CORRESPONDENCE/REBUTTAL pubs.acs.org/est
Response to Comment on “Photocatalytic Oxidation of Arsenite over TiO2: Is Superoxide the Main Oxidant in Normal Air-Saturated Aqueous Solutions?”
W
e appreciate the opportunity to respond to the comments of Choi et al.1 on our recent article.2 We believe that the comments are completely incorrect, irrelevant, and reflect a lack of understanding of our paper. The first argument in the comment is not true where it is stated that we have finally admitted the superoxide plays a significant role in the photocatalytic oxidation (PCO) of As(III) as they claimed in their previous four reports. We have never excluded that the superoxide may play a role in the system, and just disagree with their claim that it is the main oxidant because they failed to provide any convincing evidence to support this. Our conclusion is that the contribution of superoxide is not more than that of photohole even if that of its derivates is included.2 We are surprised that they said our conclusion was in agreement with their claim. The following argument in the comment where they argued that our mechanism is not a PCO mechanism and that it has a misleading title is also wrong. We stated clearly in our paper2 that the mechanism or contribution of superoxide and photohole was obtained under open-circuit (OC) and the title question was accurately answered by the results at OC. Yet this fact is overlooked, obviously reflecting a lack of understanding of our paper. Their comments about the role of potential are common sense in this field. Note that the mentioned recombination process via As(IV) does not affect the validness of the flux-matching principle (eq 5 2). Potential does not affect the validness of charge columbic efficiency (not quantity) though it affects both charge density and rate constant, as only the transferred interfacial charge was needed for deriving the efficiency. They argued that we disregarded their evidence obtained from the slurry PCO systems without showing data to disprove them. This is also not true. For example, our results in both the previous3 and the commented paper go against their arguments. Importantly, their evidence was based on the addition of competitive additives that could alter the normal mechanism and is even contradictory.2 Thus, it is invalid. Actually, all their important evidence in the four papers cited by us in ref 2 was further questioned. Particularly, they argued that their reply to our concerns to the transient diffuse reflectance (TDR) results was ignored. This is also untrue as we have presented it already.2 Again, for instance, the TDR results are even in contrast with that of photocurrent which was actually increased with As(III) concentration but the untrue photocurrent data (due to sampling time too long, with O2, and etc.) were chosen to justify their claim of As(III) acting as a charge recombination center as we commented.2 We wonder why the actual mechanism depends on research methods. Furthermore, they even used misconception of lifetime of charge to explain their TDR results (lifetime depends the extent of the absolute TDR value decays.). We do not think it is necessary to explain more here within the limited journal space. In fact, as long as only one opposite fact like the r 2011 American Chemical Society
one given above cannot be explained by their arguments their conclusions should be invalid, thus we do not need to reproduce their data. Their two specific comments about the accelerated oxidation of superoxide by illumination are outside the scope of our article as we mentioned previously, and actually it has been briefly addressed.2 For example, it is supported directly by a combination of much smaller columbic efficiency of electron (Figure 4 2), comparable current (Figure 1 2) or charge, and much smaller rate of As(III) oxidation in the dark compared to under illumination, and indirectly by the results of Figure S2.2 Importantly, these comments are irrelevant to the key point of our paper, partitioning the two mechanisms, because our conclusion does not depend how and whether it is accelerated. On the contrary, their claim that superoxide should contribute more to the oxidation than hydroxyl radicals from a small retardation of tert-butyl alcohol (TBA) under both OC and negatively biased condition is completely invalid, because it is based on (i) a misunderstanding that •OH was the only converted stronger oxidant than superoxide under a negative bias (we did not exclude others); (ii) an unproved assumption that the oxidation of TBA could fully compete with that of As(III) (probably not, possibly due to it is just a surface reaction), and the last but not the least (iii) a dogma that TBA is an •OH scavenger and consequently should retard the PCO of As(III) under OC (actually it depends on the difference in rate between that retarded by TBA through holemediated reactions and that enhanced by superoxide-mediated reactions because of an increased electron density as we mentioned2). Considering these, it is enough to conclude that their claim is wrong and even opposite to common sense. The argument is also untrue where it is stated that the photoelectrochemical system cannot faithfully represent the PCO system for As(III) oxidation, based on an observation that the oxidation was retarded by excess TBA with the film electrode but not with slurry TiO2. Reemphasizing that our mechanism is the one under no bias. Also, we argue this is an unfair comparison because an improper electrode nanostructure TiO2/FTO instead of the polycrystalline and almost compact one we chose, was employed where there were many uncertainties such as a possible difference in the O2 reduction behavior (thus affecting superoxide) over the TiO2 and FTO (exposed to solutions) that may affect the As(III) PCO, yet these factors were not considered for comparison. The lengthy comments in the last paragraph are doctrines and cannot be used either to support their proposed superoxide mechanism or as an excuse that the PCO mechanism cannot be probed by proper photoelectrochemical methods. In summary, all their comments do not undermine our conclusions at all. Published: October 26, 2011 9818
dx.doi.org/10.1021/es2036042 | Environ. Sci. Technol. 2011, 45, 9818–9819
Environmental Science & Technology
CORRESPONDENCE/REBUTTAL
Wenhua Leng,* Hui Fei, and Jianqing Zhang Department of Chemistry, Yuquan Campus, Zhejiang University, Hangzhou, Zhejiang 310027, China
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This project was supported by the National Basic Research Program of China (Grant No. 2011CB936003) and the National Science Foundation Council of China (NSFC, Grant No. 50971116). ’ REFERENCES (1) Monllor-Satoca, D.; Choi, W., Comment on “Photocatalytic oxidation of arsenite over TiO2: Is superoxide the main oxidant in normal air-saturated aqueous solutions? Environ. Sci. Technol. 2011, 45. (2) 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. (3) 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.
9819
dx.doi.org/10.1021/es2036042 |Environ. Sci. Technol. 2011, 45, 9818–9819
