Reductive dehalogenation of hexachloroethane and carbon tetrachloride by aquifer sand and humic acid

This research was conducted to: (1) identify the reactive components of the aquifer material, (2) compare the results with aquifer material to simpler laboratory systems, and (3) evaluate field observations using predictions based on laboratory studies.; Borden aquifer material was fractionated by size, magnetic properties, and by treatment with 5N HCl and 1N NaOH and each fraction was assessed to determine its reactivity. Fractions with that consisted primarily of quartz, carbonates, and feldspars, and contained 0.013% organic carbon and 184 {dollar}mu{dollar}g Fe{dollar}sp{lcub}2+{rcub}{dollar}/g sand, accounted for most of the reactivity. Acid and base extractions and subsequent solubility considerations suggested that the electron donor was associated with organic matter. HCA did not, however, disappear after 100 hours of contact with extracted organic matter. Addition of 0.5mM Fe{dollar}sp{lcub}2+{rcub}{dollar} or 0.5mM HS{dollar}sp{lcub}-{rcub}{dollar} to 25 mg/l of organic matter reduced HCA 15 and 7 times faster, respectively, relative to the rate with no organic matter.; Reductive dehalogenation of HCA, carbon tetrachloride (CTET) and bromoform (BROM) were investigated with International Humic Substances Society soil humic acid. CTET was reduced approximately 8 times slower than HCA and BROM reacted too slowly to quantify reliably. This trend is consistent with estimates based on the Marcus theory of electron transfer. It is proposed that quinone-hydroquinone couples in humic acid are important in reducing haloaliphatics because: (1) the reduction of HCA and CTET was promoted by Fe{dollar}sp{lcub}2+{rcub}{dollar} and HS{dollar}sp{lcub}-{rcub}{dollar} which react with quinones to form hydroquinones, (2) the reduction of HCA was not promoted by hydroxylamine hydrochloride which reacts with quinones to form oximes, (3) anthrahydroquinone (AHQDS) reduced both HCA and CTET, and (4) increasing pH increased the rate in both humic acid and AHQDS systems.; The reduction of HCA by the aquifer solids resulted in the quantitative formation of tetrachloroethene (PCE). Pseudo-first-order rate constants for HCA reduction were determined by fitting a diffusion-limited sorption and a pseudo-first-order reaction model to the observed HCA and PCE data. The temperature-dependence of the rate constants followed the Arrhenius relationship and the observed activation energy was 48 kJ/mol. The rate constant for the Borden field conditions extrapolated from the laboratory data gave good agreement with the observed disappearance of HCA in the field.