsorption capacity were as follows:

Phosphate adsorption capacity of unmodified bentonite and modified bentonite materials

is shown in Figure 3.24.


Figure 3.24. Dependence of phosphorus adsorption capacity of modified bentonite on time.


The results presented in Figure 3.24 show that the phosphorus adsorption process on modified bentonite occurs rapidly in the first 1 hour, then gradually decreases as time increases and reaches equilibrium after 4 hours. Unmodified bentonite (B40, B90) used as raw material to prepare modified bentonite has very little phosphorus adsorption capacity ( q e < 0.32 mg/g). Modified bentonite has a much larger phosphate anion adsorption capacity than raw bentonite, because the number of active sites in modified bentonite increases when the modifier is introduced into the middle layer of bentonite through the exchange process between them and the interlayer hydrated cations. Thus, modified bentonite has a better ability to adsorb phosphate in solution than unmodified bentonite due to the modifiers La, Al, Fe, Al/La and Al/Fe contained in the material structure. From Figure 3.24, it can be seen that the phosphate adsorption capacity decreases in the following order: BAlFe > BFe > BAl > BAlLa > B90-La > B40-La.

3.5.1.3. Effect of pH


The experiment to investigate the effect of pH on phosphorus adsorption capacity was carried out as described in section 2.5 in the experimental part. The phosphorus adsorption capacity on La, Al, Fe, Al/La and Al/Fe modified bentonite at different solution pH is shown in Figure 3.25.

Figure 3.25 . Dependence of phosphorus adsorption capacity of modified bentonite on solution pH.

From Figure 3.25, it can be seen that pH has a significant effect on the adsorption capacity of P on modified bentonite materials. According to Zhu [161] and Shin [117], pH affects the adsorption capacity of ions on the surface of the solid phase, especially the solid phase with variable surface charge, such as: aluminum oxide, iron oxide, various types of


clay minerals. P adsorption on Fe/Al oxides and clay minerals mainly occurs through ligand exchange mechanism with surface functional groups, such as Al-OH/OH2, Fe-OH/OH2, La-OH/OH2 [55, 130, 153, 157, 161].

The phosphate adsorption capacity does not change much in the pH range from 3

to 6. The suitable pH for phosphorus adsorption on modified bentonite is from 3 to

2-

6. The adsorption capacity of phosphorus decreased as the pH increased. This result showed that the pH of the solution affected the existence of phosphate ion components in the solution. Different types of phosphate ions existing in solutions with different pH were calculated from the Visual Minteq program [62] shown in Figure 3.26, showing that in the pH range from 3 to 6, mainly dihydrogen phosphate ion (H2PO4-) existed. Hydrogen phosphate ion (HPO4 ) existed at high pH from 8 to 11.

Figure 3.26. Phosphate ion composition in solutions at different pH.

The phosphorus adsorption capacity ( q e) increased with increasing H2PO4- anion concentration (Figure 3.25). This shows that the denaturing agents (La, Al, Fe, Al/La, Al/Fe) have a high affinity for dihydrogen phosphate ion (H2PO4-). At low pH (pH 3

– 6) The surface hydroxyl groups of M – OH (M is Al, Fe, La) can be protonated

protonation to form a positively charged group. The protonation process can occur according to the following equation: ≡M – OH + H+ ↔ ≡M – OH2+. Then, the M – OH group without charge forms a positively charged M – OH2+ group. This group can easily interact with the dihydrogen phosphate anion in solution [55, 157]. This

showed that monovalent phosphate ions can react with the active sites of


modified bentonite is easier to adsorb. Therefore, another reason explaining the phosphate adsorption efficiency of modified bentonite materials at low pH is due to the nature of phosphate anion species existing in the solution [55, 124, 129].

When the pH increases from 7 to 11, the phosphorus adsorption capacity decreases sharply. The reason for the influence of pH on the phosphate adsorption process on modified bentonite is explained as follows: at high pH, ​​the surface of the adsorbent material carries a more negative charge and therefore there is a stronger repulsive interaction between the negatively charged ions in the solution (phosphate anions) and the negative charge on the surface of the bentonite clay layer, leading to the ability to

adsorption decreases. Furthermore, in the high pH range, the OH- ion concentration is quite high, leading to

The adsorption competition between OH- ions and phosphate anions in solution for the adsorption sites of modified bentonite materials increased [63].

However, the adsorption of phosphate on BFe, BAlFe materials is less dependent on the pH of the solution. Compared with the maximum phosphate adsorption values ​​of BFe and BAlFe, the values ​​were approximately 10% and 38% less at different pH values. According to Zhu [161], the presence of iron polyoxocation in the interlayer of modified bentonite is likely to enhance the adsorption affinity of the active sites to phosphate anions in the solution.

3.5.1.4. Phosphorus adsorption kinetics on modified bentonite


The kinetics of phosphorus adsorption on modified bentonite were investigated as described in section 2.5 in the experimental section. The pseudo-second-order kinetic model (2.8) and the Elovich model (2.10) are commonly used to describe the kinetics of phosphorus adsorption on modified bentonite [55, 130, 148]. The linear graphs of the pseudo-second-order kinetic model (2.8) and the Elovich model (2.10) are shown in Figure 3.27.

The calculations of the parameters in the apparent second-order kinetic model and the Elovich model in Figure 3.27 are presented in Table 3.25. From the results given in Table

3.25 found that all correlation coefficients of the apparent second-order kinetic equation were larger than those of the Elovich equation, which means that the apparent second-order kinetic equation described well the kinetic law of phosphorus adsorption on modified bentonite at 25oC with a high correlation coefficient ( R 2 > 0.99). This result is consistent with the study of phosphorus adsorption kinetics on modified bentonite by some authors [130,


161]. Modified bentonite has the ability to adsorb phosphorus quickly in the first hour. In this adsorption step, the ligand exchange process occurs between the phosphate anion and the OH- group.

/OH2 of the modifying agent (La-OH-/OH2, Al-OH-/OH2, Fe-OH-/OH2), as well as the electrostatic interaction of the active site present in the modified bentonite [55, 148, 157, 161]. The phosphorus adsorption capacity varies depending on the modifying agent and the interlayer between the modified bentonite and is arranged in increasing order as follows: q e(B40-La) < q e(B90- La) < q e(BAlLa) < q e(BAl) < q e(BFe) < q e(BAlFe). The adsorption rate constants ( k ) of B90-La and B40-La are much larger than those of the other modified bentonite samples, and the BAlFe sample has the lowest adsorption rate constant ( k = 0.0026 g/mg.min).

Figure 3.27. Phosphate adsorption kinetics of modified bentonite: (a) Linear form of the pseudo-second-order kinetic equation; b) Linear form of the Elovich equation.


Adsorbent

Apparent second-order kinetic model

t = 1 t + 1

qq result2

Elovich model

q = 1 ln t + 1 ln (αβ )

t β β

k

(g/mg.min)

qe

(mg/g)

R2

α

(mg/g.min)

β

(g/mg)

R2

B40-La

0.0142

8,4459

0.9997

12,0449

0.9774

0.9326

B90-La

0.0169

9.6712

0.9999

25,5317

0.9107

0.8635

BAl

0.0064

12,5628

0.9977

4,9414

0.5466

0.9445

BFe

0.0049

13,8313

0.9998

3,3579

0.4372

0.9751

BALa

0.0055

12,1507

0.9995

4,2613

0.5657

0.9729

BAlFe

0.0026

14,2248

0.9988

1,7954

0.4194

0.9886

Table 3.25. Phosphorus adsorption kinetic parameters of modified bentonite


tee


As mentioned in Table 3.25, the parameter α is the initial adsorption rate in the Elovich model, so it can be considered as the kinetic rate in the fast adsorption step within 1 hour of the modified bentonite material. The α values ​​of B90-La and B40-La are much higher than those of BAl, BFe, BAlLa and BAlFe samples, the corresponding analysis results are shown in Figure 3.24.

3.5.1.5. Adsorption isotherm

The adsorption isotherm was investigated as described in section 2.5 in the experimental part. The adsorption conditions were at temperatures of 25oC, 30oC and 35oC, phosphorus concentration from 2.5 mg/l to 100 mg/l, adsorbent amount 1 g/l, pH 5, stirring speed 170 rpm. The adsorption isotherm of phosphorus on solid materials, phyllosilicate minerals and metal oxides [130, 148, 150, 161] is often described by the Langmuir (2.12) and Freundlich (2.13) equations. The linear graphs according to the Langmuir (2.12) and Freundlich (2.13) equations are presented in Figures 3.28 and 3.29.

The data of P adsorption isotherm on modified bentonite are consistent with Langmuir and Freundlich equations, calculating the characteristic parameters shown in Table 3.26. From the results shown in Table 3.26, it can be seen that all correlation coefficients of Langmuir equation are larger ( R 2 > 0.97) than Freundlich equation, which means that Langmuir isotherm describes the process

The adsorption of phosphorus on modified bentonite was better suited than the Freundlich equation in the temperature range of 25oC to 35oC.

The results shown in Table 3.26 show that the monolayer saturated adsorption capacity ( q m) is proportional to the temperature, meaning that the phosphate adsorption capacity on modified bentonite increases as the temperature increases and the adsorption process is endothermic. The increase in q m value is accompanied by a high energy requirement for the phosphate adsorption process on the investigated adsorbent materials. The q m and K L values ​​presented in Table 3.26 show that the maximum adsorption capacity and affinity of modified bentonite for phosphorus in solution are arranged in the order: B40-La < B90-La < BAlLa < BAl < BFe < BAlFe. The phosphorus adsorption capacity of BAl, BFe, BAlFe materials is higher than that of B40-La, B90-La and BAlLa, consistent with the results shown in the Langmuir equation. These results are consistent with previous reports on the adsorption of phosphate in aqueous solution on adsorbents containing aluminum, iron or lanthanum [41, 55, 130, 148, 161].


a)

8

7

25°C

30°C

35°C

(g/L)

6

5

C e /qe

4

3

2

1

0

0 20 40 60 80 100

Ce (mg/L)

b)


8

7

25°C

30°C

35°C

( g / L )

6

5

C e /qe

4

3

2

1

0

0 20 40 60 80 100

Ce (mg/L)



c)

6

25°C

30°C

35°C

(g/L)

5

4

C e /qe

3

2

1

0

0 20 40 60 80 100

Ce (mg/L)


d)

6

25°C

30°C

35°C

(g/L)

5

4

C e /qe

3

2

1

0

0 20 40 60 80 100

Ce (mg/L)


8 e)

7

Ce/qe (g/L)

6

25°C

30°C

35°C

5

4

3

2

1

0

0 20 40 60 80 100

Ce (mg/L)

5 f)

Ce/qe (g/L )

4

25°C

30°C

35°C

3


2

1


0

0 20 40 60 80 100

Ce (mg/L)


Figure 3.28. Linear form according to Langmuir isotherm equation of phosphate adsorption process on a) B40-La, b) B90-La, c) BAl, d) BFe, e) BAlLa, f) BAlFe.

In the Freundlich equation, K F and 1/n are constants related to the adsorption capacity and adsorption intensity, respectively. The K F values ​​of the BAl, BFe, BAlFe samples (8.41; 8.54 and 8.97 at 35oC) were larger than those of B40-La, B90-La and BAlLa (4.50; 6.51 and 5.94 at 35oC). These results indicate that the adsorption capacity of


The BAl, BFe and BAlFe samples were larger than the B90-La, B40-La, BAlLa samples with low K F values. The K F values ​​almost increased with increasing temperature, which showed that the phosphorus adsorption capacity on the modified bentonite material increased with increasing temperature. The 1/n value was between 0 and 1, indicating that the phosphorus adsorption process on the modified bentonite was highly efficient.


3

2.5

L n qe

2

1.5

1

0.5

0


3

2.5

L n q e

2

1.5

1

0.5

0

a)



25°C

30°C

35°C


-4 -2 0 2 4 6

Ln Ce


c)


25°C

30°C

35°C


-6 -4 -2 0 2 4 6

Ln Ce


3

2.5

L n q e

2

1.5

1

0.5

0


3

2.5

L n q e

2

1.5

1

0.5

0

b)


25°C

30°C

35°C


-4 -2 0 2 4 6

Ln Ce


d)


25°C

30°C

35°C


-6 -4 -2 0 2 4 6

Ln Ce



3

2.5

L n q e

2

1.5

1

0.5

0

e)


25°C

30°C

35°C


-2 0 2 4 6

Ln Ce


3

2.5

L n qe

2

1.5

1

0.5

0

f)



25°C

30°C

35°C


-6 -4 -2 0 2 4 6

Ln Ce


Figure 3.29. Linear form according to Freundlich isotherm equation of phosphate adsorption process on a) B40-La, b) B90-La, c) BAl, d) BFe, e) BAlLa, f) BAlFe.

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