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quantity will increase with the applied current density. Fig. 5 a Setup of the photoelectrocatalytic system: 1 CS300 electrochemical workstation, 2 halogen lamp, 3 ...
Qin et al. Nanoscale Research Letters (2016) 11:32 DOI 10.1186/s11671-016-1248-5

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Efficiently Visible-Light Driven Photoelectrocatalytic Oxidation of As(III) at Low Positive Biasing Using Pt/TiO2 Nanotube Electrode Yanyan Qin, Yilian Li*, Zhen Tian, Yangling Wu and Yanping Cui*

Abstract A constant current deposition method was selected to load highly dispersed Pt nanoparticles on TiO2 nanotubes in this paper, to extend the excited spectrum range of TiO2-based photocatalysts to visible light. The morphology, elemental composition, and light absorption capability of as-obtained Pt/TiO2 nanotubes electrodes were characterized by FE-SEM, energy dispersive spectrometer (EDS), X-ray photoelectron spectrometer (XPS), and UV-vis spectrometer. The photocatalytic and photoelectrocatalytic oxidation of As(III) using a Pt/TiO2 nanotube arrays electrode under visible light (λ > 420 nm) irradiation were investigated in a divided anode/cathode electrolytic tank. Compared with pure TiO2 which had no As(III) oxidation capacity under visible light, Pt/TiO2 nanotubes exhibited excellent visible-light photocatalytic performance toward As(III), even at dark condition. In anodic cell, As(III) could be oxidized with high efficiency by photoelectrochemical process with only 1.2 V positive biasing. Experimental results showed that photoelectrocatalytic oxidation process of As(III) could be well described by pseudo-first-order kinetic model. Rate constants depended on initial concentration of As(III), applied bias potential and solution pH. At the same time, it was interesting to find that in cathode cell, As(III) was also continuously oxidized to As(V). Furthermore, high-arsenic groundwater sample (25 m underground) with 0.32 mg/L As(III) and 0.35 mg/L As(V), which was collected from Daying Village, Datong basin, Northern China, could totally transform to As(V) after 200 min under visible light in this system. Keywords: Pt/TiO2 nanotubes, As(III), Anode electrolytic tanks, Photoelectrocatalytic oxidation

Background Arsenic (As) contamination is widely recognized as a global health problem. The distribution of As(III) and As(V) in natural water depends on the redox potential and pH of water [1]. Compared with As(V), As(III) is generally reported to have a low affinity to the surface of various minerals, because it mainly exists as nonionic H3AsO3 in natural water when pH PtOx (II, IV)/TiO2 > bare TiO2 [42]. When applied current density increased to 0.5 mA cm−2, the photocatalytic ability of Pt/TiO2 nanotubes for As(III) oxidation was kept stable. To the following experiments, the applied current density was kept at 0.5 mA cm−2. Photocatalytic Ability Comparison Between Naked TiO2 Nanotubes and Pt/TiO2 Nanotubes

To prove the function of Pt for photocatalysis, oxidation abilities of As(III) were compared between TiO2 and Pt/

TiO2 nanotubes under visible light irradiation or visible light irradiation with 1.2 V positive biasing. From Fig. 7a, we could find that under visible light, no As(III) was oxidized by TiO2 nanotubes, no matter if 1.2 V positive biasing was applied. While, under ultraviolet light, 82.0 % of As(III) could be oxidized to As(V) after 30 min. This means, only under ultraviolet light condition, TiO2 nanotubes have photocatalytic oxidation ability for As(III). To Pt/TiO2 nanotubes electrodes prepared at 0.5 mA cm−2 with 5 min, they displayed high photocatalytic and photoelectrocatalytic oxidation activity for As(III) (Fig. 7 b). To avoid the influence of adsorption

Fig. 7 a Comparison of As(III) photocatalytic oxidation on TiO2 nanotubes under visible light and UV. Comparison of As(III) (b) and As(V) (c) concentration in system with photocatalytic and photoelectrocatalytic process on Pt/TiO2 nanotubes. d Concentration of arsenic species in photocatalytic system and photoelectrocatalytic anodic/cathodic cells with Pt/TiO2 nanotubes after 280 min

Qin et al. Nanoscale Research Letters (2016) 11:32

effect of Pt/TiO2 nanotubes, 30 min equilibrium adsorption was first operated before catalytic experiments, which made As(III) concentration decrease from 3.41 to 3.20 mg/L. When both electrochemical and photocatalytic processes were simultaneously applied, 94.2 % of As(III) could be oxidized in 280 min. This value was 13.5 % higher than the only photocatalytic oxidation process. Fabricated Pt nanoparticles were acted as electron traps, which could enhance the separation of electron-hole pairs, and the external positive biasing drove electron (e−) to cathode, then the recombination of electron-hole pairs could be further reduced. So, more holes could cause stronger direct (h+) oxidation or indirect (HO·) oxidation for As(III) on anode Pt/TiO2 nanotubes electrode, which was the reason why the As(III) oxidation rate on anode in the photoelectrocatalytic process was higher than that in the photocatalytic process. In addition, an interesting phenomenon was found on Pt/TiO2 nanotubes, As(III) even could be oxidized in dark condition. And 17.4 % of As(III) could be converted into As(V) in 280 min, this could be induced by catalytic effect of platinum itself, and O2 activation on Pt nanoparticles might be responsible for this dark activity. When 1.2 V of positive biasing was applied under dark condition, the oxidation efficiency of As(III) to As(V) was hardly improved. With detection solution in cathode cell, it was interesting to find that As(III) was also continuously transformed into As(V) during the photoelectrocatalytic process at reduction potential. When 1.2 V of positive bias potential was applied to this system, the conversion rate in cathode cell was 66.4 % after 280 min (Fig. 7d). Effect of Initial As(III) Concentration on Photoelectrocatalytic Result of Pt/TiO2 Nanotubes Electrode

Figure 8a showed the oxidation rate of As(III) increased obviously with the rise of initial As(III) concentration.

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Furthermore, the kinetics simulation curves of As(III) photoelectrocatalytic oxidation were summarized and presented in Fig. 8b. All oxidation reactions were well fitted in the pseudo-first-order kinetics model.   Ct ¼ k app t −ln C0 where C is the concentration of As(III) (mg/L), t is the reaction time (min), and kapp is the apparent first order reaction constant (min−1). The values of kapp and the regression correlation coefficient R2 of Pt/TiO2 nanotubes for As(III) photoelectrocatalytic oxidation were listed in Table 2. Correlation coefficient (R2) values of pseudofirst-order kinetic model with different initial As(III) concentration were all more than 0.980. This meant that photoelectrocatalytic oxidation process of As(III) on Pt/TiO2 nanotubes obeyed pseudo-first-order kinetics equation. The Influence of Applied Bias Potentials on the Photoelectrocatalytic Oxidation of As(III)

The bias potential is an important parameter in the process of photoelectrocatalytic activity. Photo-generated electrons on Pt/TiO2 nanotubes electrode could be driven to the counter electrode with positive potential. So, bias potentials ranged from 0.0 to 2.0 V were monitored over 280 min in photoelectrocatalytic oxidation treatment (Fig. 9). Figure 9a illustrated that As(III) oxidation rate increased with the increase of applied positive bias. With applied 0.0 to 2.0 V positive bias, the plot of Ct versus time was fitted with exponential decay equation with all R2 exceeding 0.980. The apparent As(III) oxidation rate constants were varied from 7.2 × 10−3 to 12.4 × 10−3 min−1. Applying a positive biasing to the Pt/TiO2 nanotubes electrode can transfer the photo-generated electrons away from photo-generated

Fig. 8 a As(V) concentration in anodic cell and b kinetics simulation of As(III) oxidation in the photoelectrocatalytic oxidation process with varies initial As(III) concentration

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Table 2 Kinetic parameters of photoelectrocatalytic oxidation of As(III) with different initial As(III) concentration Initial As(III) concentration (mg/L)

C0(Exp.) (mg/L)

C0(cal.) (mg/L)

R2

kapp (min−1)

2.0

1.877

1.945

0.984

−0.0103

2.8

2.638

2.753

0.983

−0.0106

3.4

3.204

3.256

0.992

−0.0108

4.0

3.768

3.791

0.999

−0.0086

5.0

4.737

4.630

0.995

−0.0083

6.0

5.692

5.511

0.986

−0.0071

holes on the Pt/TiO2 nanotubes electrode via the external circuit; thus, the recombination of photogenerated electron-hole pairs is minimized [43, 44]. In Fig. 9a, it was found that in the range of studied positive biasing, rate constant linearly went up with the increase of applied bias voltage. With the positive bias voltage increasing, more and more photogenerated electrons moved to counter electrode. As a result, the photo-generated electrons and holes were well separated, thus more hydroxyl radicals (HO·) could be produced by H2O oxidized in the holes [45]. Which species (h+, HO· and ·O2−) was mainly responsible for the oxidation from As(III) to As(V) in the UV/TiO2 system, different opinions had been proposed on this issue. But so far, it still remained as a controversial issue [46]. So, on the anode Pt/TiO2 nanotubes electrode, it was in dispute whether h+ or HO· was responsible for As(III) oxidation. At the same time, the oxidation rate of As(III) to As(V) on graphite rod in cathodic cell was also verified to increase with applied potential (Fig. 9b). After 280 min, As(V) concentration were 0, 0.84, 0.96, 1.69, 2.15, 3.02, and 3.39 mg/L when system was applied with 0.0, 0.3, 0.6, 0.9, 1.2, 1.5, and 2.0 V positive bias

potentials. As(III) in cathodic cell could be completely transformed into As(V) after 280 min when system was applied with 2.0 V positive bias potential. Leng et al. found that the main product of oxygen reduction reaction was hydrogen peroxide (H2O2) on the cathode graphite in aqueous solution with pH from 2 to 12 [47]. Nevertheless, ·O2− was only stable in concentrated alkaline solutions or aprotic media. So, it was inferred that e- on cathode electrode could be trapped by the surface absorded O2 to generate H2O2 as Eq. (1), which had powerful oxidation ability to convert As(III) to As(V) as Eq. (2) [47]. O2 þ 2H2 O þ 2e− →H2 O2 þ 2OH−

ð1Þ

H3 AsO3 þ H2 O2 þ 2OH− →HAsO4 2− þ 3H2 O

ð2Þ

The Effect of Solution pH Value on As(III) Oxidation

The influence of initial pH on photocatalytic oxidation of As(III) in anodic cell was shown in Fig. 10a. The oxidation rate in alkaline environment was mostly a little faster than the rate in acid environment. The result differed from that of Lee and Choi [48] who found that the initial oxidation rate at pH 9 was about twice as fast as the rate at pH 3, and also differed with the result of Bissen et al. [13] and Sharma et al. [49] who both found that the small increase in the oxidation of As(III) with increasing pH was within experimental error. The potential of As(V)/As(III) couple is much less positive than the valence band potential of TiO2, so, the photo-generated holes have enough thermodynamic potential to oxidize As(III) to As(V) [50]. The potential of As(V)/As(III) couple in alkaline environment is lower than that in acid, which may be contributed to the increase of the oxidation rate with the increase of pH.

Fig. 9 a Photocatalytic oxidation kinetics simulation of As(III) at various applied bias potentials, inset was the dependence of rate constant on electrical bias potentials; b concentrations of As species in cathode cell with various applied bias potentials after 280 min

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Fig. 10 a Effect of initial pH on photoelectrocatalytic system; b comparison of pH under photocatalytic process and photoelectrocatalytic process (applied 1.2 V) with same initial 7.0 pH and 3.4 mg/L As(III)

The pH change during the photoelectrocatalytic oxidation of As(III) in aqueous solution was displayed in Fig. 10b. During the electrochemical experiment, pH decreased with irradiation time during both photocatalysis and photoelectrocatalysis experiments. The decrease of pH in the photoelectrocatalysis experiment was faster than that in the photocatalysis. The influence of pH on the speciation of arsenic oxoanions can easily be obtained from Eq. (3) to (8), acid dissociation constants are also given. −

þ

In the photocatalytic process, As(III) was oxidized to As(V), meanwhile, protons were produced, and the reaction could be described as H3 AsO3 þ H2 O → HAsO4 2− þ 4Hþ þ 2e− ðin alkaline solutionsÞ

ð9Þ

H3 AsO3 þ H2 O → H2 AsO4 − þ 3Hþ þ 2e− ðin acidic solutionsÞ

ð10Þ

In photoelectrocatalytic process, the anode oxidation caused O2 evolution in anode cell, which produced protons and contributed to the further reduce of the pH during photoelectrocatalytic activity.

H3 AsO3 ↔ H2 AsO3 þ H pKa1 ¼ 9:20

ð3Þ

H2 AsO3 − ↔HAsO3 2− þ Hþ pKa2 ¼ 12:10

ð4Þ

HAsO3 2− ↔AsO3 3− þ Hþ pKa3 ¼ 13:40

ð5Þ

H3 AsO4 ↔H2 AsO4 − þ Hþ pKa1 ¼ 2:20

Photoelectrocatalytic Oxidation of As(III) by Natural Sunlight

ð6Þ

H2 AsO4 − ↔HAsO4 2− þ Hþ pKa2 ¼ 6:94

ð7Þ

HAsO4 2− ↔AsO4 3− þ Hþ pKa3 ¼ 11:50

ð8Þ

To further extend the photoelectrocatalytic applicability of Pt/TiO2 nanotube arrays electrode in a more practical situation, the photoelectrocatalytic oxidation ability was also evaluated under natural sunlight. Figure 11a showed the evolution of As(III)/As(V) concentration

Fig. 11 a Concentration of As species and b kinetics simulation of As(III) oxidation in the process of photoelectrocatalytic oxidation under natural sunlight. Nov. 3rd, 2014, from 10:00 to 14:40, temperature is 16 °C

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Fig. 12 a Concentrations of As species and b kinetics simulation during the photoelectrocatalytic process with high arsenic groundwater sample under visible light

with irradiation time with initial 3.4 mg/L As(III) and 1.2 V positive bias potential under natural sunlight irradiation. The photoelectrocatalytic oxidation process under natural sunlight accorded with zero-order kinetics law (Ct = − 0.0114t + 2.9971(R2 = 0.973)), different with process under visible light which followed firstorder kinetics law. As(III) could be completely oxidized after 280 min under natural sunlight irradiation. While, to visible light, it needed 360 min to totally oxidize As(III). This might be because of the effect of ultraviolet light in sunlight. Photoelectrocatalytic Oxidation of As(III) in High Arsenic Groundwater Sample Under Visible Light

The photoelectrocatalytic activity of real groundwater sample with high arsenic was also investigated by Pt/ TiO2 nanotubes arrays electrode under visible light. The high arsenic underground water sample (25 m underground) was collected from Daying Village, Datong basin, Northern China, concentrations of main ions in this water sample was determined as follows: Na+ 324.2 mg/L, Ca2+ 35.8 mg/L, Mg2+ 61.5 mg/L, Cl− 297.4 mg/L, HCO3− 544.1 mg/L, SO42− 275.5 mg/L, NO2− 1.10 mg/L, Fe2+ 0.75 mg/L, NH4+ 0.81 mg/L, S2− 74.9 μg/L, As(III) 0.32 mg/L, As(V) 0.35 mg/L, 8.32 of pH, and 2.3 mS cm−2 of conductivity. After 30 min for equilibrium adsorption, 9.5 % of As(III) and 14.2 % of As(V) were adsorbed on Pt/TiO2 nanotube arrays. The conversion of As(III) during the photoelectrocatalytic process with 1.2 V of positive bias potential was found to follow a zero-order kinetics law as Ct = − 0.0015t + 0.3013 (R2 = 0.981), and As(III) in groundwater sample was totally transformed into As(V) in 200 min under visible light. This proved Pt/TiO2 nanotubes electrode was an efficient photoelectrocatalytic material for As(III) oxidation and could be used in high arsenic underground water pretreatment (Fig. 12).

Conclusions To promote photocatalytic oxidation of As(III) by TiO2 materials under visible light, Pt/TiO2 nanotubes were introduced via a constant current deposition method. First, the prepared nanotubes were detected by SEM, XPS to prove the existing of Pt, and UV-vis diffuse reflectance spectra results proved that Pt could promote separate efficiency and extend the excited spectrum range. Pt/TiO2 nanotubes presented more efficient photoelectrocatalytic oxidation performance for As(III) than TiO2 nanotubes. Even under dark condition, it was also useful in As(III) photoelectrocatalytic oxidation. Furthermore, the photoelectrocatalytic oxidation process under visible light was found to obey pseudo-first-order kinetics. The prominent conversion from As(III) to As(V) in cathodic cell also occurred because of the production of H2O2 from electrons trapping by O2 on cathode. While to natural sunlight, the oxidation of 3.4 mg/L As(III) on Pt/ TiO2 nanotubes electrode with 1.2 V applied voltage followed zero-order kinetics law, and its oxidation rate was slightly higher than that of As(III) under visible light. As(III) in real groundwater sample could be totally transformed into As(V) in 200 min by Pt/TiO2 nanotubes electrode with 1.2 V under visible light and also accorded with zero-order kinetics. Competing Interests The authors declare that they have no competing interests. Authors’ Contributions YQ prepared the nanomaterials, carried out the structural analyses of the samples, and drafted the manuscript. YC designed the experiments, YC and YL guided YQ for the experimental section, and YC edited the whole manuscript. ZT and YW took part in the experimental section. All authors read and approved the final manuscript. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 40902070), and the Fundamental Research Funds for the Central Universities (No. CUG110414).

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