Correspondence/Rebuttal Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
pubs.acs.org/est
Response to Comment on “Investigation of the Iron−Peroxo Complex in the Fenton Reaction: Kinetic Indication, Decay Kinetics, and Hydroxyl Radical Yields”
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conclusions to be valid. In the following, the existence of a plateau in observed pseudo first order reaction rate constants of FeIIIformation (k’) at elevated [H2O2] is addressed. Furthermore, we will discuss the simplifications of equation (VIII) of our article.2
e appreciate the opportunity to respond to the comment of Lee et al. (2018)1 on our recent publication.2 This comment gave us the opportunity to critically review our manuscript again and we found all of our general outcomes and
Figure 1. a) Observed pseudo first order reaction rate constants at increasing [H2O2] measured at different pH-values at T = 20 °C and [FeII]0 = 300 μM. (b), (e) Respective residuals corresponding to pH 1−4.
© XXXX American Chemical Society
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DOI: 10.1021/acs.est.8b00982 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
We agree with Lee et al. (2018)1 that not all of our data exhibit clear plateaus within the investigated [H2O2] range. However, it was not possible to determine k’ at higher [H2O2], due to the strong background absorbance of H2O2, which largely interfered with FeIII absorbance. Indeed, we already tried to calculate k13 by fitting our data with an exponential function (equation a). Thereby k13 can be derived by extrapolation to infinite [H2O2].3 k′ = k13 + A × e[H2O2]/ B
Kinetic indication, decay kinetics and hydroxyl radical yields. Environ. Sci. Technol. 2017.511432110.1021/acs.est.7b03706 (3) El Seoud, O. A.; Baader, W. J.; Bastos, E. L., Practical Chemical Kinetics in Solution. In Encyclopedia of Physical Organic Chemistry, 1st ed.; Wang, Z., Wille, U., Juaristi, E., Eds.; John Wiley & Sons, Inc.: Hoboken, 2017. (4) Connors, K. A. Chemical Kinetics - The study of Reaction Rates in Solution; VCH Publishers, Inc.: New York, 1990.
(a)
with A and B being fitting parameters. For those data, which already exhibit clear plateaus within the observed [H2O2] range (pH 3 and 4 in Figure 1a), the function describes our data very well (cf. respective residuals in Figure 1d and e). However, those data which do not exhibit a clear plateau within the investigated range of [H2O2] k’ measured at high [H2O2] are not exactly depicted by the function (see pH 1 and 2 in Figure 1a and respective residuals in Figure 1b and c). This results in values for k13, which reveal large statistical errors since the error is leveraged by the strong extrapolation to infinite [H2O2]. Hence, we did not use equation (a) for determination of k13 in our recent publication and developed another methodology to determine k13. However, we agree that also this method may not match the real value of k13 in case of k’ is not exhibiting a clear plateau at high [H2O2]. Lee et al. (2018)1 claimed that the simplification of equation (VIII) shown in our work is only valid if k−12 is much smaller compared to k12 and formulated an alternative simplification without neglecting k−12 in the denominator of equation (VIII).1 However, at very large (infinite) [H2O2] the term k12[H2O2] is larger compared to k−12 and the reaction is strongly forced toward Fe(H2O2)2+. Hence, neglecting k−12 in the denominator of equation (VIII) is legitimated. Furthermore, according to the steady state approximation FeIII-formation would have been markedly suppressed in case of k−12 being much larger than k12[H2O2].4 As all of our experiments showed fast formation of FeIII, our assumption (k12[H2O2] ≫ k−12) is in line with our experimental results.2 Lee et al. (2018)1 derived the equilibrium constant (K) for the formation of the intermediate FeII−H2O2-complex (K12) from our data presented for ksecond and k13. This and also the conclusion derived from the value of K12, that k−12 is almost equal to k12 is invalid. It must be taken into account that ksecond was determined from k’ obtained for small [H2O2], whereas k13 is only valid for infinite [H2O2].2 In other words, ksecond and k13 are reaction rate constants for two systems, which cannot be combined. We are still convinced that our published data exhibit a kinetic indication for the formation of an intermediary ironperoxo-complex in Fenton reactions and want to point out that our recently published conclusion that the intermediate forfeits stability with decreasing pH is valid regardless of the method applied for data evaluation. 3
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Hanna Laura Wiegand Klaus Kerpen Holger V. Lutze Torsten C. Schmidt REFERENCES
(1) Lee, C.; Kim, M. S.; Kim, H.-H. Comment on “Investigation of the Iron-Peroxo Complex in the Fenton Reaction: Kinetic Indication, Decay Kinetics, And Hydroxyl Radical Yields”. Environ. Sci. Technol. 2018; 10.1021/acs.est.8b00062. (2) Wiegand, H. L.; Orths, C. T.; Kerpen, K.; Lutze, H. V.; Schmidt, T. C. Investigation of the iron-peroxo-complex in the Fenton reaction: B
DOI: 10.1021/acs.est.8b00982 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
