ARTICLE pubs.acs.org/est
Photocatalytic Oxidation of Arsenite over TiO2: Is Superoxide the Main Oxidant in Normal Air-Saturated Aqueous Solutions? Hui Fei,† Wenhua Leng,*,† Xiang Li,† Xiaofang Cheng,†,§ Yiming Xu,† Jianqing Zhang,† and Chunan Cao†,‡ † ‡
Department of Chemistry, Yuquan Campus, Zhejiang University, Hangzhou, Zhejiang 310027, China State Key Laboratory for Corrosion and Protection of Metals, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
bS Supporting Information ABSTRACT: TiO2 photocatalytic oxidation (PCO) of As(III) in the normal air-saturated aqueous solutions has been widely studied. Yet no consensus has been achieved on the mechanism whether superoxide is the main oxidant, although many approaches have been taken. (Photo)electrochemical method can minimize changes to TiO2 surface and could therefore not alter the normal mechanism. In this Article, both this approach and As(III) oxidation kinetic measurements were performed to clarify the disputed mechanism. Under a sufficient cathodic bias potential, the dark oxidation of As(III) by superoxide could occur, but both the reaction rate and the columbic efficiency were rather low, suggesting that it is a weak oxidant. However, under UV light, both the reaction rate and the columbic efficiency were greatly enhanced even at potentials negative enough to eliminate photohole participation, indicating that more efficient oxidants than superoxide were produced. Under UV illumination and enough positive potential where superoxide was absent, the As(III) oxidation was the most highly efficient. The columbic efficiency of photoholes was much higher than that of superoxide. In the normal aerated aqueous solutions and at open circuit, although the total contribution of superoxide and its derivates to the PCO of As(III) was considerably high (up to 43%), it was not more than that of photohole (57%). In addition, the reported various approaches taken to elucidate the mechanism were discussed, and the resulting disputes can be clarified by these findings. It was demonstrated that (photo)electrochemical method could provide direct and undisputed evidence to reveal the truth mechanics issues.
’ INTRODUCTION Arsenic contamination of groundwater is one of the environmental problems threatening people’s health.14 Inorganic arsenic in groundwater is usually in the oxidation states of As(III) and As(V),14 among which As(III) is more toxic and mobile.3,4 Thus, the former oxidation to the latter is a desirable reaction for water treatment. Various water treatment methods for the transformation of As(III) to As(V) have been reported in the literature, including UV/Fe(III) complexes,5 Fe(II)/H2O2,6 and Fe(VI).7 TiO2 photocatalysis has been widely studied as a promising alternative method for water treatment.810 It is well recognized810 that the photogenerated hole is involved in the oxidation of pollutants, either directly or indirectly via OH radical produced by the hole oxidation of surface adsorbed OH/H2O, while the photogenerated electron is removed by adsorbed O2 to produce O2 3 and/or HO2 3 (denoted hereafter as superoxide). TiO2 photocatalysis has been shown to be effective in oxidizing As(III) to As(V) with oxygen in many studies.14,1119 However, there has been a controversy over the photocatalytic oxidation (PCO) mechanism of As(III), which originated from a claim that superoxide was the main r 2011 American Chemical Society
oxidant of As(III) in the UV/TiO2 system.1115 This argument has been doubted and refuted by other research groups.1619 However, unfortunately, almost all related studies are based on the effect of competitive additives and/or on the product analysis. The addition of additives may lead to change in mechanism, because both hole and electron reactions can initiate As(III) oxidation. Product analysis, although simple and often used, cannot distinguish whether the reaction product results from a photo-oxidation or photoreduction process. Thus, it is difficult to draw a direct and undisputed conclusion by these approaches. Recently, through photocurrent measurement and competitive effect, Choi et al.14 insisted that superoxide was the main photo-oxidant of As(III) in aerated aqueous TiO2 suspensions. However, their conclusion is weakly supported by relevant experimental data.20 Very recently, mainly by time-resolved diffuse reflectance (TDR) spectroscopy measurements, Choi Received: February 18, 2011 Accepted: April 11, 2011 Revised: April 11, 2011 Published: April 18, 2011 4532
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Environmental Science & Technology Scheme 1. (A) Schematic Representation of the Charge Carrier Fluxes in a TiO2 Particle or Film; and (B) Schematic Diagram of the Photoelectrochemical Behavior of TiO2a
a
qΦ0 and Jrec are the flux of generation and recombination of electron hole pair, respectively; Jn and Jp are the flux of electrons and holes transferred to acceptors and acceptors in the solution, respectively; Jn is the dark oxygen reduction current density versus potential; Jp is the oxidation of H2O/As(III) under UV; J is the net flux oxidation of H2O/ As(III) under illumination with oxygen; and qΦ0 equals the electron hole pair generation flux at the voltage indicated. Flux-matching condition when J < qΦ0. For more details, see the text.
et al.15 claimed that they provided direct and undisputed evidence to support that As(III) acted as an external charge recombination center, which was used to demonstrate the superoxide-mediated mechanism. However, as we commented,21 the authors neglected very important factors affecting the TDR signal, based on which their conclusion made may still be questionable. Hence, currently, the superoxide-mediated mechanism of PCO of As(III) remains as a controversial issue. (Photo)electrochemical techniques provide a powerful method for the study of charge transfer and recombination processes at semiconductor/electrolyte interfaces.9,10,22 These methods can minimize changes to TiO2 surface. It is then possible to evaluate the hole and electron reactions separately on two different electrodes. Applying an external cathodic bias in the dark or even under illumination would favor superoxidemediated reaction as photoholes will not be involved in the oxidation reaction at all, whereas hydroxyl radical- or holemediated reactions would be favored by an anodic bias potential.
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Thus, it may allow us to clarify the disputed mechanism issues. In fact, different photoelectrochemical measurements help to elucidate mechanistic details of the photocatalytic process. Photocurrent (Jph) versus potential,2226 Jph versus time9,2426 and Jph versus substrate concentration,22,24,25 have been used to better understand the photocatalytic behavior of TiO2 electrodes. In this study, the PCO of As(III) over TiO2 has been examined through both (photo)electrochemical and kinetic measurements. Under cathodic bias, superoxide was a weak oxidant, but the reaction rate was accelerated under UV light, implying more reactive species than superoxide were produced. Although the contribution of superoxide and its derivates to As(III) oxidation in the air-saturated aqueous solutions was considerably high, it was not more than that of photohole or hydroxyl radical. Theoretical Background. We present a brief description of the theoretical expectations for the (photo)electrochemical experiments. This formalism will provide a basis for the analysis of the following PCO kinetics of As(III). The main idea behind this is similar to that reported by Kesselman et al.10 for determining whether O2 reduction is rate-determining under photocatalytic conditions for the degradation of CHCl3 at TiO2 electrodes. Scheme 1A presents the four fluxes of interest in the TiO2 PCO of As(III) with oxygen. A flux of absorbed photons in terms of current, qΦ0, will create electronhole pairs in the TiO2. Generally, a part of photocarriers will recombine in TiO2, giving a recombination flux, Jrec. The interfacial charge-transfer flux of holes, Jp, includes oxidation of OH/water and direct oxidation of As(III). The interfacial charge-transfer flux of electrons, Jn, primarily involves reduction of the electron acceptors, generally considered to be O2. At open circuit (OC, steady-state), Jp must equal Jn (flux-matching equation). Using polycrystalline TiO2, it is possible to evaluate separately the potential dependence of Jp and Jn. For example, in an air-saturated solution and in the dark, where the flux Jp is not important, thus steady-state JE curves can directly give the steady-state potential dependence of the flux for O2 reduction (Jn in Scheme 1B) at TiO2. The potential dependence of Jp can be determined independently by obtaining JE data for TiO2 with As(III) under illumination and in the absence of O2 (g) (Jp in Scheme 1B). When both of the processes are present, the net charge-transfer flux across the interface would J over the time of interest, t, be the sum of Jp and Jn. Integrating R yields the passed charge, Qt = t0J dt. From the quantity of As(V) produced, we can calculate the columbic efficiency of η (electron or hole) by eq 1, where Δc is the change of As(V) concentration, V is the solution volume, F is the Faraday’s constant, and n is the charge number. η¼
ΔcV Q t =nF
ð1Þ
The intermediate of As(III) oxidation is regarded to be As(IV), which is a reducing species and reacts with O2, almost at a diffusion-limited rate.19,27 Next, it is reasonable to assume that oxidizing 1 mol of As(III) only consumes 1 mol of charge carriers. The charge number n equals 1 and 2 with and without oxygen, respectively. Because Jn and Jp can be separately measured, then the electron and hole columbic efficiencies of ηe and ηh can be calculated, which would allow one to separately evaluate the oxidative capacities of superoxide and hole. 4533
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The PCO of As(III) with O2 at a given OC potential, Voc, occurs through hole, superoxide, and its photoinducer. Let us analyze the contribution of superoxide itself first. For the reduction process at OC under illumination, the passed electron charge (Qle,OC) for a given time should be the same as that (Qe, d OC ) of the dark reduction of electron acceptors (mainly O2) under the same Voc applied because Jn is a monotonic function of potential,10 that is: Q le, OC ¼ Q de, OC
ð2Þ
As for the production of superoxide through the O2 reduction at the Voc, it should be independent of whether the electrons are from the light-induced or electro-generated in the dark and of whether they are transferred via conduction band edge or surface states (actually, it is widely accepted that almost all of the electrons are believed to transfer via conduction band edge2830). So, from eq 2, the oxidation of As(III) by superoxide itself (Δcls,OC) under illumination at the Voc should be the same as that (Δcde,OC) by the electro-generated superoxide in the dark under the same potential: Δcls, OC ¼ Δcde, OC
ð3Þ
Consequently, according to eqs 2 and 3, the columbic efficiencies of electron in both cases should also be equal: ηle, OC ¼ ηde, OC
ð4Þ
Integrating the flux-matching equation of electron and hole at OC, we can get the passed hole (Qlh,OC) across the TiO2/ electrolyte interface. Z t Z t Qhl , OC ¼ Jp ðtÞ dt ¼ Jn ðtÞ dt ¼ Q le, OC ð5Þ 0
0
According to eqs 1 and 5, the amount of oxidation of As(III) by photohole at OC (Δclh,OC) can be written as: Δclh, OC ¼
ηnh, OC Q lh, OC =nF V
¼
ηnh, OC Q le, OC =F V
ð6Þ
where ηnh,OC is the columbic efficiency of hole with oxygen at the applied Voc. The total charges across the interface at OC (Qltot,OC) are the sum of electron and hole charges passed, which can be written as eq 7 by using eqs 2 and 5. Q ltot, OC ¼ Q lh, OC þ Q le, OC ¼ 2Q de, OC
ð7Þ
Similarly, the columbic efficiency of total charges at OC under illumination (ηtot,OC) can be defined as ηtot, OC ¼
Δcltot, OC V Q ltot, OC =nF
¼
Δcltot, OC V 2Q de, OC =F
ð8Þ
where Δcltot,OC is the production of As(V) concentration at OC. Thus, the As(III) oxidation by the superoxide derivates (Δcder, l OC ) can be obtained by eq 9. Δclder, OC ¼ Δcltot, OC Δcls, OC Δclh, OC
ð9Þ
Therefore, from the As(III) oxidation initiated by photoelectrons (Δcle,OC = Δcls,OC þ Δclder,OC) and photoholes (Δclh,OC), we can judge who is the main oxidant of As(III) at OC. It is not important whether the interfacial reactions occur through one-electron intermediates or through more complicated
chemical processes, because only the potential dependences of Jn and Jp are required to perform the desired analysis. Recombination processes are implicitly included in the net JE data under illumination, so the flux-matching procedure is valid even if a significant carrier recombination occurs on the TiO2 surface.10 In addition, OC potential may fluctuate during As(III) oxidation, but the flux-matching conditions are still valid for such a quasi-steadystate process. This unfavorable situation can be minimized when the initial As(III) oxidation rates are used.
’ EXPERIMENTAL SECTION Materials and Chemicals. NaAsO2 (As(III), analytical grade, Shuikoushan Mining Bureau, Hengyang Industrial Co., Hunan, China) was used as the source of arsenic solution. As(V) was obtained by excess H2O2 oxidation of As(III). Titanium sheet (99.7%, 0.15 cm thick, Shanxi Yuanlian Rare Metal Ltd.) was used as the substrate of photoelectrode. Specpure graphite sheet (4.9 cm 4.2 cm) was used as received. All other chemicals and solvents were at least of analytical grade and used as received. HClO4 and/or NaOH were used to adjust the solution pH before the reaction. Double distilled water was used in all experiments. Preparation of Photoelectrodes. The anatase TiO2 film electrode was fabricated by dip-coating of TiO2 sol onto Ti substrate, followed by heat treatment at 500 °C for 30 min to form one layer of TiO2. Five layers of TiO2 were coated for the following film electrodes unless otherwise specified. Photoelectrodes with exposed surface area of 6 cm 4.5 cm and 0.8 cm 0.8 cm were used for the oxidation of As(III) and (photo)electrochemical measurements, respectively, and the rest was covered with epoxy resin. Further details on the preparation procedures and photocatalyst characterization can be found elsewhere.31,32 (Photo)electrochemical Measurements. Linear scan voltammograms of the TiO2 electrode were carried out in a conventional three-electrode cell with a quartz window,9 respectively. The working electrode is the TiO2/Ti thin film electrode; a platinum wire and a saturated calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. The light source was a 500 W Xe lamp (CHFXM, Beijing Chang Extension Technology Co., Ltd.). The average incident light intensity into the cell was 5.0 mW cm2 (λ = 365 nm) detected by a UV-irradiance meter (UV-A, Instruments of Beijing Normal University, China). As(III) Oxidation and Analysis. Experiments of As(III) oxidation were conducted in a photoreactor31,32 with a 45 cm3 electrolyte that holds a TiO2/Ti thin film electrode as working electrode and a saturated calomel electrode as reference electrode. A large surface area of graphite rod was used as counter electrode and placed in another container, which was connected to the photoreactor by a salt bright. The light source was the same as the above. The incident wavelengths shorter than 300 nm were cut off by Pyrex glass in front of the reactor. The average incident light intensity into the reactor was 1.6 mW cm2 (λ = 365 nm) detected by the aforementioned UV-irradiance meter. The presence and absence of oxygen in the reactor was controlled by purging air or high purity nitrogen throughout the experiments. Samples of 0.5 mL were periodically withdrawn for analysis. The concentration of As(V) in the solution was estimated colorimetrically at 630 nm (752 spectrophotometer, Shanghai) using the method of ammonium molybdate and 4534
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experiment (the adsorption of As(III) and As(V) was ca. 1.2% and 2.8%, respectively, which is very small due to a small specific surface area of planar TiO2 used). All experiments were performed on an electrochemical workstation (CHI660a, Shanghai Chenghua Corp., China) and were repeated at least twice. The relative experimental error was less than 5%. All potentials in the text are vs SCE.
’ RESULTS AND DISCUSSION JE Behavior for TiO2 Electrode under Different Atmospheres. Figure 1a displays the currentpotential curves in the
Figure 1. Linear sweep voltammograms of TiO2/Ti electrode in the dark (a) and under illumination (b) at a scan rate of 20 mV s1; (c) steady-state currentpotential in the PCO experiments. Solutions pH = 3.0, with [As(III)]0 = 500 μM or without (blank), saturated with air or N2, respectively.
malachite green described in refs 19,33. All of the oxidation experiments were conducted on athe same piece of TiO2 electrode. After use, it was soaked with 1 M K2HPO4 and then washed thoroughly with water.18 This procedure was repeated until the reproducibility of the photocurrent in the blank electrolyte was less than 10% prior to use. The tested solutions of 0.1 mol dm3 NaClO4 with a pH value of 3.0 ( 0.1 with or without 500 μM As(III) were employed in all experiments unless otherwise specified. All of the solutions were stirred for at least 20 min to obtain adsorption equilibrium before
dark with or without As(III), saturated with air or N2. As expected, the oxygen reduction occurred preferentially over proton reduction at potentials < 0.2 V. Steady-state results in the dark show similar behavior (Figure 1c). A significant increase in the anodic current was observed from ca. 0.5 V in all cases (Figure 1b), as compared to that in the dark. The difference in current between two cases gives photocurrent. The photocurrent increased with increasing the anodic bias due to a decreased recombination of photocarriers,9,31,32 and then saturated at ca. 0.0 V. The photocurrent was higher with As(III) than its absence near the onset potential (Vonset) of photocurrent, but these differences decreased with further increasing potential. The Vonset with oxygen was shifted more positive, and the photocurrent was quenched near the Vonset because of oxygen still acting an efficient electron scavenger, but the quench is not obvious when the applied potentials were >0.0 V. Essentially identical currentpotential behavior under illumination was observed at steady-state (Figure 1c). The data of Figure 1c allowed determination of the flux under photocatalytic conditions. The reduction current in the dark with air only matched the hole-transfer rate under illumination with air at 0.4 V, which is in good agreement with the actual OC potential of PCO of As(III) (Figure S1 in the Supporting Information (SI)). Therefore, at potentials < 0.4 V, almost the majority of the photoholes should recombine and not be involved in the oxidation of As(III). The matched flux was much smaller than the saturated photocurrent, indicating that substantial recombination occurred at OC.10 Oxidation of As(III) by TiO2 Electrode under Cathodic Bias. To examine whether superoxide and its derivatives are able to and efficiently oxidize As(III), experiments of As(III) oxidation were conducted under conditions of negative bias potentials both in the dark and under illumination. These experiments are appropriate to understand the role of superoxide and its derivates in the oxidation of As(III) alone, because no photoholes will be involved in the oxidation. Figure 2a shows the dark oxidation rate of As(III) on the TiO2 at negative potentials of interest in the airsaturated solutions. Obviously, the formation rate of As(V) increased in the initial stage with decreasing the applied potential. Because oxygen did not act as an efficient oxidant and the only oxidant produced here was the product of molecular oxygen reduction, superoxide, these results indicate that As(III) could be oxidized by superoxide, which is further supported by the observation that negligible As(V) was produced in the absence of oxygen as indicated in Figure 2b. However, the amount of As(III) conversion was rather small (maximum only 10%), suggesting that superoxide itself is not an efficient oxidant, which is further supported by its low columbic efficiency as will be presented later. Upon UV illumination, as shown in Figure 3a, the As(V) production was highly accelerated even if under 4535
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Figure 2. Formation of As(V) under different applied potentials and at open circuit (OC) in the dark: (a) air-saturated; (b) N2 saturated. pH = 3.0; [As(III)]0 = 500 μM.
Figure 3. Formation of As(V) under different applied potentials and open circuit (OC) under illumination: (a) air-saturated; (b) N2 saturated. pH = 3.0; [As(III)]0 = 500 μM.
negative enough potentials (
