Dechlorination of Chlorinated Methanes by Ferrous Ion Associated with Various Iron Oxide Minerals

Abstract

The dechlorination of carbon tetrachloride (CT) by Fe(II) ion in the suspensions of crystalline iron oxide minerals including goethite (α-FeOOH), hematite (α-Fe2O3), magnetite (Fe3O4), and amorphous ferrihydrite (Fe(OH)3) was investigated. Experiments were performed using 10 mM iron oxides and 3 mM Fe(II) to form surface-bound iron suspensions at pH 7.2 under anoxic conditions. Ferrous ion can rapidly sorb onto the surface of iron oxides to form surface-bound iron species. The sorption of Fe(II) onto goethite and hematite followed Langmuir isotherm, while Freundlich isotherm was observed in the magnetite and ferrihydrite suspensions. The major product of CT dechlorination in the iron oxide suspensions containing 3 mM ferrous ion was chloroform (CF) and the conversion ratio of CT to CF was in the range 14-74%, depending on the type of iron oxides. In addition, the dechlorination followed pseudo-first-order kinetics, and the rate constants (kobs) for CT dechlorination were 0.38 h-1 and 0.84 h-1 in goethite and hematite suspensions, respectively. Whereas the kobs was 0.061 h-1 for magnetite and 0.014 h-1 for ferrihydrite, which were lower than those in the highly crystalline iron oxide suspensions. However, the normalized surface area rate constants (ksa) followed the order goethite > hematite > magnetite > ferrihydrite. The solution pH values strongly influenced the rate and efficiency of CT dechlorination and both the kobs and sorbed Fe(II) concentrations were exponentially increased with increasing pH value within the range of 4-8.5, depicting that the increase in kobs is mainly attributed to the increase in the surface-bound Fe(II) concentration at high pH.