Correspondence/Rebuttal Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
pubs.acs.org/est
Response to Comment on “Visible-Light-Driven Photocatalytic Degradation of Organic Water Pollutants Promoted by Sulfite Addition” which resulted in less production of SO•− 3 , thus impeding the MO degradation. Second, even though oxygen can react with •− •− SO•− 3 to produce SO5 , the generated SO5 could react with 2− 2− •− SO3 and produce SO5 and SO3 , which is called chain 4 reaction that can regenerate SO•− 3 . Third, because of the •− strong oxidizing capability of SO4 and SO•− 5 , they are prone to react with photogenerated electrons and it is possible they are scavenged by electrons before reacting with MO. All the above discussion and the experimental results (Figure 1a) support the conclusion that in a truly aerobic system involving BiOBr/ •− sulfite/MO under photoillumination, SO•− 4 and SO5 are not be the major reactive species for MO degradation. The rationale given by Chen et al.2 does not apply to our system that is totally different from theirs. The complexity of our sulfite-enhanced photocatalytic system also prohibits the easy scrutinization of the exact contribution of SO•− and SO•− toward MO degradation using radical 4 5 scavengers (such as ethanol and diphenylamine), as suggested by Chen et al.2 This is simply because these scavengers are electron donors and can easily react with photogenerated holes, thus impeding the generation of SO•− 3 in the first place. In conclusion, we have re-examined our BiOBr/sulfite/MO photocatalytic system under truly aerobic condition by purging the solution with air, as suggested by Chen et al. in their comment.2 We have observed markedly slower kinetics in MO degradation because of the air purging. This suggests that SO•− 4 and SO•− 5 are not the major reactive species because more rapid MO degradation should be observed when they are produced more rapidly under aerobic conditions and this was not observed. On the other hand, the additional experiments conducted here reveal the adverse effect of aeration in our sulfite-enhanced photocatalytic system. This is contrary to traditional expectations in pure photocatalytic systems or in systems assisted by oxysulfur radicals where aeration in general would promote degradation of organics. This demonstrates the uniqueness of our approach of combining photocatalysis and sulfite radicals in the removal of water pollutants.
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n a recent article, we have developed a sulfite-enhanced visible-light-driven photodegradation process that is applicable as a general approach to promote the degradation of organic water pollutants and we suggested that sulfite radicals (SO•− 3 ) are the active species responsible for methyl orange (MO) photodegradation using BiOBr as the model photocatalyst under visible light illumination.1 In a comment to our article, Chen et al.2 agreed with our conclusion that sulfite radicals are responsible for MO degradation under anaerobic conditions; however, they argued that our experiments conducted open to air with constant stirring were essentially anaerobic, instead of aerobic, because both sulfite species and sulfite radicals possess strong capability to deplete dissolved oxygen, forming peroxomonosulfate radical (SO•− 5 ) and sulfate •− •− •− radical (SO•− 4 ). They believe that SO5 and SO4 , not SO3 , are responsible for MO degradation under truly aerobic conditions. In another work by Chen et al.,3 they reported that SO•− 4 is a much stronger oxidant than SO•− 3 for organic compounds such as acid orange 7 (AO7) because its reaction rate constant with 3 AO7 is at least 100 times higher than that of SO•− 3 . In their 2 comment, Chen et al. reported significantly different kinetics for MO degradation in nitrogen and oxygen purged Cu(II)/ sulfite systems. Interestingly, their results show that Cu(II)/ sulfite with O2 had much slower reaction rates for MO degradation than without O2. This experimental result •− contradicts their rationale that SO•− 4 and SO5 are the major responsible species under aerobic condition. If these radicals that are produced under aerobic conditions were the responsible species, then rates of MO degradation would be higher under aerobic conditions. We have followed the suggestion of Chen et al.2 to reevaluate our BiOBr/sulfite/MO system under truly aerobic condition by purging it with a stream of 100 mL/min air, and we have compared the MO degradation kinetics with and without air purging. As displayed in Figure 1a, it is obvious that air purging retards the degradation of MO, which is evidence •− that SO•− 4 and SO5 are not the major reactive species in our BiOBr/sulfite/MO system. We have monitored the sulfite concentration in our system and we indeed observed faster sulfite depletion with air purging (Figures 1b and 1c). This result agrees with Chen et al.2 and the literature that sulfite can be scavenged by oxygen. However, in our reaction system (BiOBr/sulfite/MO), SO•− 3 is the major reactive species even under truly aerobic conditions. Possible explanations for this are given below. First, our BiOBr/sulfite/MO system under photoillumination is much more complex than and very different from the Cu(II)/sulfite system used by Chen et al.2 Sulfite radicals are generated in our system by reaction of photogenerated holes with sulfite; hence, as long as photoillumination continues and sulfite is present, the generation of sulfite radicals will continue. The air purging depleted sulfite more rapidly (Figure 1c), © XXXX American Chemical Society
Wei Deng† Huilei Zhao† Fuping Pan† Xuhui Feng† Bahngmi Jung‡ Ahmed Abdel-Wahab‡ Bill Batchelor§ Ying Li*,† †
Department of Mechanical Engineering, Texas A&M University, College Station, Texas United States ‡ Chemical Engineering Program, Texas A&M University at Qatar, Doha, Qatar
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DOI: 10.1021/acs.est.7b06200 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
Figure 1. Kinetics of sulfite-enhanced photodegradation of MO with and without 100 mL/min air purging under visible light irradiation (a), and the − changes of UV−vis absorption band of SO2− 3 /HSO3 in the BiOBr/sulfite/MO photocatalytic system without air purging (b) and with air purging (c). The initial concentration of MO, sulfite, and BiOBr were 10 ppm, 10 mM, and 0.5 g/L, respectively. HCl was added to adjust the initial pH to 7.5. §
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Department of Civil Engineering, Texas A&M University, College Station, Texas United States
AUTHOR INFORMATION
Corresponding Author
*Phone: +1-979-862-4465; e-mail:
[email protected]. ORCID
Fuping Pan: 0000-0001-9171-0726 Bill Batchelor: 0000-0002-9885-4535 Ying Li: 0000-0002-6775-5649 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was made possible by a grant from the Qatar National Research Fund under its National Priorities Research Program award number NPRP 8-1406-2-605. The paper’s contents are solely the responsibility of the authors and do not necessarily represent the official views of the Qatar National Research Fund.
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REFERENCES
(1) Deng, W.; Zhao, H.; Pan, F.; Feng, X.; Jung, B.; Abdel-Wahab, A.; Batchelor, B.; Li, Y. Visible-light-driven photocatalytic degradation of organic water pollutants promoted by sulfite addition. Environ. Sci. Technol. 2017, 51, 13372−13379. (2) Chen, L.; Ding, W.; Wu, F. Comment on “Visible-Light-Driven Photocatalytic Degradation of Organic Water Pollutants Promoted by Sulfite Addition. Environ. Sci. Technol. 2017, 10.1021/acs.est.7b05875. (3) Zhou, D.; Yuan, Y.; Yang, S.; Gao, H.; Chen, L. Roles of oxysulfur radicals in the oxidation of acid orange 7 in the Fe (III)−sulfite system. J. Sulfur Chem. 2015, 36 (4), 373−384. (4) Das, T. N. Reactivity and role of SO5•-radical in aqueous medium chain oxidation of sulfite to sulfate and atmospheric sulfuric acid generation. J. Phys. Chem. A 2001, 105 (40), 9142−9155.
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DOI: 10.1021/acs.est.7b06200 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
