Synthesis and Characterization of Î³-CD-Modified TiO2 Nanoparticles

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Synthesis and Characterization of γ‑CD-Modified TiO2 Nanoparticles and Its Adsorption Performance for Different Types of Organic Dyes Seyed Hossein Mousavi, Fatemeh Shokoofehpoor, and Asadollah Mohammadi* Department of Chemistry, Faculty of Science, University of Guilan, Rasht, Iran

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S Supporting Information *

ABSTRACT: In this work, γ-cyclodextrin-modified titanium dioxide nanoparticles (NPs) as an efficient nanoadsorbent were synthesized and used to remove methylene blue (MB), malachite green (MG), crystal violet (CV), disperse red 1 (DR1), acid blue 113 (AB113), and congo red (CR) dyes from synthetic wastewater. Characterization of cyclodextrin-functionalized TiO2 nanoparticles using Fourier transform infrared, field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and Brunauer−Emmett−Teller (BET) analysis revealed that the TiO2 NPs were effectively modified with γ-CD. Furthermore, synthesized adsorbent with a large BET specific surface area of 383.131 m2/g showed excellent adsorption performance toward organic dyes. The dye adsorption kinetics, isotherms, and thermodynamics using TiO2/γ-CD NPs were also investigated. The results showed that pseudo-second-order model and Freundlich isotherm model well describe the dye adsorption process on the TiO2/γCD NPs. Additionally, the maximum adsorption capacity of MB, MG, CV, DR1, AB113, and CR onto TiO2/γCD NPs was found to be 134, 244, 213, 238, 157, and 5000 mg/g, respectively. On the basis of the obtained RL values, which were less than 1.0, the adsorption of all tested dyes was favorable. Furthermore, the calculated thermodynamic parameters showed that the adsorption process of the studied dyes was endothermic.

1. INTRODUCTION Environmental pollution, especially water contamination, is one of the greatest ecological issues of the day, and a major global concern. Industrial effluent is identified as the primary source of water contamination, which leads to soil pollution. Residual organic compounds from chemical industries are released into the environment through industrial effluents.1,2 Among these pollution sources, wastewaters from the textile industry play an important role in environmental pollution. The discharge of textile effluents into surface water can cause severe health and ecological problems to human beings.3−5 Furthermore, colorants may inhibit the growth of plants and animals, and they have a tendency to chelate metal ions that produce microtoxicity to fish and other organisms in natural streams. Dyes also interfere with transmission of sunlight into streams hence reducing light penetration and photosynthetic activity.6,7 More seriously, most organic dyes are toxic and cause many health problems to human beings.8−10 Organic colorants are widely used in many applications, especially in textiles. Dyes used in textile industry can be characterized as anionic, cationic, and nonionic.11,12 Congo red (CR) is a hazardous anionic dye known to metabolize to benzidine, a known human carcinogen. So, the adsorption of Congo red by effective adsorbents has been considered for several reasons, such as high solubility in water, its limited stability and biodegradability in sunlight, and complex chemical structure.13,14 Crystal violet (CV), a bacteriostatic agent, is © XXXX American Chemical Society

widely used in paints, printing ink, paper, and textile industries.15,16 Malachite green (MG) was widely used in our life and various industries. However, its high concentrations in aqueous solutions caused significant effects such as carcinogenesis, mutagenesis, chromosomal fractures, and teratogenic and respiratory toxicity on humans.17,18 Methylene blue (MB), a cationic dye, is usually applied for dyeing in textile industries. Although MB is not seriously hazardous, it does have several negative side effects.19,20 Acid blue 113 (AB113), a commercial azo dye, is used in many industries, and its discharge into water harmfully affects the ecosystems.21,22 Therefore, the removal of hazardous dyes by effective adsorbents from waste waters before discharging into water resources is very important for environmental safety. Techniques that have been widely used to treat wastewater include filtration, osmosis, solvent extraction, chemical precipitation, electrolysis, oxidation/reduction, ion exchange, and adsorption processes.23−30 Among these, the adsorption method is considered as a competitive technique for practical wastewater treatment due to its high efficiency, easy handling, and economic feasibility. Adsorbents with high performance play a vital role in the adsorption of environmental pollutants, especially dye molecules. Thus, the development of eco-friendly Received: July 26, 2018 Accepted: December 17, 2018

A

DOI: 10.1021/acs.jced.8b00656 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Scheme 1. Structure of the Investigated Organic Dyes

tion performance of the developed new nanoadsorbent to different types of organic dyes was studied using equilibrium kinetics and thermodynamics. However, the main probable disadvantage associated with this adsorbent is using several materials for multiple-step synthesis procedure.

and low-cost adsorbents is a topic of serious research. Many nanomaterials, such as nanosized TiO2 particles and its organic composites, have been designed to absorb pollutants in water and soil systems. The type of application determines the properties of TiO2 nanoparticles (NPs), such as size, surface properties, and morphologies. Among them, cyclodextrin (CD)functionalized nanomaterials, especially TiO2/CD composites, have received much attention for the removal of hazardous dyes in recent years.31,32 There are three main types of CDs that have six, seven, or eight glucopyranoside units and are named α-CD, β-CD, and γ-CD, respectively. The remarkable feature of CDfunctionalized materials is their ability to form host−guest complexes with a significant number of molecules, making them quite applicable in various fields. CDs are eco-friendly watersoluble molecules, and they can improve applied features of advanced hybrid materials.33,34 Moreover, it is well-known that CDs, as polysaccharide derivatives, are abundant, renewable, and biodegradable resources.35 Therefore, adsorption on cyclodextrin-based materials can be a cheaper procedure for the removal of environmental contaminants from solution and a useful tool for protecting the environment. Herein, γ-CD-functionalized TiO2 nanoparticles, as an efficient nanoadsorbent, were designed and prepared for the first time and utilized for the adsorption of selected organic dyes from aqueous solutions. The main objective of this study is to provide useful information about the adsorption performance of TiO2/γCD NPs and its comparison with recently introduced αand β-cyclodextrin-based materials. Moreover, the effect of environmental factors, including pH, contact time, adsorbent dosage, initial dye concentration, and temperature on the adsorption performance of the TiO2/γCD NPs were studied in detail. The structure and morphology of the developed nanoadsorbent were characterized by Fourier transform infrared (FT-IR), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energydispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and Brunauer−Emmett−Teller (BET) techniques. The adsorp-

2. EXPERIMENTAL SECTION 2.1. Reagents. γ-Cyclodextrin powder (γCD), titanium(IV) isopropoxide (TTIP), glacial acetic acid, ethylenediamine (EDA), p-toluenesulfonyl chloride (p-TsCl), epichlorohydrin (EP), acetonitrile, acetone, methanol, ethanol, hydrochloric acid, sodium hydroxide, methylene blue (MB), crystal violet (CV), acid blue 113 (AB113), malachite green (MG), disperse red 1 (DR1), and congo red (CR) were obtained from SigmaAldrich and Merck Chemical companies. Deionized water was prepared by the Milli-Q water ion-exchange system and was used in all experiments. Scheme 1 shows the structure of the studied organic dyes. 2.2. Synthesis of γCD-Modified TiO2 Nanoparticles. 2.2.1. Synthesis of Bare TiO2 Nanoparticles. The sol−gel method was used for the preparation of titanium dioxide nanoparticles (TiO2 NPs).31 Titanium(IV) isopropoxide (TTIP) was used as the precursor for titania preparation. In a typical synthesis, 10 mL of TTIP was added to glacial acetic acid (20 mL) with stirring. Deionized water (60 mL) was added to the mixture dropwise with vigorous stirring. The solution was stirred for 2 h to get a clear transparent sol. After that, the synthesized TiO2 NPs were heated for 3 h at 200 °C to eliminate adsorbed water. 2.2.2. Synthesis of γCD-TsO. Briefly, 20 mL of 2.5 M NaOH was added dropwise to 70 mL of aqueous solution containing 5 g of γ-cyclodextrin (γCD). The suspension became homogeneous and slightly yellow before the addition was complete. paraToluenesulfonyl chloride (p-TsCl) (1.3 g) in acetonitrile (10 mL) was then added very slowly. The obtained solution was stirred for 2 h at 20 °C, and then the suspension was filtered. B



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Scheme 2. Synthetic Route of TiO2/γCD NPs

After that, the filtrate was neutralized with HCl and kept overnight at 4 °C. Gross product obtained by suction filtration was washed with acetone and deionized water and then dried in vacuum at 60 °C to afford white solid powder in yield.36,37 2.2.3. Synthesis of γCD-NH2. Briefly, in order for the nucleophilic reaction to occur, the mixture of pure γCD-TsO (2.5 g) and ethylenediamine (15 mL) was heated at 80 ◦C for 24 h. After that, the temperature of the resultant mixture was cooled to 20 °C. The pure γCD-NH2 was then obtained by acetone reprecipitation method. The resultant product was then dried at 60 °C.36,37 2.2.4. Synthesis of TiO2/γCD Nanoparticles. In the final step of adsorbent preparation, the cross-linking reaction was performed using epichlorohydrin (EP) for joining γCD-NH2 to TiO2 NPs. TiO2 NPs (0.5 g) was suspended in 5 mL of deionized water followed by adding the γCD-NH2 (0.5 g) and then homogenized using the magnetic stirrer. After that, the synthesized γCD-NH2 was covalently cross-linked to the TiO2 NPs surface using EP (20 μL). The mixture was agitated at room temperature for 12 h. The solid precipitate was collected and then washed with deionized water. Then obtained yellow powder was dried in vacuum at 60 °C (TiO2/γCD NPs). Scheme 2 shows the synthetic route of TiO2/γCD NPs. 2.3. Batch Adsorption Experiment. To study the adsorption performance of the TiO2/γCD NPs, the removal of six industrial dyes from aqueous solutions was achieved using the batch method. The adsorption experiments were conducted in a set of 50 mL Erlenmeyer flasks containing 0.01 g of TiO2/ γCD adsorbent and 10 mL of MB, MG, CV, DR1, and AB113 solutions (50 mg/L) and CR solution (2000 mg/L). Variations of dye concentrations over time with respect to important factors in different levels were evaluated. In addition, the pH of samples was adjusted to optimum values by using 0.1 M NaOH or 0.1 M HCl solutions. The maximum wavelength for measurement of residual concentration of dyes in solution (supernatant) through standard curves was 650, 610, 590, 517, 590, and 490 nm, respectively. The amount of dye adsorbed was determined by using the following equations: qe =

(C0 − Ce)V m

R(%) =

100(C0 − Ce) C0

(2)

where qe (mg/g) is the adsorption capacity of adsorbent, C0 and Ce (mg/L) are concentration of the dye at the initial and equilibrium states, respectively, m is the weight of adsorbent (g), and V is the volume of the solution (L). 2.4. Desorption and Regeneration Study. Regeneration experiment was suggested to get information on the developed adsorbent reuses for potential industrial applications. Therefore, desorption capability of dye-adsorbed TiO2/γCD NPs was performed using some eluents, including 0.1 M HCl, 0.1 M NaOH, methanol, ethanol, acetonitrile, and acetone. The TiO2/ γCD NPs were then recovered by filtration after the adsorption process and washed with the optimal eluent. After that, the recovered adsorbent was dried, and the adsorption−desorption experiments were repeated seven times.

3. RESULTS AND DISCUSSION 3.1. Characterization. 3.1.1. FT-IR Characterization. The structure of intermediates and product was characterized by FTIR spectroscopy (Figure 1). The FT-IR spectra of the raw TiO2, γ-CD, and γ-CD-NH2 are exhibited in Figure 1. As can be seen

Figure 1. FT-IR spectra of pure γCD, γCD-NH2, TiO2 NPs, and TiO2/ γCD NPs.

(1) C



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Figure 2. XRD patterns of pure TiO2 NPs and TiO2/γCD NPs (a) and nitrogen adsorption−desorption isotherm of the TiO2/γCD (b), CR dyeadsorbed TiO2/γCD (c), and CV dye-adsorbed TiO2/γCD (d).

from Figure 1, all functional groups of γ-CD are correctly assigned in FT-IR spectra. The broad absorption peak at 3400 cm−1 in FT-IR spectrum of γ-CD was attributed to O−H stretching vibrations. A new peak at 3282 cm−1 in FT-IR spectrum of γ-CD-NH2 was attributed to N−H stretching vibrations. Appearance of this characteristic peak with absorption bands of γ-CD shows that the γ-CD was successfully modified with EDA.36,37 As can be seen from Figure 1, O−H stretching and bending vibrations in the FT-IR spectra of pure TiO2 were recognized by the absorption bands at 3415 and 1629 cm−1, respectively. In addition, a broad absorption peak at 657− 789 cm−1 for TiO2 NPs represented the Ti−O−Ti stretching band in the synthesized nanoparticles.31 Figure 1 also shows the FT-IR spectrum of final product (TiO2/γCD NPs). The broad absorption band at ∼3388 cm−1 in FT-IR spectrum of the TiO2/ γCD NPs is assigned to the O−H groups of γCD. The C−H stretching vibration peak in γCD appears at 2927 cm−1, and it reveals that the structure of γCD linked to the TiO2 NPs has not been damaged.31,38 In addition, the strong peaks at 1029 and 1157 cm−1 for TiO2/γCD NPs represented the antisymmetric glycosidic (C−O−C) vibrations and coupled (C−C/C−O) stretch vibration in γCD.31 3.1.2. BET and XRD Analysis of TiO2/γCD NPs. XRD patterns of pure TiO2 NPs and TiO2/γCD NPs are shown in Figure 2a. The synthesized TiO2 NPs show diffraction peaks at 2θ values of 25.3°, 37.8°, 48.1°, 53.7°, and 55.1°, which correspond to anatase (101), (004), (200), (105), and (211) crystal planes. All diffraction peaks in the XRD pattern of the pure TiO2 are in good agreement with the standard spectrum (JCPDS Card No. 21-1272).39−41 After chemical modification of the pure TiO2 by γCD, the intensity of the characteristic TiO2 peaks is decreased. This indicates that the surface of TiO2 NPs was modified with γCD molecules. In addition, the peak at 2θ value of 22.8° in TiO2/γCD XRD pattern confirmed that the γCD molecules were successfully attached onto TiO2 surface.42 Furthermore,

the broad diffraction peaks in the TiO2/γCD NPs are due to the presence of abundant oxygen-containing functional groups on TiO2 NPs.43 These results clearly exhibited that the γCD molecules were successfully cross-linked to the TiO2 NPs. According to the BET method, after adsorption of CR and CV, the specific surface area of the developed TiO2/γCD nanoadsorbent was decreased from 383.131 to 50.319 m2/g and 88.901 m2/g, respectively. This clear decrease in the BET surface area is due to the effective adsorption of CR and CV dyes by the developed TiO2/γCD. Figure 2b−d shows the BET graphs of the synthesized TiO2/γCD, CR-adsorbed TiO2/γCD, and CV-adsorbed TiO2/γCD samples. In addition, the BET specific surface area, total pore volume, and average pore diameter of samples are presented in Table 1. Accordingly, the Table 1. BET Parameters of TiO2/γCD, CR Dye-Adsorbed TiO2/γCD and CV Dye Adsorbed TiO2/γCD BET-plot

TiO2/ γCD

TiO2/γCDCR

TiO2/γCDCV

average pore diameter [nm] BET specific surface area [m2/g] total pore volume [cm3/g]

0.692 383.131 0.543

0.704 50.319 0.482

0.723 88.901 0.432

obtained TiO2/γCD nanoadsorbent can be considered as a typeIV isotherm (mesoporous structure) with a high specific surface area. Consequently, the developed nanoadsorbent with a high specific surface area (383.131 m2/g) can be applied to the treatment of synthetic and industrial wastewater samples. 3.1.3. EDX Analysis. EDX is a useful method used to determine the elemental composition of synthesized samples. The EDX images of TiO2 and TiO2/γCD were exhibited in Figure S1 (Supporting Information). In addition, EDX elemental microanalysis (wt %) of samples are listed in Table 2. As can be seen from Figure S1 and Table 2, the chemical D



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FESEM images of dye-adsorbed TiO2/γCD NPs (Figure 3c−e). Figure 4 shows the FESEM images of the TiO2/γCD NPs as nanoadsorbent. This image indicates that the synthesized TiO2/ γCD NPs was in the nano dimensions and was modified with a large number of γCDs. Therefore, these results confirming that the TiO2/γCD NPs as an efficient adsorbent can adsorb organic dyes from aqueous media. 3.1.5. Determination of Zero Charge Point pH (pHPZC). Zero charge point pH (pHPZC) of the materials depends on the nature of the surface functional groups. The pHPZC for surface of the TiO2/γCD was obtained using the solid addition method according to previous studies.44 The pHPZC measurement of γCD-functionalized TiO2 NPs was performed over the pH range from 3.0 to 12.0, and the obtained result is shown in Figure 5. According to the pH changes of the solution, the PZC of the TiO2/γCD NPs is found to be pH 8.6. This indicates that in pH values below the pH 8.6 (pHPZC), the TiO2/γCD surface is positively charged, and at pH values higher than the PZC, the TiO2/γCD surface is negatively charged. 3.2. Optimization of the Dye Adsorption Performance of TiO2/γCD NPs. 3.2.1. Effect of pH. In this study, the effect of pH on the adsorption ability of the developed TiO2/γCD was investigated over the range from 3.0 to 12.0. The obtained results were exhibited in Figure 6. For all dyes, except for CR, the adsorption capacity of the TiO2/γCD increased with the increase of pH from 3.0 to 9.0. The highest amounts of

Table 2. EDX Elemental Microanalysis (wt %) of TiO2 NPs and TiO2/γCD NPs normalized wt % sample

Ti

O

C

N

Cl

S

TiO2 NPs TiO2/ γCD NPs

45.90% 29.86%

48.98% 38.61%

5.12% 20.46%

9.30%

1.18%

0.60%

elements Ti, O, and C in the TiO2 NPs and Ti, O, C, N, S, and Cl in the TiO2/γCD NPs with acceptable percentages are observed. This analysis indicated that the γCD molecules were crosslinked to the TiO2 NPs. 3.1.4. Morphological Characterization of Samples. The surface morphology of the pure TiO2 NPs, TiO2/γCD NPs, and dye-adsorbed TiO2/γCD NPs were analyzed by FESEM images (Figure 3). From Figure 3a, the spherical morphology of pure TiO2 NPs with nano dimensions was observed clearly. In the case of TiO2/γCD nanoadsorbent, the spherical shape of TiO2 NPs disappeared, and nanomaterials with new structure and morphology were formed. As shown in Figure 3b, the obtained TiO2/γCD had cavity structures, which indicate that the TiO2 surface was modified by γCD molecules. In addition, the particle size of the TiO2/γCD NPs was in the range of 10−22 nm. After the adsorption, the morphology of TiO2/γCD NPs was clearly changed, and the cavities were smooth with dye according to the

Figure 3. FESEM images of pure TiO2 NPs (a), TiO2/γCD NPs (b), CR dye-adsorbed TiO2/γCD NPs (c), CV dye-adsorbed TiO2/γCD NPs (d), and MG dye-adsorbed TiO2/γCD NPs (e). E



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Figure 4. TEM images of the TiO2/γCD NPs.

Moreover, for CR anionic dye, the maximum adsorption was observed at acidic pH values (pHPZC), the electrostatic attraction forces are increased, which results in a higher cationic dye adsorption by TiO2/γCD NPs. This is attributed to the negatively charged sites (surface hydroxyl groups) on the TiO2/γCD.46,47 Consequently, the formation of ionic complexes between MB, MG, CV, AB113, and DR1 dyes and the anionic TiO2/γCD is responsible for the high adsorption efficiency. F



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temperature range from 15 to 55 °C. For all six dyes, the adsorption efficacy was enhanced with a rise in temperature from 25 to 45 °C and then kept constant. Therefore, the optimum value of temperature for the adsorption of all six dyes was 45 °C. The experimental results show that the present system is governed by endothermic adsorption. In addition, the increment in rate of adsorption with temperature may be due to the enhanced diffusion rate of the dyes as well as the lower solution viscosity of the TiO2/γCD particles.50 Further, at evaluated temperatures, the adsorption capacity improves with the increasing temperature. This may be a result of an increase in the mobility of the dye molecule with increasing temperature. Furthermore, it may produce a swelling effect within the internal structure of the developed TiO2/γCD, allowing the large dye to penetrate further.51 This confirms the endothermic nature of the dye uptake. 3.2.5. Effect of Initial Dye Concentration. This effect was considered by changing the initial dye concentrations in the range of 50−200 mg/L for MB, MG, CV, DR1, and AB113 and in the range from 2000 to 4000 mg/L for CR dye. Figure 8 displayed that the adsorption capacity of TiO2/γCD increased, as the initial dye concentration was increased. Correspondingly, the percentage removal of dye displayed the opposite manner and decreased, as the initial dye concentration was increased.

0.010, and 0.008 g were selected as the optimal values of TiO2/ γCD nanoadsorbent for the adsorption of MB, MG, CV, DR1, AB113, and CR dyes, respectively. 3.2.4. Effect of Temperature. Temperature is also an important factor during the adsorption processes. The influence of temperature on the adsorption efficiency of tested dyes is shown in Figure 7. The effect of temperature was studied in the

Figure 7. Influence of temperature on the dye removal by TiO2/γCD. (optimum pH, time, and adsorbent dosage, initial dye concentration: 100 mg/L for MB, MG, CV, DR1, and AB113 and 3000 mg/L for CR).

Figure 8. Effect of initial dye concentration on the dye removal by TiO2/γCD (optimum pH, time, adsorbent dosage, and temperature). G



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Table 3. Kinetic Parameters for the Adsorption of Dyes on TiO2/γCD Nanoadsorbent pseudo-first-order kinetic model

pseudo-second-order kinetic model

dyes

Qe (mg/g)

K1 (1/min)

Qcal (mg/g)

R2

Qe (mg/g)

K2 (g/(mg·min))

Qcal (mg/g)

R2

MB MG CV DR1 AB113 CR

46.360 50 50 50 44.450 2000

0.0978 0.3147 0.2746 0.2113 0.0805 0.1639

6.631 14.491 18.422 15.364 10.047 144.681

0.9560 0.9571 0.9578 0.9661 0.8985 0.9734

46.360 50 50 50

0.0454 0.0373 0.0239 0.0232 0.0245 0.0025

46.948 52.084 53.192 52.910 45.045 2000

0.9996 0.9991 0.9975 0.9955 0.9965 0.9999

44.450

Table 4. Elovich and Weber−Morris Kinetic Model Parameters Elovich kinetic model dyes

Weber−Morris kinetic model

β (g/mg)

α (mg/(g·min))

R

0.493 0.236 0.169 0.180 0.343 0.018

7.23 × 10 5.22 × 10+4 2.55 × 10+3 3.68 × 10+3 3.57 × 10+5 8.35 × 10+15

0.9745 0.9819 0.9753 0.9371 0.8794 0.8896


+8

2

(3)

t 1 t = + 2 qt q k 2qe e

(4)

Kd (mg/(g·min0.5))

R2

39.6328 37.6360 32.4881 32.8062 33.5036 1817.3547

1.5009 3.9517 5.5156 5.2494 2.2185 54.0858

0.9974 0.9966 0.9980 0.9813 0.9529 0.9471

process, the slowest step is important to predict the rate-limiting step.56,57 The Weber−Morris model is expressed in the eq 5.

Consequently, at lower concentrations, a maximum amount of dye molecules would be able to adsorb by TiO2/γCD, resulting in a higher percentage of adsorption. By contrast, at higher dye concentrations, adsorption capacity (mg/g) decreased for all six dyes.31,52 3.3. Adsorption Kinetics. For the study of the mechanism of the adsorption and rate-controlling step, several kinetics models are used to test the experimental data. In this study, the kinetic models of pseudo-first-order, pseudo-second-order, the Weber−Morris and the Elovich were used to evaluate the kinetic adsorption mechanism. The pseudo-first-order and the pseudosecond-order kinetic models are expressed in the eqs 3 and (4), respectively.53,54 ln(qe − qt) = ln qe − k1t

C (mg/g)

qt = Kdt 0.5 + C

(5)

Figure S6 (Supporting Information) exhibited the fitting plot of qt against t0.5. From Figures S2 and S6, it was observed that the plots are alienated into two straight lines showing the influence of dual steps in the adsorption of dyes by the developed TiO2/ γCD. The first part implies the transfer of dye molecules from solution to the TiO2/γCD surface, which is attributed to the diffusion on the boundary layer. The second part represents a balanced adsorption of dye molecules through the surface of TiO2/γCD, which is attributed to the intraparticle diffusion.58 Furthermore, the rate-limiting step involves intraparticle diffusion when the plots qt versus t1/2 passes through the origin. This indirectly shows the contribution of intraparticle diffusion in the adsorption of dyes by the TiO2/γCD. So, it is expected that both steps may happen together during adsorption process. The Elovich kinetic model is useful for energetically heterogeneous solid surfaces and is given in the eq 6.59−61

The linear relationships of the models of pseudo-first-order (Figure S4, Supporting Information) and pseudo-second-order (Figure S5, Supporting Information) were drawn using the data in Figure S2. Additionally, the kinetics parameters of the four models are given in Table 3. According to the results, for all six dyes, the pseudo-second-order model fitted the adsorption kinetics better than the pseudo-first-order model. On the basis of the pseudo-second-order model, the correlation coefficients (R2) values for all tested dyes are greater than 0.99, while those of pseudo-first-order are not satisfactory. In addition, the theoretical calculated qe values from the pseudo-second-order model were more consistent with the experimental qe values. Therefore, the adsorption process is a chemisorption process involving exchange or sharing of electrons between the dyes and functional groups of TiO2/γCD.55 Further, to evaluate the adsorption mechanism of dyes, intraparticle diffusion model is studied. The entire solute adsorption onto the adsorbent surface may be controlled by one or more steps. To identify the diffusion mechanism, the experimental data were then tested using the model developed by Weber and Morris.56,57 In general, the rate of the adsorption process is controlled by four steps. Additionally, in an adsorption

qt =

1 1 ln(αβ) + ln t β β

(6)

According to the Elovich equation, a plot of qt versus ln t gives 1/β and (1/β)ln(αβ) as the slope and the intercept, respectively (Figure S7, Supporting Information). As shown in Table 4, the low values of R2 show that the adsorption of AB113 and CR dyes onto TiO2/γCD does not follow the Elovich model. However, the magnitude of R2 for other tested dyes shows that the adsorption of dyes onto TiO2/γCD NPs follows the Elovich mode. Therefore, the results of Tables 3 and 4 indicate that the kinetics of dye adsorption onto TiO2/γCD nanoadsorbent follows the pseudo-second-order kinetic model. 3.4. Adsorption Isotherms. Equilibrium adsorption isotherms are useful in determining the adsorption capacity of dye onto the TiO2/γCD as adsorbent. The adsorption isotherm also evaluates the nature of adsorption process. In this study, the four isotherm models, including Langmuir, Freundlich, Dubinin−Radushkevich, and Temkin isotherms, were selected H



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Table 5. Langmuir and Freundlich Isotherm Model Parameters Langmuir constants

Freundlich constants

dyes

Qm (mg/g)

KL (L/mg)

0.9990

R2

n

1/n

KF ((mg/g) (L/mg)1/n)

R2


134 244 213 238 157 5000

0.140 0.195 0.096 0.156 0.069 0.023

0.9985 0.9949 0.9983 0.9993 0.9882 0.021

0.9855 0.9703 0.9875 0.9746 0.9940 0.9187

2.1313 3.1221 3.3921 3.2041 1.7734 4.3706

0.4692 0.3203 0.2948 0.3121 0.5639 0.2288

25.427 75.301 57.161 69.598 17.041 1160.379

0.9990 0.9985 0.9949 0.9983 0.9993 0.9882

Table 6. Temkin and Dubinin-Radushkevich Isotherm Model Parameters Temkin constants dyes MB MG CV DR1 AB113 CR

bT (J/mol) 72.130 45.455 55.633 47.949 62.181 2.856

BT 34.3488 54.5060 44.5340 51.6710 39.8443 867.4820

Dubinin−Radushkevich constants

KT (L/mg) 0.957 1.855 1.134 1.592 0.506 0.462

R

2

E (kJ/mol)

0.9868 0.9891 0.9939 0.9950 0.9881 0.9748

BT =

1 1 + C0KL

RT bT

ln qe = ln qs − βε 2

1 zyz ji ε = RT lnjjj1 + z j Ce zz{ k

(8)

R2

94 201 177 194 95 150

0.8417 0.8432 0.9098 0.8478 0.8769 0.7771

(10)

(11)

(12)

(13)

The Dubinin−Radushkevich model constants, namely, qs and β, can be determined from the intercept and slope of linear plot of ln qe versus ε2, respectively. It is described in Figure S11 (Supporting Information). The mean adsorption energy E (kJ/mol) gives information about the mechanism of the adsorption process. The E value can be calculated using the following equation.

Ho and McKay established that (1) 0 < RL < 1 for favorable adsorption; (2) RL > 1 for unfavorable adsorption; (3) RL = 1 for linear adsorption; and (4) RL = 0 for irreversible adsorption.63 The Freundlich isotherm was based on the assumption that there are multilayer as well as monolayer adsorptions in an adsorption system. Therefore, both chemical and physical adsorption should be considered when the model is used. The equation of Freundlich isotherm model could be expressed as follows:64 1 log qe = log KF + log Ce n

2.28 × 10 2.94 × 10−6 13.38 × 10−6 4.55 × 10−6 5.16 × 10−6 447.03 × 10−6

Qs (mg/g)

The Temkin parameters BT and KT were determined from a plot of qe versus ln Ce. It is described in Figure S10 (Supporting Information). The Dubinin−Radushkevich (D-R) isotherm model gives an idea about the type of the adsorption.66,67

(7)

The qm and KL values were measured from the intercept and slope of linear plots of 1/qe versus 1/Ce, respectively. It is described in Figure S8 (Supporting Information). For the investigation of whether adsorption is “favorable” or “unfavorable”, the Langmuir model can be classified by a term RL, a dimensionless constant separation factor (eq 8).63 RL =

−6

qe = BT ln KT + BT ln Ce

to correlate experimental data for dye adsorption on the TiO2/ γCD. The linear form of the Langmuir isotherm equation could be expressed as follows:62 1 1 1 = + qe KLQ mCe Qm

0.4683 0.4127 0.1933 0.3316 0.3114 0.0334

β (mol2/kJ2)

E=

1 ( −2β)1/2

(14)

The adsorption isotherm parameters are listed in Tables 5 and 6, and the fitting plots of the experimental data for the four equilibrium isotherms are shown in Figures S8−S11 (Supporting Information). The overall comparison of the isotherms using the average values of R2 shows that the experimental equilibrium data are better explained by the Freundlich model. It also seems that the obtained experimental data for the adsorption of investigated dyes were fitted well by the Langmuir isotherm model. This may be due to both homogeneous and heterogeneous distribution of active sites on the surface of the TiO2/γCD NPs. Furthermore, the adsorption takes place on heterogeneous surfaces that have different adsorption energies. The heterogeneity is caused by the presence of different functional groups on the surface and also by various mechanisms

(9)

The Freundlich isotherm constants (KF and n) were calculated from the intercept and slope of the linear plots of log qe versus log Ce, respectively. It is described in Figure S9 (Supporting Information). When the value of n < 1 or n > 1, this implies that the adsorption process is related to a chemical or favorable physical process, respectively. Furthermore, the 1/n values indicate the type of isotherm to be irreversible (1/n = 0), favorable (0 < 1/n < 1), or unfavorable (1/n > 1).64 The Temkin isotherm model assumes that the heat of adsorption of all molecules decreases linearly with coverage due to adsorbate−adsorbate interactions. The linear form of the Temkin isotherm equation could be expressed as follows:65 I



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of adsorbent−adsorbate interactions. Thus, both chemical and physical adsorptions should be considered. On the basis of the obtained RL values that were less than 1.0, the adsorption of all tested dyes was favorable.63 Furthermore, the values of n were greater than 1; this implies that the adsorption process is related to a favorable physical process.64 The results show that qm values of MB, MG, CV, DR1, AB113, and CR onto TiO2/γCD NPs are found to be 134, 244, 213, 238, 157, and 5000 mg/g, respectively. In addition, the Temkin isotherm parameters obtained showed that TiO2/γCD NPs has the highest binding constant.65 As seen from Table 6, the mean energy (E) of adsorption from D-R isotherm model were lower than 8.0 kJ/mol for all investigated dyes. It shows that the adsorption of dyes onto TiO2/γCD NPs is a physisorption controlled process.66,67 However, on the basis of the Langmuir isotherm equation, in Table 7, the maximum adsorption capacity of TiO2/γCD NPs was also compared with the other adsorbents for adsorption of corresponding dyes. The results showed that the TiO2/γCD NPs have high adsorption capacities for different types of dyes compared to many other adsorbents. Moreover, the adsorption ability of TiO2/γCD NPs toward dyes was compared to raw TiO2 NPs and γCD-NH2 samples (Figure 9). As can be seen from Figure 9, the adsorption capacities of all investigated dyes on pure TiO2 NPs or γCD-NH2 are lower than that of final TiO2/γCD sample. Therefore, the highest adsorptions were observed using the developed TiO2/γCD in similar experimental conditions. 3.5. Thermodynamic Study. The adsorption thermodynamics for adsorption of dyes on TiO2/γCD were performed with the temperature in the range of 288−328 K (Figure 7). The thermodynamic parameters such as the ΔH°, ΔG°, and ΔS° were calculated using the following equations.86 ln K C = −

ΔH ° ΔS° + RT R

ΔG° = −RT ln K C

Table 7. Comparison of TiO2/γCD NPs with Other Adsorbent for Removal of Dyes dye MB

TiO2/γCD NPs

MG

TiO2/Gly/β-CD NPs magnetic cellulose/grapheme oxide (MCGO) CMT-g-PAM/SiO2-3 sepiolite nanocrystalline cellulose (NCC) sewage sludge based granular activated carbon TiO2/γCD NPs

CV

zeolite-rGO MIL-53(Al)-NH2 oxalic acid modified rice husk (MRH) Fe3O4@MgSi rice husk activated carbon (RHAC) CFA-PEI TiO2/γCD NPs

AB113

cellulose-based adsorbent (CGS) sulfuric acid activated (RHS) zinc chloride activated (RHZ) magnetic nanocomposite coniferous pinus bark powder (CPBP) CarAlg/MMt nanocomposite hydrogels TiO2/γCD NPs

CR

TiO2/Gly/β-CD NPs activated red mud (ARM) OCSP biosorbent RTAC nano zerovalent iron (NZVI) date stones (DS) TiO2/γCD NPs

DR1

SiO2−CD β-CD polymer lignocellulosic biomass jute fiber (Jb) hierarchical hollow structure ZnO (CZ-400) Fe(OH)3@Cellulose PHFs SP-PDMS 2 nanohybrid TiO2/γCD NPs

689.65 167.45 238

TiO2/Gly/β-CD NPs

139

(15) (16)

The values of ΔH° and ΔS° were obtained using the slope and intercept of the linear plot of ln KC versus 1/T, respectively (Figure S12, Supporting Information). The calculated thermodynamic parameters for dye adsorption onto TiO2/γCD NPs are listed in Table 8. According to Table 8, the values of ΔG° were negative at all tested temperatures; it indicates that the adsorption of the investigated dyes by TiO2/ γCD was spontaneous and irreversible. The absolute values of ΔG° increased with increasing temperature, indicating that there are more available active sites on the TiO2/γCD at higher temperatures. Furthermore, the ΔG° values were within the range of −20 < ΔG° < 0 kJ mol−1, meaning that the adsorption of MB, DR1, AB113, and CR dyes was spontaneous physical adsorption. The ΔG° values for MG and CV dye were −20.35 and −21.71 kJ mol−1, showing that the adsorption of these dyes was not a single physical or chemical adsorption, but there were two types of adsorption.87 The calculated ΔH° for all tested dyes is positive, which indicates that the adsorption process was an endothermic reaction, and it is consistent with the fact that adsorption capacity was increased with increasing temperature.88 The ΔH° values (21.88 and 15.77 kJ/mol, respectively, for the MB and AB113) at the studied temperatures reveals that the adsorption process is the result of physical interactions, such as van der Waals, dipole−dipole interactions, and electrostatic forces.

qm (mg/g)

adsorbent

134

time (min) 20

82 70.03

30 950

43.86 91.90 101.16 131.80

120 1440 20 1500

244

10

ref this work 31 19 20 30 68 69 this work 17 18 70

48.60 164.90 54.02

15 500 500

125.15 49.62

90 40

71 72

174.83 213

70 10

182.15 64.87 61.57 113.31 32.78

150 120 120 135 120

73 this work 15 16 16 74 75

88.80

500

76

157 77 83.33 59.81 9.20 50.80 34.48 5000

30 60 20 40 30 40 10

70.10 36.20 27.12

120 210 30

this work 31 21 22 77 78 79 this work 80 81 82

120

83

500

20

1500 120 10 15

84 85 this work 31

Furthermore, ΔH° (109.21, 142.48, 101.35, and 85.47 kJ/mol, respectively, for the MG, CV, DR1, and CR) at the tested temperatures shows that the adsorption of dyes is the result of chemical and physical interactions, such as chemical bonding, hydrogen bonding, van der Waals, dipole−dipole interactions, and electrostatic forces. The ΔS° values were positive, which exhibited that the adsorption of six dyes on TiO2/γCD was related to the enthalpy and entropy changes of the system. Because the water molecules desorbed from TiO2/γCD were J



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Figure 9. Adsorption of dyes on the pure TiO2 NPs, γCD-NH2, and TiO2/γCD NPs (optimum conditions).

much more than dye molecules adsorbed on TiO2/γCD, the adsorption results displayed an increase in entropy.89 3.6. Desorption and Reuse. In this work, desorption study in a batch system was performed to evaluate the reusability of the developed TiO2/γCD nanoadsorbent. Different eluting agents, including methanol, ethanol, acetonitrile, acetone, 0.1 M NaOH, and 0.1 M HCl solutions were used for regeneration of the TiO2/γCD in the desorption processes. As can be seen from Figure 10a, the highest recovery efficiency for TiO2/γCD was observed in methanol and ethanol, especially methanol. Therefore, methanol as an effective elution was selected for further experiments. The effect of seven reusing times was displayed in Figure 10b. The adsorption capacity of TiO2/γCD for MG, MB, AB113, CV, DR1, and CR dyes reduced with each reusing. At the end of seventh adsorption−desorption processes, the adsorption capacity of TiO2/γCD for MB, MG, CV, DR1, AB113, and CR remained at 79, 95, 83, 89, 70, and 96%, respectively. The results show that the TiO2/γCD as an effective nanoadsorbent can be used several times in real applications. 3.7. Adsorption Mechanism. The various factors including the chemical structure of adsorbate samples, surface functional groups on the developed adsorbent, and the type of interaction between adsorbent and adsorbate are effective in the adsorption efficiency. On the basis of the adsorption kinetic, isotherm, and thermodynamic studies, clearly the developed TiO2/γCD nanoadsorbent has good adsorption efficiency for different types of dyes. For example, 10 mL of 2000 mg/L CR can be completely adsorbed by 10 mg of TiO2/γCD nanoadsorbent within 10 min. Figure S13 (Supporting Information) exhibited the UV−vis absorption spectra of CR solutions before and after adsorption by the developed TiO2/γCD. On the one hand, it was found that the CR is completely adsorbed by the developed TiO2/γCD in a short period of time. On the other hand, a large BET specific surface area of TiO2/γCD (383.131 m2/g) may be

Figure 10. (a) Desorption efficiency plots of dyes using various eluents. (b) Cycle adsorption experiments of dyes by TiO2/γCD in methanol (optimum pH, time and adsorbent dosage, initial dye concentration: initial dye concentration: 50 mg/L for MB, MG, CV, DR1, and AB113 and 2000 mg/L for CR).

one of the reasons for the high adsorption performance, but it should not be the conclusive one. The specific surface area of TiO2/γCD was decreased to 50.319 and 88.901 m2/g, after adsorption of CR and CV dyes. These results clearly prove that the adsorption of dyes by intermolecular forces (Figure 11) decreases the active empty sites of the developed TiO2/γCD. The pH of the solutions has also contributed significantly to study the adsorption mechanism. According to the pHPZC value of TiO2/γCD (8.6), at higher pH (>pHPZC), the electrostatic interactions, hydrogen-bonding, and van der Waals forces between TiO2/γCD and cationic dyes are increased (Figure 11). Consequently, the formation of ionic complexes between these dyes and the anionic TiO2/γCD is responsible for the high adsorption efficiency. However, for CR anionic dye, a positive surface charge (

Synthesis and Characterization of Î³-CD-Modified TiO2 Nanoparticles

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