Comments on “Reductive dechlorination of organochlorine pesticides in soils from an 
abandoned manufacturing facility by zero-valent iron” by Cong et al. [Sci. Tot. Environ. 
(2010) doi:10.1016/j.scitotenv.2010.04.035.] 
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Noubactep C. 
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 
 
Recently, Cong et al. (2010) published a comprehensive study on the characterization of 
reductive properties of hexachlorocyclohexanes (HCHs) and dichlorodiphenyltrichloroethane 
(DDTs) using powdered metallic iron (Fe0). Quantum chemical descriptors characterizing 
different molecular structures and physicochemical properties of the seven tested 
organochlorine pesticides (4 HCHs and 3 DDTs) were computed to discuss the major 
influential factors over their reductive dechlorination rates. A model was reported to be 
established, which correlates the reductive dechlorination properties of pollutants with their 
structural descriptors. As major result, the reductive dechlorination rate of tested 
organochlorine pesticides appears to be mainly limited by the rate of dissolution in the 
aqueous phase. It is evident that this conclusion is not specific to reduction by Fe0 but rather a 
simple solubility issue. 
The work of Cong et al. (2010) is a justified attempt to correlate the intrinsic properties of 
individual contaminants to their chemical reactivity in a aqueous systems. Accordingly, the 
authors are to be commended for having listed up to twenty two (22) structural and 
thermodynamic parameters which may influence the chemical reactivity of individual 
contaminants. Table 1 gives three structural [molecular weight (Mw) and average molecular 
polarizability (Polar), Van der waals volume of molecule (V)] and one thermodynamic 
parameters [formation free energy (ΔfG0)] representative for discussing the reactivity. For 
instance, if Mw is the most determinant parameter for the reactivity then two classes of 
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reactivity (1 and 2) should have been observed. However Cong et al. (2010) reported seven 7 
reactivity’s classes as each tested pollutant’s reactivity was significantly different from that 
from others. Table 1 shows that from the three other selected parameters each exhibit a 
different reactivity order for the pollutants. This order of reactivity was different from the one 
reported by Cong et al. (2010). The exception of V and Δ
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fG0 which yield the same order of 
reactivity should be noticed. This identity is not surprising as V is a thermodynamic constant 
used for the calculation of ΔfG0. A closed look on Tab. 1 reveals that HCHs (1, 2, 3, 4) are 
more reactive that DDTs (5, 6, 7) for all selected parameters. But for the comparison based on 
the soil contaminant concentrations ([X] in μmol/kg), this order is perturbed. Suggesting that 
relevant aspects were not purposefully considered in the experimental design. 
 
Table 1: Selected structural and thermodynamic parameters of organochlorine pesticides 
tested by Cong et al. (2010) and related relative order of reactivity. 
 
Pollutant [X] Polar V ΔfG0 Relative reactivity 
 (μmol/kg) (a.u.) (cm3/mol) (kJ/mol) present [X] Mw Polar V ΔfG0
γ-HCH 3.7 118.9 177.015 133.562 1 2 1 2 4 4 
δ-HCH 27.3 117.6 139.522 137.806 2 5 1 1 1 1 
β-HCH 92.8 121.6 170.585 132.205 3 6 1 4 3 3 
α-HCH 23.9 119.6 139.661 130.04 4 5 1 3 2 2 
o,p'-DDT 3.8 191.6 247.826 311.879 5 3 2 5 7 7 
p,p'-DDT 4.4 195.4 242.140 309.503 6 1 2 7 6 6 
p,p'-DDE 1.9 193.3 200.024 307.898 7 4 2 6 5 5 
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First, the tested HCHs (4) and DDTs (3) were not added as chemicals into the aqueous phase 
but were characterized from their leaching extent (and subsequent removal by Fe
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0) from 
contaminated soils from a former pesticide manufacturing facility. However, the leachibility 
of individual contaminants depends on their affinity to soil materials. This explains why some 
HCHs are more sensitive to water leaching that DDTs (Tab. 1). 
Second, given the difference in molecular weight of tested pollutants (287.860 and 315.938 
a.m.u) the extent of contaminant removal should have been expressed in molar unit.  
Third, Cong et al. (2010) have randomly interchanged “contaminant removal” and 
“contaminant reductive transformation” by Fe0. Aqueous Fe0 is rigorously a system in which 
the surface of iron (bare Fe0) is permanently covered with a layer of iron oxides (Statmann 
and Müller,1994; Noubactep, 2007; Noubactep, 2008). Accordingly the rate of contaminant 
removal is necessarily a function of contaminant affinity to iron oxides. This suggests that 
beside the properties of tested organochlorine pesticides, the properties of the adsorbing 
phases should have been considered as well. 
In conclusion, the experimental results of Cong et al. (2010) could be differently presented. 
The fact that Cong and his colleagues attributed contaminant removal to reductive 
dechlorination demonstrates the extent to which researchers have been driven deep into 
confusion by the concept considering Fe0 as a reducing agent. However, before introducing 
this proven inconsistent concept, no survey of available data on the Fe0/H2O system and the 
link of the data for all possible hypotheses and the consequences for contaminant removal was 
achieved (Noubactep 2007, Noubactep 2008). 
References 
Cong X, Xue N, Wang S, Li K, Li F. Reductive dechlorination of organochlorine pesticides in 
soils from an abandoned manufacturing facility by zero-valent iron. Sci. Tot. Environ., 2010, 
doi:10.1016/j.scitotenv.2010.04.035. 
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Noubactep C. Processes of contaminant removal in “Fe0–H2O” systems revisited. The 
importance of co-precipitation. 
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Open Environ. J. 2007:1, 9–13. 
Noubactep C. A critical review on the mechanism of contaminant removal in Fe0–H2O 
systems. Environ. Technol. 2008; 29: 909–20. 
Stratmann M, Müller J. The mechanism of the oxygen reduction on rust-covered metal 
substrates. Corros. Sci. 1994; 36:327–59. 
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