pH dependence of carbon tetrachloride reductive dechlorination by magnetite

Magnetite is precipitated by dissimilatory iron-reducing bacteria or forms through corrosion of zero-valent iron (ZVI) in permeable reactive barriers. Reduction of carbon tetrachloride (CCl4) by synthetic magnetite was examined in batch reactors to evaluate the pH dependence of the reaction rates and product distributions. This work presents the first data where magnetite promotes CCl4 dechlorination independent of added sorbed Fe(II) or coexisting minerals that maintained Fe2+ above the magnetite solubility limit. In this system, reaction rate constants increase with increasing pH values between 6 and 10. The pH dependence is explained by acid-base equilibrium between two surface sites, where the more deprotonated exhibits greater dechlorination reactivity. The distribution of reaction products was also found to depend on pH. The primary reaction product is carbon monoxide (CO) followed by chloroform (CHCl3). CHCl3 production is at a minimum at pH 6 but increases through pH 10. Formation rate constants for both products increase with increasing pH, but the values for CHCl3 increase at a much faster rate. A hypothesis is proposed that relates the CHCl3 rate enhancement to the reduced capacity of deprotonated surface sites to stabilize the trichlorocarbanion transition-state complex. These data form a basis to assess the natural attenuation capacity of magnetite formed under iron reducing conditions. Application of this information to permeable barrier technology suggests that, in the long term, oxidation of ZVI to magnetite may be accompanied by a shift toward more benign reaction products as well as a 2 order of magnitude decrease in reaction rate constants.