Rapid and efficient photocatalytic reduction of hexavalent chromium by using “water dispersible” TiO2 nanoparticles

Materials Chemistry and Physics 178 (2016) 190e195

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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Rapid and efficient photocatalytic reduction of hexavalent chromium by using “water dispersible” TiO2 nanoparticles Lei Wang, Shi-Zhao Kang*, Xiangqing Li, Lixia Qin, Hao Yan, Jin Mu** School of Chemical and Environmental Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai 201418, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


“Water dispersible” TiO2 nanoparticles with high photocatalytic activity.
100% Cr (VI) (10 mg L1) can be reduced within 10 min.
Obvious decrease of electrical energy consumption.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 November 2015 Received in revised form 15 April 2016 Accepted 1 May 2016 Available online 6 May 2016

In the present work, “water dispersible” TiO2 nanoparticles were prepared, and meanwhile, their photocatalytic activity was systematically tested for the reduction of aqueous Cr(VI) ions. It is found that the as-prepared “water dispersible” TiO2 nanoparticles are a highly efficient photocatalyst for the reduction of Cr(VI) ions in water under UV irradiation, and suitable for the remediation of Cr(VI) ions wastewater with low concentration. Compared with commercial TiO2 nanoparticles (P25), the “water dispersible” TiO2 nanoparticles exhibit 3.8-fold higher photocatalytic activity. 100% Cr (VI) ions can be reduced into Cr(III) ions within 10 min when the Cr (VI) ions initial concentration is 10 mg L1. Moreover, the electrical energy consumption can be obviously decreased using the “water dispersible” TiO2 nanoparticles. These results suggest that the “water dispersible” TiO2 nanoparticles are a promising photocatalyst for rapid removal of Cr (VI) in environmental therapy. © 2016 Elsevier B.V. All rights reserved.

Keywords: Inorganic compounds Semiconductors Nanostructures Surface properties

1. Introduction The presence of hexavalent chromium in the aquatic environment is an urgent problem to the ecosystem due to its high toxicity and potential carcinogenicity for human being [1]. Thus, many methods have been developed to eliminate hexavalent chromium, such as chemical reduction precipitation [2], electrochemical

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S.-Z. Kang), [email protected] (J. Mu). http://dx.doi.org/10.1016/j.matchemphys.2016.05.004 0254-0584/© 2016 Elsevier B.V. All rights reserved.

precipitation [3], ion exchange [4], membrane separation [5], adsorption [6], reverse osmosis [7], biological treatment [8], electro coagulation [9], photocatalytic reduction [10] and so on. Among these technologies, photocatalytic reduction is regarded as an ideal green technology for the removal of hexavalent chromium as it is low cost, high efficiency, simplicity and does not cause secondary pollution. To date, photocatalytic removal of hexavalent chromium has received considerable attention, and a series of efficient photocatalysts including TiO2 [11], CuI [12], a-GaOOH and a-Ga2O3 selfassembly [13], Bi nanowire networks [14], Cu2ZnSnS4 nanoparticles [15], MIL-53(Fe) ereduced graphene oxide nanocomposites [16],

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polyoxometalates/TiO2 composites [17] etc. have been reported. Nevertheless, the activity of the photocatalysts need be further enhanced from the practical point of view. Moreover, electrical energy consumption in heterogeneous photocatalytic processes is still high. Therefore, it is desirable to develop an efficient photocatalyst and decrease the electrical energy consumption in heterogeneous photocatalytic processes. Nowadays, the photocatalysts are often used in the form of suspension when hexavalent chromium is removed from aqueous solution via photocatalytic reduction. In order to ensure efficient contact between photocatalyst and wastewater, the slurry needs to be continuously stirred. As a result, the electrical energy consumption due to stirring accounts for a large portion of the total power consumption in the photocatalytic process. Moreover, because of light scattering, reflection and shading from photocatalyst particles, the light penetration distance is limited in the slurry, which leads to that a fraction of photocatalyst cannot exhibit its full photocatalytic activity. If these problems can be resolved, the efficiency of photocatalytic removal of hexavalent chromium would be further increased meanwhile the electrical energy consumption would be decreased. Therefore, for practical application of photocatalysts, it is necessary to overcome the drawbacks above. In our previous work [18], we prepared the “water-soluble” TiO2 nanoparticles. The as-prepared TiO2 nanoparticles can be easily dispersed in water to form a stable solution-like suspension. Furthermore, compared with commercial TiO2 nanoparticles (P25), the “water-soluble” TiO2 nanoparticles obtained exhibit higher photocatalytic activity for the degradation of dye in water. Based on these experimental results, we speculate that the “water-soluble” TiO2 nanoparticles would be an efficient photocatalyst for the reduction of Cr(VI) ions in water, and the electrical energy consumption in the photocatalytic processes would be decreased due to their excellent dispersibility in water. Following this idea, “water dispersible” TiO2 nanoparticles were prepared, and then their photocatalytic activity was systematically tested for the reduction of aqueous Cr(VI) ions. Our main aim of this study was focusing on enhancement of photocatalytic removal efficiency of hexavalent chromium and reduction of electrical energy consumption. Finally, a preliminary photocatalytic mechanism was discussed.

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[19]. In a typical process, 0.625 mL reaction solution was withdrawn intermittently. Then, pH was adjusted to 1 using dilute sulphuric acid. After the mixture was centrifuged and filtered to remove the TiO2 nanoparticles, the filtrate was diluted to 25 mL using deionized water. Finally, changes in Cr(VI) ions concentration were determined using the 1,5-diphenylcarbazide spectrophotometric method. The photocatalytic reduction efficiency of Cr(VI) ions was calculated according to the following equation:

Efficiency ð%Þ ¼ ðC0  CÞ=C0  100%

(1)

Where C0 represents the concentration of Cr(VI) ions after stirred in dark for 30 min, C the concentration of Cr(VI) ions after irradiation. 3. Results and discussion Fig. 1 shows the TEM images of the “water dispersible” TiO2 nanoparticles. From Fig. 1, we can clearly observe the TiO2 nanoparticles with a mean diameter of approximate 3.0 nm. In addition, any obvious agglomeration is not found. Fig. 2 shows the XRD pattern of the as-prepared TiO2 nanoparticles. As can be seen from Fig. 2, the XRD data of the as-prepared sample are in good agreement with those of anatase TiO2 (JCPDS file no. 21-1272). Moreover, no diffraction peaks from crystalline impurities can be observed. The control experimental results (supplementary Fig. S1) indicate that the as-prepared powder can be easily dispersed in water to form a stable solution-like suspension. Based on these results, it can be concluded that the “water dispersible”TiO2 nanoparticles are successfully prepared. Fig. 3 shows the relationship of irradiation time and reduction efficiency of Cr(VI) ions in the presence of the “water dispersible” TiO2 nanoparticles. For comparison, photoreduction of Cr(VI) ions

2. Experimental 2.1. Preparation of “water dispersible” TiO2 nanoparticles All reagents were of analytical grade and used as received. Deionized water was used as solvent. “Water dispersible” TiO2 nanoparticles were prepared according to the procedure reported in ref. [18]. And the as-prepared product was characterized with Xray diffraction and transmission electron microscopy. The experimental details about the preparation procedure and characterization are reported in supplementary. 2.2. Photocatalytic activity measurements A 500 W mercury lamp with an optical filter (l < 420 nm) was used as UV radiation source. In order to remove infrared light, the lamp was equipped with a water jacket. The photocatalytic experiments were carried out in a quartz reactor containing 100 mL aqueous K2Cr2O7 (40 mg L1, pH ¼ 3.0) and photocatalysts (200 mg). The distance between the lamp and the reactor was maintained to be 8 cm. The solution-like suspension of TiO2 nanoparticles was stirred in the dark for 30 min in order to reach the adsorption equilibrium prior to the photoreduction of Cr(VI) ions. After a given irradiation time, the residual concentration of K2Cr2O7 was monitored using the 1,5-diphenylcarbazide method

Fig. 1. (A) TEM image and (B) HRTEM image of the “water dispersible” TiO2 nanoparticles.

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faster photoreduction rate than P25. In addition, the control experimental results show that the reduction percentage of Cr(VI) ions and the reaction rate constant are 23% and 8  103 min1 in the presence of the anatase TiO2 nanoparticles. These results indicate that the “water dispersible” TiO2 nanoparticles are a highly active photocatalyst for photoreduction of Cr(VI) ions in water. The “water dispersible” TiO2 nanoparticles can be easily dispersed in water to form a stable solution-like suspension. The contact between photocatalyst and wastewater can be efficient and sufficient even without vigorous stirring. As a result, we speculate that the electrical energy consumption due to stirring should obviously decrease when the “water dispersible” TiO2 nanoparticles are used as a photocatalyst. In order to confirm our assumption, the electric energy per order (kW h per m3 per order, EEO) required to the photocatalytic reduction of Cr(VI) ions was calculated according to the following equations [21]:

EEO ¼ ð38:4  Pel Þ=ðV  kÞ

Photoreduction efficiency / %

Fig. 2. XRD pattern of the “water dispersible” TiO2 nanoparticles.

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Irradiation time / min Fig. 3. Time-courses of Cr(VI) ions photoreduction (a) in the presence of the “water dispersible” TiO2 nanoparticles, (b) in the presence of P25, and (c) in the absence of photocatalyst (Photocatalyst dosage: 2 g L1; initial Cr(VI) ions concentration 40 mg L1, pH: 3).

in the presence of P25 or in the absence of TiO2 nanoparticles was also measured and shown in Fig. 3. As can be seen from Fig. 3, the Cr(VI) ions is reduced by 99% in the presence of the “water dispersible” TiO2 nanoparticles, while the reduction percentage of Cr(VI) ions is only 3% in the absence of TiO2 nanoparticles. Moreover, the photocatalytic activity of the “water dispersible” TiO2 nanoparticles is much higher than that of P25. When the “water dispersible” TiO2 nanoparticles is used instead of P25, the photocatalytic reduction efficiency of Cr(VI) ions can be increased by 3.8 times. In order to quantitatively describe the activities of the photocatalysts, the reaction rate constant (k) of photoreduction of Cr(VI) ions over the “water dispersible” TiO2 nanoparticles and P25 was evaluated according to the pseudo-first-order model, respectively [20].

lnðC0 =Ct Þ ¼ kt

(2)

Where C0 represents the concentration of Cr(VI) ions after stirred in dark for 30 min, Ct the concentration of Cr(VI) ions after irradiation for t min. The reaction rate constants (k) of photoreduction of Cr(VI) ions over the “water dispersible” TiO2 nanoparticles and P25 are 0.14 min1 and 7.2  103 min1, implying that the “water dispersible” TiO2 nanoparticles present a much

(3)

Where Pel is the input electric power (kW), k the reaction rate constant of photoreduction of Cr(VI) ions (min1), V is the volume of Cr(VI) ions aqueous solution (L) in the reactor. The EEO of the “water dispersible” TiO2 nanoparticles is 1412 kW h m3 order1 while the EEO of P25 is 25,617 kW h m3 order1. In addition, the EEO of the TiO2 nanoparticles prepared in a common hydrothermal process is 21,851 kW h m3 order1. Thus it can be concluded that the electrical energy consumption in the photocatalytic process would drastically decline when the “water dispersible” TiO2 nanoparticles are used. Based on the experimental results above, a possible mechanism is suggested as follows. At first, when the “water dispersible” TiO2 nanoparticles are under irradiation, the electrons are excited from the valence band to the conduction band. And then the excited electrons transfer to the surface of the TiO2 nanoparticles. Finally, Cr(VI) ions is reduced into Cr(III) ions by the photogenerated electrons on the surface of the TiO2 nanoparticles. Herein, the dramatical photocatalytic activity of the “water dispersible” TiO2 nanoparticles may be ascribed to two causes. (1) The size of the “water dispersible” TiO2 nanoparticles is much smaller in comparison with that of general TiO2 nanoparticles. For example, the TEM images indicate that the mean diameter of the “water dispersible” TiO2 nanoparticles is about 3 nm (Fig. 1) while the size of P25 is about 28 nm [22]. As a result, the movement or transfer of electrons and holes generated inside the crystal to the surface become more efficient. (2) The “water dispersible” TiO2 nanoparticles possess excellent dispersibility in water. When they are dispersed in water, a transparent solution-like dispersion can be obtained. Therefore, the light penetration distance in this translucent solution-like dispersion is longer in comparison with those in the slurries of other TiO2 nanoparticles, which makes the light absorption more efficient in the case of the “water dispersible” TiO2 nanoparticles. In order to confirm our assumption, the UVevis transmission spectrum and the UVevis diffuse reflectance spectrum of the “water dispersible” TiO2 nanoparticles dispersion were measured. For comparison, the UVevis transmission spectrum and the UVevis diffuse reflectance spectrum of the P25 slurry were also recorded. The results are shown in Fig. 4. As can be seen from Fig. 4A, the transmittivity of the “water dispersible” TiO2 nanoparticles dispersion in the visible range is higher than that of the P25 slurry, which confirms that the light penetration distance in this translucent solution-like dispersion is longer than that in the P25 slurry. Besides, the transmittivity of the “water dispersible” TiO2 nanoparticles dispersion in the UV range is about 0, indicating that the “water dispersible” TiO2 nanoparticles can efficiently absorb UV light. In contrast, the reflectance of the P25 slurry is

L. Wang et al. / Materials Chemistry and Physics 178 (2016) 190e195

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Transmittivity / %

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secondary pollution would not be caused due to the application of the “water dispersible” TiO2 nanoparticles. Fig. 5 shows the effect of pH on the photocatalytic activity of the “water dispersible” TiO2 nanoparticles. Moreover, the effect of pH on the EEO of the “water dispersible” TiO2 nanoparticles is also shown in Fig. 5. From Fig. 5A, it can be observed that the reduction ratio of Cr(VI) ions decreases gradually with increasing the pH of the solution from 2 to 11. The pseudo-first-order rate constants are 0.33 min1, 0.14 min1, 0.042 min1, 0.018 min1, 0.015 min1 and 0.0088 min1 when pH values are 2, 3, 5, 7, 9 and 11, respectively. Meanwhile, the EEO of the “water dispersible” TiO2 nanoparticles increases from 608 kW h m3 order1 to 22,601 kW h m3 order1. It is well known that the reduction potential of Cr(VI) to Cr(III) (E0Cr(VI)/Cr(III)) decreases with increasing pH. The difference between E0Cr(VI)/Cr(III) and the conduction band energy level of TiO2 decreases from 1.43 V to 0.84 V when pH increases from 1 to 7 [23]. Therefore, we speculate that the difference between E0Cr(VI)/Cr(III) and the conduction band energy level of TiO2 would decrease with increasing pH from 2 to 11, which leads to high photocatalytic activity of the “water dispersible” TiO2 nanoparticles in acidic condition. In addition, the deposition of Cr(OH)3 on the surface of TiO2 nanoparticles at pH values above 5 may also cause a decrease in photocatalytic activity [1,24]. Fig. 6A shows the effect of initial Cr(VI) ions concentration on the photoreduction efficiency of Cr(VI) ions over the “water

Photoreduction efficiency / %

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Wavelength / nm Fig. 4. (A) UVevis transmission spectra of (a) the “water dispersible” TiO2 nanoparticles dispersion and (b) the P25 slurry; (B) UVevis diffuse reflectance spectra of (a) the “water dispersible” TiO2 nanoparticles dispersion and (b) the P25 slurry.

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higher than that of the “water dispersible” TiO2 nanoparticles dispersion. The reflectance of the P25 slurry is 7.5% at 340 nm while the reflectance of the “water dispersible” TiO2 nanoparticles dispersion is only 0.4%. The transmittivities of the “water dispersible” TiO2 nanoparticles dispersion and the P25 slurry are 0.7% and 0.1% at 340 nm, respectively. More than that, the sum of the reflectance and the transmittivity of the P25 slurry is less than 100% in the visible range. One possible explanation is that there exists relatively strong light scattering because the size of P25 is bigger. These results confirm that the “water dispersible” TiO2 nanoparticles dispersion can absorb UV light more efficiently compared with the P25 slurry. From the practical point of view, the recovery of the “water dispersible” TiO2 nanoparticles is important, because they may cause a secondary pollution if not properly handled. We find that the precipitation of the “water dispersible” TiO2 nanoparticles would happen when the pH value of the “water dispersible” TiO2 nanoparticles dispersion is adjusted to 13 (Fig. S2). Thus, after adjusted the pH value, the “water dispersible” TiO2 nanoparticles can be recovered from the dispersion through filtration. Fig. S3 shows the UVevis spectra of the “water dispersible” TiO2 nanoparticles dispersion and the filtrate after recovery treatment. As can be seen from Fig. S3, the absorbance of the filtrate is about zero in the range of 250 nme400 nm while the dispersion exhibits a strong absorption band with an edge at approximately 380 nm. This phenomenon indicates that the “water dispersible” TiO2 nanoparticles can be recovered from the dispersion, suggesting that the

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pH Fig. 5. (A) Effect of pH on the photocatalytic activity of the “water dispersible” TiO2 nanoparticles; (B) effect of pH on the EEO of the “water dispersible” TiO2 nanoparticles (Photocatalyst dosage: 2 g L1; initial Cr(VI) ions concentration: 40 mg L1, irradiation time: 20 min).

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remediation of Cr(VI) ions wastewater with low concentration. Fig. 7A shows the effect of photocatalyst dosage on the photoreduction efficiency of Cr(VI) ions over the “water dispersible” TiO2 nanoparticles. Fig. 7B shows the effect of photocatalyst dosage on the EEO of the “water dispersible” TiO2 nanoparticles. As can be seen from Fig. 7A, the photocatalytic reduction of Cr(VI) ions is increased from 26.8% to approximate 100% with increasing the photocatalyst dosage from 0.5 g L1 to 4 g L1 after irradiation for 10 min. The pseudo-first-order rate constants are 0.021 min1, 0.067 min1, 0.32 min1, 0.44 min1, and 0.69 min1 when the photocatalyst dosages are 0.5 g L1, 1 g L1, 2 g L1, 3 g L1 and 4 g L1, respectively. Meanwhile, the EEO of the “water dispersible” TiO2 nanoparticles decreases from 9549 kW h m3 order1 to 286 kW h m3 order1. It is interested that the photocatalytic reduction efficiency of Cr(VI) ions over the “water dispersible” TiO2 nanoparticles increases with the photocatalyst dosage increasing. Generally, the photocatalytic efficiency will increase with increasing photocatalyst dosage at the beginning. Then, further increment in the dosage will result in no significant increase in the photocatalytic efficiency due to light scattering, reflection and shading from photocatalyst particles. This phenomenon has been reported by many researchers [1,19,26,27]. The control experimental results show that all of the “water dispersible” TiO2 nanoparticles dispersions are transparent and solution-like when the

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Fig. 6. (A) Effect of initial Cr(VI) ions concentration on the photocatalytic activity of the “water dispersible” TiO2 nanoparticles; (B) effect of initial Cr(VI) ions concentration on the EEO of the “water dispersible” TiO2 nanoparticles (Photocatalyst dosage: 2 g L1; pH: 5, irradiation time: 10 min).

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dispersible” TiO2 nanoparticles. Fig. 6B shows the effect of initial Cr(VI) ions concentration on the EEO of the “water dispersible” TiO2 nanoparticles. It can be found from Fig. 6A that the photocatalytic reduction of Cr(VI) ions is increased from 18.9% to approximate 100% with decreasing the initial Cr(VI) ions concentration from 80 mg L1 to 10 mg L1 after irradiation for 10 min. The pseudofirst-order rate constants are 0.92 min1, 0.32 min1, 0.042 min1, 0.014 min1, and 0.0057 min1 when the initial Cr(VI) ions concentrations are 10 mg L1, 20 mg L1, 40 mg L1, 60 mg L1 and 80 mg L1, respectively. One possible explanation is that higher concentration of the substrate will result in more Cr(VI) ions absorbed on the TiO2 nanoparticles, which makes the adsorption sites decrease so as to cause an inhibitive effect on further photocatalytic reduction of Cr(VI) ions [1]. Moreover, increasing the initial Cr(VI) ions concentration might lead to that the light absorbed by the Cr(VI) ions solution is more than that by the catalyst [25]. Thus, higher concentration of the solution is not beneficial to the photoreduction of Cr(VI) ions. Moreover, we can also find that the EEO of the “water dispersible” TiO2 nanoparticles sharply increases from 215 kW h m3 order1 to 34,755 kW h m3 order1 with increasing the initial Cr(VI) ions concentration from 10 mg L1 to 80 mg L1. These results indicate that the “water dispersible” TiO2 nanoparticles are suitable for the

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Fig. 7. (A) Effect of photocatalyst dosage on the photocatalytic activity of the “water dispersible” TiO2 nanoparticles; (B) effect of photocatalyst dosage on the EEO of the “water dispersible” TiO2 nanoparticles (initial Cr(VI) ions concentration: 20 mg L1, pH: 5, irradiation time: 10 min).

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content of TiO2 nanoparticles is in the range of 0.5e4 g L1. Therefore, the difference in the effects of photocatalyst dosage may be ascribed to elongation of light penetration distance. Based on these experimental results, it can be concluded that the inhibitive effect due to shading from photocatalyst particles can be avoided using the “water dispersible” TiO2 nanoparticles.

[9]

[10]

[11]

4. Conclusion In conclusion, the as-prepared “water dispersible” TiO2 nanoparticles is a highly efficient photocatalyst for the reduction of Cr(VI) ions in water under UV irradiation. Meanwhile, the electrical energy consumption in the photocatalytic processes can be obviously decreased taking advantage of their excellent dispersibility in water. These results provide us with new possibility for finding an efficient photocatalyst for rapid removal of Cr (VI) ions in environmental therapy.

[12]

[13]

[14]

[15]

Acknowledgements [16]

This work was financially supported by the Open Project of Bejing National Laboratory for Molecular Sciences (No. 20140163) and the National Natural Science Foundation of China (No. 21301118).

[17]

[18]

Appendix A. Supplementary data [19]

Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2016.05.004

[20]

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