Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Colloidal Properties and Stability of Graphene Oxide Nanomaterials in the Aquatic Environment”
W
e appreciate the opportunity to clarify issues raised in the comment of Zhang et al.1 on our paper2 describing the colloidal properties and stability of graphene oxide (GO) nanomaterials in the aquatic environment. Zhang et al.1 commented on our findings regarding the pH effect on the physicochemical properties of GO. Zhang et al.1 conducted similar GO stability experiments using similar solution chemistry which yielded results (unpublished) that were very close to those we observed for NaCl solution chemistry. Our comment on pH in our paper is based on NaCl conditions, and we made this clear in our paper by stating “Because we did not observe notable changes in electrokinetic or hydrodynamic properties of GO over this range, it is quite likely that pH will have minor effects on fate and transport of GO in the aquatic environment” (page 6291). The natural aquatic environment is complex and consists of various ions and organic matter. Zhang et al.1 investigated the pH effect in a mix of CaCl2 and NaCl and found that hydrodynamic diameter of GO increased with pH. They observed similar effects for aged tap water. They attributed this increased diameter to Ca2+ ions, which neutralize GO surface charge by adsorbing on GO. Since Zhang et al.1 did not provide any surface charge data and detailed information on aged tap water (filtered or not, other ions, organic matter content) in the comment, it is difficult to assess the veracity of this conclusion. However, other studies have shown that pH can affect the protonation and deprotonation properties of GO.3−5 Whitby et al.4 found that acidic groups on GO can be protonated increasing the hydrophobicity of GO at low pH, while acidic groups become deprotonated at high pH resulting higher hydrophilicity. Szabó et al.5 delineated the protonation and deprotonation mechanisms of graphite oxide because of pH change. This study found that increased pH and salt concentration can enhance the dissociation of acidic functional groups on graphite oxide. Another study3 investigated the pHdependent behavior of GO using experimental and molecular dynamics simulation study and determined the aggregate structure at different pH. These surface functionality changes on GO due to pH can influence the interactions of GO with divalent ions (Ca2+) and organic matter. Since pH effect on GO is well investigated, we primarily focused our study on the other environmental parameters including ionic strength, ion valence, and organic matter. Our comment on pH in our paper is based on monovalent salt (NaCl) condition, which will be appropriate where monovalent salts are dominating in the aquatic environment. For complex environmental conditions, interactions of divalent ions and organic matter with GO can be pH dependent.
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† National Research Council Research Associate, Athens, Georgia 30605, United States ‡ Departments of Material Science and Engineering, Chemistry, and Medicine, Northwestern University, Evanston, Illinois 60208, United States § National Exposure Research Laboratory, Ecosystem Research Division, United States Environmental Protection Agency, Athens, Georgia 30605, United States
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: 706-355-8333. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Zhang, J.; Terracciano, A.; Meng, X. Comment on “Colloidal Properties and Stability of Graphene Oxide Nanomaterials in the Aquatic Environment. Environ. Sci. Technol. 2013, DOI: 10.1021/ es40912p. (2) Chowdhury, I.; Duch, M. C.; Mansukhani, N. D.; Hersam, M. C.; Bouchard, D. Colloidal Properties and Stability of Graphene Oxide Nanomaterials in the Aquatic Environment. Environ. Sci. Technol. 2013, 47 (12), 6288−6296. (3) Shih, C.-J.; Lin, S.; Sharma, R.; Strano, M. S.; Blankschtein, D. Understanding the pH-Dependent Behavior of Graphene Oxide Aqueous Solutions: A Comparative Experimental and Molecular Dynamics Simulation Study. Langmuir 2011, 28 (1), 235−241. (4) Whitby, R. L. D.; Korobeinyk, A.; Gun’ko, V. M.; Busquets, R.; Cundy, A. B.; Laszlo, K.; Skubiszewska-Zieba, J.; Leboda, R.; Tombacz, E.; Toth, I. Y.; Kovacs, K.; Mikhalovsky, S. V. pH-Driven Physicochemical Conformational Changes of Single-Layer Graphene Oxide. Chem. Commun. 2011, 47 (34), 9645−9647. (5) Szabó, T.; Tombácz, E.; Illés, E.; Dékány, I. Enhanced Acidity and pH-Dependent Surface Charge Characterization of Successively Oxidized Graphite Oxides. Carbon 2006, 44 (3), 537−545.
Indranil Chowdhury†,§ Matthew C. Duch‡ Nikhita D. Mansukhani‡ Mark C. Hersam‡ Dermont Bouchard*,§
© XXXX American Chemical Society
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dx.doi.org/10.1021/es405531h | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
