Response to Comment on “Redox-Active Oxygen-Containing

Correspondence/Rebuttal Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Response to Comment on “Redox-Active Oxygen-Containing Functional Groups in Activated Carbon Facilitate Microbial Reduction of Ferrihydrite” O, COOH, and finally CO2.3 Hence, the new C−OH, CO, and − COOH groups are introduced upon HNO3 treatment of AC, and the existing C−OH and CO groups are consecutively oxidized to COOH groups. The introduction of new oxygen-containing functional groups and consecutive oxidation of existing ones occur concurrently, while the increase of COOC, COOH, and CO groups content but the decrease of C−OH groups content indicate the reactions progressed to different extents.7 Ternero-Hidalgo et al.10 reported low concentration of nitro groups (−NO2) was formed on the carbon surface after HNO3 treatment, but no obvious peak originated from nitrogen-containing functional groups was observed in the XPS spectra for the AC with HNO3 treatment in our study (Figure R1). The inconsistence of the XPS and FTIR results may result from the different oxidation degree of the carbon at the surface and in the bulk, as characterized by XPS and FTIR.

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e appreciate the concerns that Ghysels and Ronsse1 raised on our paper regarding the quantification of surface oxygen-containing functional groups of activated carbon (AC).2 It is generally supported that the C−OH peak appears in the XPS C 1s spectra corresponds to both phenolic and alcohol groups.3 The presence of alcohol groups in carbonaceous materials has not been reported or verified by other analytical methods, that is, temperature-programmed desorption4 and near-edge X-ray absorption fine structure,5 although these methods are effective in characterizing oxygen-containing functional group in carbonaceous materials. But the presence of phenolic groups is widely accepted and observed to be redox active.6 Hence, the phenolic groups is discussed represent for C−OH groups in our paper. The type and quantity of surface oxygen-containing functional groups in AC characterized by XPS, FTIR, and Boehm titration are methods dependent upon several factors, which together contributed to the inconsistent results between XPS and FTIR or Boehm titration in our and many other studies.2,7 The XPS analysis quantifies the oxygen-containing functional groups of a few atomic layers at the AC exterior surface. But the FTIR characterizes the bulk content of oxygen-containing functional groups, while Boehm titration only determines the solvent-accessible surface. Therefore, only partial of the oxygenated functional groups in AC detected by XPS analysis can be assessed by Boehm titration analysis, which was 1.77− 9.19% in our previous study,7 and 31−47% reported by Otake et al.8 Moreover, the content of carbonyl groups in ACs was less than 100 μmol g−1 (data not shown) in the Boehm titration analysis because some of the carbonyl groups may not react with NaOC2H5.9 The Boehm titration analysis dramatically underestimates the content of oxygen-containing functional groups in AC due to its inherent limitations. But the surfacesensitive XPS quantifies these different types of surface oxygencontaining functional groups with a high resolution. Furthermore, the exterior surface rather than the interior surface in the micropore and mesopore of AC is more available to the Shewanella oneidensis MR-1 cells and ferrihydrite particles in this study (discussed in our paper). Therefore, the XPS data rather than Boehm titration data were used for the correlation analysis in our study. Both the verification of redox active quinone/hydroquinone in cyclic voltammetry results2 and the identification of oxygen-centered free radicals in electron paramagnetic resonance results7 indicate the part of oxygencontaining functional groups, i.e. C−OH and CO groups fitted in XPS results, is probably redox active. The correlation analysis was therefore performed between rate of microbial reduction of ferrihydrite and electron exchange capacity and content of C−OH and CO groups. For the part concerning on the evolution of different types of oxygen-containing functional groups, the progressive oxidation mechanism for carbon surface is as follows: C−H, C−OH, C © XXXX American Chemical Society

Figure R1. XPS spectra of AC-X, AC-X-N2, and AC-X-N4.

The decrease of aromatic C content and the increase of Cdefects content indicated the destruction of the graphitic integrity and subsequent formation of small graphitic fragments.11 Although the increase of surface COOH group content has beneficial effect on the electrical conductivity with the mild chemical oxidation,12 the destruction of graphitic structure may be the main reason that lead to the drop of electrical conductivity of AC with aggressive HNO3 oxidation at high temperature.13,14 It is possible that the addition of AC suppresses the synthesis or secretion of flavins by bacteria since AC can act as an electron-shuttle itself. But it is impossible that the production of flavins by bacteria was completely ceased. Because, the synthesis of flavins is a must for cell metabolize, and both the synthesis and secretion of flavins are regulated by the gene

A

DOI: 10.1021/acs.est.8b01263 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

Correspondence/Rebuttal

expression in Shewanella oneidesis MR-1.15,16 Moreover, the flavins have been detected with the presence of graphite17 or ferrihydrite, respectively. Therefore, we stated in the manuscript as follows: this intriguing observation could possibly result from the removal of FAD, FMN, and RF compounds due to adsorption onto AC surfaces.

(12) Tantang, H.; Ong, J. Y.; Loh, C. L.; Dong, X.; Chen, P.; Chen, Y.; Hu, X.; Tan, L. P.; Li, L.-J. Using oxidation to increase the electrical conductivity of carbon nanotube electrodes. Carbon 2009, 47 (7), 1867−1870. (13) Polovina, M.; Babić, B.; Kaluderović, B.; Dekanski, A. Surface characterization of oxidized activated carbon cloth. Carbon 1997, 35 (8), 1047−1052. (14) Kim, Y. J.; Shin, T. S.; Choi, H. D.; Kwon, J. H.; Chung, Y.-C.; Yoon, H. G. Electrical conductivity of chemically modified multiwalled carbon nanotube/epoxy composites. Carbon 2005, 43 (1), 23−30. (15) Brutinel, E.; Gralnick, J. Shuttling happens: soluble flavin mediators of extracellular electron transfer in Shewanella. Appl. Microbiol. Biotechnol. 2012, 93 (1), 41−48. (16) Kotloski, N. J.; Gralnick, J. A. Flavin electron shuttles dominate extracellular electron transfer by Shewanella oneidensis. mBio 2013, 4 (1).e00553-1210.1128/mBio.00553-12 (17) Velasquez-Orta, S. B.; Head, I. M.; Curtis, T. P.; Scott, K.; Lloyd, J. R.; von Canstein, H. The effect of flavin electron shuttles in microbial fuel cells current production. Appl. Microbiol. Biotechnol. 2010, 85 (5), 1373−1381.

Song Wu†,‡ Guodong Fang† Cun Liu† Dongmei Zhou*,† †

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Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, P.R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P.R. China

AUTHOR INFORMATION

Corresponding Author

*Phone: +86 25 86881180; fax: +86 25 86881000; e-mail: [email protected]. ORCID

Guodong Fang: 0000-0002-3837-6279 Dongmei Zhou: 0000-0002-7917-7954 Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.est.8b01263 Environ. Sci. Technol. XXXX, XXX, XXX−XXX