Correspondence/Rebuttal pubs.acs.org/est
Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Comment on “Investigation of the Iron−Peroxo Complex in the Fenton Reaction: Kinetic Indication, Decay Kinetics, and Hydroxyl Radical Yields”
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n a recently published paper1, Wiegand et al. claimed that they have presented the first evidence for the existence of Fe(II)−peroxo complexes in the Fenton reaction by kinetic experiments. Although Wiegand et al.’s approach is interesting, we believe that their data interpretation has critical problems for the following reasons: Our question began with the fact that the plots of k′ versus [H2O2] do not appear to exhibit clear plateaus, which is different from the explanation given in the text as well as the scheme presented in the graphical abstract (which shows a perfect plateau). Wiegand et al.1 stated that “k13 was derived f rom these data by linear regression analysis of k′ versus [H2O2] in the plateau with a slope of 0”. However, such a perfect plateau with a zero slope is hardly seen in their data (Figures 1 and 2 in ref 1). This discrepancy results from their own definition of the plateau as described in the statement that “The plateau was defined as the range starting f rom the highest [H2O2] until no overlap of determined standard errors could be observed between k′ at a certain [H2O2] and k′ at the next lower [H2O2]”. However, this definition seems to be unreasonable. Based on such a definition, the plots exemplified in Figure 1 can be misinterpreted as plateaus.
each other. The average value of k13 determined (Table 1 in ref 1) is 54.2 s−1. In addition, they determined the ksecond values from the linear regression of k′ versus [H2O2] plots at low [H2O2] ranges where k′ linearly increases with increasing [H2O2], and the average value of ksecond determined (Table 1 in ref 1) is 67 M−1 s−1. According to eq 3, ksecond can be expressed by k13K12. (eq 3). Therefore, K12 should be ksecond/k13 = 1.24 M−1. This value of K12 does not satisfy the assumption used in eq 2 (i.e., 1 ≪ K12[H2O2]) given that [H2O2] used to determine k13 ranges from 0.75−1.25 M. After all, the model calculation using the determined values of k13 and K12 does not fit the experimental data (refer to the blue lines in Figure 2). k′ =
(1) k′ =
k13K12[H 2O2 ] ≈ k13(if1 < >K12[H 2O2 ]) 1 + K12[H 2O2 ]
(3)
We believe that eq 1 should have been directly used to model k′ as a function of [H2O2]. In principle, both k13 and K12 can be determined by model fitting of experimental data; the plot of 1/ k′ versus 1/[H2O2] will give a linear line. Since it is difficult for us to perform the accurate model fitting, we simply depicted the lines of our calculation over the figures presented by Wiegand et al. (Figure 2). Based on the result in Figure 2, it appears that the best-fit value of k13 ranges from approximately 150−300 s−1, which is much higher than the values reported by Wiegand et al.1 In summary, Wiegand et al.1 unreasonably defined the plateau in the plots of k′ versus [H2O2] (without a doubt, the most important data in their study), weakening the legitimacy of their claim that they presented the first experimental evidence for the formation of Fe(II)−peroxo complexes. The authors overlooked the fact that the equilibrium constant for the formation of Fe(II)-complexes could be low (in this case, the plateau can hardly be observed in the plots of k′ versus [H2O2]). Resultantly, the authors have misused the approximation for the kinetic expression of k′, thus reporting the k13 values contradictory to those of ksecond.
Figure 1. Examples of plots defined as the plateau based on the definition provided by Wiegand et al.
Wiegand et al.1 formulated the kinetic expression for k′ in terms of [H2O2] and relevant rate constants (eq 1), which seems acceptable if the effect of •OH is neglected (as stated by the authors). To explain the plateau, they simplified the expression for k′ at high [H2O2] (eq 2). However, this approximation is valid only if k−12 is much lower than k12, in other words, only if the equilibrium constant for the formation of the Fe(II)−peroxo complex (K12 = k12/k−12) is high enough (i.e., 1 ≪ K12[H2O2]). However, no rationale for such an assumption could be found in the study by Wiegand et al. As a result, the data presented in their study are contradictory with © XXXX American Chemical Society
k13k12[H 2O2 ] k13k12[H 2O2 ] k K [H O ] ≈ = 13 12 2 2 k −12 + k13 + k12[H 2O2 ] k −12 + k12[H 2O2 ] 1 + K12[H 2O2 ]
Changha Lee* Min Sik Kim Hak-Hyeon Kim
School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulju-gun, Ulsan 44919, Republic of Korea
A
DOI: 10.1021/acs.est.8b00062 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
Figure 2. Model calculations (lines) of k′ as a function of [H2O2] (eq 1 was used for the calculation, and the lines were displayed over Figures 1 and 2 in ref 1. Blue lines indicate the calculations using k13 and ksecond (K12 = ksecond/k13) reported by ref 1; the average values at pH 3 in Table 1 in ref 1 were used.).
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AUTHOR INFORMATION
Corresponding Author
*Phone: 82-52-217-2812; fax: 82-2-450-3542; e-mail: clee@ unist.ac.kr. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Wiegand, H. L.; Orths, C. T.; Kerpen, K.; Lutze, H. V.; Schmidt, T. C. Investigation of the iron−peroxo complex in the Fenton reaction: Kinetic indication, decay kinetics, and hydroxyl radical yields. Environ. Sci. Technol. 2017, 51, 14321−1432.
B
DOI: 10.1021/acs.est.8b00062 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
