Cr-Doped ZnO Nanoparticles: Synthesis, Characterization, Adsorption

Research Article www.acsami.org

Cr-Doped ZnO Nanoparticles: Synthesis, Characterization, Adsorption Property, and Recyclability Alan Meng,*,† Jing Xing,† Zhenjiang Li,‡,§ and Qingdang Li§ †

State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, 53 Zhengzhou Road, Qingdao 266042, Shandong, P. R. China ‡ Key Laboratory of Polymer Material Advanced Manufacturings Technology of Shandong Provincial, Qingdao University of Science and Technology, Qingdao 266061, Shandong, P. R. China § College of Sino-German Science and Technology, Qingdao University of Science and Technology, Qingdao 266061, Shandong, P. R. China S Supporting Information *

ABSTRACT: In this paper, a mild solvothermal method has been employed to successfully synthesize a series of Cr-doped ZnO nanoparticles (NPs) with different Cr3+ contents, which is a kind of novel and high-efficiency absorbent for the removal of acid dye methyl orange (MO) from aqueous solution. The as-prepared products were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), Brunauer, Emmet, and Teller (BET), and Zeta potential measurements. In accordance with the adsorption capacity of the products, the obtained optimal Cr/Zn molar ratio is 6%. The adsorption process of MO on Cr-doped ZnO was investigated by kinetics, thermodynamics, and isotherm technologies, which, respectively, indicated that the adsorption was fast (adsorption reached equilibrium in 2 h) and followed a pseudo-second-order model, that the adsorption process was spontaneous and endothermic, and that it agreed well with the Langmuir isotherm with a maximum adsorption capacity of 310.56 mg g−1. Moreover, a reasonable mechanism was proposed to elucidate the reasons for their adsorption behavior. In addition, a simple and low-cost chemical method was developed to separate and recycle ZnO and MO from the used adsorbent, effectively avoiding the secondary pollution. This work can not only describe efficient experimental approaches for obtaining novel adsorbents and recycling them but also offer valuable clues for the preparation and property study of other semiconductor adsorbents. KEYWORDS: Cr-doped ZnO, adsorption, methyl orange, kinetics, isotherm

1. INTRODUCTION

wastewater containing dyes is becoming more and more urgent in contemporary environmental research.7 Technologies with high efficiency and low cost to remove MO from wastewater are urgently needed. So far, removal methods of MO and other azo dyes have been developed, including photocatalysis,12,13 coagulation,14 adsorption,15 and etc.16−20 Among these methods, adsorption is one of the best choices because it is nontoxic and has low cost and high efficiency.21−24 Conventional absorbents such as active carbon25,26 and alumina27 have been used as adsorbents for the treatment of wastewater; however, their adsorptive capacity is not as high as expected, and they are expensive.2,28 Therefore, it is needed to find a low-cost adsorbent with fast kinetics and high adsorption capacity. Recently, nanomaterials, as a new kind of adsorbent,

Dyes are widely used in industries such as the textile, paper, leather, printing, food, and plastics industries.1−4 However, some dyes containing special compounds are considered toxic to both humans and animals even at very low concentrations. Generally most of these compounds cause mutagenic, teratogenic, and carcinogenic effects, which subsequently lead to the generation of health disorders such as dysfunction of the kidney, reproductive system, liver, brain, and central nervous system.5,6 Methyl orange (MO) belongs to the family of azo dyes, which represent around 50% of all dyes used in the textile industry.8,9 Due to the existence of the −NN− group, azo dyes are highly toxic and cause various diseases.10,11 As an anionic dye, it is possible for MO to leach into the soil and contaminate groundwater, which is seriously harmful for human health and the environment. Therefore, it is necessary to remove MO and other azo dyes from wastewater before it is discharged into bodies of freshwater, and treatment of © 2015 American Chemical Society

Received: October 3, 2015 Accepted: November 24, 2015 Published: November 24, 2015 27449

DOI: 10.1021/acsami.5b09366 ACS Appl. Mater. Interfaces 2015, 7, 27449−27457

Research Article

ACS Applied Materials & Interfaces

Figure 1. (A) UV−vis spectrum of MO solutions after adsorption and (B) the percentage of MO removal on Cr-doped ZnO NPs with different Cr raw material. MO concentration: 100 mg/L, adsorbent dosage: 1.0 g/L, contact time: 20 min. Insets: (1) photos of initial MO solution and the solution treated with 6% Cr-doped ZnO NPs and (2) molecular structure of MO.

autoclave of 100 mL capacity, sealed, and heated at 120 °C for 12 h in an oven. After the reaction system cooled down, the asformed precipitates were centrifuged, washed with deionized water and ethanol, and finally dried in air at 80 °C for 12 h. 2.2. Adsorption Experiments. All the experiments were carried out using a ca. 250 mL flask. For each experiment, 0.2 g of Cr-doped ZnO NPs was added to 200 mL MO solutions with different initial concentrations and continuously stirred for 24 h. In the adsorbing stage, 5 mL of solution was sampled at specified intervals up to 24 h and then treated with centrifugation for solid−liquid separation. The MO concentration was measured by using an UV−vis spectrophotometer at its maximum wavelength (λ) of 463 nm. All adsorption experiments were repeated 3 times. The amount of MO adsorbed on Cr-doped ZnO NPs, qe, was calculated by eq 1

have attracted worldwide attention due to their small particle sizes, large specific surface areas, and accessible active sites. For example, Su et al.29 reported that zirconium oxide nanoparticles had strong adsorption for phosphate. Chen et al.30 had synthesized self-assembled Fe3O4-layered double hydroxide colloidal nanohybrids, which have excellent performance for treatment of organic dyes in water. ZnO-based nanoadsorbent, due to its nontoxicity, large adsorption capacity, simple preparation, and recyclability, has a great application prospect in the treatment of dye wastewater. Herein, a series of Cr-doped ZnO NPs with different Cr3+ contents have been successfully prepared by a simple, mild, and eco-friendly solvothermal method, and under the optimal Cr/ Zn molar ratio (6%), the as-fabricated products exhibited a remarkable adsorption capacity of 310.56 mg g−1 and fast adsorption rate for the removal of MO from aqueous solution. The adsorption process with spontaneity and endothermicity agreed well with the pseudo-second-order kinetics model and Langmuir isotherm. More importantly, we developed a facile and cost-effective chemical approach for the recycle of the used adsorbent, hindering the pollution of nanoparticles to the environment. Additionally, a rational mechanism of Cr-doped ZnO NPs presenting outstanding adsorption property was discussed in detail. The intriguing preparation and recycle processes of Cr-doped ZnO NPs with large adsorption capacity and fast adsorption rate for removing azo dyes make the synthesized products serve as promising candidates as adsorbents for wastewater treatment in industrial development.

qe =

(C0 − Ce) ·V M

(1)

and the dye removal of MO was calculated as follows Dye removal (%) =

C0 − Ce × 100% Ce

(2)

−1

where C0 and Ce (mg L ) are the liquid-phase concentrations of dye at the initial point and equilibrium, respectively; V (L) is the volume of dye solution; and M (g) is the mass of the adsorbent used. 2.3. Separation and Recycle. An amount of 1.3 g of Crdoped ZnO NPs which had reached maximum adsorption capacity was dissolved in 25 mL of 1 M HNO3, forming Zn2+ and Cr3+, and then 25 mL of 1 M NaOH was added drop by drop into the above solution. Atomic absorption spectroscopy (AAS, TAS-990, China) was used to measure the concentrations of Zn 2+ and Cr 3+ in the liquid phase after centrifugation, and the precipitate was washed, dried, and calcined at 180 °C for 2 h. 2.4. Characterization. The morphology and composition of the products were characterized by TEM (JEM-2100, Japan) and SEM (JSM-6460LV, Japan) equipped with an energydispersive spectrometer (EDS). XRD of the as-prepared samples was performed by using an X-ray diffractometer (D/ MAX-2500/PC, Japan) with Cu Kα radiation. XPS (ESCALAB 250, U.S.A.) was used to ascertain the presence of Cr and its oxidation state in ZnO. FT-IR was recorded on a Bruker VECTOR-22 IR spectrometer. BET surface area analysis was performed on an automated surface area analyzer (ASAP 2020,

2. MATERIALS AND METHODS 2.1. Preparation of Cr-Doped ZnO NPs. All of the chemical reagents used in this work, including zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Sinopharm Chemical Reagent Co., Ltd., 99.5%), chromic nitrate nonahydrate (Cr(NO3)3· 9H2O, Sinopharm Chemical Reagent Co., Ltd., 99.8%), ethanol (CH3CH2OH, Laiyang Chemical Works, 99.7%), and sodium hydroxide (NaOH, Sinopharm Chemical Reagent Co., Ltd., 96%), are A.R. grade. The Cr-doped ZnO NPs were prepared by a mild solvothermal method.31,32 An amount of 2.0 mmol of Zn(NO3)2·6H2O and a certain amount of Cr(NO3)3·9H2O were dissolved in 40 mL of ethanol, and the solution was stirred at room temperature for 20 min. Then 40 mL of 4.0 M NaOH ethanol solution was added drop by drop into the above solution with stirring. After being stirred at room temperature for 1 h, the mixture was transferred into a Teflon-lined stainless 27450


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treatment systems. The effect of contact time on MO adsorption is shown in Figure 3. It can be seen that adsorption

U.S.A.) by means of nitrogen adsorption−desorption. Zeta potential was measured at room temperature by a Zeta Potential Analyzer (Nano-ZS90, English).

3. RESULTS AND DISCUSSION 3.1. Determination of the Optimal Cr/Zn Molar Ratio. In order to determine the optimal Cr/Zn molar ratio, a series of Cr-doped ZnO NPs with different dosages of Cr raw materials were investigated as adsorbent for MO removal. From Figure 1, an apparent increase in the adsorption amount of MO can be noted with the increase of Cr raw materials from 0 to 6%. However, the adsorption amount of MO decreased with increasing content of Cr raw materials to 8% and 10%, which may be because when Cr raw material is 6% the doping amount of Cr3+ has reached saturation and when Cr raw material is relatively high (>6%) the surplus Cr3+ may generate other products (shown in Figure 5e), causing a negetive effect on the adsorption capacity of the products. The optimal Cr/Zn molar ratio is 6%. In this paper, we choose 6% Cr-doped ZnO NPs as adsorbent. 3.2. Effect of Initial MO Concentration. The adsorption capacity of Cr-doped ZnO NPs to different initial concentrations of MO can be used to reflect the actual usage of the adsorbent. The variation of the MO adsorption capacity and removal efficiency with the initial concentration of MO were investigated by changing the initial MO concentration from 50 to 600 mg L−1 with an adsorbent dose of 1 g L−1. From Figure 2A, it is clear that the amount of MO adsorbed on Cr-doped

Figure 3. Effect of contact time on the adsorption of MO by Cr-doped ZnO NPs.

is fast in the early stages of the adsorption process until it gradually approached a plateau. Further increase in contact time does not cause any appreciable rise in adsorption efficiency due to the equilibrium of the adsorbent sites.34 The adsorption equilibration time is less than 2 h in the MO concentration of 150−300 mg L−1, and the short contact time is beneficial for the practical application. 3.4. Effect of Temperature. In order to obtain the additional in-depth information regarding the inherent energetic changes involved during adsorption, the adsorption studies were carried out for the initial MO concentration of 300 mg L−1 at 298, 308, and 318 K. The equilibrium adsorption capacity of MO increased from 286.52 to 293.21 mg g−1 with the increase in temperature from 298 to 318 K as displayed in Table 4. The changes in thermodynamic parameters of free energy (ΔG0), enthalpy (ΔH0), and entropy (ΔS0) were calculated by using the following equations ΔG 0 = −RT ln K q K= e Ce

ln K = Figure 2. Effect of initial MO concentration on (A) absorption capacity and (B) removal efficiency.

(3)

(4)

ΔS 0 ΔH 0 − R RT

(5) −1

−1

where R is the universal gas constant (8.314 J mol K ); K is the thermodynamic equilibrium constant; and T is the absolute temperature (K). ΔH0 and ΔS0 were calculated from the slope and intercept of ln K versus 1/T (shown in Figure 4a). The results of ΔG0, ΔH0, and ΔS0 are listed in Table 4. The change in ΔG0 for physisorption is between −20 and 0 kJ mol−1; the physisorption together with chemisorptions is at the range of −20 to −80 kJ mol−1; and chemisorption is at the range of −80 to −400 kJ mol−1. The values of ΔG0 (−7.57 to −9.96 kJ mol−1) for the adsorption of MO on Cr-doped ZnO were in the range of physisorption and confirm the spontaneous nature and feasibility of the adsorption process. The positive value of ΔH0 (27.93 kJ mol−1) reveals that the adsorption process is endothermic and goes through physisorption. The positive value of ΔS0 indicates the increased randomness of the entire system during the adsorption of MO on Cr-doped ZnO NPs. Meanwhile, the Arhenius equation was applied to investigate the activited energy (Ea) for the adsorption process.

ZnO NPs increased with the increase of initial concentration of MO from 50 to 300 mg L−1. This is due to that the higher initial MO concentration provides higher driving force for the dye ions from the solution to the surface of Cr-doped ZnO NPs, resulting in more collisions between dye ions and active sites on the Cr-doped ZnO NPs.33 Between 300 and 600 mg L−1, the adsorption amount remains constant at approximately 302 mg g−1. This is because the adsorption of MO on Cr-doped ZnO NPs reaches adsorption saturation. The removal efficiency is an important parameter in the practical process of wastewater treatment. The dye removal rate decreases with the increase of initial MO concentration, as shown in Figure 2B. When the concentration is in the range of 50−300 mg L−1, more than 95% of MO is removed, which is high enough for practical applications. 3.3. Effect of Contact Time. Equilibrium time is one of the most important parameters affecting the design of wastewater 27451


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Figure 4. Effect of temperature on the adsorption of MO on Cr-doped ZnO: (a) plots of ln K vs 1/T and (b) plots of ln K2 vs 1/T.

Figure 5. (a) SEM images of 6% Cr-doped ZnO NPs (inset f: the EDS spectrum of 6% products); (b, c) TEM images and (d) HRTEM image of 6% Cr-doped ZnO, and the two insets are the lattice fringes (g) and selected area electron diffraction (h); and (e) XRD patterns of undoped ZnO and 6% and 10% Cr-doped ZnO NPs.

ln k = ln A +

Ea RT

is the HRTEM image of as-prepared samples, which shows lattice fringes with 0.266 nm spacing for Cr-doped ZnO NPs, corresponding to the (002) planes of wurtzite-phase ZnO crystals. The XRD patterns of undoped ZnO and 6% and 10% Crdoped ZnO NPs are shown in Figure 5e. All peak positions of undoped ZnO as well as 6% Cr-doped ZnO correspond to the standard diffraction patterns of the wurtzite hexagonal structure of ZnO (JCPDS Card No. 36-1451). As the EDS has confirmed the presence of Cr and there is no secondary phase in the XRD pattern of 6% Cr-doped ZnO NPs, we conclude that Cr3+ had entered into the lattices of ZnO substituting of Zn2+. Moreover in the XRD pattern of 10% Cr-doped ZnO, in addition to the diffraction peaks of ZnO, there is a secondary phase ZnCrO4 (JCPDS Card No. 19-1456, marked with “”), which may be the reason that the adsorption capacity of Cr-doped ZnO NPs decreases when the Cr raw material is, respectively, high (>6%). The shift of corresponding diffraction peaks is shown in the

(6)

where k refers to the pseudo-second-order rate constant (g mg−1 min−1); Ea is the activation energy of MO adsorption (kJ mol−1); A is the pre-exponential factor (frequency factor); R is the gas constant (8.314 J mol−1 K−1); and T is the adsorption temperature (K). From the linear relationships between ln K2 and 1/T (Figure 4b), for the process of adsorbing MO on the Cr-doped ZnO, Ea was found to be 4.60 kJ/mol ( 1 indicate a normal Langmuir isotherm and cooperative adsorption, respectively.44,45 The plot of qe versus Ce is shown in Figure 9. For our study, the values of 1/n = 0.18638 indicate that the normal Langmuir isotherm is favorable. The correlation coefficient is R2 = 0.77592, which indicates that the Freundlich model has lower efficiency compared to the Langmuir model. The adsorption isotherm parameters are presented in Table 3. 3.8. Adsorption Mechanism. As shown in Figure S2 in Supporting Information, the as-prepared adsorbent maintains a positive charge, while MO maintains a negative charge. The adsorption mechanism may rely on the ionic interactions between the protonation hydroxyl (−OH2+) of Cr-doped ZnO and sulfonate groups (−SO3−) of MO, as is shown in Figure 10. Because of the Cr3+ dopant, there are more defects in the ZnO lattice, which caused the increase of the surface hydroxyl groups (−OH), and under experimental conditions, −OH can be 27454


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ACS Applied Materials & Interfaces Table 3. Isotherm Parameters for Adsorption of MO on Cr-Doped ZnO NPs at Room Temperature Langumir

Freundlich

qm/(mg g−1)

KL

R2

RMSE

KF

1/n

R2

310.60

0.3347

0.98067

12.67317

116.35

0.18638

0.77592

Figure 10. Schematic diagram for the adsorption of MO on Cr-doped ZnO NPs.

protonated into −OH2+; so, the Cr-doped ZnO can adsorb acid dyes from aqueous solution by ionic interactions with the sulfonate groups (−SO3−) of MO. Although the as-prepared Cr-doped ZnO NPs possesss a large adsorption capacity and the adsorption process has a high rate, their application is limited in the removal of acid dyes due to the Cr-doped ZnO NPs possessing a positive charge. 3.9. Recycle and Reuse. The results of AAS showed that c(Zn2+) in solution was 6.5 × 10−6 mol L−1. As is known, it can be considered that ions have precipitated completely when their concentration in the solution is below 10−5 mol L−1. Due to Ksp,Zn(OH)2 ≫ Ksp,Cr(OH)3, Cr3+ transformed into precipitation first. It can be assumed that Cr3+ had precipitated completely (images of this process are shown in Figure S3), and no Cr3+ was detected. The crystalline structure of the product after calcination was characterized by XRD, and all peak positions correspond to the standard diffraction patterns of wurtzite hexagonal structure (JCPDS Card No. 36-1451, shown in Figure S4). The obtained ZnO, although it has lost adsorption properties, can be reused as an industrial raw material in other fields. In this way, it is possible to avoid the secondary pollution of nanomaterials to the environment. In addition, MO has not changed in this process (UV−vis spectrum of MO solution is shown in Figure S5) and can be reused in the fields of printing, biological stains, etc. after purification.

experimental parameters such as initial MO concentration, contact time, and temperature on the MO removal percentage were investigated in detail, and the analysis results illustrated that the adsorption of MO on Cr-doped ZnO NPs was spontaneous and endothermic (Table 4), that the adsorption Table 4. Thermodynamics Parameters for Adsorption of MO on Cr-Doped ZnO NPs T

ΔG0

qe −1

ΔH0 −1

ΔS0 −1

(K)

(mg g )

(kJ mol )

(kJ mol )

298 308 318

286.52 290.47 293.21

−7.57 −8.75 −9.96

27.93

−1

(J mol

Ea −1

K )

119.12

(kJ mol−1) 4.60

process was rapid, reaching equilibrium in 2 h, and that it was in accord with pseudo-second-order kinetics model and Langmuir isotherm with a maximum adsorption capacity of 310.56 mg g−1 (Table 5). Furthermore, the reason for Cr-doped ZnO NPs exhibiting prominent adsorption property was that the surfaces of the fabricated products maintained a positive charge to Table 5. Comparison of Adsorption Capacity of Various Adsorbents adsorbent

4. CONCLUSIONS Cr-doped ZnO NPs with splendid adsorption property were successfully achieved via a mild, high-efficiency, and environmentally benign solvothermal process and which could be recycled through a simple chemical treatment, protecting the environment from pollution of the NPs. The influences of

HA-Fe3O4 MgO hierarchical hollow Fe2O3 activated carbon Cr-doped ZnO 27455

adsorbate

adsorption capacity (mg g−1)

MB Congo Red Congo Red

93.08 131 160

46 47 48

MB MO

185 310.65

43 this work

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adsorb acid dye MO by electrostatic attraction. Consequently, it is entirely reasonable for us to believe that such a highly efficient, technically feasible, mild, and cost-effective adsorbent could be applied in dye effluent treatment.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09366. Additional Figures S1−S5 and additional Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86 532 88956228. Tel.: +86 532 88956228. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work reported here was supported by the National Natural Science Foundation of China under Grant No. 51572137, 51272117, 51172115, the Natural Science Foundation of Shandong Province under Grant No. ZR2015PE003, ZR2011EMZ001, ZR2011EMQ011, ZR2013EMQ006, the Research Award Fund for Outstanding Young Scientists of Shandong Province Grant No. BS2013CL040, the Specialized Research Fund for the Doctoral Program of Higher Education of China under Grant No. 20123719110003, the Tackling Key Program of Science and Technology in Shandong Province under Grant No. 2012GGX1021, the Application Foundation Research Program of Qingdao under Grant No. 13-1-4-117-jch, 14-2-4-29-jch, Shandong Province Taishan Scholar Project and Overseas Taishan Scholar Project. We express our grateful thanks to them for their financial support.

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Cr-Doped ZnO Nanoparticles: Synthesis, Characterization, Adsorption

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