Carbon Tetrachloride Reduction by ${\mathrm{Fe}}^{2+}\text{,}$ $S^{2-}\text{,}$ and FeS with vitamin $B_{12}$ as Organic Amendment

The reductive dechlorination of carbon tetrachloride (CT) was examined in the presence of free iron ions $\left({\mathrm{Fe}}^{2+}\right)\text{,}$ sulfide ions $\left(S^{2-}\right)\text{,}$ and freshly precipitated ferrous sulfide (FeS) as reducing agents, and vitamin $B_{12}$ as organic amendment. The reductive dechlorination of CT by the reducing ions ${\mathrm{Fe}}^{2+}$ and $S^{2-}$ 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 $\left({\mathrm{FeCl}}_2\right)$ solutions (200 mM) where ${\mathrm{Fe}}^{2+}$ 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 $\left({\mathrm{Na}}_{2}S\right)$ solutions (110 and 200 mM) with $S^{2-}$ 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 $B_{12}$ (0–4 mM) resulted in an enhancement of the dechlorination reaction. When 4 mM of vitamin $B_{12}$ 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 $B_{12}\text{,}$ 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.