Correspondence Comment on “Photocatalytic Oxidation of Arsenite on TiO2: Understanding the Controversial Oxidation Mechanism Involving Superoxides and the Effect of Alternative Electron Acceptors” The article “Photocatalytic Oxidation of Arsenite on TiO2: Understanding the Controversial Oxidation Mechanism Involving Superoxides and the Effect of Alternative Electron Acceptors,” by Choi et al. (1) appearing in a recent issue of Environmental Science & Technology, discusses the photocatalytic oxidation (PCO) mechanism of As(III). By carrying out photoelectrochemical measurements and investigating the effects of competitive substrates on As(III) PCO, the authors concluded that superoxides are the main oxidant in aqueous TiO2 suspensions with dissolved O2 as a sole electron acceptor. As interested yet critical readers of the article, we must raise serious doubts on the appropriateness and validity of the interpretations of the experimental data. Comments on three specific issues presented in the paper follow. First, the authors claimed that the photocurrent monitored should indicate whether a substrate is oxidized (electrondonating) or reduced (electron-accepting), and that an electron-donating process occurring on the TiO2 electrode should increase the photocurrent. These statements are not really true. In fact, photocurrent depends on many factors, such as charge-transfer rate constant, concentration of reactant, and recombination of photogenerated carriers, etc. (2). For simplification but without losing universality, we can assume that photogenerated holes on the surface can transfer directly to any surface species such as OHsurfand/or water k1
• h+ VB + OHsurf and/or H2O98OH
v1 ) k1[h+ VB]surf[OH ]surf (1)
k2
OH• + eCB 98OHs
v2 ) k2[OH•]surf [eCB]surf
(2)
and/or other electron donors such as As(III) k3
3+ v3 ) k3[h+ VB]surf[As ]surf
(3)
k4
v4 ) k4[As4+]surf[eCB]surf
(4)
3+ h+ 98 As4+ VB + As 3+ As4+ + eCB 98 As
The photocurrent may be written as eq 5 in bank electrolyte and eq 6 in the presence of As(III) without subtracting the part resulting from the reduction of oxygen. Iblank ) F(v1 - v2) ) Fk1[h+ ph VB]surf[OH ]surf -
Fk2[OH•]surf [eCB]surf (5)
+ 3+ IAs ph ) F(v3 - v4 + v′1 - v2) ) Fk3 [hVB]surf[As ]surf + Fk4[As4+]surf[eCB]surf + Fk1[hVB]surf[OH ]′surf -
Fk2[OH•]surf [eCB]surf (6)
All the signs above have their usual meanings. Note that the surface hydroxyl groups concentration with and without 10.1021/es070349n CCC: $37.00 Published on Web 07/31/2007
2007 American Chemical Society
As(III) may be different. So judging only from the relative and IAs magnitude of Iblank ph ph, one cannot evaluate whether As(III) adsorbed on the surface of TiO2 is oxidized or reduced. In other words, an electron-donating process occurring on the electrode does not necessarily increase the photoanodic current, and also an observed photocurrent drop on addition of substance in solution does not necessarily indicate it is an electron acceptor. The authors have found that As(III) is oxidized in the experiments, and that the addition of As(V) hardly influenced the photocurrent, then it may be described as + 3+ IAs ph ) F(v3 + v′1 - v2) ) F[hVB]surf {k3[As ]surf +
Fk1[OH-]′surf} - Fk2[OH•]surf [eCB]surf (7)
Comparing eqs 7 and 5 we can still not make sure that blank IAs ph is larger than Iph , since the relative magnitude between k3[As3+]surf + k1[OHs-]′surf and k1[OHs-]surf is uncertain. The paradox given by the authors that As(III) is oxidized but its presence decreases the photocurrent can be at least rationalized by considering the above equations. In addition, we found (not shown) that the dark current of the TiO2 electrode under cathodic polarization is slightly larger with As(III) than in the air-saturated blank electrolyte, which suggests that electron transfer is faster to the solution, thus may decrease more photocurrent with As(III). So, the authors’ statement that if As(III) adsorbed on the surface of TiO2 is mainly oxidized by valence band (VB) holes, the photocurrent should be enhanced when As(III) is added into the solution, is loose logic, and their claim that the observed photocurrent drop indicates that As(III) oxidation cannot be induced by a direct hole transfer, should be baseless. Second, they employed an observation that the photocurrent with formate dropped when As(III) was spiked, to explain that photocatalytic degradation of benzoic acid (BA) and formate were delayed in the presence of As(III), thus to support their claim that such a delay was because OH radicals (or VB holes) are depleted through the charge-recombining null cycles as long as As(III) is present on the TiO2 surface. We wish to point out that such a photocurrent change cannot possibly support their claim. We conducted a similar photoelectrochemical test with formate and BA, and the results are shown in Figure 1. The photocurrent with formate dropped when As(III) was spiked, but it increased with further increasing the concentration of As(III). However, the photocurrent with BA increased when As(III) was added. According to their views the photocurrent in this case should decrease as As(III)/As(IV) acts as a charge recombination center. In fact, another explanation, not taken into account by the authors, that might be invoked to explain the decrease in photocurrent with formate, is that addition of As(III) might inhibit the effect of photocurrent-doubling since formate is a current doubling agent (3). So their claim that hydroxyl radicals were not the main oxidant of As(III) PCO seems to be weakly supported. Third, to account for the mentioned paradox, the authors argued that with As(III) adsorbed on TiO2, the redox couple of As(III)/As(IV) rapidly mediated the recombination of charge pairs while depleting VB holes and adsorbed OH radicals in null cycles with electrons, and therefore that under such condition the main oxidant of As(III) PCO was superoxides. This is also hard to believe. Since the photogenerated electrons and holes are in pairs produced by irradiation, if most of the electrons are consumed by As(III)/As(IV) charge VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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the VB edge at TiO2 surface and As(III)/As(IV) (E° ) 2.4 VNHE) (1), is so favorable that the holes can in theory transfer efficiently to As(III). So, As(III) PCO can preferentially occur even with BA or formate. The energy level of the most efficient surface recombination center is in theory located in the middle of energy band edges of semiconductors (6), since the location of level of the redox is close to the VB edges, and from this point of view, the adsorbed As(III)/As(IV) cannot possibly act as the most efficient charge recombination center.
Literature Cited
FIGURE 1. Change of photocurrent generated with a TiO2/ITO electrode immersed in the solution of formate and benzoic acid upon spiking an aliquot of As(III). The experimental conditions were similar to those in the paper on which we are commenting: [LiClO4] ) 10 mM, applied potential ) +0.1 V (vs SCE), pHi ) 3, and airequilibrated. recombination center, the question remains, then, how are superoxides formed? In summary, the employed photoelectrochemical approach is, in our opinion, lacking proper design to allow postulation of superoxides being the main oxidant in aqueous TiO2 suspensions. As long as other more plausible explanations like the ones given above are not considered such a statement is invalid and bare of fundamental proof. The findings of the commented article should, hence, be taken with care, especially if these data are intended to be used for mechanism assessment purposes. So far we call most attention to the use of photoelectrochemical methods and reliable interpretations deduced thereof. Although superoxides were involved in the As(III) PCO process as suggest by the authors. However, we wish to tentatively point out that holes may be involved in its oxidation as suggested by Yoon et al. (4). It is well accepted that the electron tunneling process must take place between states of equal energy (5). The overlap of energy level between
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(1) Ryu, J.; Choi, W. Photocatalytic oxidation of arsenite on TiO2: Understanding the controversial oxidation mechanism involving superoxides and the effect of alternative electron acceptors. Environ. Sci. Technol. 2006, 40, 7034-7039. (2) Mandelbaum, P. A.; Regazzoni, A. E.; Blesa, M. A.; Bilmes, S. A. Photo-electro-oxidation of alcohols on titanium dioxide thin film electrodes. J. Phys. Chem. B 1999, 103, 5505-5511. (3) Byrne, J. A.; Eggins, B. R.; Mailley, S. L.; Dunlop, P. S. M. Analyst 1998, 123, 2007-2012. (4) Yoon, S.-H.; Lee, J. H. Oxidation mechanism of As(III) in the UV/TiO2 System: Evidence for a direct hole oxidation mechanism. Environ. Sci. Technol. 2005, 39, 9695-9701. (5) Gerischer, H. Electron-transfer kinetics of redox reactions at the semiconductor/electrolyte contact. A new approach. J. Phys. Chem. 1991, 95, 1356-1359. (6) Lewis, N. S.; Rosenbluth, M. In Photocatalysis: Fundamentals and Applications; Serpone, N., Pelizzetti, E., Eds.; John Wiley & Sons: New York, 1989; p 45.
W. H. Leng* and X. F. Cheng Department of Chemistry, Yuquan Campus, Zhejiang University, Hangzhou, 310027, China
J. Q. Zhang and C. N. Cao Department of Chemistry, Yuquan Campus, Zhejiang University, Hangzhou, 310027, China, and State Key Laboratory for Corrosion and Protection of Metals, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China ES070349N
