Carbon and hydrogen isotope fractionation during aqueous diffusion of benzene

Abstract

Compound-specific isotope analysis is widely used for evaluating the degradation of organic compounds in aquifers. We investigated the potential of aqueous diffusion of benzene under aquifer conditions (310 K, 70 bars) to cause significant isotope fractionation that might bias the estimates of biodegradation. Diffusion coefficients were computed using external field – non-equilibrium molecular dynamics simulations for benzene isotopologues with fictive masses to investigate the influence of molecular mass on aqueous diffusion. Our findings reveal variable power-law relations between molecular masses and diffusion coefficients depending on mass distribution, none of them being consistent with the kinetic theory often used to address isotopic effects on solute diffusion, even in dense solvents. Fictive isotopologues with mass substitutions not affecting the symmetry of the molecule consistently align with a power-law exponent β_a = 0.038. In contrast, mass substitutions on a single atom within the benzene molecule (either hydrogen or carbon) result in a smaller mass dependency with βs = 0.021. This result implies that isotopically-labelled molecules such as perdeuterated benzene might not behave similarly to naturally-occurring mono-substituted molecules, an observation that is of importance for the use of these compounds in laboratory experiments. The resulting fractionation factor for mono-substituted benzene isotopologues is −0.3 ‰, and hardly contributes to the variations in benzene isotope composition under aquifer conditions, which validates the use of isotope ratios for evaluating biodegradation of organic compounds in aquifers.