Variability in the carbon isotope fractionation of trichloroethene on its reductive dechlorination by vitamin B_(12)

摘要

Stable carbon isotope fractionation through the reductive dechlorination of trichloroethylene by vitamin B_(12) was determined to assess the possibility of using stable carbon isotope analysis to determine the efficacy of remediation of trichloroethylene using vitamin B_(12). We elucidated the effects of environmental conditions, including the pH, reaction temperature, and vitamin B_(12) concentration, on the carbon isotope enrichment factor (ε). The ε values were relatively insensitive to the reaction temperature and vitamin B_(12) concentration, ranging from -15.7‰ to -16.2‰, with a mean of -15.9 ± 0.2‰, at different temperatures and vitamin B_(12) concentrations. Such a reproducible s value could be particularly useful for estimating the extent of degradation in reactions in which a mass balance is difficult to achieve. However, changing the initial solution pH from 6.5 to 9.0 caused a notable change in the e values, from -14.0‰ to -18.0‰. Reactions were investigated by calculating the apparent kinetic isotope effects for carbon, which, at 1.029-1.037, were smaller than the kinetic isotope effect values previously found for C-Cl bond cleavage. This indicates that a reaction other than the elimination of chloride may be a competitive degradation pathway. The dominant degradation pathway may be different for different initial solution pH values, and this will clearly influence carbon isotope fractionation. Therefore, if the e value varies with reaction conditions, such as the solution pH, the calculations should take into account the actual environmental conditions that affect the rate limiting pathways.