Comment on “Coupled acidification and ultrasound with iron enhances 
nitrate reduction” by Tsai et al. J. Hazard. Mater. 163 (2009) 743 
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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, Tsai et al. [1] reported on their investigation of the effect of solution pH 
(H+) and ultrasound (US) on the destruction of passive oxide on metallic iron (Fe0). The aim 
of the study was to sustain Fe0 reactivity upon nitrate reductive degradation. Their results 
showed that ultrasound, used alone (US/H2O system) or with Fe0 (US/Fe0/H2O system) could 
not induce nitrate (NO3-) reductive degradation. Enhanced NO3- degradation could be 
observed at low pH values (Fe0/H+ system) and the degradation was further enhanced by 
sonication (US/Fe0/H+ system). Tsai et al. [1] concluded that US was helpful in destroying or 
preventing the formation of passive film on Fe0 in acidic solution. However, there is strong 
evidence that these conclusions are not supported by own experimental data. 
First, the fact that no significant difference between the systems US/H2O and US/Fe0/H2O 
could be observed indicates that something was wrong in the experimental design. The 
reaction time (≤ 4 h) was likely too short for the used US power (150 W) and frequency (43 
kHz). Enhancing the power and the frequency of US or lengthen the experimental duration 
(e.g. > 4 h) would have certainly yield better results. A complementary Fe0/H2O system 
should have been investigated to limit speculations. Rigorously, Tsai et al. [1] have performed 
their experiments in undisturbed systems (very low mixing conditions). Powdered Fe0 was 
condensed at the bottom of the vials. Therefore, NO3- transport to Fe0 is influenced by gravity 
or US radiation. Obviously, the used power and the frequency of US radiation were too low to 
induce noticeable NO3- transport within 4 h. The combined effect of the mixing intensity and 
the reaction time were comprehensively discussed by Noubactep et al. [2]. 
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Second, Tsai et al. [1] intended to “investigate the effects of ultrasound and pH on the 
destruction of passive oxide film”. However, they investigated at most the effects of 
ultrasound and pH in avoiding the formation of passive oxide films on Fe
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0. In fact, in order to 
investigate the destruction of any film, one has to first favour its formation. For example by 
pre-equilibration Fe0/H2O systems at neutral pH value and then add acid or apply ultrasonic 
vibrations. At two of the three tested initial pH values (2 and 4) iron is readily soluble and 
oxide films will not formed immediately after the start of the experiments. Keeping the pH 
low will avoid the formation of the film. 
Third, the increase in pH value is used to support degradation pathway while adsorption an 
co-precipitation are not mentioned [3, 4]. However, the results of Tsai et al. [1] showed 
clearly that Fe0 oxidation was not mediated by NO3-. In fact, when the initial pH was 2.0, the 
final pH values varied from 3.42 to 3.83. This corresponds to a protons consumption ([H+] = 
10-pH) of about 390 μmoles in 40 mL solution. The initial concentration of NO3- (100 mg/L) 
corresponds to 65 μmoles of NO3- in 40 mL solution. The resulting molar ratio H+/NO3- of 6 
suggest in agreement with well documented results from the corrosion science, that Fe0 is 
oxidized by acidic dissolution [5]. Moreover this large molar ratio indicate that enough iron 
oxides was generated in the system to co-precipitate possibly reduced NO3-. Here, removed 
NO3- is not necessarily reduced and reduced NO3- is not necessarily transformed by electrons 
from Fe0 (direct reduction) [3, 4]. 
Fourth, the statement that “minor changes in pH indicate that the chemical reduction process 
by Fe0 did not progress significantly” contradicts well-documented results from the iron 
corrosion. In fact, oxygen reduction is the major cathodic reaction during the immersed and 
atmospheric Fe0 corrosion of iron for pH values > 4. For example, Stratmann and Mueller [6] 
analysed the relation between the kinetics of the O2 reduction and the reduction of the oxide 
film. Their results showed that oxygen is predominantly reduced (by FeII) within the oxide 
film and not at the Fe0/H2O interface. Due to the low solubility of iron at pH > 5, the oxide 
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film is generated in the vicinity of Fe0. Therefore, the discussion of the redox processes within 
the oxide film can not be based on changes in pH alone. 
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In conclusion the work of Tsai et al. [1] illustrates how a single mistake (reaction time) in the 
experimental set up can yield to erroneous results. The results are compared to other 
published data [7] and discrepancies are explained for example by differences in the power 
and the frequency of applied ultrasound radiation [1]. It is obvious, that the results of Tsai et 
al. [1] can not be considered for a broad-based understanding of iron barrier technology. A 
unified procedure for the investigation of processes in Fe0/H2O systems is needed to minimize 
time and resources loss by other research groups and to quickly achieve progresses in 
understanding this proven effective technology. 
References 
[1] Y.-J. Tsai, F.-C. Chou, T.-C. Cheng, Coupled acidification and ultrasound with iron 
enhances nitrate reduction, J. Hazard. Mater. 163 (2009) 743–747. 
[2] C. Noubactep, A.-M.F. Kurth, M. Sauter, Evaluation of the effects of shaking intensity on 
the process of methylene blue discoloration by metallic iron. J. Hazard. Mater. 
doi:10.1016/j.jhazmat.2009.04.046   (Available online 19 April 2009). 
[3] C. Noubactep, Processes of contaminant removal in “Fe0–H2O” systems revisited. The 
importance of co-precipitation. Open Environ. J. 1 (2007) 9–13. 
[4] C. Noubactep, A critical review on the mechanism of contaminant removal in Fe0–H2O 
systems. Environ. Technol. 29 (2008) 909–920. 
[5] G.W. Whitman, R.P. Russel, V.J. Altieri, Effect of hydrogen-ion concentration on the 
submerged corrosion of steel. Indust. Eng. Chem. 16 (1924) 665–670. 
[6] M. Stratmann, J. Müller, The mechanism of the oxygen reduction on rust-covered metal 
substrates. Corros. Sci. 36 (1994) 327–359. 
[7] H. Zhang, L. Duan, Y. Zhang, F. Wu, The use ultrasound to enhance the decolorization of 
the C.I. Acid Orange 7 by zero-valent iron, Dyes Pigments 65 (2005) 39–43. 
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