Correspondence/Rebuttal Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
pubs.acs.org/est
Comment on “Redox-Active Oxygen-Containing Functional Groups in Activated Carbon Facilitate Microbial Reduction of Ferrihydrite” Moreover, the sentence ‘‘The oxidation of ACs by HNO3 led to the significant increase of electron-accepting CO groups, whereas the electron-donating C−OH groups remained rather constant [0.19−0.80 versus 0.29−0.90 mM e− (g AC)−1].’’ indicates that electron donating/accepting capacities and electron donating/accepting groups might have been mixed up. Measured moles of electrons donated and accepted might not necessarily correspond to moles of electron donating and accepting functional groups. Electrons might, for example, also be donated from persistent free radicals within the activated carbon.5 Moreover, the recorded amount of electrons is also depending on the type of electron transfer mediator during measurements of electron donating/accepting capacities.9 Next to this, the C−OH groups measured through XPS did not “‘remain constant”’ upon HNO3 treatment as previously stated. Instead, their quantity systematically decreased.1 What did remain constant was the electron donating capacity. Under hypothesis 2 an increased number of phenolic−OH groups upon HNO3 treatment can coexist with a decreased number of C−OH groups. If phenolic−OH groups are generated while other C−OH are lost to a greater extent, the phenolic−OH content increases, while the overall C−OH content decreases. It is up to the authors to assess whether or not this is plausible, taking into account their data. It is also worth considering that not all C−OH groups are necessarily redox-active.
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recently published paper1 investigated microbial ferrihydrite reduction in the presence of modified activated carbon (AC) with different types and quantities of surfaceoxygen groups. Although we greatly appreciate this work, we would like to point out some apparent contradictions in the interpretation of the results. Resolving the herein outlined comments is both scientifically valuable and of relevance to possible applications of activated carbon in redox-mediated reactions. Redox-active surface functional groups on activated carbon induces reducing and oxidizing capacities. If properly understood and used, such activated carbons with redox capacities could be deployed to shuttle electrons from microbial metabolism to the environment for example in applications such as degradation of chlorinated contaminants2,3 and of azo dyes.4
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QUANTIFICATION OF SURFACE-OXYGEN GROUPS Activated carbons were oxidized with HNO3 to increase the amount of surface-oxygen groups. The types and quantities of those functional groups were assessed through Fouriertransform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Based on FTIR, multiple oxygen groups were identified (carboxylic−OH at 3435 cm−1, lactonic −COO− at 1715 cm−1, carbonyl CO at 1590 cm−1 and phenolic−OH at 1165 cm−1). From FTIR intensities, the authors report that the quantity of all those groups increases upon HNO3 oxidation. On the other hand, the percentage of C−OH groups as quantified by XPS was reported to decrease upon HNO3 treatment. Two hypotheses could be proposed to reconcile both observations: 1. Hypothesis 1: the C−OH groups quantified through XPS are identified as phenolic−OH groups. 2. Hypothesis 2: the C−OH class includes groups other than phenolic−OH groups. Under hypothesis 1, measurements of XPS and FTIR appear inconsistent. Moreover, some of the same authors of this paper previously demonstrated that HNO3 treatment increases the quantity of phenolic−OH groups as determined by Boehm titration.5 The Boehm titration data can be directly compared to the XPS and FTIR results, as it was performed on the same activated carbons. Both papers1,5 also reported identical C−O% and CO% values. Despite the apparent contradiction under hypothesis 1, both the title of the article and various statements in the article (e.g., ‘‘Together, these results suggested that redox-active oxygencontaining functional groups (i.e., quinone/hydroquinone) of activated carbon played a crucial role in facilitating electron transfer during microbial reduction of ferrihydrite’’ and ‘‘The C−OH functional group is redox-active and known for its electron-donating capacity.’’) directly accept hypothesis 1. Indeed, redox-active, electron-donating C−OH groups are phenolic−OH groups (hydroquinone-like structures) according to the cited literature of the authors.6−8 © XXXX American Chemical Society
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IMPLICATIONS AND THOUGHTS Because the authors appeared to favor hypothesis 1, a contradiction arose between the FTIR data,1 the XPS data1 and previous work.5 If hypothesis 1 is to be maintained, a thorough argument has to be made to explain how those apparent contradictions arose and how they can be resolved. Alternatively, the authors might wish to consider whether other types of redox-active C−OH groups exist on the activated carbons. Whichever conclusion is reached, we believe the following questions ought to be considered: (i) are new phenolic−OH, other C−OH, CO, and −COO− groups being introduced upon HNO3 treatment? (ii) Are existing phenolic−OH, other C−OH, and CO groups on ACs being consecutively oxidized to carboxylic acid groups? (iii) Do introduction and consecutive oxidation occur simultaneously and to a similar extent? (iv) Are nitro groups being formed on activated carbon upon HNO3 treatment10,11 and if so, do they interfere with measurements? (v) The authors state: ‘‘original C−OH and newly generated C−OH tended to be oxidized when the ACs are treated with HNO3’’. How was this proven and how does this match with FTIR1 and titration data5 under hypothesis 1? Upon HNO3 treatment, the electrical conductivity of activated carbon systematically dropped. To provide novel
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DOI: 10.1021/acs.est.8b00453 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
carbons by HNO3 treatment: Influence of phosphorus surface groups. Carbon 2016, 101, 409−419.
insights, the authors might wish to assess whether or not this has been caused by conjugated π−π networks being disrupted upon introduction of new oxygen functionalities. The observed decrease of aromatic C by XPS5 suggests this might be the case. During incubation tests with activated carbon, microbial electron-shuttling compounds were undetected. While this was attributed to adsorption onto the activated carbon surface, we wonder if it is possible that those compounds simply were not produced at all. Does activated carbon “suppress” electronshuttle synthesis in micro-organisms by acting as an electronshuttle itself?
Stef Ghysels* Frederik Ronsse
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Department of Green Chemistry and Technology, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, Ghent, Belgium
AUTHOR INFORMATION
Corresponding Author
*Phone: +32 (0) 479 78 32 78; e-mail: Stef.Ghysels@UGent. be. ORCID
Stef Ghysels: 0000-0002-6957-725X Frederik Ronsse: 0000-0002-3290-9177 Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Wu, S.; Fang, G.; Wang, Y.; Zheng, Y.; Wang, C.; Zhao, F.; Jaisi, D. P.; Zhou, D. Redox-Active Oxygen-Containing Functional Groups in Activated Carbon Facilitate Microbial Reduction of Ferrihydrite. Environ. Sci. Technol. 2017, 51, 9709−9717. (2) Fang, G.; Liu, C.; Gao, J.; Dionysiou, D. D.; Zhou, D. Manipulation of Persistent Free Radicals in Biochar To Activate Persulfate for Contaminant Degradation. Environ. Sci. Technol. 2015, 49, 5645−5653. (3) Yu, L.; Yuan, Y.; Tang, J.; Wang, Y.; Zhou, S. Biochar as an electron shuttle for reductive dechlorination of pentachlorophenol by Geobacter sulfurreducens. Sci. Rep. 2015, 5.10.1038/srep16221 (4) van der Zee, F. P.; Bisschops, I. A. E.; Lettinga, G.; Field, J. A. Activated Carbon as an Electron Acceptor and Redox Mediator during the Anaerobic Biotransformation of Azo Dyes. Environ. Sci. Technol. 2003, 37, 402−408. (5) Fang, G.-d.; Liu, C.; Gao, J.; Zhou, D.-m. New Insights into the Mechanism of the Catalytic Decomposition of Hydrogen Peroxide by Activated Carbon: Implications for Degradation of Diethyl Phthalate. Ind. Eng. Chem. Res. 2014, 53, 19925−19933. (6) Klüpfel, L.; Keiluweit, M.; Kleber, M.; Sander, M. Redox Properties of Plant Biomass-Derived Black Carbon (Biochar). Environ. Sci. Technol. 2014, 48, 5601−5611. (7) Montes-Morán, M.; Suárez, D.; Menéndez, J.; Fuente, E. On the nature of basic sites on carbon surfaces: an overview. Carbon 2004, 42, 1219−1225. (8) Kobayashi, K.; Nagao, M.; Yamamoto, Y.; Heo, P.; Hibino, T. Rechargeable PEM Fuel-Cell Batteries Using Porous Carbon Modified with Carbonyl Groups as Anode Materials. J. Electrochem. Soc. 2015, 162, F868−F877. (9) Prévoteau, A.; Ronsse, F.; Cid, I.; Boeckx, P.; Rabaey, K. The electron donating capacity of biochar is dramatically underestimated. Sci. Rep. 2016, 6.10.1038/srep32870 (10) Lakshminarayanan, P. V.; Toghiani, H.; Pittman, C. U. Nitric acid oxidation of vapor grown carbon nanofibers. Carbon 2004, 42, 2433−2442. (11) Ternero-Hidalgo, J. J.; Rosas, J. M.; Palomo, J.; Valero-Romero, M. J.; Rodríguez-Mirasol, J.; Cordero, T. Functionalization of activated B
DOI: 10.1021/acs.est.8b00453 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
