Mechanistic insights into the formation of chloroform from natural organic matter using stable carbon isotope analysis

Abstract

Chloroform can be naturally formed in terrestrial environments (e.g. forest soils, peatland) by chlorination of natural organic matter (NOM). Recently, it was demonstrated that natural and anthropogenic chloroform have a distinctly different carbon isotope signature that makes it possible to identify its origin in soil and groundwater. In order to evaluate the contribution of different functional groups to chloroform production and factors controlling the isotopic composition of chloroform, carbon isotope trends during chlorination of model compounds, soil organic matter (SOM) and humic acids were evaluated, and apparent kinetic isotope effects (AKIEs) quantified. Phenol and propanone were selected as model compounds representing common functional groups in NOM. Chlorination was induced by hypochlorous acid to mimic natural chlorination. The pH ranged between 4 and 8 to cover typical soil conditions. For each model compound and pH, different AKIEs were observed. For phenol, the AKIE was normal at pH 4 (1.0156±0.0012) and inverse at pH 8 (0.9900±0.0007). For 2-propoanol, an opposite pH dependence was observed with an inverse AKIE at pH 4 (0.9935±0.0007) and a normal AKIE at pH 8 (1.0189±0.0016). The variations of the AKIE values suggest that the rate-limiting step of the reaction is either the re-hybridization of the carbon atom involved in chloroform formation or the hydrolysis of trichloroacetyl intermediates depending on the nature of functional group and pH. The chloroform formation from humic acid and SOM gives rise to small isotope variations. A comparison of the isotopic trends of chloroform formed from humic acid and SOM with those found for the model compounds suggest that opposed AKIE associated with the chlorination of phenolic and ketone moieties of NOM partly compensate each other during chlorination of NOM indicating that different types of functional groups contribute to chloroform formation.