We demonstrated adsorption and reduction of cobalamin(III) (Co(III)) on nano-mackinawite (nFeS) surface and their impact on reductive dechlorination of tetrachloroethene (PCE). The adsorption of Co(III) on the nFeS surface followed Langmuir isotherm and the reduction of Co(III) provided different reactive surface chemical species on nFeS surface. Content of Fe2+-S on nFeS surface decreased (45.9-14.5%) as Fe2+-S was oxidized to Fe3+-S and Fe3+-O coupled with the surface reduction of Co(III) to cobalamin(II) (Co(II)). S2- and S-n(2-) contents on the nFeS surface also decreased by 48.5% and 82.3%, respectively during the formation of sulfidecobalamin(II) ( S2--Co(II)) by the reactive surface sulfur. PCE was fully degraded in nFeS-Co(III) suspension at pH 8.3 in 120 h. The dechlorination kinetic rate constant of PCE in the nFeS-Co(III) suspension (k(FeS-Co(III)) = 0.188 +/- 0.003 h(-1)) was 145 times greater than that in nFeS suspension, showing a potential role of S2--Co(II) as an electron transfer mediator to shuttle electrons for the enhanced reductive dechlorination. PCE was transformed to acetylene and 1,3-butadiene as major products via reductive beta-elimination and isomerization reactions, respectively. The experimental findings can provide basic knowledge to identify a reaction mechanism for the enhanced reductive dechlorination of chlorinated organic by biogeochemical reactions possibly observed in natural reducing environments. (C) 2012 Elsevier B.V. All rights reserved.