Electric-Field-Enhanced Photocatalytic Removal of Cr(VI) under

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Electric-field-enhanced photocatalytic removal of Cr(VI) under sunlight of TiO2 nanograss mesh with nondestructive regeneration and feasible collection for Cr(III) Si Chao Xu, Hualin Ding, Shusheng Pan, Yuanyuan Luo, and Guanghai Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01831 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 15, 2016

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ACS Sustainable Chemistry & Engineering

Electric-field-enhanced photocatalytic removal of Cr(VI) under sunlight of TiO2 nanograss mesh with nondestructive regeneration and feasible collection for Cr(III) S. C. Xu,*a H. L. Ding,a S. S. Pan,a Y. Y. Luo,a and G. H. Li*a,b a

Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P.R China b

University of Science and Technology of China, Hefei 230026，P. R. China

Abstract: Rutile TiO2 nanograss was synthesized by a hydrothermal method on Ti mesh. The nanograss is composed of thin TiO2 nanorods with abundantly exposed {110} facets. The TiO2 nanograss on Ti mesh (TNTM) shows an excellent photocatalytic removal of Cr(VI) ions from plating wastewater under sunlight, and the removal capacity can reach as high as 143.8 mg/g. It was found that an external electric field can greatly enhance the removal efficiency of the TNTM, in which the electrons provided by the electric field can effectively neutralize the photoholes and inhibit the recombination of photoelectrons and photoholes. The TNTM can be removed easily after photocatalytic reaction and can be nondestructively regenerated by an electrolyzing process. The Cr(III) ions on the TNTM can be collected by the electrolyzation, effectively avoiding a secondary pollution. Keywords: TiO2, electric field enhancement, nondestructive regeneration, Cr(VI) removal, Cr(III) collection

*

Corresponding author: Fax:+86-551-65591434, E-mail address: [email protected], [email protected]

1 ACS Paragon Plus Environment


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1. Introduction Chromium pollution is one of the most frequent and toxic contaminant in wastewaters arising from various industrial processes like electroplating, mining and leather tanning.1,2 Chromium in water generally exists in two stable oxidation states: hexavalent [Cr(VI)] and trivalent [Cr(III)], and the former is very dangerous than the latter because of its high toxicity, extreme solubility and mobility. Cr(VI) is carcinogenic and potentially mutagenic to human being,3–6 while the Cr(III) is less toxic and can be readily precipitated in neutral or alkaline solution.7 The preferred treatment of Cr pollutant is simultaneous detoxification of Cr(VI) to Cr(III) and adsorption of the latter from water.8,9 Among different detoxifying treatments, semiconductor photocatalysis technology, making use of the clean, safe and “free” solar energy, has received considerable attention in reducing Cr(VI) to Cr(III).10,11 Nevertheless, previous studies focus mainly on semiconductor nanostructures such as nanoparticles, nanocomplex and other disperse powders that require a complex and time consuming separation process to remove photocatalysts from the wastewater.12 The integration of the nanostructured photocatalyst with magnetic substance can facilitate the separation though, the fabrication process generally involves relatively a complex process. On the other hand, the methods on the desorption of Cr(III) ions from photocatalysts and the regeneration of the photocatalysts are also imperfect. The general acid or alkaline treatment will partly destroy the photocatalyst and deteriorate the photocatalytic ability, and the desorbed Cr ions are generally still inside the treating solution, resulting in a secondary pollution.


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Therefore, the development of a simple and easy collection technique for the adsorbed Cr ions and a nondestructive regeneration for the photocatalysts is very important for practical photocatalytic treatment of Cr ions contaminated water. Being having a favorable chemical stability, high photocatalytic activity, low cost and safety to environment, TiO2 has become one of the most frequently used photocatalysts.13–17 Previous studies focus more on anatase TiO2 than rutile as the former is generally accepted having a better photocatalytic activity.18 Low efficiency in using visible light and high recombination rate of photoelectrons and holes that drastically reduces the quantum efficiency are the main drawbacks of anatase TiO2 photocatalyst.19,20 Comparatively, rutile TiO2, with a band gap (3.0 eV) narrower than anatase (3.2 eV), can absorb certain visible light. It was found that rutile TiO2 crystals with an ultra-fine size are photoactive and show a better photocatalytic activity than anatase on the total degradation of phenol.21,22 Our previous study also found that the urchin-like rutile TiO2 with ultrathin nanorods and exposed {110} facets demonstrates an excellent photocatalyst in the removal of Cr(VI) ions.23 Different crystal facets have different energy levels of the conduction and valence band,24 and such energy level difference will drive the electrons and holes to different crystal facets.25 Generally, the photocatalytic activity of a catalyst greatly depends on the separation efficiency of photogenerated electron−hole pairs,26 and the energy level of conduction band electrons plays key roles in the photocatalytic reduction reactions by promoting the transfer of photogenerated electrons from the excited photocatalyst and inhibiting the



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recombination of those photogenerated charge carriers.27,28 The rutile {110} facets have a lower electronic energy level compared with other facets and can serve as effective reduction sites by facilitating photoelectrons transfer to the surface, which can effectively enhance the photocatalytic activity.29,30 In our previous work, the urchin-like rutile TiO2 powders synthesized by a hydrothermal method were dispersed in Cr(VI) solution under sunlight irradiation, the photoelectrons can easily tranfer to the low energy exposed {110} facets of the surface nanorods and effectively reduce the Cr(VI) ions. Unfortunately, continuous agitation is needed in the photocatalytic reaction to prevent the TiO2 powders from precipitating, and a time consuming centrifuging separation process is required to remove the dispersed powders from the wastewater. In addition, many ultrathin TiO2 nanorods will break off from the urchin-like rutile TiO2 powders in following recycle after rough acid treatment, and thus some of the desorbed Cr ions will remain in the treating solution inevitably, resulting in a secondary pollution and drop in the photocatalytic ability of the photocatalyst. Therefore, the photocatalyst with high efficiency, chemical durability, environment-friendly and easy accessibility is essential for commercial applications in the solar-powered sewage treatment.31 In this paper, we report the hydrothermal synthesis and photocatalytic activity of TiO2 nanograss grown on Ti mesh (TNTM). The nanograss composed of separated rutile TiO2 nanorods has plenty of the exposed {110} facets. The TNTM shows an excellent photocatalytic removal of Cr ions from plating wastewater under sunlight and the removal capacity can be


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enhanced by an external electric field. Specific purposes of the present work are three folds. First, to simplify separation process of photocatalysts from the wastewater. Second, to realize nondestructive regeneration and recycle of the photocatalysts. Third, to develop a feasible collection technique for the Cr ions desorbed from the photocatalysts after photocatalytic reaction.

2. Experimental 2.1. Preparation of TNTM photocatalysts All the chemicals were used as-received without further purification. TiOSO4·2H2O (tech. 95 %), HNO3 (68 wt %), absolute alcohol and glycerol (both analytical grade) are purchased from Tianjin Guangfu Fine Chemical Research Institute, China. In a typical procedure, an emulsion containing TiOSO4·2H2O (2 g), absolute alcohol (28.7 mL), deionized water (5.68 mL, purified through a Millipore system) and glycerol (21.6 mL) was stirred for 1 hr and transferred into a 70 mL Teflon-lined autoclave. The Ti mesh (purity: 99.9 % with mesh number of 20, purchased from Baoji soldiers nickel titanium Co., LTD, China) was cut into a rectangle with 3.9 cm in width and 100 cm in length and soaked in 20 % HNO3 solution for 10 hrs. After the Ti mesh was washed with deionized water and absolute alcohol, the rectangle mesh was rolled into a cylinder and placed into the autoclave and immerged inside the emulsion. The autoclave was kept at 180 °C for 10 hrs and then naturally cooling down to room temperature. Finally, the TNTM was taken



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out from the autoclave, washed in an ultrasonic bath with alcohol and dried at 70 °C. The long mesh was cut into short rectangles for the photocatalytic experiments. 2.2. Characterization The final products were characterized by field emission scanning electron microscopy (FESEM, Hitachi SU8020), high resolution electron microscopy (HRTEM, JEM 2010), Grazing Incidence X-ray Diffraction ( GIXRD, X’Pert Pro MPD) and the X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250). The Cr ions concentration was analyzed by ICP emission spectrometer (ICP 6000), and the remaining Cr ions in the reaction solutions were evaluated by UV-Vis adsorption spectrum (SHIMADZU UV-3600). All the photocatalytic reactions were performed under sunlight and the average luminance was evaluated by a digital light meter (TES-1332A). 2.3. Photocatalytic reactions The photocatalytic reactions were carried out using the plating wastewater from a local electroplate plant in Hefei, Anhui province. The plating wastewater mainly contains H2SO4 with a concentration about 7.4 mM (initial pH value of about 1.85) and Cr ions with a concentration of about 6137 ppm (about 5 % Cr(III), measured by precipitating Cr(III) as Cr(OH)3)).15 The TNTM with the size of 3.9 cm in width and 6.2 cm in length (the same size was used hereafter) was added in a quartz glass tube containing 10 mL diluted plating wastewater (25 ppm with pH value


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of 3.16). The solution was irradiated under sunlight (with an average luminance of about 8.45×104 Lux) with tender agitation. After irradiation for a set time, 2 mL solution was temporarily taken out for an immediate UV-Vis spectrum measurement to evaluate the remaining Cr ions and then put back to the solution. To study the influence of pH value on the removal capacity of Cr(VI) ions, five pieces of the TNTMs were respectively added in five quartz glass tubes, each containing 10 mL plating wastewater (25 ppm) with different initial pH values adjusted by either 10 mM H2SO4 or 0.5 M NaOH solution. After photocatalytic reaction for 3 hrs under sunlight (the average luminance of about 7.46×104 Lux), the TNTMs were taken out and the solutions were evaluated by UV-Vis spectrum measurements. The removal capacity was evaluated by using different initial Cr concentrations. Five quartz tubes each containing 10 mL plating wastewater (with Cr ion concertation of 25, 50, 100, 500 and 1000 ppm) were prepared, and after adjusting the initial pH values to about 3, the TNTMs were added. After sunlight illumination for 6 hrs (the average luminance of about 7.84×104 Lux), the TNTMs were taken out and the solutions were used for ICP measurements. To study the recycle ability, the TNTMs after photocatalytic reactions were soaked in a solution containing 50 mM NaCl and electrolyzed at voltage of 0.5 V for 15 min, then washed with deionized water for several times. The removal capacity of the regenerated TNTM was evaluated and compared with the fresh one under the same experimental conditions. The solution



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after electrolyzation was measured by UV-Vis spectrum measurements, and the precipitate in the electrolyte and the powder scraped from the TNTM after photocatalytic reaction were analyzed by X-ray photoelectron spectroscopy. The electric-field-enhanced photocatalytic reactions were performed by applied an external electric field between an enamelled copper wire situated in the center of the rolled TNTM (as anode) and the TNTM itself (as cathod). To optimize the applied voltage, five quartz tubes each containing 10 mL plating wastewater (50 ppm) were prepared, and five voltages were applied. After sunlight illumination for 1 hr (the average luminance of about 8.02×104 Lux), the TNTMs were taken out and the solutions were measured by UV-Vis spectrum measurements. The removal capacity of the electric-field-enhanced photocatalytic at 2 V (kept in dark) was compared with that of the photocatalytic reaction with the initial Cr concentration of 25 ppm.

3. Results and discussion 3.1. Morphology and Structure Fig. 1a shows a typical digital photo of the TiO2 nanograss grown on Ti mesh after 10 hrs hydrothermal reaction. The Ti mesh is composed of cross connected Ti wires with about 250 µm in diameter and 1.2 mm in wire interspace (Fig. 1b). The relative large wire interspace ensures adequate absorption of sunlight of the TiO2 nanograss and free flow of the solution in the TMTM during the photocatalytic reaction. From the corresponding magnified image (Fig. 1c), one can


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see the Ti wires are covered by dense grass-like TiO2 nanorods of about 8 nm in diameter. The SAED pattern of the TiO2 nanorods (the inset in Fig. 1d) can be well-indexed to rutile phase TiO2, and the corresponding HRTEM analysis of the nanorods (Fig. 1d) and the GIXRD pattern of the TNTMs in which all the diffraction peaks can be indexed to rutile phase TiO2 apart from the intensive Ti diffraction peaks from Ti mesh, further prove that the TiO2 nanorods are single crystalline and have abundant exposed {110} facets.

Fig. 1. (a) Digital photo of TiO2 nanograss grown on Ti mesh, FESEM images of a single Ti wire (b) and TiO2 nanograss (c), and (d) HRTEM image of a single nanorod (the inset is the corresponding SAED pattern of the nanograss), (e) GIXRD pattern of the TNTMs.

3.2 Kinetics of the photocatalytic removal of Cr(VI) Time evolution of the photocatalytic removal of Cr(VI) by the TNTM is characterized by



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optical absorption measurements, as shown in Fig. 2. One can see the intensity of Cr ions absorption peaks decreases rapidly and remarkably at the initial period and then further decreases steadily with increasing irradiation time, and almost disappears after 5.5 hrs. The final concentration of the Cr ions remained in the solution after 5.5 hrs sunlight irradiation drops to 0.027 ppm from the initial 25 ppm, and the solution becomes colorless, indicating that the TNTM can almost thoroughly remove Cr(VI) ions from the plating wastewater.

Fig. 2. Absorption spectra of the Cr(VI) solution after photocatalytic reaction for different times, the inset is the corresponding digital photo of the solutions before (1) and after 5.5 hrs (2) photocatalytic reaction.

3.3 Influence of pH value on photocatalytic activity Fig. 3 shows the influence of pH value on the photocatalytic activity of the TNTM. One can see the photocatalytic removal ability of Cr(VI) ions depends strongly on the solution pH value and decreases with increasing pH value. The removal ability changes slightly with increasing pH value from 2.55 to 3.16, and decreases rapidly with further increasing pH value to 4.25. The photocatalytic removal of Cr ions depends on the photocatalytic reduction of Cr(VI) and


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adsorption of the Cr(III) ions. The Cr(VI) ions exist mainly in the form of CrO42−, Cr2O72− or HCrO4− at a low pH value less than 6.5 according to Carlos E. Barrera-Díaz,32 and the surface of TiO2 nanograss is highly protonated and has a strong affinity towards those anions, and thus can enhance the photocatalytic removal of the Cr(VI) ions. With increasing pH value, the surface of TiO2 nanograss will become less positively charged and even negatively charged, and the absorption of CrO42−, Cr2O72− or HCrO4− ions onto TiO2 surface become difficult, leading to the decrease in the photocatalytic removal ability of Cr ions. The optimal pH value of the solution for the removal of Cr(VI) is about 3.

Fig. 3. Absorption spectra of Cr(VI) solution after 3 hrs photocatalytic reaction at different initial pH values.

3.5 Evaluation of Cr(VI) removal capacity of the TNTM Removal capacity was evaluated with different initial Cr(VI) concentrations at pH value of 3 after 6 hrs sunlight illumination. The removal capacity is determined by the following equation: Q = m r /m = (C 0 − C t ) × V/m


(1)


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where Q is the amount of Cr ions reduced and removed by a unit mass TiO2 nanograss (expressed in mg g-1), mr the mass of Cr ions reduced and removed (mg), m the photocatalyst mass (g), C0 the initial concentration of Cr ions (ppm or mg L-1), Ct the concentration of Cr ions (ppm or mg L-1) after 6 hrs photocatalytic reaction and V (L) the volume of the solution from which the photocatalytic reaction takes place, and the result is shown in Fig. 4. The catalyst mass (m) in equation (1) contains only the TiO2 nanograss (without the Ti mesh) that taking part in the photocatalytic reaction and is calculated from the weight difference of the Ti mesh before and after 10 hrs hydrothermal reaction (about 12.5 mg TiO2 nanograsses on the snipped TMTN with 3.9 cm in width and 6.2 cm in length). From Fig. 4 one can see the removal capacity firstly increases rapidly with increasing the Cr(VI) concentration at relatively low level, and then steady increases with Cr(VI) concentration at high level. The removal quantity of Cr ions can reach as high as 143.8 mg/g for the initial Cr concentration of 1000 ppm after 6 hrs sunlight illumination. The nonlinear fitting of the data shown in Fig. 4 obeys the following equation: Q = A exp (－ C 0 /B) + Q max

(2)

where the parameters of A, B and Qmax are displaced in Fig. 4. The fitting result demonstrates that the theoretical maximum removal capacity is about 156.7 mg/g. The high removal capacity is considered firstly due to the high utilization of the photocatalyst, in which the thin TiO2 nanograss almost straightly grows out from the Ti wires and has enough interspaces between individual TiO2 nanorods, ensuring completely contact of every TiO2 nanorod with the Cr ions.


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Second, the exposed {110} facets of the rutile TiO2 nanorods have a lower electronic energy level compared with the {001} facets which directly grow on the Ti mesh and can serve as effective reduction sites by promoting the transfer of photoelectrons to surface, and the ultrathin single crystalline structure of the nanorods further facilitates the separation of electrons and holes by accelerating the electrons transfer to the {110} facets passing through such a narrow interspace, preventing them from recombination, and leading to a structurally enhanced photocatalytic redution.23 On the other hand, the relatively large wire interspaces of the Ti mesh, which guarantees not only adequate absorption of sunlight of every TiO2 nanorod and but also free flow of Cr ions solution among the TiO2 nanograss, also contributes to the enhanced removal capacity of the Cr ions.

Fig. 4. Cr ions removal capacity of the TNTM after 6 hrs photocatalytic reaction with different initial Cr(VI) concentrations at initial pH value of about 3.

3.6 Recycle ability of TNTM The stability and regeneration of the TNTM is very important for practical application in the



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Cr(VI) removal. Ideal regenerating process of the photocatalyst should be nondestructive and easily performed. The XPS analysis provides a helpful hint in answering this demand. The wide peaks of Cr 2p adsorbed on the TNTM surface could be fitted with two strong peaks at binding energy of ∼577.9 and ∼586.8 eV as well as two weak peaks at ∼581.3 and ∼591.7 eV (Fig. 5a), assigned to Cr(III) and Cr(VI), respectively, suggesting that most of the adsorbed Cr ions are Cr(III) ions. To desorb these Cr cations the photocatalyzed TNTM was electrolyzed in a NaCl aqueous solution at a voltage of 0.5 V. The color of the electrolytic solution changes from colorless to pale green after 15 min electrolyzation (Figs. 5c and 5d) and finally to darkness after electrolyzation for several times. The UV-Vis measurements of the electrolytic solution demonstrate that the Cr ions absorption peaks begin to appears after first time electrolyzation and become strong after following repeat usage compared to the initial electrolytic solution (Fig. 5e), indicating the adsorbed Cr ions can be desorbed from the TNTM and resolve or deposit in the electrolytic solution (Fig. 5d). The XPS analysis reveals that most of the precipitate in the electrolytic solution are also Cr(III) ions, as shown in Fig. 5b, indicating that the Cr ions are removed from the TNTM without being reduced. This result indicates that the Cr ions on the TNTM can be easily collected by electrolyzation.


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Fig. 5. (a) and (b) XPS spectra of Cr 2p absorbed on the surface of the TNTM and the precipitate in the electrolyte, (c) and (d) digital photoes of the electrolyte solutions before and after five times electrolysis, (e) absorption spectra of electrolyte solution before and after different circular electrolysis.

The removal ability of the regenerated TNTM as a function of the electrolyzing time was evaluated, as shown in Fig. 6a, in which the removal ability increases with increasing the electrolyzing time and almost reaches a constant value after 15 min electrolyzation. The removal ability of the regenerated TNTM is only slightly lower than that of the fresh one, even after five cycles, as shown in Fig. 6b. The recovery in the removal ability of the regenerated TNTM is due to our friendly regenerating process that only releases the adsorbed Cr ions without destroying the structure of the TiO2 nanograss. The FESEM observations before and after five cycles shown in Figs. 6c and 6d further proved that no morphologic changes occur in the TiO2 nanograss, indicating the electrolyzation is a nondestructive process, and the TNTM has a high durability and can be regenerated by the electrolyzation process.



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Fig. 6. (a) Absorption spectra of Cr(VI) solution after 3 hrs photocatalytic reaction by the regenerated TNTM with different electrolyzing time, (b) absorption spectra of Cr(VI) solution after 4 hrs photocatalytic reaction of the regenerated TNTM in different cycles, (c) and (d) FESEM images of the TiO2 nanograss before and after five cycles photocatalytic reaction.

Fig. 7. Absorption spectra of Cr(VI) solution after 1 hr electric-field-enhanced photocatalytic reaction with different applied voltage (the inset is the corresponding digital photo of the electric-field-enhanced photocatalytic reaction device).

The electric-field-enhanced photocatalytic reaction was performed as shown in the inset of Fig. 7. When an external voltage was applied to the TNTM, the intensity of Cr ions solution absorption peaks becomes weaker than that without voltage at the same reaction time, indicating


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an enhanced photocatalytic removal ability with the applied voltage, as shown in Fig. 7. From Fig. 7 one also can see the intensity of Cr ions absorption peaks gradually decreases with increasing the applied voltage and reaches a minimum value at the voltage of 2 V, and then increases with further increasing the voltage, indicating that there is an optimal applied voltage. The applied voltage affects the photocatalytic reduction of the Cr(VI) ions and adsorption of the Cr(III) ions. Unlike most of the reported photoelectrocatalytic reduction of Cr(VI) based on TiO2, where the UV radiation source was necessary due to the limitation of the applied anatase TiO2 band gap and the photoelectrons in the conduction (CB) of the TiO2 photoanode are transferred through an external circuit to the cathode by the applied voltage and then reduce the Cr(VI),33-35 the TNTM covered with rutile TiO2 can makes use of the visible light and connects directly with the cathode. As discussed in the introduction part, the rutile TiO2 {110} facets have a lower electronic energy level compared with other facets, especially with the {001} facets, such energy level difference will apt to drive more photoelectrons trasnfer to the {110} facets and photoholes to {001} facets which directly grow on the Ti mesh. The electrons from the cathode (the TNTM) upon an applied voltage can neutralize some of those photoholes and inhibit them recombining with photoelectrons, leading to an enhancement in the photocatalytic reduction of the Cr(VI) ions. The higher the applied voltage the more the electrons, and thus the higher the photocatalytic removal ability. On the other hand, with increasing the applied voltage, the TiO2 nanograss will become more negatively charged, which will prevent CrO42−, Cr2O72− or HCrO4− ions from being



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absorbed onto the surface of the TNTM, and as a result, the photocatalytic removal capacity decreases at eccessive high applied voltage. The optimal voltage is about 2 V.

Fig. 8. Absorption spectra of the Cr(VI) solution of the TNTM with a 2 V applied voltage under sunlight and in darkness with different times.

Time evolution of the electric-field-enhanced photocatalytic removal of Cr ions by TNTM is characterized by optical absorption measurements, as shown in Fig. 8. The intensity of Cr ions absorption peaks decreases rapidly with time and almost disappears just after 2 hrs for the electric-field-enhanced photocatalytic reaction, while the purely photocatalytic reaction needs nearly 5.5 hrs to achieve a complete removal of Cr ions (Fig. 2). The removal efficiency of Cr(VI) ions for the purely photocatalytic reaction is also remarkably at the initial time, because of the existence of abundant {110} facets in the TiO2 nanograsses that serves as effective reduction and adsorption sites, but drops obviously with time in comparison with that for the photocatalytic reaction enhanced by an external electric-field. This result clearly demonstrates that an applied external voltage can greatly enhance the photocatalytic removal ability of the TNTM. On the


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other hand, the electrons from the applied voltage also contribute to the reduction of the Cr(VI) ions, as can be seen from Fig. 8, in which the intensity of Cr ions absorption peaks also slightly decreases with increasing applied voltage when the test is performed in darkness.

4. Conclusion In summary, rutile TiO2 nanograss composed of thin TiO2 nanorods with exposed {110} facets has been synthesized by a hydrothermal method on Ti mesh. The TNTM shows an excellent photocatalytic removal of Cr ions from plating wastewater under sunlight and has a romoval capacity of as high as 143.8 mg/g. The high removal capacity is due to the special morphology and structure of the TNTM, which can highly utilize the photocatalyst and sunlight, and provides abundantly exposed {110} facets serving as effective reduction sites and facilitating transfer of the photoelectrons to the surface. The removal capacity of the TNTM can be enhanced by an external electric field by providing electrons to neutralize photoholes and to inhibit the recombination of photoelectrons and photoholes. After photocatalytic reaction, the TNTM can be nondestructively regenerated by a facile electrolyzation method. The Cr ions on the TNTM can be collected easily, avoiding a secondary pollution. The easy fabrication, high removal capacity of Cr(VI) ions, electric-field-enhanced photocatalyzation and simple nondestructive regeneration as well as facile collation of Cr(III) ions will make TNTM a very suitable photocatalytic architecture for practical application in Cr ions wastewater treatment.



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Acknowledgments The authors gratefully acknowledge the financial support from National Basic Research Program of China (Grant no. 2013CB934304) and National Natural Science Foundation of China (Grant no. 51502298).

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Electric-field-enhanced photocatalytic removal of Cr(VI) under sunlight of TiO2 nanograss mesh with nondestructive regeneration and feasible collection for Cr(III) S. C. Xu, H. L. Ding, S. S. Pan, Y. Y. Luo, and G. H. Li

TOC

Synopsis TNTM can be easily separated and nondestructively regenerated after photocatalytic reaction and Cr(III) ions can be collected without secondary pollution.


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Fig. 1. (a) Digital photo of TiO2 nanograss grown on Ti mesh, FESEM images of a single Ti wire (b) and TiO2 nanograss (c), and (d) HRTEM image of a single nanorod (the inset is the corresponding SAED pattern of the nanograss), (e) GIXRD pattern of the TNTMs. Fig. 1

ACS Paragon Plus Environment


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Fig. 2. Absorption spectra of the Cr(VI) solution after photocatalytic reaction for different times, the inset is the corresponding digital photo of the solutions before (1) and after 5.5 hrs (2) photocatalytic reaction. Fig. 2


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Fig. 3. Absorption spectra of Cr(VI) solution after 3 hrs photocatalytic reaction at different initial pH values. Fig. 3 59x45mm (600 x 600 DPI)



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Fig. 4. Cr ions removal capacity of the TNTM after 6 hrs photocatalytic reaction with different initial Cr(VI) concentrations at initial pH value of about 3. Fig. 4 59x45mm (600 x 600 DPI)


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Fig. 5. (a) and (b) XPS spectra of Cr 2p absorbed on the surface of the TNTM and the precipitate in the electrolyte, (c) and (d) digital photoes of the electrolyte solutions before and after five times electrolysis, (e) absorption spectra of electrolyte solution before and after different circular electrolysis. Fig. 5



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Fig. 6. (a) Absorption spectra of Cr(VI) solution after 3 hrs photocatalytic reaction by the regenerated TNTM with different electrolyzing time, (b) absorption spectra of Cr(VI) solution after 4 hrs photocatalytic reaction of the regenerated TNTM in different cycles, (c) and (d) FESEM images of the TiO2 nanograss before and after five cycles photocatalytic reaction. Fig. 6


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Fig. 7. Absorption spectra of Cr(VI) solution after 1 hr electric-field-enhanced photocatalytic reaction with different applied voltage (the inset is the corresponding digital photo of the electric-field-enhanced photocatalytic reaction device). Fig. 7



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Fig. 8. Absorption spectra of the Cr(VI) solution of the TNTM with a 2 V applied voltage under sunlight and in darkness with different times. Fig. 8 59x45mm (600 x 600 DPI)


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TNTM can be easily separated and nondestructively regenerated after photocatalytic reaction and Cr(III) ions can be collected without secondary pollution. TOC.tif


Electric-Field-Enhanced Photocatalytic Removal of Cr(VI) under

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