CORRESPONDENCE/REBUTTAL pubs.acs.org/est
Comment on “Photocatalytic Oxidation of Arsenite over TiO2: Is Superoxide the Main Oxidant in Normal Air-Saturated Aqueous Solutions?” ecently, Fei et al.1 reported a photoelectrochemical (PEC) study on the arsenite (As(III)) oxidation using illuminated TiO2 electrodes under aerated conditions, concluding that under open-circuit conditions both the photogenerated holes and superoxide radicals participate in the global photooxidation process, but holes have a higher contribution (57%) than superoxide and its derivatives (43%). After a thorough reading of the paper, we would like to comment on the following points. We are happy to see that the authors finally admitted that superoxide plays a significant role in the photocatalytic oxidation (PCO) of As(III), as we claimed in our previous reports.2 4 However, in this paper1 and their previous comments,5,6 the authors seem to be unduly biased with PEC data in discussing the PCO mechanism in the TiO2 slurry system. The commented paper is entirely about a PEC study of arsenite oxidation but has a misleading title that states “photocatalytic oxidation...in airsaturated aqueous solution”. The photocatalytic and PEC reactions are closely related but they can be very different in cases like the As(III) oxidation in which the biased potential sensitively influences the interfacial charge transfer (especially the critical recombination step: As(IV) + ecb f As(III)). Changing the electrode potential would eventually change both the oxidant and charge carriers concentrations (electrons, holes, superoxide, and derived radicals) and the kinetic constants of the elemental charge transfer steps to alter the rate determining step and hence the overall mechanism. What Fei et al.1 studied and concluded is about the PEC mechanism, not PCO mechanism. The PEC investigation provides useful data for understanding photocatalysis mechanism, but it does not accurately represent the photocatalytic system itself. Fei et al.1 failed to provide a balanced view on the PCO mechanism of As(III) with strong bias toward PEC data but against more direct photocatalytic evidence. They simply disregarded numerous evidence obtained from the slurry PCO systems2 4 without showing any solid scientific data to disprove them. For example, the direct evidence of transient spectroscopic data supporting the role of As(III) as a charge recombination center2 was simply dismissed as “questionable” despite our reply to their concerns.7 We wonder how they can be so sure about the credibility of our data even without trying to reproduce them. The claim by Fei et al. against our data and interpretation should be based on more complete scientific reasonings. A full mechanistic assessment should take into account all evidence coming from PCO experimental data, which Fei et al. failed to do. More specific comments are as follows. The authors compared the effect of superoxide radicals in the dark and under illumination in Figures 2a and 3a,1 concluding that the photooxidation under illumination was accelerated by the presence of more efficient oxidants than superoxide. This is not a fair comparison, as the concentration of superoxide should not be the same in the dark and under illumination. The claim
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Figure 1. The time profiles of As(III) oxidation to As(V) on UVirradiated TiO2 electrode, in the presence or absence of 0.1 M TBA (OH radical scavenger), at open-circuit condition and biased at 0.6 V vs SCE. Working electrode: P25 TiO2 deposited on 4 cm2 FTO. Counter electrode: Pt coiled wire. Reference electrode: SCE. Electrolyte: airequilibrated 0.5 M NaClO4 solution, buffered at pH 3; [As(III)]0 = 500 μM.
can be justified only if both systems contain a similar concentration of superoxides. Second, they claim that under illumination, the production of As(V) in aerated conditions (Figure 3a1) was accelerated even at negative bias (few holes available), due to the probable conversion of superoxide to hydroxyl radicals. To investigate the role of hydroxyl radicals in this case, we performed the similar PEC experiments in the presence and absence of tertbutyl alcohol (TBA, an OH radical scavenger). As shown in Figure 1, the presence of excess amount of TBA retarded the production of As(V) only by 34% and 19% (after 120 min irradiation) in the open-circuit and negatively biased condition, respectively, which indicates that superoxides should contribute more to the oxidation of As(III) than OH radicals. With a positive bias, the electrons are scavenged and the PEC oxidation of As(III) should be mediated by holes in the absence of O2 (Figure 3b1). On the other hand, it should be noted that the effect of TBA in this PEC system is quite different from the slurry Published: October 17, 2011 9816
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Environmental Science & Technology PCO system in which the presence of excess TBA does not retard the oxidation rate at all.2 Even at the open-circuit condition which is the closest to the PCO system, a measurable inhibition effect of TBA was observed (Figure 1, upper panel). This observation clearly indicates that the PEC system cannot faithfully represent the PCO system in the present case of As(III) oxidation. A direct comparison between the photocatalytic mechanism in slurry and PEC mechanism on the electrode should be very carefully carried out, considering that the photocatalytic mechanism sensitively changes depending on many experimental parameters (especially in the case of As(III) PCO) as we demonstrated in the previous reports.2 4 Between PCO and PEC systems, there are many differences in the number of irradiated particles, the number of charge carriers per particle, the available surface area, the mass transfer rate, the interparticle electron transfer rate, and the substrate/surface ratio, which eventually influence the overall mechanisms. As such conditions are difficult to be balanced between both systems, all mechanistic studies must be taken with care as their nature is not universally conclusive. Hence, we strongly disagree with Fei et al.1 in their claim that their proposed PEC method could provide direct and undisputed evidence to reveal the true mechanism in PCO. The mechanism drawn from the PEC investigation cannot be always applied to the PCO system.
CORRESPONDENCE/REBUTTAL
(6) Leng, W. H.; Li, H.; Fei, H.; Zhang, J. Q.; Cao, C. N. Comment on “Photocatalytic oxidation mechanism of As(III) on TiO2: Unique role of As(III) as a charge recombinant species”. Environ. Sci. Technol. 2011, 45, 2028–2029. (7) Monllor-Satoca, D.; Tachikawa, T.; Majima, T.; Choi, W. Response to comment on photocatalytic oxidation mechanism of As(III) on TiO2: Unique role of As(III) as a charge recombinant species. Environ. Sci. Technol. 2011, 45, 2030–2031.
Damian Monllor-Satoca and Wonyong Choi* School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea
’ AUTHOR INFORMATION Corresponding Author
*Fax: +82-54-279-8299, e-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the KOSEF NRL program (No. R0A-2008-000-20068-0) and KOSEF EPB center (Grant No. R11-2008-052-02002) ’ REFERENCES (1) Fei, H.; Leng, W.; Li, X.; Cheng, X.; Xu, Y.; Zhang, J.; Cao, C. Photocatalytic oxidation of arsenite over TiO2: Is superoxide the main oxidant in normal air-saturated aqueous solutions? Environ. Sci. Technol. 2011, 45, 4532–4539. (2) Choi, W.; Yeo, J.; Ryu, J.; Tachikawa, T.; Majima, T. Photocatalytic oxidation mechanism of As(III) on TiO2: unique role of As(III) as a charge recombinant species. Environ. Sci. Technol. 2010, 44, 9099–9104. (3) 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. (4) Ryu, J.; Choi, W. Effects of TiO2 surface modifications on photocatalytic oxidation of arsenite: The role of superoxides. Environ. Sci. Technol. 2004, 38, 2928–2933. (5) Leng, W. H.; Cheng, X. F.; Zhang, J. Q.; Cao, C. N. Comment on “Photocatalytic oxidation of arsenite on TiO2: Understanding the controversial oxidation mechanism involving superoxides and the effect of alternative electron acceptors”. Environ. Sci. Technol. 2007, 41, 6311–6312. 9817
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