A laboratory investigation was conducted to measure the reaction of several types of zero-valent iron (ZVI) with groundwater containing high levels of chlorinated solvents, particularly carbon tetrachloride (CT) at 71 ppm, chloroform (CF) at 34 ppm, tetrachloroethene (PCE) at 32 ppm, and trichloroethene (TCE) at 18 ppm. The concentrations of chloride and sulfate were 3,000 and 600 ppm, respectively. The purpose of the study was to document the rate and extent of reaction between ZVI and individual chlorinated compounds, and to identify products formed as a result of the reaction. Five types of ZVI with different surface coating and particle sizes were studied. All experiments were performed in 160 cc serum bottles containing 100 ml of groundwater. Decreases in initial contaminants and formation of products including chlorinated daughter compounds as well as molecular hydrogen, methane, ethene, ethane, low molecular weight hydrocarbons, carbon monoxide, and carbon dioxide were made by withdrawing 100 mL headspace samples and analyzing by direct injection into a gas chromatograph. Measurements were made at time intervals from 0.5 to 200 hr.
The data showed that ZVI caused extensive degradation of chlorinated methanes and ethenes. Type 2 ZVI reacted most extensively, exhibiting the following half-lives: CT, 0.5 hr; TCE, 24 hr; PCE, 97 hr; and CF, 128 hr. All of the contaminants except CT were directly dechlorinated to non-chlorinated products. The appearance of non-chlorinated alkanes and alkenes was strongly correlated with the destruction of the chlorinated methanes and chlorinated ethenes. In the case of CT, 9 % to 43 % was recovered as CF. CT was degraded completely by all types of ZVI, although some types reacted with high selectivity toward that compound.
The reaction of ZVI with chlorinated compounds appears to involve multiple mechanisms including catalytic formation of alkanes and alkenes via a free radical mechanism, and reductive dechlorination. It is important to note that ZVI continued to react for over 200 hr. Although the destruction of chlorinated compounds was only partially accounted for by the formation of alkanes and alkenes, particularly the series from methane through butenes, it is likely that additional breakdown products (such as higher molecular weight alkanes and carbon monoxide) are present as water-soluble constituents.
The fact that chloroform was not stoichiometrically formed during the reaction of ZVI with CT is important, since CF is a strong microbiological inhibitor. Because CT and CF are inhibitors of dehalorespirers, rapid destruction of CT by ZVI is of particular importance to sites undergoing in-situ bioremediation that contain both chlorinated methanes and ethenes. The observed rapid reactivity of ZVI with CT may permit CT hot spots to be treated in-situ or in above ground reactors. The release of large amounts of molecular H2 from the reaction between ZVI and water could serve as an electron donor for dehalorespirers.