We demonstrated that the addition of vitamin B-12 can significantly enhance the reductive dechlorination of tetrachloroethene (PCE) by nano-sized zero valent iron (nZVI). A remarkable reductive dechlorination of PCE (0.25 +/- 0.01 h(-1)) was observed in nZVI suspension (0.05 g/24 mL) with 0.5 mM vitamin B-12 in 6 h, while no significant reductive dechlorination of PCE was observed in the nZVI suspension without vitamin B-12. Vitamin B-12r of which central metal is Co2+ was identified as predominant specie during the dechlorination of PCE. The dechlorination kinetic rate constant of PCE in nZVI/vitamin B-12 system was significantly dependent on the suspension pH and concentrations of vitamin B-12, nZVI, and PCE. An enhancement in PCE dechlorination kinetic (0.180 +/- 0.006-0.709 +/- 0.004 h(-1)) due to the formation of hydroxovitamin B-12r was observed by increasing the suspension pH (5-9). The dechlorination kinetic rate constant of PCE increased proportionally (0.0010 +/- 0.0009-1.3660 +/- 0.2940 h(-1)) as the concentration of vitamin B-12 increased (5 nM to 5 mM). As the concentration of nZVI increased by 50 times (0.01-0.50 g), the dechlorination kinetic increased approximately 137 times (0.007 +/-0.001-0.961 +/- 0.182 h(-1)). However, the dechlorination kinetic decelerated (0.184 1 +/- 0.010-0.039 +/- 0.006 h(-1)) as PCE concentration increased (0.005-1.0 mM). This study provides basic understanding on the role of vitamin B-12/vitamin B-12r as an electron mediator in the enhanced reductive dechlorination of PCE by nZVI occasionally observed in remediation sites. (C) 2011 Elsevier B.V. All rights reserved.