Comment on “Colloidal Properties and Stability of Graphene OxideNanomaterials in the Aquatic Environment”

Chowdhury et al.1 measure hydrodynamic diameter ofgraphene oxide (GO) nanoparticles in a 10 mM NaCl

solution at different pH and conclude that “pH does notnotably affect the physicochemical properties of GO, indicatingpH will have a minor influence on the fate of GO in the aquaticenvironment.” Then, other tests are conducted at fixed pH 5.5to evaluate the effect of Ca2+, Mg2+, and Na+ on GO stability.We conduct similar GO stability tests to Chowdhury et al.1

using NaCl and CaCl2−NaCl solutions and aged tap water. GOcontent in the solutions is 10 mg/L. The tap water is aged in aclosed container for several days to remove free chlorine in it.The solution pH is adjusted to desired levels using NaOH andHCl solutions. After 15 min of mixing, 2 mL of samples aretaken for analysis of hydrodynamic diameter of GO nano-particles using a Nano ZetaSizer ZEN3600 (Malvern Instru-ment, U.K.).Figure 1 shows that the diameter of GO is not affected by pH

in the 10 mM NaCl from pH 4−10, which is consistent with

Chowdhury et al.1 However, the diameter increases moderatelyand dramatically with increasing pH in the CaCl2−NaClsolution and aged tap water, respectively. Since the ionicstrength in both NaCl and CaCl2−NaCl is 10 mM, theincreased diameter is attributed to the effect of Ca2+. It ispossible that more Ca2+ is adsorbed on GO with increasing pH,which neutralizes more negative surface charge on GO andreduces the electrostatic repulsion between the GO nano-particles. Zhao et al.2 report that adsorption of Cd2+ and Co2+

by GO increases dramatically with increasing pH.Ca concentration in the aged tap water is only 0.4 mM. The

more dramatic increase in the diameter of GO with increasingpH in the tap water than in the CaCl2−NaCl solution suggeststhat in addition to Ca, other solutes in the tap water alsosignificantly influent the stability of GO. The results in Figure 1indicate that pH significantly influences the stability of GO inthe aquatic environment.

Jianfeng Zhang†

Amalia Terracciano‡

Xiaoguang Meng*,‡†School of Environmental and Municipal Engineering,Xi’an University of Architecture and Technology, Xi’an710055, China‡Center for Environmental Systems, Stevens Institute ofTechnology, Hoboken, New Jersey 07030, United States

■ AUTHOR INFORMATIONCorresponding Author*Tel: +1 201 2168014. Fax: +1 201 2188303. E-mail: xmeng@stevens.edu.NotesThe authors declare no competing financial interest.

■ REFERENCES(1) Chowdhury, I.; Duch, M. C.; Mansukhani, N. D.; Hersam, M. C.;Bouchard, D. Colloidal properties and stability of graphene oxidenanomaterials in the aquatic environment. Environ. Sci. Technol. 2013,47 (12), 6288−6296.(2) Zhao, G.; Li, J.; Ren, X.; Chen, C.; Wang, X. Few-layeredgraphene oxide nanosheets as superior sorbents for heavy metal ionpollution management. Environ. Sci. Technol. 2011, 45 (24), 10454−10462.

Figure 1. Hydrodynamic diameter of GO as a function of final pH inthree different solutions. GO content = 10 mg/L.

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