Adsorption of Congo red from aqueous solution using ...

Journal of Molecular Liquids 249 (2018) 772–778

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Adsorption of Congo red from aqueous solution using ZnO-modified SiO2 nanospheres with rough surfaces Jiajin Zhang, Xinlong Yan ⁎, Mengqing Hu, Xiaoyan Hu, Min Zhou Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), School of Chemical Engineering & Technology, China University of Mining and Technology, XuZhou 221116, PR China

a r t i c l e

i n f o

Article history: Received 2 September 2017 Received in revised form 7 November 2017 Accepted 18 November 2017 Available online 21 November 2017 Keywords: ZnO SiO2 Rough surface Adsorption Congo red

a b s t r a c t As one of the toxic dyes, Congo Red (CR) is considered as emerging pollutant in surface water, thus, the development of novel adsorbent for its efficient removal is necessary. Here, ZnO/SiO2 nanosphere composites with rough surfaces were synthesized by a facile wet impregnation method. The structure and morphology of as-prepared composites were characterized and their performances toward CR adsorption were evaluated. The ZnO/SiO2-1 composite exhibited high adsorption capacity of ~83 mg/g and fast adsorption kinetic for removal of CR from aqueous solution. The adsorption kinetic and isotherm data could be well interpreted by the Pseudo-secondorder rate equation and Langmuir adsorption isotherm, respectively. Moreover, the ZnO/SiO2-1 could be easily regenerated by calcination and the adsorption capacity could be maintained at high level (up to 83%) for at least 4 adsorption-regeneration cycles. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Organic dyes are extensively used in the textile, plastics, printing and cosmetic industries. The discharge of dye contaminants into natural or engineered water bodies has caused serious environmental problems in recent years, as many of the dyes are highly toxic to aquatic life and hard to be degraded in natural environments [1,2]. Therefore, it is essential to remove these dyes from wastewater. The most commonly used techniques for dye removal are catalytic oxidation, ion-exchange and adsorption [3–6]. Among them, adsorption has become the most competitive method for wastewater treatment due to its easy and inexpensive operation, high efficiency and low energy consumption [7]. Various adsorbents, including activated carbon [8, 9], clays [10,11], and metal oxides and hydroxides [12,13], have been studied for the adsorptive removal of dyes from contaminated water. However, many of these adsorbents fail to fully meet the requirements, such as good adsorption selectivity or high adsorption capacity. Thus, for the practical application of adsorption, exploring effective adsorbents is still needed. As a common metal oxide, zinc oxide (ZnO) has been applied in many fields because of its low cost, good stability, nontoxicity and

⁎ Corresponding author. E-mail address: [email protected] (X. Yan).

https://doi.org/10.1016/j.molliq.2017.11.109 0167-7322/© 2017 Elsevier B.V. All rights reserved.

environmental-friendly properties [14–16]. To date, different methods have been used for the production of ZnO particles, such as precipitation [17,18], hydrothermal [19–21] and ultrasonic assisted methods [22,23]. In general, most of the methods have been developed for synthesis of ZnO nanoparticles, as nanomaterials often have larger specific surface areas and feature better adsorption performance than bulk materials. However, ultrafine particles tend to aggregate in water and are difficult to separate. Dispersion of ZnO onto a substrate with relatively large size may solve the above problem. On one hand, impregnation of ZnO particles on a substrate with large internal surface area, such as porous material, may improve adsorption performance. However, introducing ZnO might occlude the pores of the substrate [24], which would be difficult to be used for the adsorption of large molecules. On the other hand, dispersion of ZnO onto a particle with large external surface, such as particles with rough surface, may be beneficial for adsorption of large molecules, but it has been rarely reported. Recently, novel SiO2 spheres with rough surfaces have been successfully synthesized and showed remarkable properties [25–27], which makes it possibly be a good substrate for ZnO dispersion. In this work, ZnO was dispersed onto SiO2 spheres with rough surfaces by wet impregnation successfully. The ZnO-SiO2 composites were characterized by different techniques and evaluated for the adsorption performances toward Congo red (CR). The results show that simple impregnation method led to good dispersion of ZnO onto the SiO2 substrate. The adsorption capacities of the ZnO/SiO2 composites for CR were as high as ~83 mg/g at 30 °C.

103

110

ZnO/SiO2-1.5 ZnO/SiO2-1 ZnO/SiO2-0.5

SiO2 10

20

30

40

50

60

70

2 theta (degree) Fig. 1. XRD patterns of the pure SiO2 sample and ZnO-SiO2 samples.

2. Experimental

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120 100

3

Intensity (a.u.)

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ZnO/SiO2-2

Amount adsorbed (cm /g, STP)

100 002 101

J. Zhang et al. / Journal of Molecular Liquids 249 (2018) 772–778

80 60

ZnO/SiO2-2 ZnO/SiO2-1.5 ZnO/SiO2-1 ZnO/SiO2-0.5 SiO2

40 20 0 0.0

0.2

0.4

0.6

0.8

1.0

P/P0 Fig. 3. Nitrogen adsorption-desorption isotherms of the pure SiO2 sample and ZnO/SiO2 samples.

2.1. Materials All reagents used were of analytical grade and were purchased from the Sino-reagent Company (Shanghai, China), including Zn(NO3)2·6H2O (N99%), ZnO (N99%), tetraethoxysilane (TEOS, N28.4% SiO2), resorcinol (N 99.5%), formaldehyde (N 37%), ammonia aqueous solution (N 28%), CR (N98%) and ethanol (N99.7%).

water under stirring. The mixture was stirred continuously for 6 h at room temperature, followed by addition of 2.4 mL of TEOS and stirring for 8 min. Then 2.24 mL of formaldehyde and 1.6 g of resorcinol were added and the solution was stirred for another 2 h. Finally, the material was obtained by repeated filtration, washing with ethanol and subsequently drying in air at 50 °C overnight and calcination at 550 °C for 5 h.

2.2. Preparation of silica spheres

2.3. Preparation of the ZnO-modified silica spheres

The synthesis of silica spheres followed a reported procedure with minor revision [25]. Typically, 0.6 g of resorcinol, 0.84 mL of formaldehyde and 12 mL of aqueous ammonia solution were successively added into a solution composed of 140 mL of ethanol and 20 mL of

In a typical synthesis, 0.6 g of the dry silica spheres were suspended in 80 mL of Zn(NO3)2 solution with different concentrations (0.5, 1.0, 1.5 or 2.0 mol/L). Then, the suspension was treated by sonication for 30 min and the resulting paste was separated by centrifugation. Afterwards, the

Fig. 2. SEM images of (a) SiO2, (b) ZnO/SiO2-0.5, (c) ZnO/SiO2-1, (d) ZnO/SiO2-1.5 and (e) ZnO/SiO2-2. (f) TEM image of the ZnO/SiO2-1.5 sample. (g–i) EDX mapping of the ZnO/SiO2-1.5 sample (Magnification 7686).

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Table 1 Surface area and porosity of the ZnO/SiO2 composites.

50

ZnO/SiO2-1

40 30

ZnO/SiO2-1.5 ZnO/SiO2-2 Pure ZnO

20

ZnO/SiO2-0.5

3

Samples

SBET m /g

Vt cm /g

SiO2 ZnO/SiO2–0.5 ZnO/SiO2–1 ZnO/SiO2–1.5 ZnO/SiO2–2

46.5 35.5 34.5 27.5 24.7

0.16 0.17 0.16 0.14 0.11

paste was dried at 50 °C and then calcined at 550 °C for 2 h. The obtained powders were labeled as ZnO/SiO2-x, where x represents the concentration of the Zn(NO3)2 solution used during synthesis.

qt (mg/g)

2

10

SiO2

0 0

20

40

60

80

100

120

140

160

t (min)

2.4. Characterization Inductively coupled plasma optical emission spectrometry (ICPOES) analyses were conducted on a Varian 720-ES spectrometer. Powder X-ray diffraction patterns were obtained by using a Bruker D8 ADVANCE diffractometer with Cu Kα radiation (40 kV, 30 mA). The surface area and adsorption/desorption isotherm measurements were carried out with an IQ2 Quantachrome porosimeter after sample degassing at 100 °C overnight. The morphology of the powders was observed by transmission electron microscopy (TEM) (FEI Tecnai G2 F20) and scanning electron microscopy (SEM) (FEI Quanta 400 FEG). The Fourier transform infrared (FT-IR) spectra of the pure and functionalized silica spheres were measured with a Nicolet iS5 spectrometer.

Fig. 5. Time-dependent adsorption of CR on the pure SiO2, pure ZnO and ZnO/SiO2 samples at 30 °C. Adsorbent, 200 mg/L; CR, 50 mg/L.

2.5. Adsorption experiments

The phase structures of the ZnO-modified SiO2 samples were analyzed by powder XRD, as shown in Fig. 1. The pure SiO2 sample shows only one broad peak at approximately 22°, which could be assigned to the amorphous structures of SiO2 [28,29]. After impregnation by a low concentration Zn(NO3)2 solution, no diffraction peaks indicative of ZnO can be seen, which suggests that the ZnO is highly dispersed or present in insufficient amounts to be detected by this method. A further increase in the concentration of the Zn(NO3)2 solution used (from 0.5 to 1 mol/L) for impregnation resulted in well-resolved diffraction peaks at 2θ of 32.0°, 34.6°, 36.4°, 47.7°, 56.8° and 63.0° that could be observed for all the ZnO-SiO2 samples, which can be indexed to the (100), (002), (101), (102), (110) and (103) planes of hexagonal wurtzite ZnO (JCPDS No. 36-1451), respectively [21,30]. No other peaks can be observed in the patterns, indicating that Zn precursor was totally converted to ZnO phase after calcination. This result implies that ZnO was successfully introduced onto the SiO2 substrate. With increasing Zn(NO3)2 concentration, amount of ZnO in the composite improved, as the peak intensity for ZnO slightly increased. The actual amount of Zn in the composite samples was determined by ICP to be 6.3 wt%, 12.8 wt%, 13.1 wt% and 15.3 wt% in the ZnO/SiO2-0.5, ZnO/SiO2-1, ZnO/SiO2-1.5 and ZnO/SiO2-2 samples, respectively.

For the adsorption isotherm study, 10 mg of the adsorbent was added into 50 mL of solutions with different CR concentrations under stirring for 24 h at 30 °C. Afterwards, the sample was withdrawn, and the adsorbent was separated using a syringe filter (PTFE, 0.22 μm). For the adsorption kinetic measurements, 20 mg of the adsorbent was added to 100 mL of CR solution (50 mg/L) under stirring, and approximately 2.5 mL of the mixture was withdraw at a pre-determined time, and the solids were separated. The subsequent filtrate was collected and analyzed using a UV–Vis spectrophotometer (TU-1810, Persee Co., China) at 495 nm. The adsorption capacities of the pure SiO2 and ZnO/SiO2 composites were calculated by the following equation: qt ¼

ðC 0 −C t ÞV m

in which qt is the adsorption capacity, C0 and Ct are the concentrations of Congo red before and after adsorption, respectively, V is the solution volume (L), and m is the weight of the ZnO/SiO2 composite used (g).

Desorption experiment was performed using a thermal regeneration method. After the CR adsorption, the ZnO/SiO2 sample was collected by filtration, calcination at 550 °C for 2 h and then reused.

3. Results and discussion 3.1. Materials characterization

Fig. 4. FT-IR spectra of CR, ZnO/SiO2-1 and SiO2 sample before and after CR adsorption.


100

1.5

(a) 80

0.5

qe (mg/g)

log (qe-qt)

1.0

0.0 -0.5 -1.0

ZnO/SiO2-1

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ZnO/SiO2-1.5

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ZnO/SiO2-2 0

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60 40 20 0

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ZnO/SiO2-1

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ZnO/SiO2-1.5

0.0

ZnO/SiO2-2 0

200

300

400

500

Fig. 7. Adsorption isotherm of CR on the ZnO/SiO2-1 sample at 30 °C. Adsorbent, 200 mg/L; initial CR concentration, ~10–450 mg/L. (Ce represents the equilibrium concentration and qe is the equilibrium adsorption capacity.)

4.0 3.5

100

Ce (mg/L)

t (min)

t/qt

775

20

40

60

80

100

120

140

t (min) Fig. 6. Plots of (a) pseudo-first-order kinetics and (b) pseudo-second-order kinetics of CR adsorption over the ZnO/SiO2 samples.

The morphologies of the SiO2 and ZnO-modified SiO2 were characterized by SEM and TEM. From Fig. 2a, it can be seen that a well-dispersed silica sphere is covered by nanosized silica spikes with a diameter of ~200–500 nm. The uniformity of the sphere size is not as good as that previously reported, perhaps due to the scaled-up preparation [25]. After modification with ZnO, the morphology of silica remains unchanged (Fig. 2b–e). The Zn distribution on the surface of silica spheres was investigated by EDX mapping. As shown in Fig. 2g–i, a homogeneous Zn coverage on silica can be clearly seen, indicating that Zn was uniformly spread on the silica using this soaking treatment. Furthermore, solid spheres with rough surfaces can be clearly seen for the ZnO/SiO2 sample in the TEM image described in Fig. 2f. The nitrogen adsorption-desorption isotherms of the pure SiO2 and ZnO/SiO2 samples are shown in Fig. 3, and the resulting porosity parameters are compiled in Table 1. Similar shapes were observed for all the isotherms, which were found to be of Type IV with H3 hysteresis loops, indicating the presence of mesoporous structure. Due to the dense structure of the ZnO and SiO2 prepared, all the samples exhibit

relatively low surface area. The pure SiO2 sphere has high surface area and pore volume of 46.5 m2/g and 0.16 cm3/g, respectively. After modification by ZnO, the surface area and pore volume of the composite gradually decreased, which implies that the surface of silica spheres was covered by ZnO. This result is consistent with XRD and SEM analyses. The pore size analyzed by NLDFT method can be found in Fig. S1. The pore size increased after impregnation of ZnO, due to the blockage of micropores of SiO2 substrate by ZnO. However, increasing ZnO impregnated led to little change in pore size distribution. The isotherm of CR-adsorbed ZnO/SiO2–1 (Fig.S3) revealed very low adsorption capacity toward nitrogen, suggesting that most of the pores in ZnO/SiO2–1 were occupied by CR. The surface chemistries of ZnO/SiO2-1 and SiO2 before and after CR adsorption were analyzed by FT-IR. As shown in Fig. 4, the bands of pure SiO2 show little change after CR adsorption, possibly because of the low amount of CR adsorbed. For ZnO/SiO2 and CR adsorbed ZnO/ SiO2 composite, strong and broad adsorption bands at ~ 3440 cm−1 could be assigned to stretching vibrations of the O\\H groups, arising from the water molecules adsorbed on interlayer and surface of the composite [31,32]. In the low frequency region, the band at 1114 cm−1 could be ascribed to asymmetric Si-O-Si stretching [33,34]. Interestingly, a band at 668 cm−1 in the spectrum of CR can also be observed in the spectrum of CR-adsorbed ZnO/SiO2 composite, indicating the incorporation of CR onto ZnO/SiO2 due to the interaction between the dye and the adsorbent. Note that, the adsorption of CR by ZnO was proved to be an electrostatic attraction in the first place [21], resulting from the anionic nature of CR and positive zeta potential of ZnO. The physical interaction between the chemical functional groups of the dye and the oxygen groups on ZnO also occurred during the adsorption [35].

3.2. Adsorption kinetics and isotherms The effects of contact time on adsorption of CR onto SiO2, pure ZnO and the as-prepared ZnO/SiO2 samples were evaluated, and the results

Table 2 Kinetic parameters for the adsorption of CR onto the ZnO/SiO2 samples. Samples

qe,exp (mg/g)

Pseudo-first-order kinetic model k1 (1/min)

ZnO/SiO2-1 ZnO/SiO2-1.5 ZnO/SiO2-2

49.0 38.3 35.2

0.0322 0.0299 0.0438

qe (mg/g) 6.8 13.1 23.9

Pseudo-second-order kinetic model R

2

0.908 0.280 0.930

k2 (g/mg min) −3

14.4 × 10 3.42 × 10−3 3.87 × 10−3

qe (mg/g)

R2

49.5 40.0 37.0

0.999 0.997 0.998

776


Table 3 Comparison of the adsorption capacities for CR of different ZnO-based adsorbents. Adsorbent

Adsorption capacity (mg/g)

References

ZnO/activated carbon ZnO/brick grain particles ZnO/SnO2 hetero-nanofibers ZnO nanorods/activated carbon Commercial ZnO ZnO/SiO2-1

65 35 85.8 142 29.6 83

[38] [38] [39] [40] This work This work

are shown in Fig. 5. The adsorption capacity increases rapidly at the initial stage for all the samples and reaches equilibrium quickly. The SiO2 and ZnO/SiO2-1 exhibited the fastest adsorption rates among all the samples, taking only approximately 20 mins to reach equilibrium, compared with nearly 40 mins for the other four samples. The equilibrium adsorption capacities are 9.8 mg/g, 29.6 mg/g, 28.0 mg/g, 48.5 mg/g, 37.1 mg/g and 35.0 mg/g for the SiO2, ZnO, ZnO/SiO2-0.5, ZnO/SiO2-1, ZnO/SiO2-1.5 and ZnO/SiO2-2 samples, respectively. Among the ZnO/ SiO2 composites, ZnO/SiO2-0.5 possesses the lowest adsorption capacity, since the low amount of ZnO on the surface of SiO2 provided limited adsorption sites for CR. By contrast, the ZnO/SiO2-1 sample shows the highest adsorption capacity, possibly attributed to its large surface area and sufficient ZnO onto the surface of SiO2. Further increase in the amount of ZnO caused dramatic decrease of surface area, which in turn, led to the decreased adsorption capacity.

6

(a)

5

Ce / q e

4 3 2 1

Table 4 Adsorption isotherm model parameters for the adsorption of CR by the ZnO/SiO2-1 sample. Langmuir

Freundlich

qm (mg/g)

KL (L/mg)

R2

KF (mg/g)

n

R2

90.1

0.032

0.989

22.6

4.6

0.920

The adsorption kinetics was further analyzed using pseudo-firstorder and pseudo-second-order kinetic models (see the Supporting information). The linear fitting results are shown in Fig. 6, and the calculated kinetic constants and correlation coefficients are listed in Table 2. Apparently, pseudo-second-order kinetic model, with a higher correlation coefficient (R2 N 0.99), gave a better fit than pseudo-first-order model. The calculated value of qe is very close to the experimental data, indicating that the pseudo-second-order model is appropriate to predict the kinetic process. The pseudo-second-order model is based on the hypothesis that chemisorption is the rate-limiting factor in the adsorption process. Therefore, it can be concluded that the adsorption of CR onto the ZnO/SiO2 composites should be a chemical process, which involves electron sharing or electron transfer [36,37]. ZnO/SiO2-1 was selected for study on adsorption isotherm, for its highest adsorption capacity and fast adsorption kinetics. The results shown in Fig. 7 demonstrates that the adsorption capacity increases with CR concentration at the initial stage and gradually achieves equilibrium, with a value of ~83 mg/g. Compared with other ZnO-based adsorbents (Table 3), such a high adsorption capacity suggests that ZnO/SiO21 can be potentially an effective adsorbent for CR. As evidence, the photograph and UV–vis spectra of CR solutions before and after treatment by ZnO/SiO2-1 was provided in Fig. S2. The adsorption isotherm data were further analyzed with the two most commonly used isotherm model, Langmuir and Freundlich models (see the Supporting information). The former presumes monolayer adsorption of the adsorbate over a homogeneous surface of the adsorbent, while the latter proposes multilayer adsorption of the adsorbate on a heterogeneous surface of the adsorbent. The resulting fits could be seen in Fig. 8, and the corresponding results are summarized in Table 4. The results indicated that the Langmuir isotherm equation can well describe the adsorption process of CR over ZnO/SiO2-1, owing to the high R2 value fitted by the equation.

0 0

100

200

300

400

500

Ce (mg/L) 4.4

(b)

ln qe

4.2 4.0 3.8 3.6 3.4 1

2

3

4

5

6

ln Ce Fig. 8. (a) Langmuir and (b) Freundlich isotherm model fitting curves of CR adsorption on the ZnO/SiO2-1 sample.

3.3. Effect of solution pH, temperature and adsorbent dosage Fig.9 describes the effects of solution pH, temperature and adsorbent dosage on adsorption of CR by ZnO/SiO2-1. The adsorption capacity decreased monotonically with improved pH value of solution in the range of 4 to 12. The higher adsorption capacity at lower PH value should be attributed to the electrostatic attraction between positively charged ZnO/SiO2-1 and negatively charged sulfonated group (−SO3− Na+) of the dye molecule in acidic solution. However, in a solution of high pH value, the negatively charged surface of ZnO/SiO2-1, resulting from excessive OH– ions over the surface, would hinder the adsorption by electrostatic repulsion force, which reduced the adsorption capacity [41,42]. The influences of temperature on adsorption of CR onto ZnO/SiO2-1 were studied at 30 °C, 50 °C and 70 °C, and the results are presented in Fig. 9b. The adsorption capacity decreased with increasing temperature. This suggests that the adsorption process is exothermic and is favored at lower temperature. As for the effects of adsorbent dosage shown in Fig. 9c, it could be obviously found that the adsorption capacity decreased with increase of adsorbent dosage. This can be explained by the fact that increasing adsorbent in a settled volume provides a large excess of active sites leading to a lower utility of the sites at a certain concentration of CR solution [43,44].


777

Fig. 9. Effects of solution pH (a), temperature (b) and adsorbent dosage (c) on CR adsorbed by ZnO/SiO2-1.

3.4. Reusability

Appendix A. Supplementary data

Reusability is a crucial factor for the application of an adsorbent in terms of cost and emissions. Here, reusability of the adsorbent was evaluated by repeated regeneration via calcination in air, and the results are shown in Fig. 10. The capacity of ZnO/SiO2-1 sample was slightly influenced after the first adsorption-regeneration cycle. With further increasing number of cycles, the adsorbent still had high adsorption capacity, with a value of about 83% of the original capacity. The results revealed that ZnO/SiO2-1 could be easily regenerated and the adsorption capacity could be maintained for at least 4 cycles.

Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2017.11.109.

4. Conclusions In this work, ZnO/SiO2 composites with rough surface were prepared by a wet impregnation method and used as adsorbents for CR removal from water. The adsorption process, including the adsorption kinetics and isotherms, factors that affected the adsorption, were investigated in detail. The results showed that the process of CR removal via adsorption by the ZnO/SiO2 could be well described by pseudo-secondorder rate equation and Langmuir isotherm model. The adsorption capacity of the ZnO/SiO2-1 sample was as high as 83 mg/g at 30 °C. Moreover, the ZnO/SiO2-1 sample could be effectively regenerated by calcination and used for at least 4 cycles without significant loss of adsorption capacity (~83% of the original capacity). Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (No. 2017XKQY069) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

40

qe (mg/g)

30

20

10

0 1

2

3

Number of Cycle Fig. 10. Recyclability of ZnO/SiO2-1 for CR adsorption.

4

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