Environ. Sci. Technol. 2010, 44, 9099–9104
Photocatalytic Oxidation Mechanism of As(III) on TiO2: Unique Role of As(III) as a Charge Recombinant Species W O N Y O N G C H O I , * ,† J I M A N Y E O , † JUNGHO RYU,‡ TAKASHI TACHIKAWA,§ AND TETSURO MAJIMA§ School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea, Green Growth Laboratory, Korea Electric Power Research Institute, Daejeon 305-706, Korea, and The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan
Received July 23, 2010. Revised manuscript received October 26, 2010. Accepted October 28, 2010.
Using TiO2 photocatalyst, arsenite, As(III), can be rapidly oxidized to arsenate, As(V), which is less toxic and less mobile in the aquatic environment. Therefore, the TiO2/UV process can be employed as an efficient pretreatment method for arsenic contaminated water. Since we first reported in 2002 that the superoxide (or hydroperoxyl radical) plays the role of main oxidant of As(III) in the TiO2/UV process, there has been much debate over the true identity of the major photooxidant among superoxides, holes, and OH radicals. The key issue is centered on why the much stronger OH radicals cannot oxidize As(III), and it has been proposed that the unique role of As(III) as an external charge recombination center on the UV-excited TiO2 particle is responsible for this eccentric mechanism. Although the proposed mechanism has been supported by many experimental evidences, doubts on it were not clearly removed. In this study, we provided direct and undisputed evidence to support the role of As(III) in the charge recombination dynamics using time-resolved transient absorption spectroscopy. The presence of As(III) indeed mediated the charge recombination in TiO2. Under this condition, the role of the OH radical is suppressed because of the null cycle, and the weaker oxidant, superoxide, should prevail. The role of the superoxide has been previously doubted on the basis of the observation that the addition of excess formic acid (hole scavenger that should enhance the production of superoxides) inhibited the photocatalytic oxidation of As(III). However, this study proved that this was due to the photogeneration of reducing radicals (HCO2 · ) that recycle As(V)/As(IV) back to As(III). It was also demonstrated that the 4-chlorophenol/TiO2 system under visible light that cannot generate neither OH radicals nor valence band holes converted As(III) to As(V) through the superoxide pathway.
* Corresponding author fax: +82-54-279-8299; e-mail: wchoi@ postech.edu. † Pohang University of Science and Technology (POSTECH). ‡ Korea Electric Power Research Institute. § Osaka University. 10.1021/es102507u
2010 American Chemical Society
Published on Web 11/09/2010
Introduction Arsenic contamination in groundwater has emerged as an important environmental issue since it poses significant health risks in many countries (1-4). Since the European Union and the U.S. Environmental Protection Agency recently lowered the maximum contaminant level for total arsenic to 10 µg/L in drinking water, intensive efforts have been made to develop effective methods to remove arsenic from drinking water (2, 3, 5). Arsenic in groundwater exists in its common oxidation state of As(III) (arsenite) and As(V) (arsenate) (6). Compared with As(V), As(III) is more toxic and mobile and more difficult to remove by coagulation/precipitation or adsorption (7). Therefore, the preoxidation of As(III) to As(V) is highly desired to enhance the removal efficiency of total arsenic in water treatment. A variety of treatment methods which oxidize As(III) to As(V) have been reported (7-16) among which the photocatalytic oxidation (PCO) of As(III) using TiO2 has been considered as a viable method (7-9). The UV/TiO2 process has demonstrated its efficiency in the photoconversion of As(III) to As(V) and has the practical merits in that it does not require any chemical oxidant except the dissolved O2, it can utilize solar light, and the titania catalyst material is cheap, nontoxic, and easily available. A particular attention in the study of this process has been devoted to a mechanistic issue regarding the true identity of the major oxidant of As(III). We have previously reported that superoxide plays the primary role as an oxidant of As(III) in the UV/TiO2 system (17-20); however, it has been questioned and refuted by other research groups (21-25). The key issue is centered on the comparison between the weak oxidant, superoxide, and the strong oxidant, hydroxyl radical or valence band (VB) hole, all of which are generated on the UV-illuminated TiO2 surface. Our claim is certainly contradictory to the intuition that the stronger oxidant (OH radicals) should be the main oxidant. To explain why the OH radicals are not the main oxidant in this specific PCO, we proposed that the adsorbed As(III) on TiO2 serves as an external charge-recombination center where the reaction of As(III) with an OH radical (or hole) is immediately followed by a conduction band (CB) electron transfer to make a null cycle (eq 1 + eq 2) (19) + As(III) + •OH/hVB f As(IV)
(1)
As(IV) + eCB f As(III)
(2)
Although many experimental evidences to explain why OH radicals should not be the main oxidant and to support the role of As(III) as a charge-recombination center were presented in our previous studies (17-19), the debate continues. Yoon et al. (25) recently reported the observation that PCO of As(III) was greatly inhibited and H2O2 generation was promoted in the presence of excess formic acid and methanol as a scavenger of VB hole (or OH radical) in UV/ TiO2. Although the presence of hole scavengers should prolong the lifetime of electrons and consequently should produce more superoxide and H2O2, the photooxidation of As(III) was inhibited. Based on this observation, they claimed that the superoxide has little role in the oxidation of As(III), and the adsorbed •OH and/or VB hole are the main oxidants of As(III) in the TiO2/UV process. In this study, we aimed to clear out the doubts centered on the superoxide-dominated PCO mechanism of As(III) by carrying out the time-resolved transient spectroscopic measurements and some reaction kinetics experiments. The role VOL. 44, NO. 23, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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of As(III) as an external charge recombination center was clearly proved by observing that the charge recombination in TiO2 was accelerated in the presence of As(III). This provides an undisputed evidence to support the proposed mechanism (eqs 1 and 2). In addition, it was found that the evidence that Yoon et al. (25) provided to disprove the superoxide mechanism was actually a result of misinterpretation of the experimental data. The PCO reactions of formic acid and methanol generate highly reducing radical species and these reducing radicals reconvert As(IV)/As(V) back to As(III). The experimental results and additional evidence were discussed in detail to clarify the disputed mechanistic issues. This study will help understand the PCO mechanism of As(III) beyond dispute.
Experimental Section Chemicals and Materials. NaAsO2 (As(III), 96.7%, Aldrich) and Na2HAsO4 · 7H2O (As(V), 99%, Kanto) were used as the arsenic source. Other chemicals used include formic acid (FA, 99%, Samchun), methanol (MeOH, 99.8%, Samchun), tert-butyl alcohol (TBA, 99%, Samchun), and 4-chlorophenol (4-CP, 99%, Sigma), all of which were of reagent grade and used as received. Commercial TiO2 sample, Degussa P25, was used as a photocatalyst. N2 (Linde-Gases, 99.999%) gas was used for purging out dissolved O2 when the anoxic condition was required. Deionized water used was ultrapure (18 MΩ · cm) and prepared by a Barnstead purification system. Photolyses and Analyses. TiO2 powder was dispersed in distilled water (0.5 g/L) by simultaneous sonication and shaking for 30 s in an ultrasonic cleaning bath. [As(III)]0 was fixed at 200 µM in most experiments, and the solution pH was adjusted with HClO4 or NaOH standard solution. The solution volume in a photoreactor was 30 mL. The solutions were equilibrated in the dark for 30 min prior to illumination and were stirred magnetically throughout the photolysis. Photoirradiation employed a 300-W Xe arc lamp (Oriel) as a light source. Light passed through a 10-cm IR water filter and a UV cutoff filter (λ > 300 nm for UV irradiation and λ > 420 nm for visible irradiation), and then the filtered light was focused onto a 30-mL Pyrex reactor with a quartz window. The reactor was open to the ambient air. Sample aliquots were withdrawn from the reactor by a 1-mL syringe intermittently during illumination and filtered through a 0.45µm poly(tetrafluoroethylene) syringe filter (Millipore) to remove TiO2 particles. For the N2-saturated system, the reactor was closed with a rubber septum and purged with N2 gas continuously before and during the illumination. Multiple photolysis experiments were performed under the identical reaction condition to confirm the reproducibility. The incident light intensity was measured by using ferrioxalate actinometry (26) and measured to be about 3.46((0.26) × 10-3 einstein/L · min (λ > 300 nm). Quantitative analysis of As(V) was performed using an ion chromatograph (IC, Dionex DX-120), which was equipped with a Dionex IonPac AS14 (4 mm ×250 mm) column and a conductivity detector. The eluent solution was 3.5 mM Na2CO3/1 mM NaHCO3. Time-Resolved Diffuse Reflectance Measurements. To monitor the transient absorption in UV-excited TiO2 slurry, the time-resolved diffuse reflectance (TDR) spectroscopic measurements were performed. The third harmonic generation (355 nm, 1.5 mJ/pulse, 5 ns full width at half-maximum) from a Q-switched Nd3+: YAG laser (Continumm, Surelite II-10) was employed for the excitation operated with temporal control by a decay generator (Stanford Research Systems, DG535) (27-29). The reflected analyzing light from a pulsed 450-W Xe arc lamp (Ushio, UXL-451-0) was collected by a focusing lens and directed through a grating monochromator (Nikon, G250) to a silicon avalanche photodiode detector 9100
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FIGURE 1. (a) Time traces of transient absorption observed at 700 nm during the 355-nm laser photolysis (1.5 mJ pulse-1) of TiO2 slurry (20 g L-1) in the presence of As(III) in air-saturated acidic water (pH 3, HClO4). (b) The initial %abs values as a function of [As(III)]. The %absorption (%abs) is given by the equation %abs ) (R0-R)/R0 × 100, where R and R0 represent the intensities of the diffuse reflected monitor light with and without excitation, respectively. (Hamamatsu Photonics, S5343). The TDR signals were recorded by a digitizer (Tektronix, TDS 580D) to monitor the transient absorption of photogenerated charge carriers in TiO2 and to obtain information about the role of As(III) in TiO2 photocatalysis. The TDR measurements do not follow the redox conversion of As(III) and As(V) directly. The reported signals were the averages of 30-100 events. The aqueous slurry (pH 3, air-equilibrated) for the TDR measurements contained the catalyst powder (20 g/L) and As(III) (0.1-10 mM). The TiO2 slurry containing As(V) was also compared as a control TDR test to confirm that the presence of As(V) does not influence the charge recombination dynamics in TiO2. All experiments were carried out at room temperature.
Results and Discussion The Role of As(III) Probed by Time-Resolved Diffuse Reflectance Spectroscopy. According to the previously proposed mechanism (19) in which the As(III)/As(IV) redox couple works as an external recombination center (eqs 1-2), the presence of As(III) on TiO2 should accelerate the charge pair recombination. On the other hand, if As(III) is not working as a recombination center, its presence should retard the recombination because As(III) should scavenge VB holes (eq 1) with making the lifetime of CB electrons longer. Therefore, the proposed mechanism can be supported or rejected by monitoring the charge recombination dynamics in TiO2 in the presence of As(III). To follow the charge recombination dynamics, the transient absorption time profiles of trapped eCB- (550-800 nm) (30-33) in TiO2 were monitored during the laser excitation at 355 nm. Figure 1a compares the time profiles of the transient absorption decay at 700 nm (trapped electron) in the laserphotolyzed TiO2 slurry with varying the concentration of As(III) from 0.1 to 10 mM. The initial absorption signal intensity, though weak, decreased with increasing the
SCHEME 1. Schematic Illustration of the Unique Role of As(III) as an External Charge Recombination Center on the UV-Excited TiO2a
a
When excess formic acids are present as hole scavengers, their reaction with holes generates highly reducing radicals (HCO2 · ) which reconvert As(IV)/As(V) back to As(III).
concentration of As(III) (Figure 1b). Since the recombination is so fast that the charge recombination dynamics in the presence and absence of As(III) does not show significant difference in the µs domain, this indicates that the As(III)mediated recombination takes place within the laser pulse duration (20 µs) and small absorbance (even in the presence of As(III)), which implies that the trapped electrons can be slowly transferred to O2 producing the superoxide/hydroperoxyl radicals (HO2 · as the main species at pH 3 on the basis of pKa ) 4.8) (eq 3). On the other hand, the remaining trapped holes/ · OH react with in situ generated H2O2 or self-recombine to generate H2O2. The further reactions of the superoxide/hydroperoxyl radicals lead to the photocoversion of As(III) f As(V) through electron transfers (eqs 4 and 5) (Scheme 1) O2 + etr- f O2 ·/HO2 · (slowly beyond the µs domain) (3) O2
As(III) + O2 ·/HO2· f As(IV) 98 As(V)
(4)
As(III) + 2HO2· f As(V) + 2OH- + O2
(5)
The superoxide-mediated oxidation of As(III) may occur either on the surface or in the solution. Whether the reaction between superoxide/hydroperoxyl radicals and As(III) is limited on or close to the surface of TiO2 remains unclear. Those radicals may desorb from the surface and react with As(III) in the solution bulk. Considering that the homogeneous bimolecular rate constant between superoxide and As(III) is 3.6 × 106 M-1s-1 (21) and the observed PCO rate of As(III) is about 0.1 µM s-1 (under the condition of [As(III)]0 ) 200 µM, air-equilibrated, [TiO2] ) 0.5 g/L, pH 3), the required concentration of superoxide should be around 1 × VOL. 44, NO. 23, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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10-10 M provided that the oxidation reaction occurs in the solution. If this PCO reaction occurs largely on the surface, however, the corresponding reaction rate constant between adsorbed species and their surface concentrations are unknown. Although we cannot be quantitative about the heterogeneous PCO kinetics, the qualitative nature regarding the main oxidant species is now quite clear. Effects of Formic Acid and Methanol on PCO of As(III). The recent observation (25) that the presence of excess formic acid and methanol actually inhibited the PCO of As(III) seems to be contradictory to the proposed superoxide mechanism since the production of superoxides (eq 3) should be enhanced in the presence of hole scavengers (formic acid and methanol). The production of H2O2 was enhanced in the presence of excess hole scavengers, which implies the enhanced production of superoxides. The PCO of As(III) should increase under this condition if the superoxidemediated oxidation of As(III) is the dominant mechanism. Based on this argument, Yoon et al. (25) rejected the superoxide mechanism. However, they failed to take an important factor into account. Formic acid and methanol are oxidized to generate intermediate radicals that are highly reducing (eqs 6 and 7) + f HCO2· + H+ HCO2H + hVB
(6)
+ CH3OH + hVB f ·CH2OH + H+
(7)
The reduction potential of HCO2 · /CO2 and · CH2OH/ HCHO couple is -1.82 VNHE (36) and -0.97 VNHE (37), respectively, whereas that of TiO2 CB is only -0.28 VNHE (at pH 3). Considering the redox couple of As(IV)/As(III) (Eo ) 2.4 VNHE) (38) and As(V)/As(III) (Eo ) 0.4 VNHE at pH 3) (17), the reducing potential of the intermediate radicals ( · CH2OH and HCO2 · ) can be strong enough to reduce As(IV) and As(V) back to As(III). Such an interfering reaction of the reducing radicals would apparently inhibit the oxidation of As(III) by recycling As(V)/As(IV) back to As(III) (eqs 8 and 9) (see Scheme 1) 2HCO2· + As(V) f CO2 + As(III) + 2H+
(8)
HCO2· + As(IV) f CO2 + As(III) + H+ 2·CH2OH + As(V) f H2CO + As(III) + 2H+
(9)
·CH2OH + As(IV) f H2CO + As(III) + H+ To investigate whether the reducing radicals are really interfering, the effect of formic acid and methanol on the PCO of As(III) was compared with that of TBA (tert-butyl alcohol) that does not generates the reducing radical intermediate. Figure 3 compares the time profile of As(V) generation in the UV-irradiated suspension of TiO2 with varying the concentration of the hole scavenger. The inhibition of PCO by formic acid is significant with 2 mM and almost complete with 20 mM (Figure 3a), which is consistent with the previous report (25). Methanol retarded PCO only moderately (Figure 3b). The fact that formic acid more efficiently inhibited the PCO of As(III) than methanol at similar concentrations indicates that the surface processes are important. Formic acid is strongly adsorbed on TiO2, while methanol is not. Therefore, the reduction by HCO2 · might predominantly occur on or close to the surface. On the other hand, TBA that was used as a control hole scavenger because of its inability to generate the reducing radical did not inhibit PCO at all even with 200 mM (Figure 3c), which reconfirms our previous observation (17). This clearly verifies that formic acid and methanol generate reducing radicals 9102
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FIGURE 3. Effect of (a) formic acid (FA), (b) methanol (MeOH), and (c) tert-butyl alcohol (TBA) on the photocatalytic oxidation of As(III) in the aqueous suspension of TiO2. The experimental conditions were as follows: air-equilibrated, [TiO2] ) 0.5 g/L, pHi ) 3.0, [As(III)]0 ) 200 µM, λ > 300 nm.
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(through eqs 6 and 7) which immediately reduce As(V)/As(IV) back to As(III) (eqs 8 and 9). No effect of TBA even at 200 mM surely dismisses the possible involvement of OH radicals or VB holes. The reducing radical involvement in the PCO process was directly verified by monitoring the photoreduction of As(V) in the presence of the three hole scavengers used in Figure 3. The time-dependent removal profiles of As(V) in the UVirradiated suspension of TiO2 with the hole scavenger are shown in Figure 4. The air-equilibrated and N2-saturated suspensions are compared since dissolved O2 should scavenge the organic reducing radicals and inhibit their reaction. As mentioned previously, As(V) cannot be reduced nor oxidized on the UV-irradiated TiO2. However, 20 mM formic acid could reduce As(V), and the removal was markedly enhanced in the absence of O2. Methanol (200 mM) did not reduce As(V) at all in the air-equilibrated condition but did reduce As(V) in the absence of O2. On the other hand, TBA showed no activity regardless of the presence of O2. The present observation confirms that the organic radicals derived from formic acid and methanol can directly reduce As(V) to As(III). Therefore, it is verified that the previous claim of Yoon et al. (25) that is based on the effect of formic acid and methanol on PCO of As(III) is resulted from the misinterpretation of experimental data without realizing the role of reducing organic radicals. Incidentally, it should be mentioned that the role of reducing radicals tested in this study has little relevance to the real photocatalytic oxidation process in which such high concentrations do not exist. The pho-
FIGURE 5. Oxidation of As(III) in the suspension of 4-CP/TiO2 under visible light irradiation. The experimental conditions were as follows: air-equilibrated or N2-saturated, [TiO2] ) 0.5 g/ L, pHi ) 3.0, [As(III)]0 ) 200 µM, [4-CP]0 ) 400 µM, [TBA]0 ) 200 mM, λ > 420 nm.
FIGURE 4. Effect of FA, MeOH, and TBA on the photocatalytic reduction of As(V) in the aqueous suspension of TiO2. The experimental conditions were as follows: (a) air-equilibrated or (b) N2-saturated, [TiO2] ) 0.5 g/L, pHi ) 3.0, [As(V)]0 ) 200 µM, λ > 300 nm. The initial level of the equilibrated concentration of As(V) highly varied depending on the kind of hole scavengers because of the mutual interaction between the organic adsorbates and As(V). tocatalytic experiments using excessive concentrations of formic acid and methanol were carried out only to confirm that the previous experiments done by Yoon et al. (25) produced a misleading conclusion. Photooxidation of As(III) on Surface-Complexed TiO2 under Visible Light. Here we present a novel system where As(III) can be oxidized under visible light to confirm the role of superoxides. 4-Chlorophenol (4-CP) can form a surface complex on TiO2 (eq 10) which weakly absorbs visible light up to 500 nm through a ligand-to-metal charge transfer (LMCT). We previously demonstrated that 4-CP on TiO2 can be degraded under visible light through LMCT mechanism (eq 11) (39) ≡Ti - OH + 4 - CP T ≡ Ti - O - C6H4Cl + H2O
(10)
+ (4 - CP)·+ ≡Ti - O - C6H4Cl + hv(λ > 420 nm) f eCB (11) f OO2 + eCB 2 ·/HO2·
(12)
The electrons injected into TiO2 CB through LMCT subsequently react with O2 to generate superoxide/hydroperoxyl radicals (eq 12), which should react with As(III) through eq 4. It should be noted that neither OH radicals nor VB holes are generated under visible light irradiation. However, Figure 5 demonstrates that As(III) is oxidized to As(V) in the visible light irradiated 4-CP/TiO2 system. The only oxidant that can be generated under this condition is the superoxide. The photooxidation was completely inhibited accordingly in the absence of dissolved O2. TBA had no influence on the photooxidation, which indicates that OH radicals are not involved. This supports that the superoxide photogenerated on TiO2 is able to oxidize As(III) to As(V). The debates on the PCO mechanism of As(III) on TiO2 have continued since we first proposed the superoxide-
mediated oxidation mechanism in 2002 (17). This case presented an interesting question since no other studies of TiO2 photocatalytic systems paid attention to the role of the superoxide which is a much weaker oxidant than the hydroxyl radical and the VB hole. To support or refute the proposed mechanism, many mechanistic investigations have been carried out by adding external chemicals as a mechanistic probe that works as either an electron donor or acceptor (9, 18-25). However, most probe chemicals were added with concentrations which are high enough to interfere with the mechanism. Probing the PCO mechanism with adding chemicals may perturb and change the mechanism itself, which may induce misleading conclusions (21-23, 25). Here we investigated the charge recombination dynamics of TiO2 in the presence of As(III) only and nothing else and confirmed that As(III) has a unique role of a charge recombination center to diminish the role of hydroxyl radicals and VB holes (see Scheme 1). Under such condition, the weaker oxidants, superoxide and hyroperoxyl radicals, prevail. The present study should be clear enough to remove all doubts centered on the superoxide-mediated PCO mechanism.
Acknowledgments This work was supported by the KOSEF NRL program (No. R0A-2008-000-20068-0), KOSEF EPB center (Grant No. R112008-052-02002), and KCAP (Sogang Univ.) funded by MEST through NRF (NRF-2009-C1AAA001-2009-0093879).
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