Environ. Sci. Technol. 2009, 43, 3966–3967
Response to Comment on “Oxidative Degradation of Organic Compounds Using Zero-Valent Iron in the Presence of Natural Organic Matter Serving as an Electron Shuttle” We reply to the comments from Noubactep (1) regarding our recent paper on the oxidative reactions occurring on zerovalent iron (ZVI). We have recently reported an accelerated oxidative degradation of organic compounds [4-chlorophenol (4-CP) and clofibric acid (CA) as test substrates] on ZVI in the presence of natural organic matters (NOMs) and proposed their role as an electron shuttle that mediates the electron transfer from the surface of ZVI to O2 (2). The NOMmediated electron transfer on ZVI led to generating more Fe2+ and H2O2, which subsequently initiated the OH-radical oxidation of organic compounds through the Fenton reaction. Noubactep raised four main issues on the above work and the itemized replies follow. First, Noubactep questioned the premise that ZVI-induced contaminant removal is initiated by the direct electron transfer from Fe0 to substrates and added that “the premise was already questioned and/or proven inconsistent” with citing only his own papers (3, 4). This argument is hardly acceptable since the role of the direct electron transfer in ZVI-mediated reactions is well established and generally accepted among the research community. Noubactep stressed the importance of the sequestration of organic substrates within the matrix of in situ generated iron corrosion products, and maintained that the oxide film on Fe0 should inhibit the action of electron shuttles. However, it is obvious that the simple sequestration of organic compounds cannot explain the concurrent production of chlorides with the removal of 4-CP. More importantly, 4-CP and CA were not removed at all in the deaerated suspension of ZVI even though the oxide layers on Fe0 were present in the as-received iron powder. These results clearly rule out the possible role of adsorption and/or coprecipitation on the corrosion products oxide layer in the overall removal mechanism. As a control test, we tried to remove 4-CP using wu ¨ stite (FeO) and magnetite (Fe3O4) instead of ZVI under the otherwise identical experimental condition. The removal of 4-CP was negligible regardless of the presence of NOMs. The adsorption/precipitation mechanism may contribute in some cases depending on the kind of substrates and the experimental conditions but it should be negligible in our study (2). Incidentally, it should be mentioned that the NOM electron-shuttling mechanism does not require the direct electron transfer from Fe0 to organic compounds. In the proposed mechanism, the direct electron transfer takes place from Fe0 to O2 (not organic compound!) and the Fenton-mediated oxidation of organic compounds can occur in the solution bulk. Therefore, the adsorption on the ZVI surface or the sequestration of organic compounds within the oxide matrix is not even necessary for initiating the oxidative degradation. The comment of Noubactep is rather surprising considering that a vast amount of experimental evidence supporting that ZVI-mediated reactions are largely based on the direct electron transfer on Fe0 is available in the literature (5, 6). Although the surface of Fe0 is always covered with oxide layer, the surface is highly nonuniform (oxide films with lots of localized pits), porous, and heterogeneous in its composition. 3966
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Therefore, despite the presence of the passive oxide layer, the electron transfer reactions occur mainly at pits (5). To take an example, our previous work utilized polyoxometalate (POM) as an electron shuttle in the ZVI suspension for the oxidative degradation of 4-CP (7). The removal of 4-CP was accompanied by the rapid reduction of POM with the appearance of blue color in the suspension. The blue coloration is a direct indicator of the formation of the reduced POM (POM + e- f POM-). This verified that the direct electron transfer occurred from Fe0 to POM although the as-received iron powder (without acid washing) was used in the study. This clearly indicates that the presence of oxide films on Fe0 does not prohibit the direct electron transfer. Second, it is mentioned that the electron shuttle chemistry suggested from our study was already known in the Becher process (a mineral processing method that extracts metallic iron from reduced ilmenite to produce synthetic rutile) that utilized anthraquinone-based redox catalysts (8). I do not feel this argument is well justified. Redox catalysts are common in many chemical processes and the electron shuttling properties of NOMs and quinone derivatives are widely known. No research work is completely new and unrelated from other studies. An ilmenite processing method and a ZVI-based water treatment method are drastically different from each other and hardly comparable. The role of the redox catalyst in Becher process is to accelerate the iron dissolution from the reduced ilmenite whereas the role of NOMs in ZVI process is to mediate the electron transfer from Fe0 to O2 for the enhanced generation of OH radicals. The two methods are totally different in their objectives and very different in their chemical nature although both have in common utilizing redox catalysts in the Fe0-involved process. Third, it was pointed out that different amounts of Clwere generated according to the presence of fulvic acid (FA) or humic acid (HA), while the kinetics of the 4-CP removal were similar in both HA and FA systems. Although both cases of 4-CP and CA were mentioned with this comment, it should be noted that only 4-CP case showed the difference. The mismatch between the 4-CP removal and the chloride generation was probably caused by the production of chlorine-containing intermediates. The removal rate of 4-CP and the production rate of Cl- do not have to be the same when the intermediates are involved although we did not carry out the analysis of the intermediates. It seems that the chlorine-containing intermediates were generated in the 4-CP/FA/Fe0 system. The chlorine-containing intermediates, if any, should be eventually degraded with releasing chlorides. Note that the chloride generation profile in the case of 4-CP/ FA/Fe0 steadily rose with approaching the saturated Cl- level in 4-CP/HA/Fe0 (see Figure 1a in ref 2). Finally, it was mentioned that the pH variation was not recorded during the reaction and the effects of HA and FA might be related with their impact on the system pH. We agree that the presence of NOMs has an effect on the system pH. In aqueous ZVI suspensions, the pH change is always accompanied as a result of the iron dissolution (reactions 1 and 2). 2Fe0 + O2 + 2H2O f 2Fe2+ + 4OH-
(1)
2Fe0 + 2H2O f 2Fe2+ + H2 + 2OH-
(2)
The pH should increase with time in ZVI suspension while the following Fenton reaction is strongly favored at acidic condition. Since NOMs have a pH-buffering capacity, their 10.1021/es900569n CCC: $40.75
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Published on Web 04/10/2009
presence should retard the variation of pH. In our study, the ZVI oxidation reactions were carried out in unbuffered solutions since we did not want to add any chemical buffers that might interfere with the surface chemical and/or radical reactions. However, this pH effect does not seem to be significant in the present case. The NOM-enhanced rate of the oxidation in ZVI suspension was observed from the very initial stage in which the pH variation from the initial pH is insignificant (see Figure 1a in ref 2). The ZVI reactions were carried out at pH 2.5 because the NOM-enhanced effect was clearly observed only at such acidic condition. We agree that such condition is unrealistic and this NOM-enhanced oxidation effect “may not have a great practical value in the real life systems” as we mentioned in the paper (2). It should be noted that the main objective of the study was focused on the fundamental understanding of the electron shuttling role of NOMs in ZVI-oxidation process, not on exploiting this phenomenon for practical water treatment.
Literature Cited (1) Noubactep, C. Comment on “Oxidative degradation of organic compounds using zero-valent iron in the presence of natural organic matter serving as an electron shuttle”. Environ. Sci. Technol. 2009, 43, doi/10.1021/es900076m. (2) Kang, S.-H.; Choi, W. Oxidative degradation of organic compounds using zero-valent iron in the presence of natural organic matter serving as an electron shuttle. Environ. Sci. Technol. 2009, 43, 878–883.
(3) Noubactep, C. A critical review on the process of contaminant removal in Fe0-H2O systems. Environ. Technol. 2008, 29, 909– 920. (4) Noubactep, C. Process of contaminant removal in “Fe0-H2O” systems revisited: The importance of co-precipitation. Open Environ. J. 2007, 1, 9–13. (5) Gaspar, D. J.; Lea, A. S.; Engelhard, M. H.; Baer, D. R.; Mier, R.; Tratnyek, P. G. Evidence of localization of reaction upon reduction of carbon tetrachloride by granular iron. Langmuir 2002, 18, 7688–7693. (6) Weber, E. J. Iron-mediated reductive transformations: Investigation of reaction mechanism. Environ. Sci. Technol. 1996, 30, 716–719. (7) Lee, J.; Kim, J.; Choi, W. Oxidation on zerovalent iron promoted by polyoxometalate as an electron shuttle. Environ. Sci. Technol. 2007, 41, 3335–3340. (8) Bruckard, W. J.; Calle, C.; Fletcher, S.; Horne, M. D.; Sparrow, G. J.; Urban, A. J. The application of anthraquinone redox catalysts for accelerating the aeration step in the Becher process. Hydrometallurgy 2004, 73, 111–121.
Seung-Hee Kang and Wonyong Choi* School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea * Corresponding author e-mail:
[email protected]; fax +8254-279-8299.
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