Degradable Organic Contaminants by ZVI

ZVI and other metals (e.g., Zn, Cu, and Al) are strong reducing agents at ambient temperature, capable of reducing many halogenated methanes, ethanes, and ethenes and other halogenated organic compounds (Sweeny, 1979; Senzaki and Kumagai, 1988; Gillham and O'Hannesin, 1994; Matheson and Tratnyek, 1994; Burris et al., 1995; Roberts et al., 1996). To illustrate, some of the halogenated compounds degradable in the presence of ZVI are listed in Table 1. The list includes many compounds with significant environmental concerns, as indicated by the fact that eight of them are among the 25 most frequently detected ground water contaminants at hazardous waste sites (NRC, 1994).

The rates of reductive dechlorination reactions differ significantly among various compounds. Chlorinated ethanes and methanes are degraded very quickly, with the half-lives less than an hour for most of the compounds (Gavaskar et al., 1998). For chlorinated ethylenes, the half-lives of degradation normalized to 1 m2 iron surface/ml solution ranges from less than an hour to several days (Johnson et al., 1996; Gavaskar et al., 1998). These degradation rates are orders of magnitude greater than the natural rates of dehalogenation (Vogel et al., 1987). Nevertheless, there are a few compounds that do not demonstrate noticeable degradation in the presence of ZVI, including chloromethane, dichloromethane, 1,2-dichloroethane, and l,4-dichlorobenzene(EPA, 1998; Gavaskar et al., 1998).

Table 1. Degradability of halogenated organic compounds in the presence of zerovalent iron (Adapted from Gavaskar et al., 1998 and EPA, 1998). Compounds indicated in bold face are among the 25 most frequently detected groundwater contaminants at hazardous waste sites (NRC, 1994).___ _

Methanes

Carbon Tetrachloride; Trichloroethane; Bromoform

Ethanes

Hexachloroethane; 1,1,2,2-Tetrachloroethane; 1,1,1,2-

Tetrachloroethane; 1,1,1-Trichloroethane; 1,1-

Dichloroethane

Ethylenes

Tetrachloroethylene (PCE); Trichloroethylene (TCE); 1,1-

Dichloroethylene; trans-l,2-Dichloroethylene; cis-1,2-

Dichloroethylene; Vinyl Chloride

Propanes

1,2,3-Trichloropropane; 1,2-Dichloropropane; 1,3-

Dichloropropane

Other Deeradable

1,1,2-Trichlorotrifluoroethane; 1,2-Dibromo-3-chloropropane;

ComDOunds

1,2-Dibromoethane; n-Nitrosodimethylamine (NDMA);

Nitrobenzene

2.2 Dechlorination Pathways

In order to apply ZVI-based dechlorination for site remediation, we have to be sure that no toxic reaction products are produced in the end, and toxic intermediates, if there are any, are degradable. Product distribution is controlled by reaction mechanisms. For the reduction of chlorinated methanes such as carbon tetrachloride (CT), hydrogenolysis, where chlorines are sequentially replaced by hydrogen, takes place in the presence of ZVI (Vogeletal., 1987; Matheson and Tratnyek, 1994; Glod etal, 1997):

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