Variability in the carbon isotope fractionation of trichloroethene on its reductive dechlorination by vitamin B12

Abstract

Stable carbon isotope fractionation through the reductive dechlorination of trichloroethylene by vitamin B₁₂ was determined to assess the possibility of using stable carbon isotope analysis to determine the efficacy of remediation of trichloroethylene using vitamin B₁₂. We elucidated the effects of environmental conditions, including the pH, reaction temperature, and vitamin B₁₂ concentration, on the carbon isotope enrichment factor (ε). The ε values were relatively insensitive to the reaction temperature and vitamin B₁₂ concentration, ranging from −15.7‰ to −16.2‰, with a mean of −15.9 ± 0.2‰, at different temperatures and vitamin B₁₂ concentrations. Such a reproducible ε 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 ε 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 ε 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.