Correspondence/Rebuttal pubs.acs.org/est
The Confounding Effects of Dissolved Humic Acid on the Oxidation of Simple Substituted Phenols by Permanganate: Comment on “Reinvestigation of the Role of Humic Acid in the Oxidation of Phenols by Permanganate”
I
n a recent article,1 Sun et al. have reinvestigated the effect of dissolved humic acid (HA) on the oxidation of simple substituted phenols (SSPs) by permanganate (Mn(VII)). They concluded that dissolved HA could significantly accelerate the oxidation dynamics of SSPs by Mn(VII) by acting as both reductant and host. In other words, both the enhanced formation of MnO2 colloids as a mild oxidant upon Mn(VII) reduction by HA and the noncovalent interactions between SSPs and HA resulted in the observed oxidation enhancement. However, for the following reasons we believe that these conclusive statements are not justified by the experiments presented in that paper.
(II). HOW STRONG WAS THE BINDING OF SSPS TO HA? The very weak interactions (probably the π−π interactions) of SSPs including phenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, and 2,4,6-trifluoropehnol with HA have recently been demonstrated with NMR technique,5,6 as reflected by the extremely small binding constants (Ka) of 0.06−10.04 M−1. These values also meant that the percentage (ρ) of HAassociated SSPs at HA of 10 mg C/L (i.e., 8.33 × 10−4 M of organic carbon) was 66% of phenol was associated with HA at 10 mg C/L.1 If the original data were reliable, Ka was calculated to be >2.3 × 103 M−1 (or >1.94 × 105 L/kg C). Although the binding of SSPs to HA is dependent upon the physicochemical properties of HA as well as the solution chemistry,6 the Ka value of several orders of magnitude higher than that (0.57 M−1) for phenol obtained in literature6 seems very unreasonable. In fact, the negligible binding of phenol to Aldrich-Sigma HA (the same source as in ref 1) as well as to Suwannee River HA and a soil HA7 was also confirmed in our preliminary study by using a complexationflocculation method,8,9 where the contaminant was allowed to interact with dissolved HA (complexation stage) and then a coagulant was added, resulting in precipitation of the dissolved HA and the associated contaminant (flocculation stage).
(III). COULD OZONE AT AN EXTREMELY LOW DOSE CAUSE THE APPRECIABLE CHANGE OF HA CHEMICAL PROPERTIES? The change of HA chemical properties such as the electron donating capacity, aromaticity, and O/C ratio was suggested by Sun et al.1 to explain “the weakening of the promotive ef fects of HA after preozonation” at a dose of ∼6.07 × 10−3 mmol O3/ mmol C. However, we notice that this value is about 2 orders of magnitude lower than the specific oxidant doses (mmol O3/ mmol C) reported in literature,10 where the appreciable change of optical and antioxidant properties of natural organic matter (NOM) such as Suwannee River HA and fulvic acid and Pony Lake fulvic acid induced by ozone can be achieved. Published: May 16, 2014 6518
dx.doi.org/10.1021/es501737q | Environ. Sci. Technol. 2014, 48, 6518−6519
Environmental Science & Technology
Correspondence/Rebuttal
(5) Šmejkalová, D.; Piccolo, A. Host-guest interactions between 2,4dichlorophenol and humic substances as evaluated by 1H NMR relaxation and diffusion ordered spectroscopy. Environ. Sci. Technol. 2008, 42, 8440−8445. (6) Šmejkalová, D.; Spaccini, R.; Fontaine, B.; Piccolo, A. Binding of phenol and differently halogenated phenols to dissolved humic matter as measured by NMR spectroscopy. Environ. Sci. Technol. 2009, 43, 5377−5382. (7) Ma, J.; Jiang, J.; Pang, S.-Y. Adsorptive fractionation of humic acid at air-water interfaces. Environ. Sci. Technol. 2007, 41, 4959−4964. (8) Laor, Y.; Rebhun, M. Complexation-flocculation: A new method to determine binding coefficients of organic contaminants to dissolved humic substances. Environ. Sci. Technol. 1997, 31, 3558−3564. (9) Rebhun, M.; Meir, S.; Laor, Y. Using dissolved humic acid to remove hydrophobic contaminants from water by complexationflocculation process. Environ. Sci. Technol. 1998, 32, 981−986. (10) Wenk, J.; Aeschbacher, M.; Salhi, E.; Canonica, S.; von Gunten, U.; Sander, M. Chemical oxidation of dissolved organic matter by chlorine dioxide, chlorine, and ozone: Effects on its optical and antioxidant properties. Environ. Sci. Technol. 2013, 36, 11147−11156.
(IV). COULD NOT MNO2 ENHANCE THE OXIDATION OF REFRACTORY NITROPHENOLS BY MN(VII) AT ACIDIC PH? In a previous study,2 the role of MnO2 as a catalyst rather than as an oxidant at acidic pH in promoting the oxidation of SSPs by Mn(VII) was suggested based on the finding that MnO2 colloids ex situ prepared could significantly accelerate the oxidation dynamics of p-nitrophenol, which otherwise showed negligible reactivity toward Mn(VII) and MnO2, respectively. However, Sun et al. stated that they did not observe the oxidation enhancement in the case of o-nitrophenol and thus ruled out the catalytic effect of MnO2.1 The discrepancy is believed to be mainly due to the significant difference in the experimental conditions between two studies (i.e., MnO2 in situ formed upon the reduction of Mn(VII) by HA of 1 mg C/L and the reaction time of several tens of minutes in ref 1 versus MnO2 colloids of 120 μM and the reaction time of several hours in ref 2). In our preliminary study, it was confirmed again that both p-nitrophenol and o-nitrophenol could be appreciably oxidized by Mn(VII) in the presence of MnO2 colloids under the identical conditions used in the previous study.2 Su−Yan Pang† Qiang Wang† Jin Jiang*,‡ † Key laboratory of Green Chemical Engineering and Technology of College of Heilongjiang Province, College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150040, China ‡ State Key Laboratory of Urban Water Resource and Environment, School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China
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AUTHOR INFORMATION
Corresponding Author
*Phone: 86−451−86283010; fax: 86−451−86283010; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
We gratefully acknowledged the financial support by the National Natural Science Foundation of China (51008104 and 51208159), the Postdoctoral Science Foundation and the Special Financial Grant of China (20110490106, 2012T50365, and 2013M541402), and the Foundation for the Author of National Excellent Doctoral Dissertation of China (201346).
(1) Sun, B.; Zhang, J.; Du, J.; Qiao, J.; Guan, X. Reinvestigation of the role of humic acid in the oxidation of phenols by permanganate. Environ. Sci. Technol. 2013, 47, 14332−14340. (2) Jiang, J.; Pang, S.-Y.; Ma, J. Oxidation of triclosan by permanganate (Mn(VII)): Importance of ligands and in situ formed manganese oxides. Environ. Sci. Technol. 2009, 43, 8326−8331. (3) Jiang, J.; Pang, S.-Y.; Ma, J. Role of ligands in permanganate oxidation of organics. Environ. Sci. Technol. 2010, 44, 4270−4275. (4) Jiang, J.; Pang, S.-Y.; Ma, J.; Liu, H. Oxidation of phenolic endocrine disrupting chemicals by potassium permanganate in synthetic and real waters. Environ. Sci. Technol. 2012, 46, 1774−1781. 6519
dx.doi.org/10.1021/es501737q | Environ. Sci. Technol. 2014, 48, 6518−6519
