Position-specific Carbon Isotope Fractionation gives Insights into Mechanistic Models for Evaporation of Organic Liquids in the Environment

Abstract

This work describes position-specific isotope effects (IE) measured by iq 13C-NMR during the evaporation of 8 different liquids of different polarities under 4 evaporation modes (passive evaporation, air-vented evaporation, low pressure evaporation and distillation). The observed effects are used to assess the question whether for carbon isotopes in organic liquids the use of the Craig-Gordon isotope model is valid. It is shown for five liquids that the overall IE include vapour-liquid IE which are strongly position-specific in polar compounds and less specific in apolar compounds, and a diffusive effect for which is not position-specific. The diffusive effect is diminished under wind and low pressure. After evaporation, the position-specific isotope pattern created by liquid-vapour IE is still quite reasonably preserved in these five liquids, which have, like water, an air-side limitation for volatilisation. Three other liquids in this study with limitations rather or fully on the liquid side differed from this pattern. It is concluded that evaporation of organic pollutants creates a unique position-specific isotope pattern that may be used to assess the progress of remediation or natural attenuation, and that the Craig-Gordon isotope model is valid for the volatilization of organic liquids with air-side limitation of the volatilization rate.