Publication

Journal	Environmental Science and Technology
Volume (Issue)	48(3)
Page(s)	1592 - 1600
Title of proceedings	Environmental Science and Technology
DOI	10.1021/es403879e

Chloroperoxidase (CPO) is suspected to play an important role in the biosynthesis of natural chloroform. The aims of the present study are to evaluate the variability of the δ37Cl value of naturally produced chloroform and to better understand the reaction steps that control the chlorine isotope signature of chloroform. The isotope analyses have shown that the chlorination of the humic substances (HS) in the presence of high H3O+ and Cl– concentrations induces a large apparent kinetic isotope effect (AKIE = 1.010–1.018) likely associated with the transfer of chlorine between two heavy atoms, whereas in the presence of low H3O+ and Cl– concentrations, the formation of chloroform induces a smaller AKIE (1.005–1.006) likely associated with the formation of an HOCl–ferriprotoporphyrin IX intermediate. As the concentration of H3O+ and Cl– in soils are generally at submillimolar levels, the formation of the HOCl–ferriprotoporphyrin IX intermediate is likely rate-limiting in a terrestrial environment. Given that the δ37Cl values of naturally occurring chloride tend to range between −1 and +1‰, the δ37Cl value of natural chloroform should vary between −5‰ and −8‰. As the median δ37Cl value of industrial chloroform is −3.0‰, the present study suggests that chlorine isotopic composition of chloroform might be used to discriminate industrial and natural sources in the environment.