Correspondence/Rebuttal pubs.acs.org/est
Comment on “Activation of Persulfate by Graphitized Nanodiamonds for Removal of Organic Compounds”
S
ince the discovery of graphene,1 carbon nanotubes,2 and nanodiamonds3 as peroxymonosulfate (PMS)/persulfate (PS) activators for aqueous-phase oxidation by our group, immense research interests have been aroused in environmental remediation with the state-of-the-art carbocatalysis. Recently, Lee et al.4 reported an article entitled “Activation of Persulfates by Graphitized Nanodiamonds for Removal of Organic Compounds”, in which the authors utilized graphitized nanodiamonds (G-NDs) as carbocatalysts for persulfate activation. Despite a similar material preparation and experimental design to our previous study,5 the authors proposed a nonradicaldominated mechanism (unfortunately our original reports6,7 on nonradical reactions of carbons were not cited). Since the mechanism on carbon-catalyzed persulfate activation remains controversial, we would like to raise some neglected points in Lee et al. study.
dimethylpyrrolidone-2-(oxy)-(1) (DMPOX). In terms of carbon-catalyzed systems from other studies, both sulfate radicals (DMPO−SO4) and hydroxyl radicals (DMPO−OH) were obviously revealed in quinone/PS,11 graphene/PS,12 nanodiamond/PS,5 CNTs/PS,13,14 and biochar/PS.15 Therefore, the EPR tests in Lee’s study need to be reconsidered.
2. EVOLUTION OF SINGLET OXYGEN Lee et al. determined that no noticeable singlet oxygen (1O2) was generated in G-ND/PS and that it was not accounted for phenol oxidation. However, the addition of L-histidine and azide ions obviously inhibited the oxidation with much lower rate constants (roughly estimated from the original data by pseudo-first-order kinetics). In contrast, the quenching experiments implied that singlet oxygen was produced and partially contributed to the organic removal. We further performed EPR tests and confirmed that singlet oxygen was indeed captured by 2,2,6,6-tetramethyl-4-piperidinol (TMP) with an increased intensity with the reaction proceeded (data not shown here due to page limit).
1. THE ROLE OF REACTIVE RADICALS The authors utilized radical scavengers to screen the effects of SO4•− and •OH on phenol oxidation and believed that the radicals presented a marginal effect. However, there are several tricky points in the experimental design in Lee’s study. First, a very low dosage of radical quenching agents (200 times of PS) were applied into the G-ND/PS system, which dramatically slowed down the complete phenol oxidation from 10 (control experiment) to 30 min for both methanol and dimethyl sulfoxide. The differences in reaction rates measured in the absence versus presence of radical quenching agents would have been more appropriately illustrated by including a plot (bar chart) comparing the measured rate constants. The GND/PS system in Lee’s study is intrinsically distinct from the exclusively nonradical-based systems of CuO/PS8 and CNT/ PS9 that alcohols hardly affected the catalytic performance. Moreover, ultrahigh loading of the oxidant (1 mM PS) was applied, which was almost 100 times of the target organic compound (0.01 mM phenol), whereas complete phenol mineralization only requires 0.14 mM PS. Thus, despite that the radical pathway was terminated by the quenching agents, phenol could still be oxidized via the nonradical pathway in the presence of excessive PS. The radical quenching tests in Lee’s study cannot rule out the crucial role of free radicals in phenol oxidation. Besides, Lee and co-workers employed in situ electron paramagnetic resonance (EPR) to capture the free radicals during the PS activation. Strangely, the EPR spectra presented in Lee’s Supporting Information did not present any signal in G-ND/PS and then Lee et al. claimed that no measurable radicals were produced in the nonradical process. However, the EPR response in Lee’s experiment is conflicting with the recently reported nonradical-based systems investigated by Lee’s group for carbon nanotubes (CNTs)/PS9 and Pd− Al2O3/PMS,10 in which 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) would be directly oxidized by the nonradical processes to exhibit strong characteristic peaks of 5,5© XXXX American Chemical Society
3. REACTION PATHWAYS AND INTRINSIC ACTIVE SITES In Lee’s study, a nonradical mechanism was proposed in which phenol and PS were simultaneously bonded with the G-ND surface to facilitate the electron transfer from the organic (electron donor) to the oxidant (electron acceptor). Ahn et al. previously revealed a similar nonradical process that Pd nanoparticles (Pd−Al2O3) worked as an electron-tunnel for oxidative degradation, and PMS could rarely be activated without organic pollutants.10 However, a high PS decomposition efficiency was still achieved on G-NDs and CNTs without the presence of phenol, implying that G-NDs served as a catalyst rather than ’just an electronic bridge for persulfate activation. Lee et al. also did not provide reasonable explanations for the results that graphene and carbon nanotubes, with highly graphitic frameworks and superior electronconductivity, exhibited inferior catalytic activities than G-NDs, which is not supportive for their mechanism of “electron-transfer mediator”. We suggest that the charge conductivity of the carbon materials was not the key factor for PS-driven oxidation and G-NDs might not simply engage as a mediator for electron-transfer. PS activation with carbocatalysis may still rely on certain active sites, possibly the structure defects (dangling bonds, vacancies, and edging sites) formed during the high-temperature annealing. Lastly, we would like to provide clarification regarding some incorrect conclusions about our earlier paper5 that were reported in Lee’s paper. Lee et al. stated our previous study of PS/G-ND was a hydroxyl radical-based system, which is definetely not the case. In our study,5 we clearly illustrated that
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DOI: 10.1021/acs.est.7b00399 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
carbocatalysts for aqueous and nonaqueous catalytic oxidation. Appl. Catal., B 2016, 188, 98−105. (8) Zhang, T.; Chen, Y.; Wang, Y.; Le Roux, J.; Yang, Y.; Croué, J. P. Efficient peroxydisulfate activation process not relying on sulfate radical generation for water pollutant degradation. Environ. Sci. Technol. 2014, 48, 5868−5875. (9) Lee, H.; Lee, H. J.; Jeong, J.; Lee, J.; Park, N. B.; Lee, C. Activation of persulfates by carbon nanotubes: Oxidation of organic compounds by nonradical mechanism. Chem. Eng. J. 2015, 266, 28− 33. (10) Ahn, Y. Y.; Yun, E. T.; Seo, J. W.; Lee, C.; Kim, S. H.; Kim, J. H.; Lee, J. Activation of peroxymonosulfate by surface-loaded noble metal nanoparticles for oxidative degradation of organic compounds. Environ. Sci. Technol. 2016, 50, 10187−10197. (11) Fang, G. D.; Gao, J.; Dionysiou, D. D.; Liu, C.; Zhou, D. M. Activation of persulfate by quinones: free radical reactions and implication for the degradation of PCBs. Environ. Sci. Technol. 2013, 47, 4605−4611. (12) Duan, X. G.; Sun, H. Q.; Kang, J.; Wang, Y. X.; Indrawirawan, S.; Wang, S. B. Insights into heterogeneous catalysis of persulfate activation on dimensional-structured nanocarbons. ACS Catal. 2015, 5, 4629−4636. (13) Zhang, X. L.; Feng, M. B.; Qu, R. J.; Liu, H.; Wang, L. S.; Wang, Z. Y. Catalytic degradation of diethyl phthalate in aqueous solution by persulfate activated with nano-scaled magnetic CuFe2O4/MWCNTs. Chem. Eng. J. 2016, 301, 1−11. (14) Feng, M. B.; Qu, R. J.; Zhang, X. L.; Sun, P.; Sui, Y. X.; Wang, L. S.; Wang, Z. Y. Degradation of flumequine in aqueous solution by persulfate activated with common methods and polyhydroquinonecoated magnetite/multi-walled carbon nanotubes catalysts. Water Res. 2015, 85, 1−10. (15) Fang, G. D.; Liu, C.; Gao, J.; Dionysiou, D. D.; Zhou, D. M. Manipulation of persistent free radicals in biochar to activate persulfate for contaminant degradation. Environ. Sci. Technol. 2015, 49, 5645− 5653. (16) Wang, X. B.; Qin, Y. L.; Zhu, L. H.; Tang, H. Q. Nitrogendoped reduced graphene oxide as a bifunctional material for removing bisphenols: Synergistic effect between adsorption and catalysis. Environ. Sci. Technol. 2015, 49, 6855−6864. (17) Duan, X. G.; Ao, Z. M.; Sun, H. Q.; Zhou, L.; Wang, G. X.; Wang, S. B. Insights into N-doping in single-walled carbon nanotubes for enhanced activation of superoxides: A mechanistic study. Chem. Commun. 2015, 51, 15249−15252.
the G-ND/PS system was not effective for degradation of benzoic acid and nitrobenzene, the hydroxyl radical probes, suggesting that SO4•− was still the dominating radicals accounting for organic oxidation. The excess ethanol (500− 2000 times of PS) drastically slowed down the oxidation, which implied the crucial contribution of the radical-induced oxidation. Moreover, we proposed that the low EPR intensity of DMPO−SO4 and remaining oxidation potential under exccesive radical scavengers were due to the formation of surface-confined sulfate radicals,5 which was similar to Wang’s study16 using nitrogen-doped graphene as a PS activator. However, we still believe that the partial water oxidation on carbon surface may be experienced which has be witnessed by the in situ EPR detection and predicted by the theoretical calculations.17 Additionally, Lee et al. also omitted an important factor that the carbon structures (onion-like carbon1 and sp2/ sp3 nanohybrid5) in these two studies are intrinsically different, not to mention that the original materials are from two distinct sources with completely different size, surface chemistry, and crystalinity.
Xiaoguang Duan† Hongqi Sun*,‡ Shaobin Wang*,†
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† Department of Chemical Engineering, Curtin University, GPO Box U1987, WA 6845, Australia ‡ School of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, Perth, WA 6027, Australia
AUTHOR INFORMATION
Corresponding Authors
*(H.S.) Phone: +61 8 6304 5067; e-mail:
[email protected]. *(S.W.) Phone: +61 8 9266 3776; e-mail: : shaobin.wang@ curtin.edu.au. ORCID
Hongqi Sun: 0000-0003-0907-5626 Shaobin Wang: 0000-0002-1751-9162 Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acs.est.7b00399 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
