Carbon Tetrachloride Reduction by Fe2+,S2-, and FeS with vitamin B12 as Organic Amendment

Abstract

The reductive dechlorination of carbon tetrachloride (CT) was examined in the presence of free iron ions (Fe2+), sulfide ions (S2-), and freshly precipitated ferrous sulfide (FeS) as reducing agents, and vitamin B12 as organic amendment. The reductive dechlorination of CT by the reducing ions Fe2+ and S2- in homogeneous phase resulted in the formation of variable amounts of the mono- and di-dechlorination products chloroform (CF) and dichloromethane (DCM). In the ferrous chloride (FeCl2) solutions (200 mM) where Fe2+ was the only electron source, 76% of the original CT was depleted within 1/2 h and about 28% and 8% went to CF and DCM, respectively, at a pH of 3.1 and with no buffer present. These same dechlorination products were observed in unbuffered sodium sulfide (Na2S) solutions (110 and 200 mM) with S2- as the electron source at pH 13.4. Dechlorination products were also observed in heterogeneous systems where FeS (75–200 mM) acted as the bulk reductant and the addition of vitamin B12 (0–4 mM) resulted in an enhancement of the dechlorination reaction. When 4 mM of vitamin B12 were added to 200 mM FeS, CT was removed continuously and the amount of CF and DCM formed increased significantly over time, yielding a mass recovery of 40% and higher after 1 h and a pseudofirst-order rate constant of 1.91 h−1. The reductive dechlorination of CT in the absence of vitamin B12, resulted in a slower disappearance of CT and the formation of smaller amounts of CF and DCM accounting for only 1% of the mass loss in the 75 mM FeS suspensions to 15% in the 200 mM FeS suspensions after 2 h.