Correspondence/Rebuttal pubs.acs.org/est
Comment on “Zirconium−Carbon Hybrid Sorbent for Removal of Fluoride from Water: Oxalic Acid Mediated Zr(IV) Assembly and Adsorption Mechanism”
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from 3.848% on ZrOx-AC to 0.862% on ZrOx-AC+F, which proved chloride participating or involving in this anion exchange process with fluoride. Usually fluoride is believed to replace singly coordinated hydroxyl or water groups from the surface of metal oxides. In this work, the chloride also takes part in the anion exchange with fluoride leading to improved adsorption capacity.3 Currently in my group, FeOOH@Graphene oxide (FeOOH@GO) and FeOOH+AA@GO are synthesized by FeCl3 hydrolysis in absence or presence of acetic acid (AA) and fluoride uptake capacity and mechanism are also investigated by XPS, respectively. Adding acetic acid in the system limits the growth and size of FeOOH on the surface of GO (shown in Figure 1) and increases the fluoride adsorption capacity by FeOOH+AA@GO comparing with FeOOH@GO (shown in Table 1). Figure 2 and Table 1 also show the At % changes of chloride and fluoride, which prove the chloride taking part in this anion exchange with fluoride. At the same
elazquez-Jimenez L.H. and coauthors reported that ZrOx-AC increased the fluoride adsorption capacity by a factor of 3 oversimple Zr-AC in which added oxalic acid (Ox) mediated to demonstrate a positive effect on controlling nucleation and limiting growth and size of Zr(IV) oxide nanoparticles so as to Increases the zirconium active adsorption sites. They propose that fluoride adsorption occurs in the ZrOx-AC surface sites with −OH displacement and/or interacts with positive charge of zirconium to form −COOH groups in the oxalate acid, see Figure 3C. As author pointed out that ZrO2 can develop di-, tri-, tetra-, and penta-fluoro zirconium species without the presence of a complexing agent such as oxalic acid. If the fluoride concentration is high enough, it would completely desorb oxalate acid from surface of ZrOx-AC.1 The presence of chloride on ZrOx-AC and ZrOx-AC+F was proved by XPS shown in Table S4 and Figure S7, which indicate chloride is coprecipitated into ZrOx-AC.2 In Table S4, the Atomic surface concentrations (At %) of chloride decreasing
Figure 1. TEM images of FeOOH@GO and FeOOH+AA@GO.
Figure 2. XPS of FeOOH@GO and FeOOH+AA@GO before and after adsorption of fluoride.
© XXXX American Chemical Society
A
DOI: 10.1021/acs.est.5b03792 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
Table 1. Summary of XPSa Cl 2p At(%)
position
At(%)
197.8 197.8 198.4 198.5
2.07 1.08 1.02 0.52
ND 684.2 ND 684.2
ND 0.90 ND 1.50
FeOOH@GO FeOOH@GO+F FeOOH+AA@GO FeOOH+AA@GO+F a
qmax
F 1s
position
mg/g 16 26
ND: no detection.
time we also detect the presence of acetic acid in the supernatant after fluoride adsorption by FeOOH+AA@GO, which show that acetic acid is desorbed in this anion exchange process. The same result would be expected to appear on ZrOx-AC. Based on these findings, we suggest Velazquez-Jimenez L.H. and coauthors should further consider or add the exchangeable role of chloride and oxalic acid in the proposed fluoride adsorption mechanism.
Yaping Zhao*,† †
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School of Ecological and Environmental Science, East China Normal University, Shanghai, 200241, China
AUTHOR INFORMATION
Corresponding Author
*Phone.: -86-21-54341241; e-mail:
[email protected]. Funding
The author thanks the finance support by National Natural Science Foundation of China (no. 21377039). Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Velazquez-Jimenez, L. H.; Hurt, R. H.; Matos, J.; Rangel-Mendez, J. R. Zirconium−Carbon Hybrid Sorbent for Removal of Fluoride from Water: Oxalic Acid Mediated Zr(IV) Assembly and Adsorption Mechanism. Environ. Sci. Technol. 2014, 48 (27), 1166−1174. (2) Gordon, L.; Salutsky, M. L.; Willard, H. H. Precipitation from Homogeneous Solution; Wiley: New York, 1959. (3) Chen, Y. Q.; Zhang, Q. K.; Chen, L. B.; Bai, H.; Li, L. Basic aluminum sulfate@graphene hydrogel composites: preparation and application for removal of fluoride. J. Mater. Chem. A 2013, 1, 13101− 13110.
B
DOI: 10.1021/acs.est.5b03792 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
