Reduction of polyhalogenated methanes by surface-bound Fe(II) in aqueous suspensions of iron oxides

Uptake of ferrous iron from aqueous solution by iron oxides results in the formation of a variety of reactive surface species capable of reducing polyhalogenated methanes (PHMs). Pseudo-first-order reaction rate constants, k(obs), of PHMs increased in the order CHBrCl2 < CHBr2Cl < CHBr3 < CCl4 < CFBr3 < CBrCl3 < CBr2Cl2. The k(obs) values increased with the exposure time, t(eq), of Fe(II) to suspended iron oxides which was attributed to the rearrangement of initially sorbed Fe(II) species to more reactive surface species with time. At pH 7.2, the k(obs) values of PHMs also increased with the concentration of surface-bound ferrous iron, Fe(II)(sorb), particularly when Fe(II)(tot) was increased to concentrations where surface precipitation becomes likely. At fixed total Fe(II) concentrations, k(obs) values increased exponentially with pH. The highest reactivities were associated with pH conditions where surface precipitation of Fe(II) is expected. Fe(II)(sorb) and pH, however, had opposite effects on the product formation of PHMs. At pH 7.2, the formation of formate from CX4 (X = Cl, Br) increased with Fe(II)(sorb), whereas increasing pH favored the format ion of CHX3. The ratio of halogenated products and formate formed is indicative of the relative importance of initial one- or two-electron-transfer processes, respectively, and was found to depend on the type of iron oxide mineral also. Our data form a basis to assess the importance of chemical reactions in natural attenuation processes of PHMs in environmental systems under iron-reducing conditions.