A novel visible light-driven TiO2 photocatalytic reduction for hexavalent chromium wastewater and mechanism

Titanium dioxide (TiO2) photocatalyst was prepared with a sol-gel method and its characterizations were analyzed TiO2 photocatalytic reduction of Cr 6þ was investigated in visible light irradiation and reduction mechanisms were calculated. Prepared TiO2 is anatase with a bandgap of about 2.95 eV. Experimental results display that almost 100% of Cr6þ is removed by visible light-driven TiO2 photocatalytic reduction after 120 min when Cr2O7 2 initial concentration is 1.0 mg·L , TiO2 dosage is 1.0 g·L , and pH value is 3. In acidic aqueous solution, HCrO4 is the dominant existing form of Cr6þ and is adsorbed by TiO2, forming a complex catalyst HCrO4 /TiO2 with an increase in wavelength to the visible light zone, demonstrated by UV–Vis diffuse reflection spectroscopy. Based on X-ray photoelectron spectroscopy data, it can be deduced that Cr6þ is adsorbed on the surface of TiO2 and then reduced to Cr3þ in situ by photoelectrons. Self-assembly of HCrO4 /TiO2 complex catalyst and self-reduction of Cr6þ in situ are the key steps to start the visible light-driven TiO2 photocatalytic reduction. Furthermore, TiO2 photocatalytic reduction of Cr 6þ fits well with pseudo-first-order kinetics and has the potential application to treat chemical industrial wastewater.


INTRODUCTION
Chromium pollutants mainly come from the industries of mining, metallurgy, electroplating, leather manufacture, bichromate, and chromium slag treatment (Zhang et al. ; Yang et al. ). However, they are used so widely that many tremendous environmental contaminations (Gheju & Balcu ; Ravindra & Mor ; Wang et al. ) are caused. Cr 6þ is the dominant existing form of chromium compounds and is easily absorbed by bodies. Many international investigations have found that some serious pathological lesions occur not only in the skin (Yu et al. ) but also in other organs caused when people are exposed to Cr 6þ -rich environment for a long time (Guo et al. ; Yoshinaga et al. ). There are some methods for wastewater containing Cr 6þ , such as adsorption or biosorption (Deveci & Kar ; Vendruscolo et al. ; Yao et al. ; Ayub et al. ), membrane separation (Habibi et al. ), electrolysis (Sarahney et al. ), and chemical reagents (Sheikhmohammadi et al. ). Adsorption is a convenient way for Cr 6þ removal (Gong et al. ), but the trace of absorbents should be noticed; otherwise, secondary pollution may be more serious. To avoid the potential risk of absorption, biosorption-biotransformation integrated processes where concentrated Cr 6þ is reduced to non-toxic Cr 3þ are developed (Jobby et al. ). However, the running environment and technical parameters are more strict than those of other technologies. Membrane separation can efficiently remove Cr 6þ from wastewater; however, membrane assembly is easily contaminated or blocked, and so how to efficiently, safely, and economically remove Cr 6þ becomes a research focus.
Semiconductor photocatalytic reaction is valued for its powerful redox ability, whether oxidation of most organics (Zhao et al. ) or reduction of variable valence heavy metal ions. Titanium dioxide (TiO 2 ) photocatalytic reduction for trace Cr 6þ has received researchers' attention (Testa et al. ; Yang et al. ) for its favorable chemical property, high stability, and low cost (Hoffmann et al. ). The absorption bandgap of pure TiO 2 (E g ¼ 3.2 eV) lies in the ultraviolet light zone (λ ¼ 1240/E g ¼ 385 nm) (Fujishima et al. ), and it has no ability to oxidize or reduce pollutants in visible light irradiation. TiO 2 modification is a good way to achieve the visible light-driven photocatalytic reaction because the spectrum scope of modified TiO 2 transfers from the ultraviolet light region to the visible light zone. Modified TiO 2 photocatalyst and its application in wastewater treatments have been greatly developed recently. Doping non-ions elements and constructing heterojunctions are both effective methods to achieve TiO 2 photocatalytic performance in visible light irradiation. Chatterjee & Dasgupta () discussed TiO 2 visible lightassisted degradation of organic pollutants and mechanism, and there is more research into TiO 2 photocatalytic degradation for organics in visible light irradiation. There are many investigations into reduction of Cr 6þ with TiO 2 ultraviolet photocatalysis (Zhou et al. ) or oxidation for organics with TiO 2 or modified TiO 2 (Asgari et al. ) or TiO 2 -integrated process (Zhao et al. , , ); however, the study about direct reduction of Cr 6þ with pure TiO 2 in visible light is scarce. Our previous experiments have shown that in visible light irradiation, Cr 6þ is reduced to Cr 3þ by TiO 2 prepared by a sol-gel method. To investigate TiO 2 photocatalytic reduction of Cr 6þ , process reactions and reduction mechanisms are analyzed in this work.

Synthesis of TiO 2
TiO 2 was prepared via a sol-gel method where titanium butoxide (Ti(OCH 2 CH 2 CH 2 CH 3 ) 4 ) was used as the precursor. Detailed process is as follows: 100 mL ethanol and 10 mL precursor were mixed together for 20 min at room temperature, forming a homogeneous mixture; 15 mL H 2 O and 4 mL CH 3 COOH were simultaneously added into the above-mentioned mixture, stirred for 40 min at a constant speed to obtain a mixture with a pH of about 4. The mixture was put into the oven and dried for a set time at 105 C, to obtain the amorphous TiO 2 . Finally, to reach the crystal phase transformation, amorphous TiO 2 was put into a boxtype resistance furnace where a gradient temperature was preset. Temperature was raised from room temperature to 250 C at a heating rate of 5 C·min À1 and kept for 1 h at 250 C, and then increased from 250 C to 600 C at the same heating rate and maintained for 1 h at 600 C.

TiO 2 photocatalytic reduction of Cr 6þ
TiO 2 photocatalytic reduction of Cr 6þ was conducted in a cylindrical reactor equipped with a 150 W spherical xenon lamp (Shellett Photoelectric Technology Co. Ltd, China). A filter plate was used to remove ultraviolet light and 80 mW·cm À1 irradiation intensity was measured using an illumination photometer. Then 200 mL of aqueous solution containing 1.0 mg·L À1 Cr 2 O 7 2À and 1.0 g·L À1 TiO 2 powder was transferred into the reactor and stirred at a speed of 100 r·min À1 for 60 min, to obtain Cr 6þ adsorption equilibrium on the surface of TiO 2 and the inside of the reactor in a dark environment. After adsorption equilibrium, the light resource was turned on and photocatalytic reduction was started. Samples were removed from the same place in reactor at given time intervals and the concentration of Cr 6þ was analyzed using diphenylcarbazide colorimetric method controlled by a UV-Vis spectrophotometer (UV-1900, Shimadzu). Total chromium was analyzed with a graphite furnace atomic absorption spectroscopy (AA4590, Shimadzu).

Characterizations of TiO 2
To determine the crystal phase of as-prepared TiO 2 , XRD pattern was performed and results are shown in Figure 1 (301) planes of anatase phase, suggesting that as-prepared TiO 2 is clear and perfect, which is in accordance with the research reported by Wang et al. (). SEM images of as-prepared TiO 2 are shown in Figure 1(b). It seems that TiO 2 looks like some uniform microspheres. The average size of as-prepared TiO 2 is about 78 nm, according to Debye-Scherrer formula (D ¼ Kλ/βcosθ), where K is a constant of 0.59, λ is irradiation wavelength of 0.15406 nm, β is half-maximum of (101) obtained via XRD software, and θ is the diffraction angle. Furthermore, the surface of TiO 2 is not smooth but layered, which is good for the adsorption and photocatalytic reduction of Cr(VI), compared to the smooth surface.

Effects of TiO 2 dosage and Cr 2 O 7 2À concentration
The effect of TiO 2 dosage on photocatalytic reduction of Cr 6þ was investigated and results are displayed in Figure 2(a). It is observed that photocatalytic reduction of Cr 6þ increases initially and then decreases with the enhancement of TiO 2 , and the optimum is 1.0 g·L À1 . The reason is that when TiO 2 dosage is lower than 1.0 g·L À1 , more Cr 6þ ions are adsorbed on the TiO 2 surface with the enhancement of catalyst dosage, resulting in high reduction; however, when the dosage is higher than 1.0 g·L À1 , the homogeneous solution becomes murky with the increase in catalyst dosage, leading to a reflection of light, meaning that only the surface layer Cr 6þ aqueous solution receives light irradiation, resulting in low photocatalytic reduction efficiency. The effect of Cr 2 O 7 2À concentration on photocatalytic reduction of Cr 6þ was studied and results are shown in Figure 2 anions participating in photocatalytic reduction reduces for the serious scattering of visible light.

Effect of pH
The effect of pH on photocatalytic reduction of Cr 6þ was studied under visible light and results are displayed in Figure 3(a). It is observed that photocatalytic reduction efficiency decreases with the enhancement of pH value. In TiO 2 photocatalytic reduction of Cr 6þ , pH not only affects the charge property of TiO 2 but also influences the existing form of Cr 6þ species. Acharya et al. () found that the existing form of Cr 6þ strictly depends on pH value: H 2 CrO 4 molecule (pH < 2), HCrO 4 À (2 < pH < 7) and Cr 2 O 7 2À (pH >7). The isoelectric point of photocatalyst varies depending on preparation conditions, and is particularly affected by pH value (Gumy et al. ). The isoelectric point of as-prepared TiO 2 was tested using an electrochemical method controlled by a zeta potentiometer and was found to be about 5. The surface of TiO 2 is positively charged when the pH value is lower than 5 and negatively charged when the pH value is higher than 5. In TiO 2 photocatalytic reduction, the main form of Cr 6þ is HCrO 4 À when the pH value ranges from 2 to 5. In this situation, the TiO 2 surface is positive, and more HCrO 4 À anions are adsorbed onto the TiO 2 surface due to coulomb attraction forces when the pH value declines gradually. The adsorbed HCrO 4 À ions are then reduced to Cr 3þ in situ by reductive substances. When the pH value increases from 5 to 7, the dominant existing form of Cr 6þ is still HCrO 4 À ; however, the negative charge property of the TiO 2 surface becomes stronger, which results in the adsorption of HCrO 4 À anions on the catalyst surface and so the photocatalytic reduction declines. Adsorption is a key step for photocatalytic reactionsthe stronger the adsorption, the higher the photocatalytic reduction efficiency.
HCrO 4 À ions are reduced to Cr 3þ in TiO 2 photocatalytic reduction, consuming lots of hydrogen ions, as the following equations demonstrates: Figure 3(b) records the changes of pH value and Cr 6þ concentration. From this figure, it can be seen that the pH value enhances from 3.0 to 3.3 and the Cr 6þ concentration declines from 0.35 to 0.017 mg·L À1 after 120 min. The concentration of generated Cr 3þ ions is 0.35 mg·L À1 at the same reaction time, theoretically. In fact, the actual Cr 3þ may be lower than the theoretically concentration because some produced Cr 3þ is still adsorbed on the TiO 2 surface, as shown by XPS analysis. On the other hand, dissolved Cr 3þ may be precipitated to Cr(OH) 3 by OH À anions if the mathematical product of Cr 3þ and (OH À ) 3 is higher than the solubility of Cr(OH) 3 (K sp ¼ 6.31 × 10 À31 ), as in the following chemical reaction: In this experiment, it is presumed that all HCrO 4 À ions are reduced to Cr 3þ and released into solution, the maximum concentration of Cr 3þ reached is at 2=294 × 10 À3 mol·L À1 (initial concentration of K 2 Cr 2 O 7 is 1.0 mg·L À1 ). At the same time, if the H þ ions are assumed to be about 2 × 7=294 × 10 À3 mol·L À1 according to Equation (R1), then the OH À concentration increases to 10 À14 =(10 À3 À 2 × 10 À3 × 7=294) mol·L À1 according to the ion product constant of water. The product of Cr 3þ and (OH À ) 3 is 7 × 10 À39 , which is much lower than 6.31 × 10 À31 . This means that Cr 3þ cannot be precipitated in the form of Cr(OH) 3 in this experiment.

Effect of atmospheres and scavengers
Effects of N 2 , O 2 , and air bubbles on TiO 2 photocatalytic reductions were studied and the results are shown in Figure 4(a). It is observed that the order of reduction ability of Cr 6þ is N 2 > air > O 2 , suggesting that N 2 is positive for reduction and O 2 is negative for it. This is probably because the amount of O 2 molecules consume photoinduced electrons to produce a superoxide radical · O 2 À , displayed in the following reaction: The production of ·O 2 À aggravates a tense competition between O 2 and Cr 6þ with photoinduced electrons, and only some of the photoinduced electrons are involved in the reduction of Cr 6þ , inhibiting photocatalytic reduction efficiency. On the contrary, bubbling N 2 can greatly drive dissolved O 2 which is an effective photoinduced electron scavenger from aqueous solution, and as a result, more photoinduced electrons can take part in the reduction of Cr 6þ in this situation, compared with those in O 2 or air atmospheres. In fact, oxidation-reduction is a concurrent reaction in the TiO 2 photocatalytic process. In this work, TiO 2 photocatalytic reduction conducted by photoinduced electrons was used to treat Cr þ6 . A partial oxidation reaction caused by photoinduced holes or hydroxyl radicals occurs, which consumes a lot of photoinduced electrons if there is no additional scavenger of holes, leading to low photocatalytic efficiency.
To inhibit the recombination of photoinduced carriers and deduce photocatalytic reduction pathway, methanol acting as a scavenger of holes was added to system. The influence of methanol on TiO 2 photocatalytic reduction was determined and the results are displayed in Figure 4(b). It is observed that the photocatalytic reduction efficiency of Cr 6þ increased and it increases with an increase in methanol dosage. Well known, photoinduced holes are easily captured by methanol (Gumy et al. ), hence more photoinduced electrons can participate in the reduction of Cr þ6 instead of recombination of carriers, compared with those in the absence of methanol, which verifies that Cr 6þ is removed via photocatalytic reduction channel.

Photocatalytic reduction mechanism
The HCrO 4 À anion is the dominant existing form when the pH is between 2 and 7. In this situation, adsorption of HCrO 4 À spontaneously happens on the surface of TiO 2 , forming a narrow bandgap HCrO 4 /TiO 2 complex catalyst, which is proven by UV-Vis DRS analysis. Photoinduced electrons are generated on the HCrO 4 À /TiO 2 conduction band in visible light irradiation and are involved in the reduction of Cr 6þ . To verify this hypothesis, XPS and UV-Vis DRS were both conducted and the results are shown in Figure 5. Figure 5(a) displays the XPS spectra of three samples. The first sample is as-prepared TiO 2 powder, named as TiO 2 ; the second is TiO 2 powder which is soaked in 1.0 mg·L À1 Cr 2 O 7 2À aqueous solution for 1 h in dark and then dried at 100 C, named as TiO 2 before reaction; and the third is TiO 2 powder which is recovered from the reduction system after 120 min, named as TiO 2 after reaction. It is apparent that there are three characteristic peaks in the spectrum of TiO 2 with binding energies of 530.2 eV, 459.5 eV, and 285.7 eV, and they are assigned to O1s, Ti2p, and C1s elements, respectively. The peak of C1s is possibly caused by pollution. It is found that Cr2p intensity in the spectrum of TiO 2 before reaction (blue line) is stronger than that of TiO 2 (black line), suggesting that some HCrO 4 À ions are adsorbed onto the TiO 2 surface in dark conditions. However, it is surprising that the Cr2p peak almost disappears in the spectrum of TiO 2 after reaction (red line), revealing that adsorbed HCrO 4 À ions are reduced under visible light irradiation. Three continuous steps including adsorption, reduction, and desorption are involved in the reduction of Cr 6þ . These steps continue until almost all HCrO 4 À anions are reduced. ] ¼ 1.0 mg·L À1 , pH ¼ 3, P ¼ 150 W.
Figure 5 | (a) XPS spectra of three samples (TiO2, TiO2 before, and TiO2 after reaction); (b) Ti2p XPS spectrum and separation peak at the binding energy of 459.5 eV for two samples (TiO2 before and TiO2 after reaction); (c) Cr2p XPS spectrum and separation peak at the binding energy of 579.7 eV for the sample (TiO2 after reaction), and the inset is the Cr2p XPS spectrum of the sample (TiO 2 before reaction); (d) O1s XPS spectra and separation peak at the binding energy of 530.2 eV for two samples of TiO 2 after reaction and TiO 2 before reaction (the inset picture); (e) UV-Vis diffuse reflectance spectra of TiO 2 and TiO 2 before reaction.
The peak of Ti2p binding at 459.5 eV was fitted using XPS software and results are shown in Figure 5(b). One peak appears at the binding energy of 464.4 eV which is caused by Ti2p 1/2 and the other appears at 458.8 eV which is caused by Ti2P 3/2 . It is found that Ti 4þ is the only chemical form during the whole photocatalytic reduction, demonstrating that TiO 2 keeps a good structural stability. The fitting of Cr2p binding at 579.7 eV was also analyzed and results are shown in Figure 5(c). The spectrum of Cr2p marked with red diamonds is divided into two individual spectra marked with green and blue curves, respectively. The blue line represents the spectrum of Cr2p 1/2 and it belongs to Cr 6þ with a characteristic peak at 579.7 eV. The green line represents the spectrum of Cr2p 3/2 and is assigned to Cr 3þ with the characteristic peak at 576.6 eV. It is concluded that Cr 3þ is the only reductive product. At the same time, the fitting of Cr2p in the spectrum before photocatalytic reaction was simulated, but there is only one Cr2p 1/2 peak caused by Cr 6þ , shown in Figure 5(c). This shows that HCrO 4 À is intitially adsorbed on the surface of TiO 2 before photocatalytic reaction and is then reduced in situ to Cr 3þ in visible light irradiation. The quantities of Cr 6þ and Cr 3þ adsorbed on the TiO 2 surface are calculated and exported using software. Molar ratio of Cr 6þ :Cr 3þ is about 1:4, indicating that HCrO 4 À is reduced to Cr 3þ and that Cr 3þ ions still adsorb on the surface of TiO 2 catalyst. Figure 5(d) displays the fitting of O1s spectra in two samples of TiO 2 after and before reaction (inset figure), respectively. It is observed that O1s binding at 530.2 eV can be divided into two peaks. One peak binding at 530.0 eV (red curve) is assigned to the lattice oxygen coming from TiO 2 and HCrO 4 À , and the other binding at about 531.4 eV (green curve) probably belongs to hydroxyl oxygen coming from the chemical adsorption of oxygen, which is in accordance with the conclusion by Yang et al. (). It is obvious that the hydroxyl oxygen is higher on the TiO 2 surface after the reaction than before reaction, proving that O 2 or OH À or produced ·O 2 À adsorbs on the TiO 2 surface during photocatalytic reduction. Figure 5(e) shows the absorption bandgap of pure TiO 2 and TiO 2 before photocatalytic reaction. The absorption of pure TiO 2 focuses on the ultraviolet zone and that of TiO 2 /HCrO 4 À increases to visible light scope, proving that TiO 2 /HCrO 4 À can produce photoinduced carriers irradiated by visible light. Based on the above analysis, visible light-driven TiO 2 photocatalytic reduction of Cr 6þ was explored. The mechanism involves four major pathways: (1) adsorption of HCrO 4 À on the surface of TiO 2 ; (2) production of photoinduced electrons under visible light irradiation; (3) reduction of Cr 6þ to Cr 3þ by photoinduced electrons; and (4) desorption of Cr 3þ . The cycle of adsorption ! reduction ! desorption continues until almost all the HCrO 4 À is reduced.

Life span, kinetics and application
TiO 2 was reused eight times under the same experimental conditions and the results are shown in Figure 6(a). It is observed that the TiO 2 photocatalytic reduction efficiency does not decrease with recycled times, showing that TiO 2 has a satisfied structural stability and a potential application for use in wastewater treatment. Photocatalytic reduction kinetics of Cr 6þ with TiO 2 was investigated and results are shown in Figure 6(b) and 6(c). The photocatalytic reduction fits well with pseudo-first-order kinetics when Cr 2 O 7 2À initial concentration ranges from 0.5 to 5.0 mg·L À1 , and rate constants decrease with the enhancement of Cr 2 O 7 2À initial concentration. The functional relationship between first-order kinetic rate constants and Cr 2 O 7 2À initial concentration was simulated and shown in Figure 6(c). Pseudo-first-order kinetics constant (k) can be described as: k ¼ 0.023 C À1.42 , where C is the Cr 2 O 7 2À initial concentration. Hence, we can obtain the corresponding k value at a certain Cr 2 O 7 2À initial concentration according to this simulation.
To investigate the application of TiO 2 photocatalytic reduction in real industrial wastewater containing Cr 6þ , a small-scale static test was carried out in the same reactor. Properties, including Cr 2 O 7 2À initial concentration and pH of chemical wastewater, are listed in Table 1, and experimental results are shown in Figure 6(d). It is observed that removal efficiencies of Cr 6þ in the simulation experiment and small-scale static test are 100% and 93%, respectively, suggesting that TiO 2 photocatalytic reduction can be used to treat wastewater containing Cr 6þ . The slight decline of Cr 6þ removal may be caused by co-existing ions in chemical wastewater.

CONCLUSIONS
Almost 100% of Cr 6þ is reduced to Cr 3þ in situ by visible light-driven TiO 2 photocatalytic reduction under the optimal experimental conditions. Photoinduced electrons are the major reductive substance of Cr 6þ , and the mechanism involves adsorption of HCrO 4 À , reduction of Cr 6þ , and desorption of Cr 3þ . The TiO 2 photocatalyst has a reliable lifespan for reduction Cr 6þ under visible light irradiation and its  Na þ NO3 À Cl À 1 mg·L À1 5 4 20 C 160 m·sm À1 10 À3 mg·L À1 10 À3 mg·L À1 10 À3 mg·L À1 10 À3 mg·L À1