Theoretical and Experimental Studies of the Dechlorination Mechanism of Carbon Tetrachloride on a Vivianite Ferrous Phosphate Surface

Abstract

Chlorinated organics are the principal and most frequently found contaminants in soil and groundwater, generating significant environmental problems. Over the past several decades, Fe-containing minerals naturally occurring in aquatic and terrestrial environments have been used as natural electron donors, which can effectively dechlorinate a variety of chlorinated organics. However, a full understanding of the reaction mechanism of the dechlorination pathway cannot be obtained by experimental investigations alone, due to the immeasurability of chemical species formed over a short reaction time. In this report, we describe experiments and density functional theory (DFT) calculations carried out to investigate the complex reduction pathway of carbon tetrachloride (CT) on a vivianite (Fe^(II)_3(PO_4)_2·8H_2O) surface. Our results indicate that chloroform (HCCl_3) and formate are the primary transformation products. The experimental results reveal that the reduction kinetics of CCl_4 can be dramatically accelerated as the pH is increased from 3 to 11. On the basis of the DFT calculations, we found that HCCl_3 can be formed by ^•CCl_3 and :CCl_3^(–*) on a deprotonated vivianite surface (an adsorbate on vivianite is denoted using an asterisk). In addition, :CCl_3^(–*) can be nonreductively dechlorinated to form :CCl_2^* followed by sequential nucleophilic attack by OH^(–*), resulting in the formation of :CCl(OH)^* and :C(OH)_2^*, which are responsible for production of CO and formate, respectively. The results obtained from this study can facilitate the modeling of systems of other halogenated species and minerals, which will provide fundamental insight into their corresponding reaction mechanisms.

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