Comment 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”

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 Fe–H2O systems or field Fe 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 Fe PRB is governed by the natural hydraulic gradient of the contaminated groundwater. Under these conditions the Fe 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 Fe–H2O systems. Therefore, the premise that some classes of inorganic and organic contaminants are mostly removed by reductive processes through electrons from Fe materials (direct reduction) should have been carefully demonstrated. This