Reductive dechlorination of carbon tetrachloride by zero-valent iron and related iron corrosion

https://doi.org/10.1016/j.apcatb.2009.06.012Get rights and content

Abstract

Electrochemical corrosion behavior of iron in aqueous solutions with and without carbon tetrachloride (CT) was investigated in a wide pH range from 0.4 to 14 using steady-state polarization curves and electrochemical impedance spectroscopy (EIS). It was found that the presence of CT significantly accelerated the hydrogen evolution reaction (HER) on the iron surface in strong acidic solutions, causing severe corrosion of iron; in return, the iron corrosion was helpful for the reductive dechlorination of CT. The inherent relationship between the dechlorination of CT and the corrosion of iron is attributed to the fact that the adsorbed hydrogen atoms produced during the iron corrosion process are necessary for the dechlorination process of CT. As a result, the removal efficiency of CT is strongly dependent on the extent of iron corrosion in aqueous solutions at different pH values.

Introduction

Carbon tetrachloride (CT), one of the commonly used chlorinated hydrocarbons, is a toxic pollutant to human health and environment [1], [2]. It is reported that CT is not only a suspected human carcinogen but also an ozone depleting substance [3], [4]. CT is formerly widely used in extinguisher and refrigeration, but largely abandoned now due to the toxicity. However, this substance is still used as a raw material in many industrial applications or as an industrial solvent at the present time. As a result, it is often found in groundwater or in public water systems [5], [6]. In order to eliminate the threat of CT and other chlorinated hydrocarbons to both human health and environment, much effort has been devoted to the development of low-cost, high-efficiency dechlorination methods.

Reductive dechlorination of chlorinated hydrocarbons with zero-valent iron (Fe0, ZVI) has received increasing attention in recent years since metallic iron was utilized for in situ passive groundwater remediation [7], [8], [9]. For example, permeable reactive barriers (PRBs) containing zero-valent iron have been used as an innovative method to remove chlorinated hydrocarbons in groundwater [10], [11], [12], [13]. When wastewater passes through the barrier, the chlorinated hydrocarbons in the water will be reduced by the iron [14]. In contrast to other wastewater treatment methods, such as activated carbon adsorption, air stripping and biotransformation process, some distinct advantages of the reductive dechlorination by ZVI include mild reaction conditions, rapid reaction rate, low apparatus cost, and especially the green process technology that does not need additional chemical additives [6], [15], [16].

The reduction of chlorinated hydrocarbons by ZVI is essentially a surface-mediated, electrocatalytic reaction, which involves oxidation of iron, dissociation of water and reductive dechlorination of organic compounds. The possible dechlorination pathways include: (i) direct electron transfer from iron to chlorinated hydrocarbons; (ii) reduction with ferrous iron; and (iii) electrocatalytic reduction with hydrogen [2], [17]. So far there has been a lot of controversy about the dechlorination mechanism of chlorinated hydrocarbons with ZVI. Because the rate of the second reaction pathway (i.e. the direct reaction between chlorinated hydrocarbons and ferrous iron produced by the iron corrosion) is quite slow, most researchers are inclined to accept that the reductive dechlorination of chlorinated hydrocarbons proceeds via the first or the third pathway [1], [18]. Some authors believe the chlorinated hydrocarbons are mainly degraded through the direct reaction of chlorinated hydrocarbons with Fe0 (ZVI) at the iron surface [7], [18]. However, the viewpoint contradicts with the experimental results that the rate of reductive dehalogenation of halohydrocarbons is correlated to the pH or hydrogen ion concentration [8], [18], [19], [20], [21], [22]. No matter whether chlorinated hydrocarbons are dechlorinated through the direct reduction by Fe0 or through the indirect reduction by hydrogen species, the iron will suffer more severe corrosion unavoidably in the solutions containing chlorinated hydrocarbons. In this sense, there exists inherent relation between the corrosion of iron and the dechlorination of chlorinated hydrocarbons although there have been few studies in this aspect [22]. It is possible to give an in-depth understanding of electrochemical reductive dechlorination of chlorinated hydrocarbons by studying the corrosion behavior of iron in the presence of chlorinated hydrocarbons.

In this paper, we studied the corrosion of iron in the presence of CT and the electrochemical reductive dechlorination of CT at the iron electrode in solutions with different pH values to explore the link between the two reaction processes. The obtained results clearly indicate that the dechlorination of CT is coupled with the hydrogen evolution reaction (HER) at the iron surface. This new finding is of fundamental importance to better understand the reductive dechlorination mechanism of chlorinated organic compounds by ZVI.

Section snippets

Experimental

A 2.0 mm-diameter iron rod (Aldrich 99.999%) was employed to prepare the working electrode. The iron rod specimen was embedded in epoxy resin mould, leaving its cross-section only exposed to the electrolyte solutions. Before use, the iron electrode was polished with emery papers of decreasing particle size to #2000 finish at first, then rinsed with ultrapure water and degreased with acetone.

The chemicals used in the experiments, carbon tetrachloride (CT), sulfuric acid, sodium sulfate and sodium

Influence of CT on the corrosion behavior of iron in acidic solutions

Fig. 1 shows a set of steady-state polarization curves for the iron electrode in strong acid solutions with and without CT. In the solution with low pH (pH 0.4), the cathodic current was obviously enhanced by CT, while the anodic current was almost not affected by the presence of CT. Interestingly, CT did not cause any obvious change in cathodic Tafel slope, and two cathodic polarization curves gave the almost identical slope of ∼120 mV decade−1 between −0.59 and −0.67 V, which implies the

Conclusions

The reductive dechlorination of CT with ZVI (Fe0) is essentially a catalytic reaction between CT molecules and the adsorbed hydrogen atoms. Because this reaction is a parallel, competitive reaction that is coupled with electrochemical desorption reaction of adsorbed hydrogen atoms, the presence of CT significantly accelerates the rate of the HER in acidic solutions, especially in the strong acidic solutions, thereby causing the more serious corrosion of iron than in blank solutions.

Acknowledgments

This work was supported by the National 973 Program Projects of China (2006CB605004, 2007CB936602), the National Natural Science Foundation of China (20673067) and China High-tech Research and Development Plan (2008AA06Z310).

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