Factors influencing rates and products in the transformation of carbon tetrachloride by zerovalent iron metal

Abstract

Batch experiments were conducted to study the influence of iron purity and solution chemistry, i.e., pH and common groundwater species such as HS-, HCO3- and Mn(II), on the anaerobic degradation of carbon tetrachloride (CT) by iron metal (Fe0) . CT degradation experiments by four commercial irons at pH values of 7 and 9.3 indicated that iron purity and extent of oxidation affect the rate of CT transformation. In contrast, the product distribution was not significantly affected by iron purity, with chloroform (CF) being the major product in all cases. Surface area normalized rate constants (kobs,sA), backscattered electron images, and elemental composition analysis of the untreated metals indicate that the highest purity and least oxidized Fe0 was the most reactive in transforming CT. 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, obtained by depth profiling analysis with X-ray photoelectron spectroscopy (XPS), for Peerless Fe0 filings equilibrated at pH 7 and 9.3 suggested that the oxide layer is thicker at pH 9.3 , which may explain the slower rates of CT transformation as pH increases. Ground water constituents such as HS-, HCO3- and Mn(II) affected the rates of CT by a high-purity Fe0, but did not strongly influence product distribution, except for HS-, where less CF was produced, possibly due to the formation of carbon disulfide (CS2). The results have practical implications for in situ remediation of groundwater contaminated with chlorinated organic compounds by iron permeable reactive barriers.