Effects of Iron Purity and Groundwater Characteristics on Rates and Products in the Degradation of Carbon Tetrachloride by Iron Metal

Carbon tetrachloride (CT) batch degradation experiments by four commercial irons at neutral pH indicated that iron metal (Fe⁰) purity affected both rates and products of CT transformation in anaerobic systems. Surface-area-normalized rate constants and elemental composition analysis of the untreated metals indicate that the highest-purity, least-oxidized Fe⁰ was the most reactive on a surface-area-normalized basis in transforming CT. There was also a trend of increasing yield of the hydrogenolysis product chloroform (CF) with increasing Fe⁰ purity. Impurities such as graphite in the lower purity irons could favor the alternate CT reaction pathway, dichloroelimination, which leads to completely dechlorinated products. High pH values slowed the rates of CT disappearance by Peerless Fe⁰ and led to a pattern of decreasing CF yields as the pH increased from 7 to 12.9. The Fe/O atomic ratio vs depth for Peerless Fe⁰ filings equilibrated at pH 7 and 9.3, obtained by depth profiling analysis with X-ray photoelectron spectroscopy, indicated differences in the average oxide layer composition as a function of pH, which may explain the pH dependence of rate constants and product yields. Groundwater constituents such as HS^-, HCO₃^-, and Mn²⁺ had a slight effect on the rates of CT degradation by a high-purity Fe⁰ at pH 7, but did not strongly influence product distribution, except for the HS^- amended Fe⁰ where less CF was produced, possibly due to the formation of carbon disulfide (CS₂).