Comment on ”Oxidative degradation of organic compounds using zero-valent iron in 
the presence of natural organic matter serving as an electron shuttle” 
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C. Noubactep 
Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D - 37077 Göttingen, Germany. 
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e-mail: cnoubac@gwdg.de; Tel. +49 551 39 3191, Fax. +49 551 399379 
 
In a recent study, Kang and Choi (1) investigated the oxidative degradation of 4-chlorophenol 
(4-CP) and clofibric acid (CA) by metallic iron (Fe0) as promoted by natural organic matters 
acting as electron shuttle to mediate electron transfer from the surface of Fe0 to dissolved O2. 
Their results indicated that both humic acid (HA) and fulvic acid (FA) can serve as electron 
shuttle, while enhancing the production of FeII and H2O2 that subsequently initiates the OH 
radical mediated oxidation of 4-CP and CA through Fenton reaction. However, there is strong 
evidence that the conclusion of Kang and Choi are not supported by own experimental data.  
First, the work of Kang and Choi (1) is based on the premise that Fe0-induced contaminant 
removal is “initiated by the direct electron transfer from Fe0 to substrates”. This premise was 
already questioned or/and proven inconsistent (2, 3). In fact, organic substances (4-CP and 
CA) are primarily sequestrated in the matrix of in-situ generated iron corrosion products and 
may be further reduced and or oxidized by several mechanisms. On the other hand the HA 
and FA can only act as electron shuttle only if the oxide-film on Fe0 is conductive (e.g. Fe3O4 
– ref. 4) or rendered conductive by sequestrated electron shuttle (including HA and FA). 
However, an electronic conductive oxide film can not be expected as the rule when the 
oxidant is O2, because the quantitative oxidation to FeIII species is thermodynamically 
favourable at non acidic pH. Note that the experiments of Kang and Choi (1) are performed at 
an initial pH of 2.5, suggesting that depending on the extend of Fe0 consumption (final pH 
value), FeII may be stable in the course of the experiment but quantitative FeIII production will 
occur with increasing pH (5). 
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Second, the system Fe0/H2O/O2 and the impact of electron shuttle on it has been extensively 
investigated in the aeration step of the Becher Process (ref. 4 and references therein). The 
Becher process is an environmentally friendly, cost-effective extraction method for upgrading 
ilmenite (FeTiO
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3) to synthetic rutile (SR: 94 % TiO2). The aeration step involves agitating the 
reduced ilmenite (RI) in a NH4Cl solution while air is sparged through the pulp. The metallic 
iron in the RI is dissolved as Fe2+ which is then oxidised to a variety of FeII/FeIII 
oxide/hydroxides during the aeration reaction. The efficiency of the aeration step is limited in 
that there is little control over the types of iron oxides that are formed. To improve the 
efficiency of the aeration step, the use of various catalysts has been examined. A class of 
redox catalysts, based on anthraquinone derivatives was identified as being very effective. It 
could be shown that the addition of these reagents was able to halve reaction times without 
influencing the quality of the SR, and to guarantee the formation of the preferred conductive 
iron oxide (Fe3O4) product. With other words, the system investigated by Kang and Choi (1) 
is not new, they should have used well-documented results from hydrometallurgy to discuss 
contaminant removal (and degradation). 
Third, the chloride ion (Cl-) production is used to support degradation pathway (vs. 
adsorption). However, the results of Kang and Choi (1) showed significant differences in the 
kinetic and the extend of Cl- production in the presence of HA and FA (figure 1a and 1c – 
only for review) but no difference in the kinetics of organic compounds removal 
(“degradation”). If Cl- production was really coupled to 4-CP (or CA) degradation, the 
difference in the kinetics of Cl- production should be reflected in the kinetics of contaminant 
degradation.  
Fourth, Kang and Choi (1) performed their experiments with a starting pH value of 2.5 and 
did not record pH evolution during their experiments. Because the system was not buffered, it 
is likely the discussed differential behaviour in the presence of HA and FA are due to their 
differential impact on the pH of the system. While working at an initial pH value of 2.5, Kang 
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and Choi (1) compared their results to those of Tratnyek et al. (6) for instance. Tratnyek et al. 
(6) performed all their experiments in buffer systems at pH values > 5.6. Therefore, 
“conflicting” results are expected as the experimental conditions are largely different. 
Moreover, it is difficult to presage which natural systems are mimicked by Kang and Choi (1) 
with and initial pH value of 2.5. 
In conclusion the work of Kang and Choi (1) is an illustration of the quality of published 
works on the topic of “remediation with Fe0” in so many peer-reviewed scientific journals. 
Are member of the Fe0 remediation community matured enough to admit the evidence (or the 
possibility) that their work has followed a wrong direction, since the seminal works of 
Matheson and Tratnyek (7) and Weber (8)? This maturity is the premise to rectify the made 
errors and quickly achieve progresses in understanding this proven effective technology. 
Beside the inconsistent premise on the mechanism of contaminant removal in Fe0/H2O 
systems, it is evident that neither available results from hydrometallurgy nor the complex pH 
dependence behaviour of iron species (reactivity of oxides, solubility of individual species) 
were properly considered by Kang and Choi (1). Furthermore, a proper analysis of 
investigated system is missed: the rationale for the used pH is not given and the evolution of 
the pH during the experiment is not considered in the discussion.  
Considering the nature of the problem, it can be presumed that some environmental scientists 
are overwhelmed with understanding the subtlety and scientific diversity of the physical and 
chemical processes which are involved in iron oxidative dissolution (iron corrosion) 
depending on their background. To solve this problem a unified experimental procedure for 
contaminant removal experiments should be introduced. This is a problem which cannot be 
resolved by a few research groups. The developed experimental procedure should be 
presented in public panels. 
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(1) 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. 
(2) Noubactep, C. Processes of contaminant removal in “Fe0–H2O” systems revisited. The 
importance of co-precipitation. Open Environ. J. 2007, 1, 9–13. 
(3) Noubactep, C. A critical review on the mechanism of contaminant removal in Fe0–H2O 
systems. Environ. Technol. 2008, 29, 909–920. 
(4) Bruckard, W.J.; Calle, C.; Fletcher, S.; Horne, M.D.; Sparrowa, G.J.; Urban, A.J. The 
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(6) Tratnyek, P.G.; Scherer, M.M.; Deng, B.L.; Hu, S.D. Effects of natural organic matter, 
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valent iron. Water Res. 2001, 35, 4435–4443. 
(7) Matheson, L.J.; Tratnyek, P.G. Reductive dehalogenation of chlorinated methanes by iron 
metal. Environ. Sci. Technol. 1994 28, 2045–2053. 
(8) Weber, E.J. Iron-mediated reductive transformations: investigation of reaction 
mechanism. Environ. Sci. Technol. 1996, 30, 716–719. 
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Figure 1 from Kang and choi (Only for review). 99 
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