γ-al o nanofibers - shodhganga : a reservoir of indian theses...

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Chapter 5

Adsorption of dyes from aqueous solution on

γ-Al2O3 nanofibers

Alumina is made from bauxite, a naturally occurring ore containing variable amounts of

hydrous aluminum oxides. Free Al2O3 occurs in nature as the mineral corundum and its

gemstone forms, sapphire and ruby. Some alumina is still produced by melting bauxite in

an electric furnace, but most of the alumina is extracted from bauxite by Bayers method.

This method results in the production of several grades of granular or powdery alumina,

including activated alumina, smelter-grade alumina and calcined alumina [1]. Alumina

has many appealing properties which makes the material interesting for applications in

many different areas. Activated alumina is a porous, granular substance that is used as a

substrate for catalysts and as an adsorbent for removing water from gases and liquids [2].

Smelter-grade alumina accounts for 90% of all alumina produced and it is electrolyzed to

produce aluminum metal. Calcined alumina is made into a variety of ceramic products,

including spark-plug insulators, integrated circuit packages, bone and dental implants,

laboratory ware, sandpaper grits, grinding wheels and refractory linings for industrial

furnaces. These products exhibit the properties for which alumina is well known,

including low electrical conductivity, resistance to chemical attack, high strength,

extreme hardness and high melting point.

Alumina exists in a number of crystalline phases (polymorphs), three of the most

important being γ, θ, and α. The α structure is thermodynamically stable at all

temperatures up to its melting point, but the metastable phases (γ and θ) still appear

http://www.britannica.com/EBchecked/topic/56513/bauxite

http://www.britannica.com/EBchecked/topic/138974/corundum

http://www.britannica.com/EBchecked/topic/523735/sapphire

http://www.britannica.com/EBchecked/topic/512038/ruby

http://www.britannica.com/EBchecked/topic/4518/activated-alumina

http://www.britannica.com/EBchecked/topic/88894/calcined-alumina

Chapter – 5 Adsorption of dyes on γ-Al2O3 nanofibers

125

frequently in alumina growth studies. The common alumina polymorphs can all be

formed within typical synthesis temperatures, i.e., from room temperature to about

1000 °C [2].

Metal oxide nanoparticles, including nanosized ferric oxides, zinc oxides,

aluminum oxides, titanium oxides, magnesium oxides and cerium oxides [3] have drawn

great interest in recent years due to their increasing and diversified applications [4].

Because of their large specific surface area and high potential adsorptive capacity are

classified as promising adsorbents for both organic [5, 6] and inorganic pollutants

[3, 7-9].

Alumina (Al2O3) nanoparticles are widely used as coatings and abrasives due to

their excellent dielectric and abrasive properties [10, 11]. The γ-Al2O3 has been widely

used as a catalyst and catalyst support and it also has extensive applications in structural

composites and ceramic industry. Recently, nanostructured γ-Al2O3 has been prepared for

its application in the biochemical engineering. Moreover, γ-Al2O3 is commercially used

as a component in cosmetics, pharmacy and pigment, after its biocompatibility been

improved [12].

In the present method γ-Al2O3 nanofibers were synthesized according to the

mercury-mediated method reported by Q. Yang et al. [13]. The synthesized material was

characterized by XRD, FESEM, N2 adsorption studies and FTIR analysis. The adsorption

behavior of γ-Al2O3 nanofibers has been tested towards the removal of three dyes (IB,

LFR and AR 112) from aqueous solution. Batch adsorption experiments were carried out

in an incubator shaker at constant temperature. The effect of various parameters such as


126

contact time, temperature and pH were studied and optimized. The kinetics and

thermodynamics of adsorption has been also studied.

5.1 Synthesis and characterization of γ-Al2O3 nanofibers

Figure-5.1: Images showing the various stages of γ-Al2O3 formation on aluminium plate.

In the present study, facile mercury mediated synthetic pathway was adopted for

the bulk synthesis of alumina nanofibers in the absence of surfactants. From the images

(Figure 5.1) it can be seen that the growth of white alumina fibres starts as soon as the

(a) (b)

(c)

(d)


127

aluminium plate is removed from HgCl2 solution and exposed to the air. The grown

alumina fibres are clearly observed along the plate edges (Figure 5.1d) which ultimately

fall down after reaching a short length.

According to Q. Yang et al., the growth mechanism of alumina nanofibers is

similar to voluminous oxidation of aluminium by continuous dissolution in a wetting

mercury film [14]. The reactions involved are as shown below:

32 2332 AlHgHgAl (5.1)

HOHAlOHAl 3)(3 32

3 (5.2)

HgOnHOAlOnHOAlHg 4.23)(4 23222 (5.3)

Although the natural alumina film offers some protection for bulk aluminium, it

contains surface defects [15]. Therefore, HgCl2 solution can penetrate, react with the bulk

aluminium, and mercury will form by metathesis reaction. Because protective alumina

film is removed, aluminium atoms continuously dissolve into mercury and form

amalgam. After aluminium strip is moved from HgCl2 solution and exposed in air,

aluminium atoms diffuse from the amalgamated layer near bulk aluminium to the

amalgam/air interface and react with oxygen and water in air to form alumina according

to equation 5.3. Alumina particles assemble by preferred orientation to form nanofibers

[16]. As the Al atoms are depleted, more bulk aluminium dissolves into the mercury and

the process continues until mercury volatilize completely.


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5.1.1 X-ray diffraction studies

The powder X-ray diffraction patterns of the synthesized product and the product

calcined at 850 °C for 2h are shown in Figure 5.2. The XRD pattern of the as-grown

product indicated that the product consists of amorphous alumina (Figure 5.2a). After

calcined at 850 °C for 2 h, the amorphous alumina transferred into γ-Al2O3 (Figure 5.2b).

All the diffraction peaks matched well with those of γ-Al2O3 according to JCPDS card

No. 10-0425. The major peaks at 2θ values of 37.4º, 39.4º, 45.8º, 60.8º and 67.0º can be

indexed to the lattice planes of (311), (222), (400), (551) and (440), respectively.

Furthermore, no characteristic peaks from other impurities were detected by XRD

suggesting that the product was of pure alumina.

Figure-5.2: The XRD patterns of (a) as-grown product and (b) product calcined at 850 °C

for 2h.


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5.1.2 FESEM and surface area analysis

The surface and textural morphology of the product was studied using FESEM analysis.

The as-grown sample was collected and subjected to FESEM analysis. From Figure 5.3a

it can be observed that the sample consists of clusters of alumina nanofibers. They appear

to be fused all along their length resulting in a porous structure. The low density sample

showed peaks for aluminium and oxygen in the EDS spectrum and further no other

elements were found in EDS which evidenced that the product was of pure Al2O3.

Figure-5.3: (a) The FESEM micrographs and (b) EDS spectrum of γ-Al2O3 nanofibers.

(b)

(a)


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The N2 adsorption–desorption isotherm for the prepared γ-Al2O3 sample is shown

in Figure 5.4. The nitrogen adsorption isotherm for the prepared sample could be

classified as type IV isotherms with type H3 hysteresis loops, which are often associated

with the formation of aggregates with slit-shaped pores according to Brunauer–Deming–

Deming–Teller (BDDT) classification [17]. The type H3 hysteresis loop, which does not

clearly show any adsorption plateau at relative pressures close to unity, is usually related

to the existence of slit-shaped pores [18]. The pore size measurements from BJH (Barrett-

Joyner-Halenda) method revealed that the sample consists of mesopores in the range of

30 – 40 nm. From BET analysis, the specific surface area of γ-Al2O3 sample was found to

be 147.7 m2 g

-1 with a corresponding total pore volume of 1.08 cm

3 g

-1 (p/p0=0.987).

Figure-5.4: The N2 adsorption–desorption isotherm for alumina nanofibers.


131

5.1.3 Fourier transform infrared spectroscopy (FTIR)

The FTIR spectrum of γ-Al2O3 was recorded by making pellet with KBr. The stronger

broad band at 3451 cm–1

may be attributed to the AlO-H stretching vibration of surface

adsorbed water molecules in the product [19]. The band near 1643 cm-1

may be attributed

to H-O-H bending vibrations of adsorbed water molecules. The stronger broadening

band in the range of 1000 – 400 cm–1

corresponds to Al-O vibrations [20, 21].

Figure-5.5: The FTIR spectrum of γ-Al2O3 nanofibers.

5.2 Adsorption studies

The synthesized nano aluminium oxide was employed for the adsorption of three textile

dyes namely, Levafix fast red (LFR), Indanthren blue BC (IB) and Acid red 112 (AR

112). Batch experiments were carried out for the adsorption of these dyes from their

aqueous solutions. The effect of adsorption parameters such as contact time and initial

solution pH were studied and optimized. The adsorption kinetics and thermodynamics


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have been also studied and the calculated kinetic and thermodynamic parameters have

been presented.

5.2.1 Effect of contact time

Figure-5.6: Effect of contact time on the adsorption capacity of nano Al2O3 towards (a)

LFR (b) IB and (c) AR 112.

The variation of adsorption capacity of nano alumina towards textile dyes with

time is depicted in Figure 5.15. It is evident from all the three figures that the adsorption

was rapid initially and the dye removal percentage improved slowly with time and finally

reached almost a constant value at equilibrium. The initial rapid adsorption rate is due to

the presence of large number of active sites on the surface of alumina and the high

(a) (b)

(c)


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concentration gradient. Adsorption equilibrium was attained within 120 min of contact

time for LFR, 90 min for IB and 60 min for AR 112. The presence of plateau after the

equilibrium might be attributed to the saturation of adsorption sites on alumina

nanofibers.

5.2.2 Effect of pH

Figure-5.7: Influence of pH on the adsorption capacity of nano Al2O3 towards the dyes

studied.

It is well established that, pH is an important parameter which affects the

adsorption performance of oxide materials in aqueous medium. In order to study the

effect of pH, adsorption equilibrium studies were carried out at 25 °C and at varying pH

(2.0-11.0) for a series of dye solutions (100 ppm) containing a fixed amount of alumina

nanofibers. The variation of equilibrium adsorption capacity (qe) of γ-Al2O3 towards the

three dyes (LFR, IB and AR 112) is shown in Figure 5.7. From the Figure it can be seen

that the values of qe decrease with increase in the pH for LFR and AR 112. However, the


134

variation of solution pH did not influence the adsorption of IB and hence the qe values

were found to be almost constant in the entire pH range.

The adsorption performance of nano alumina towards the chosen dyes may be

understood by considering the surface charge of γ-Al2O3 under different pH conditions.

From literature, it is well known that the point of zero charge (pHZPC) for γ-Al2O3 is 7.9

[22]. Thus the surface of γ-Al2O3 is positively charged in acidic, neutral and slightly basic

environments (below pH 8.0). However, the surface is negatively charged under basic

conditions (above pH 8.0). The higher qe values observed for LFR and AR 112 at acidic

pH conditions may be explained by considering the surface charge on γ-Al2O3 and dye

molecules. The charge of the AR 112 molecules is highly negative due to the presence of

four SO3-

groups on each molecule. Similarly, the reactive dye LFR is also negatively

charged due to the presence of negatively charged reactive groups. Since γ-Al2O3 is

positively charged below pH 8.0, the negatively charged dye molecules are strongly

attracted towards the adsorbent surface. Thus, electrostatic force of attraction between the

positively charged γ-Al2O3 and negatively charged dye molecules is responsible for the

high adsorption capacity which was observed upto pH 8.0. However, a drastic decrease in

the dye removal efficiency was observed for both LFR and AR 112 after pH 8.0. This is

because the surface of γ-Al2O3 acquires positive charge which causes repulsion with the

dye molecules [22].

From Figure 5.7, it can also be observed that there was no appreciable change in

the adsorption capacity for IB in the entire pH range studied. Indanthren Blue BC (IB) is

insoluble but highly dispersible in water and thus it remains non-ionic in aqueous

solution. Although solution pH influences the surface charge of adsorbent, the surface


135

properties of IB remain unchanged. Thus the change in solution pH did not influence the

adsorption of IB.

5.2.3 Adsorption isotherms

Adsorption isotherms are the equilibrium relations between the concentration of

adsorbate on the solid phase and its concentration in the liquid phase. In this study,

adsorption isotherm studies were carried out to obtain the maximum adsorption capacity

of γ-Al2O3 nanofibers for various dyes. The isotherm studies were carried out as batch

experiments by taking a series of dye solutions of various concentrations in the range of

25 – 200 mg L-1

at pH 2.0. For each dye, the batch experiments were carried out at 298,

308, 318 and 328 K respectively. The data obtained from isotherm studies provide

information on the capacity of the adsorbent or the amount of adsorbent required to

remove a unit mass of pollutant under the system conditions.

Figure-5.8: Langmuir adsorption isotherm plots at different temperatures for the

adsorption of LFR onto γ-Al2O3 nanofibers.


136

Table-5.1: The parameters of Langmuir model for the adsorption of LFR onto γ-Al2O3

nanofibers at different temperatures.


adsorption of AR 112 onto γ-Al2O3 nanofibers.

T (ºC) qmax (mg g-1

) KL (L mg-1

) R2 RL

25 131.75 0.582 0.995 0.007-0.035

35 142.65 0.347 0.997 0.012-0.057

45 152.21 1.626 0.994 0.003-0.013

55 163.13 1.416 0.996 0.003-0.007


137

Table-5.2: The parameters of Langmuir model for the adsorption of AR 112 onto γ-Al2O3



) KL (L mg-1

) R2 RL

25 139.47 0.899 0.985 0.012-0.037

35 207.04 5.367 0.997 0.002-0.006

45 260.42 1.280 0.994 0.008-0.015

55 304.88 2.827 0.995 0.004-0.007


adsorption of IB onto γ-Al2O3 nanofibers.


138

Table-5.3: The parameters of Langmuir model for the adsorption of IB onto γ-Al2O3



) KL (L mg-1

) R2 RL

25 88.97 0.270 0.997 0.018-0.129

35 106.9 0.196 0.995 0.025-0.169

45 125.5 0.283 0.995 0.017-0.124

55 143.7 0.155 0.988 0.031-0.205

In this study, equilibrium data were analyzed using the Langmuir and Freundlich

adsorption isotherm models. The plots of Langmuir isotherm model obtained for the

adsorption of LFR, AR 112 and IB at different temperatures are shown in Figures 5.8, 5.9

and 5.10 respectively. The calculated Langmuir model parameters for the three dyes are

summarized in Tables 5.1, 5.2 and 5.3. From the tables it can be seen that high

correlation coefficient (R2) values were obtained for all the three dyes at all the

temperatures studied, indicating a good agreement of the data. However, very poor data

fit with low correlation coefficient (R2) values were observed for the Freundlich model.

Thus it may be concluded that Langmuir model is a suitable isotherm model to describe

the adsorption of all the three dyes onto γ-Al2O3 nanofibers. The agreement of data with

Langmuir model for all the three dyes emphasizes the formation of monolayer coverage

of the dye molecules on the surface of adsorbent. At 298 K, the maximum adsorption

capacities (qmax) of γ-Al2O3 were found to be 131.75, 139.47 and 88.97 mg g-1

for LFR,

AR 112 and IB respectively. Comparatively low qmax values obtained for IB may be

attributed to its insolubility and non-ionic nature in aqueous medium, while the high qmax

values observed for LFR and AR 112 may be due to the strong electrostatic attractive


139

forces between the negatively charged dye molecules and positive surface of γ-Al2O3.

From tables 4.3 and 4.8 (shown in the previous chapter), it is clear that the adsorption

capacity of γ-Al2O3 nanofibers is relatively high than the other adsorbents reported

earlier. The high adsorption capacity of γ-Al2O3 nanofibers towards the dyes is due to the

high specific surface area coupled with electrostatic forces of attraction involved during

adsorption.

The RL vaues within the range 0 < RL < 1 indicate a favorable adsorption. In the

present study, RL values obtained were in the range of 0.003 – 0.057, 0.004 – 0.037 and

0.017 – 0.205 for LFR, AR 112 and IB respectively. This clearly indicated favorable

adsorption of all the three dyes on γ-Al2O3 nanofibers.

5.2.4 Adsorption thermodynamics

The effect of temperature on the adsorption capacity of γ-Al2O3 nanofibers towards the

chosen dyes was studied by performing the adsorption experiments at four different

temperatures (298, 308, 318 and 328 K). Batch experiments were performed separately

for LFR, IB and AR 112 at pH 2.0.

The plots showing the variation of adsorption capacity of γ-Al2O3 nanofibers for

the three dyes at different temperatures are given in Figures 5.11a, 5.12a and 5.13a. The

results revealed that the adsorption capacity increased with increase in temperature from

298 to 328 K for all the three dyes. This indicated that the adsorption of all the three dyes

is endothermic. This supports chemisorption of dyes where there is an increase in the

number of dye molecules acquiring sufficient energy to undergo chemical reaction with

the adsorbent at higher temperatures [23].


140

Figure-5.11: (a) Effect of temperature on the adsorption capacity and (b) van’t Hoff’s plot

for the adsorption of LFR onto γ-Al2O3 nanofibers.


for the adsorption of IB onto γ-Al2O3 nanofibers.

(a) (b)

(a) (b)


141


for the adsorption of AR 112 onto γ-Al2O3 nanofibers.

Figure-5.14: Plots of ln(1-θ) versus 1/T for (a) LFR (b) IB and (c) AR 112.

(a) (b)

(a) (b)

(c)


142

Table-5.4: Thermodynamic parameters for the adsorption of LFR, IB and AR 112 on

alumina nanofibers at pH 2.0.

Dye T

(°C)

ΔG°

(kJ mol−1

)

ΔS°

(J K−1

mol−1

)

ΔH°

(kJ mol−1

)

Kc Ea

(kJ mol−1

)

S*

LFR 25 0.669 25.01 8.123 0.767 3.83 0.121

35 0.419 0.858

45 0.169 0.914

55 -0.081 1.046

IB 25 1.129 111.2 34.27 0.652 18.68 3.3×10-4

35 0.0174 0.963

45 -1.0946 1.556

55 -2.2066 2.224

AR

112

25 -1.903 369.8 108.3 2.310 99.0 1.5×10-18

35 -5.601 9.468

45 -9.299 28.98

55 -12.99 129.5

The van’t Hoff’s plots for the adsorption of LFR, IB and AR 112 onto γ-Al2O3

nanofibers are shown in the Figures 5.11b, 5.12b and 5.13b respectively. The

corresponding values of thermodynamic parameters obtained from these plots are given

in Table 5.4. The values of ΔH° were found to be positive for all the three dyes which


143

indicated that the adsorption process is endothermic. Also, the positive values of ΔS°

revealed that adsorption of dyes on nano alumina leads to the increase in randomness of

the system [24]. Among the three chosen dyes, the values of ΔH° and ΔS° were found to

be maximum for AR 112. In case of LFR, the values of free energy (ΔG°) were slightly

positive at lower temperatures and became negative at higher temperatures. However, the

ΔG° values were found to be negative at all the temperatures for the other two dyes (IB

and AR 112). The negative values of ΔG° indicated that the process of adsorption is

spontaneous. Further, the values of ΔG° became more negative with increase in

temperature for all the three dyes, suggesting that the adsorption of dye molecules

became more favourable at higher temperatures.

Energy of activation

The values of activation energy (Ea) and sticking probability (S*) were calculated using

modified Arrhenius equation. The values of Ea were calculated from the slope of the plot

of ln(1−θ) versus 1/T (Figure 5.14). In the present case, the values of Ea were found to be

3.83, 18.68 and 99 kJ mol-1

for LFR, IB and AR 112 respectively. Higher values of Ea

indicated more favourable adsorption of dyes and it also suggested the involvement of

chemisorption on γ-Al2O3 nanofibers.

5.2.5 Adsorption kinetics

The kinetics of adsorption was studied by employing pseudo-first-order and pseudo-

second-order models. The pseudo-first-order and pseudo-second-order plots for the

adsorption of LFR, IB and AR 112 on γ-Al2O3 nanofibers are shown in Figures 5.15, 5.16

and 5.17 respectively.


144

Figure-5.15: (a) Pseudo-first-order and (b) pseudo-second-order plots for the adsorption

of LFR onto γ-Al2O3 nanofibers.


of IB onto γ-Al2O3 nanofibers.


of AR 112 onto γ-Al2O3 nanofibers.

(a) (b)

(a) (b)

(a) (b)


145

Table-5.5: The pseudo-first-order and pseudo-second-order kinetic parameters for the

adsorption of LFR, IB and AR112 onto γ-Al2O3 nanofibers (Dye: 50 ppm; pH: 2.0).

The values of rate constants and calculated equilibrium adsorption capacity

(qe(cal)) obtained for pseudo-first-order and pseudo-second-order models, and the values of

experimental equilibrium adsorption capacity (qe(exp)) are listed in Table 5.5. From the

table it can be seen that the values of correlation coefficient (R2) were high for both

pseudo-first-order and pseudo-second-order kinetic models. The R2 values for the

pseudo-second-order model were slightly higher than those of the pseudo-first-order

model for all the three dyes. Moreover, the qe values obtained from pseudo-first-order

model were too small compared to the experimental value. However, the qe values

calculated from pseudo-second-order kinetic model are close to the experimental value

for all the three dyes. Therefore, it can be concluded that pseudo-second-order equation is

better in describing the adsorption kinetics of the chosen dyes on γ-Al2O3 nanofibers.

Dye

qe(exp)

(mg g-1

)

Pseudo-first-order model Pseudo-second-order model

qe(cal)

(mg g-1

)

k1

(min-1

)

R2 qe(cal)

(mg g-1

)

k2

(g mg-1

min-1

)

R2

LFR 140.1 96.82 12.8×10-3

0.963 146.4 1.32×10-3

0.994

IB 93.34 43.01 16.76×10-3

0.979 93.63 1.97×10-3

0.997

AR112 172.7 57.23 5.50×10-2

0.976 182.1 2.39×10-3

0.999


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According to pseudo-second-order model the adsorption depends on the adsorbate as well

as the adsorbent, and it involves chemisorption.

Figure-5.18: Intra-particle diffusion model for the adsorption of (a) LFR (b) IB and (c)

AR 112 onto γ-Al2O3 nanofibers.

Table-5.6: The Intraparticle diffusion model parameters for the adsorption of LFR, IB

and AR 112 onto γ-Al2O3 nanofibers (Dye: 50 ppm; pH: 2.0).

Dye kid (mg g-1

min-0.5

) C (mg g-1

) R2

LFR 8.952 70.17 0.970

IB 5.199 45.73 0.980

AR 112 9.541 116.4 0.953

(c)

(a) (b)


147

In order to further analyze the kinetic results, the intra-particle diffusion model

proposed by Weber and Morries was employed. The plots of qt versus t0.5

for LFR, IB

and AR 112 are shown in Figure 5.18. The values of kid and C were calculated from the

slope and intercept of plots of qt versus t0.5

and are summarized in Table 5.6. All the three

plots were found to be linear with high correlation coefficient values, however they do

not pass through the origin. Therefore it may be concluded that the boundary layer (film)

diffusion is the rate controlling step in the process of dye adsorption by γ-Al2O3

nanofibers.

5.2.6 Recyclability of adsorbent

Regeneration of adsorbent is one of the important factors in order to reduce the cost

incurred during pollutant adsorption. The nano alumina adsorbent could be recovered by

combustion at 800 °C and reused. Thus regenerated alumina could remove similar

amounts of dyes even after the second and third regenerations.

5.3 Conclusions

The γ-Al2O3 nanofibers were synthesized by mercury mediated method. The low density

nano alumina was found to posses high adsorption capacity for textile dyes. The

adsorption was found to be maximum at pH 2.0 for LFR and AR 112, which is due to

strong electrostatic forces of attraction between γ-Al2O3 and dye molecules. However, the

solution pH did not influence the adsorption of IB. Langmuir isotherm was found to be

suitable to describe the adsorption mechanism, suggesting monolayer coverage of dye

molecules on γ-Al2O3 nanofibers. Chemisorption was found to be the operating

mechanism and qmax values for LFR, AR 112 and IB were found to be 131.75, 139.47 and

88.97 mg g-1

respectively. Thermodynamic studies revealed that the adsorption of all the


148

dyes was endothermic and spontaneous. Further, the adsorption of dyes followed pseudo-

second-order kinetics. The high adsorption capacity of γ-Al2O3 nanofibers towards

different classes of textile dyes suggests that it can act as a potential adsorbent for the

removal of dyes from textile wastewater.


149

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