Comments on “1,1,2,2-Tetrachloroethane Reactions with OH , Cr(II), Granular Iron, 
and a Copper-Iron Bimetal: Insights from Product Formation and Associated Carbon 
Isotope Fractionation.” By 
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Elsner et al. 2007 (Environ. Sci. Technol. 41, 4111). 
C. Noubactep 
Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D - 37077 Göttingen, Germany. 
e-mail: cnoubac@gwdg.de; Tel. +49 551 39 3191, FAX: +49 551 399379
 
The paper by Elsner et al. (2007) is a justified attempt to contribute to the elucidation of the 
mechanism of reductive dehalogenation of organohalides in real-world permeable reactive 
barriers (field Fe0–H2O systems or field Fe0 PRBs) by means of a very well-established 
scientific tool (carbon isotope fractionation). For this purpose, the authors used well-mixed 
batch experiments that are unfortunately known not to be representative for real-world 
conditions. Therefore, using column experiments rather than batch systems would have 
closely simulated field conditions (Henderson and Demond 2007). Moreover, it is yet to be 
proven that abiotic contaminant reduction in laboratory batch experiments and in the field 
occurs with the same mechanism, since the hydrodynamic regimes in both systems are largely 
different. 
The hydrodynamic regime in a field Fe0 PRB is governed by the natural hydraulic gradient of 
the contaminated groundwater. Under these conditions the Fe0 surface is covered with an 
oxide-film at an earlier time scale of the barrier life before the occurrence of quantitative 
contaminant inflow (Scott et al. 2005). Various types of reducible contaminants (Henderson 
and Demond 2007), electrochemically non-reducible contaminants like zinc (Ludwig and 
Jekel 2007), and biological agents like viruses (You et al. 2006) are successfully removed 
from aqueous solutions in Fe0–H2O systems. Therefore, the premise that some classes of 
inorganic and organic contaminants are mostly removed by reductive processes through 
electrons from Fe0 materials (direct reduction) should have been carefully demonstrated. This 
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has not been the case as the premise is of pure thermodynamic origin and is based on the low 
redox potential of the couple FeII/Fe0 (Matheson and Tratneyk 1994, Powell et al. 1995). The 
efficiency of various other reductants (FeII(aq), FeII(s), H/H2) at reducing contaminants in the 
presence of oxide surfaces (largely present in Fe0–H2O systems) has been demonstrated 
(Matheson and Tratnyek 1994, White and Paterson 1996). Because these secondary redox 
couples are partly thermodynamically more powerful at reducing contaminants than the 
couple FeII/Fe0, there is no reason why direct reduction should be a priori considered as the 
major mechanism path of abiotic contaminant reduction as observed in column studies and in 
field Fe0 PRBs. In field Fe0 PRBs microbially mediated processes may play an important role 
in reducing contaminant. 
From the huge available literature on iron corrosion it is known that under environmental pH 
conditions, the Fe0 surface is always covered by an oxide-film (Cohen 1959, Schmuki 2002, 
Stratmann and Müller 1994, Wilson 1923). Thus, investigations regarding the processes of 
contaminant removal in Fe0–H2O systems should be performed under conditions favouring 
the formation of oxide-films on Fe0 materials. In particular mixing operations should be 
justified as they have been proven disturbing for the processes of oxide generation and 
transformation (Büchler et al. 1998, Tomashov and Vershinina 1970). In the reactive wall 
literature mixing operations (shaking, stirring) are generally considered as an important tool to 
facilitate the transport of contaminant to the Fe0 surface, and therefore, speed up the reduction 
kinetics (Agrawal and Tratnyek 1996). This was also the intention of Elsner et al. (2007) as 
their systems were “sufficiently mixed at all times to ensure rapid solid/water mass transfer”. 
However, mixing operations enhance Fe0 oxidation and hinder or delay the formation of 
oxide-film on Fe0 or in its vicinity. Therefore, mixing intensities above natural turbulences 
should be avoided. Even though Elsner et al. (2007) have not specified their mixing intensity, 
high mixing intensities are used in referenced works. For example, Cwiertny and Roberts 
(2005) used mixing intensities varying from 20 to 250 rpm. While using such mixing 
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intensities it is possible that reductive conditions are generated and/or maintained, that will 
not occur in field Fe0 PRBs. Therefore, it is possible that contaminant reduction under the 
experimental conditions of Elsner et al. (2007) are not representative for field conditions. This 
remark is a strong criticism of the majority of works dealing with the investigation of 
processes in Fe0–H2O systems, in particular for mechanistic investigations. 
Before using well-established scientific tools to investigate reductive processes in Fe0–H2O 
systems one should make sure that: (i) reduction occurs, and (ii) the experimental conditions 
mimic field conditions as closely as possible. Therefore, Elsner et al. (2007) could have made 
isotopic investigations for parallel experiments under different mixing regimes (including 
non-disturbed systems). 
Another work recent work has used isotope fractionation to investigate the mechanism of 
uranium removal in Fe0–H2O systems (Rademacher et al. 2006) and reported different 
conclusions than the original work (no U(VI) reduction) of Gu et al. (1998). The reports of Gu 
et al. (1998) were mostly based on the results of fluorescence spectroscopic studies. This 
contradiction should have been discussed by Elsner et al. (2007) as the work of Rademacher 
et al. (2006) was published in the same journal four months before their manuscript 
submission. These two examples (organohalides and uranium) show that something is wrong 
in the conception of experiments to investigate processes in Fe0–H2O systems. Diversities in 
the large array of contaminants and in the reactivity of used Fe0 materials (Miehr et al. 2004, 
Noubactep et al. 2005) are certainly among the reasons for these observations, but should not 
explain why two well-established scientific methods (fluorescence spectroscopic and isotope 
fractionation) deliver different results for the same contaminant. 
In conclusion, the work of Elsner et al. (2007) certainly reiterates the importance of isotope 
fractionation for the elucidation of reaction mechanisms. However, because used 
hydrodynamic regimes were not pertinent to environmental conditions, it is difficult to see 
how these results can enable a better prediction of the formation of harmful versus benign 
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organohalide reaction products. A unified procedure for the investigation of processes in Fe0–
H2O systems is needed. 
 
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