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Response to Comment on “Activation of Persulfate by Graphitized Nanodiamonds for Removal of Organic Compounds”

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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
*(C.L.) Phone: +82-52-217-2812; fax: +82-52-217-2809; e-mail: [email protected]
*(J.-H.K.) Phone: +1-203-432-4386; fax: +1-203-432-4387; e-mail: [email protected]
Cite this: Environ. Sci. Technol. 2017, 51, 9, 5353–5354
Publication Date (Web):April 14, 2017
https://doi.org/10.1021/acs.est.7b01642
Copyright © 2017 American Chemical Society
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We 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.

Radical Species

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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 efficiency 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.’ study (5) 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 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.

Figure 1

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).

Singlet Oxygen

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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.

Others

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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 G-ND is not a catalyst. We agree with their claim, “charge conductivity of the carbon materials was not the key factor for PS-driven 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.

Author Information

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  • Corresponding Authors
    • Changha Lee - 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 Email: [email protected]
    • Jae-Hong Kim - Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United Stateshttp://orcid.org/0000-0003-2224-3516 Email: [email protected]
  • Author
    • Hongshin Lee - Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States
  • Notes
    The authors declare no competing financial interest.

References

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This article references 6 other publications.

  1. 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. 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 DOI: 10.1021/acs.est.6b02079
  3. 3
    Duan, X. G.; Sun, H. Q.; Wang, Y. X.; Kang, J.; Wang, S. B. N-doping-induced nonradical reaction on single-walled carbon nanotubes for catalytic phenol oxidation ACS Catal. 2015, 5, 553 559 DOI: 10.1021/cs5017613
  4. 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 DOI: 10.1016/j.carbon.2016.03.034
  5. 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 DOI: 10.1016/j.apcatb.2016.04.043
  6. 6
    Goldstein, S.; Meyerstein, D.; Czapski, G. The Fenton reagents Free Radical Biol. Med. 1993, 15, 435 445 DOI: 10.1016/0891-5849(93)90043-T

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  • Figure 1

    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).

  • References

    ARTICLE SECTIONS
    Jump To

    This article references 6 other publications.

    1. 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. 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 DOI: 10.1021/acs.est.6b02079
    3. 3
      Duan, X. G.; Sun, H. Q.; Wang, Y. X.; Kang, J.; Wang, S. B. N-doping-induced nonradical reaction on single-walled carbon nanotubes for catalytic phenol oxidation ACS Catal. 2015, 5, 553 559 DOI: 10.1021/cs5017613
    4. 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 DOI: 10.1016/j.carbon.2016.03.034
    5. 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 DOI: 10.1016/j.apcatb.2016.04.043
    6. 6
      Goldstein, S.; Meyerstein, D.; Czapski, G. The Fenton reagents Free Radical Biol. Med. 1993, 15, 435 445 DOI: 10.1016/0891-5849(93)90043-T