Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Activation of Persulfate by Graphitized Nanodiamonds for Removal of Organic Compounds”
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radical species on phenol oxidation. Also note that the EPR analysis using spin-trapping agents is often imperfect to identify radical species because the spin adduct can be formed through different pathways.6 As described in our study,2 the negligible oxidation of benzoic acid (its minor removal was found to be due to the adsorption) indicates that the radical pathway is not important. Duan et al. also observed that the removal efficiency of benzoic acid is very low in their system;5 this minor removal might be also due to the adsorption onto the carbon material. Additional experimental data showed that the oxidation of benzoic acid by the UV−C/persulfate system (a well-known SO4•− source) proceeded at a similar rate to that of phenol (Figure 1a). If sulfate radical were a predominant oxidant as Duan et al. claimed, one should observe similar oxidation of benzoic acid in G-ND/persulfate system.
e appreciate the opportunity to respond to the comments of Duan et al.1 on our recent article.2 The issues regarding roles of radical species and singlet oxygen raised by Duan et al.1 are addressed below with additional experimental data, but other comments of minor importance are not dealt with here due to the page limit.
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RADICAL SPECIES Regarding the controversial issues on the mechanism of persulfate activation by carbon materials (i.e., radical versus nonradical pathways), either of the two pathways cannot be completely excluded. What matters is which pathway is more dominant. After a comprehensive review of our experimental data,2 we have concluded that the nonradical pathway is the dominant mechanism of persulfate activation by graphitized nanodiamonds (G-NDs). About the effects of radical scavengers, Duan et al.1 criticized that the dose of radical scavengers used in our study (200 mM) was too low. They particularly pointed out that the concentration of radical scavenger was only 200 times that of persulfate; but what matters here is the concentration relative to, not persulfate, but radicals which is at many orders of magnitude lower concentration. We believe that the scavenger dose is sufficient to quench most of radicals, also considering its diffusion-limited reaction kinetics with radicals. Whether the observed inhibition of phenol oxidation by radical scavengers is significant or not may need further discussion, since that there are indeed some uncertainties about the effects of high-dose radical scavengers on the nonradical oxidation of phenol. Related to this discussion, we note that Duan et al. made inconsistent interpretations on the inhibitory effects of radical scavengers in their own studies. For example, in one study on N-doped CNTs, they emphasized the nonradical mechanism and stated that “NoCNT-350 and NoCNT-700 still maintained excellent phenol degradation ef f iciency at a high concentration of radical quenching agent with PMS”,3 whereas they suggested a radical mechanism in another study on N-modified nanodiamonds, stating that “the remarkable decrease in the rate constants in this study suggests that the reactive radicals played dominant roles in phenol degradation”.4 However, comparing the experimental results of those two studies, the inhibitory effects of radical scavengers were in fact greater for N-doped CNTs than N-modified NDs.3,4 We have no explanation for the discrepancy in electron paramagnetic resonance (EPR) data between ours and Duan et al.’s.2,5 It may be attributed to the differences in experimental conditions (including possible differences in carbon materials). It should be noted that the detection of radical signals does not guarantee that the radical pathway prevails against the nonradical pathway. We note that the EPR data in Duan et al.’ study5 did not show any significant difference in the signal intensity between annealed and pristine nanodiamonds even though the rates of phenol oxidation on the two materials were immensely different, which raises a question about the role of © XXXX American Chemical Society
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SINGLET OXYGEN Duan et al. claimed that the EPR analysis confirmed the generation of singlet oxygen (1O2).1 However, once again, the detection of the reactive species does not necessarily mean that it is majorly responsible for the target compound oxidation. To provide more insight into the contribution of 1O2 for the phenol oxidation by the G-NDs/persulfate system, additional experiments were conducted to compare the oxidation kinetics of phenol and furfuryl alcohol (a 1O2 probe compound) with G-NDs/persulfate and photosensitized rose bengal (a benchmark source of 1O2) systems (Figure 1b and c). Only photosensitized rose bengal oxidized furfuryl alcohol (Figure 1b), whereas the G-NDs/persulfate system selectively oxidized phenol over furfuryl alcohol (Figure 1c), supporting that the role of 1O2 in the G-NDs/persulfate is most likely insignificant.
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OTHERS Duan et al. claimed that the substantial persulfate decomposition without phenol implies “G-NDs served as a catalyst rather than just a bridge for persulfate activation”.1 However, we believe that the persulfate decomposition without phenol does not prove the generation of reactive oxidants either; persulfate may be directly decomposed to sulfate ion. In addition, we do not clearly understand the difference between “catalyst” and “bridge” mentioned by Duan et al. We have not stated that GND is not a catalyst. We agree with their claim, “charge conductivity of the carbon materials was not the key factor for PSdriven oxidation”, but note that we have not mentioned this in our study.2 We believe that the mechanism of persulfate activation on carbon materials cannot be delineated by a few simple factors, and further clarifications are needed to elucidate it.
Hongshin Lee† Changha Lee*,‡
A
DOI: 10.1021/acs.est.7b01642 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
Figure 1. (a) Oxidaition of phenol and benzoic acid by the UV−C/PDS system: [Phenol]0 = [Benzoic acid]0 = 10 μM, [PDS]0 = 0.1 mM, pH 7 (1 mM phosphate buffer), UV−C illumination (from low pressure Hg lamps). Competitive oxidation of phenol and furfuryl alcohol by the photosensitized rose bengal (b) and G-NDs/PDS (c) systems: [Phenol]0 = [Furfuryl alcohol]0 = 10 μM (an equimolar mixture of the two compounds was used), [G-ND]0 = 0.1 g/L, [PDS]0 = 1 mM, [Rose bengal]0 = 5 μM, pH 7 (1 mM phosphate buffer), visible light illumination (from fluorescence lamps with a λ > 400 nm long pass filter).
Jae-Hong Kim*,†
(6) Goldstein, S.; Meyerstein, D.; Czapski, G. The Fenton reagents. Free Radical Biol. Med. 1993, 15, 435−445.
†
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Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States ‡ School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulju-gun, Ulsan 689-798, Republic of Korea
AUTHOR INFORMATION
Corresponding Authors
*(C.L.) Phone: +82-52-217-2812; fax: +82-52-217-2809; email:
[email protected]. *(J.-H.K.) Phone: +1-203-432-4386; fax: +1-203-432-4387; email:
[email protected]. ORCID
Jae-Hong Kim: 0000-0003-2224-3516 Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Duan, X. G.; Sun, H. Q.; Wang, S. B. Comment on “Activation of Persulfate by Graphitized Nanodiamonds for Removal of Organic Compounds. Environ. Sci. Technol. 2017, DOI: 10.1021/acs.est.7b00399. (2) Lee, H.; Kim, H.; Weon, S.; Choi, W.; Hwang, Y. S.; Seo, J.; Lee, C.; Kim, J. H. Activation of persulfates by graphitized nanodiamonds for removal of organic compounds. Environ. Sci. Technol. 2016, 50, 10134−10142. (3) Duan, X. G.; Sun, H. Q.; Wang, Y. X.; Kang, J.; Wang, S. B. Ndoping-induced nonradical reaction on single-walled carbon nanotubes for catalytic phenol oxidation. ACS Catal. 2015, 5, 553−559. (4) Duan, X. G.; Ao, Z. M.; Li, D. G.; Sun, H. Q.; Zhou, L.; Suvorova, A.; Saunders, M.; Wang, G. X.; Wang, S. B. Surface-tailored nanodiamonds as excellent metal-free catalysts for organic oxidation. Carbon 2016, 103, 404−411. (5) Duan, X. G.; Su, C.; Zhou, L.; Sun, H. Q.; Suvorova, A.; Odedairo, T.; Zhu, Z. H.; Shao, Z. P.; Wang, S. B. Surface controlled generation of reactive radicals from persulfate by carbocatalysis on nanodiamonds. Appl. Catal., B 2016, 194, 7−15. B
DOI: 10.1021/acs.est.7b01642 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
