Isotope fractionation due to aqueous phase diffusion – What do diffusion models and experiments tell? – A review

Magnitude of Diffusion- and Transverse Dispersion-Induced Isotope Fractionation of Organic Compounds in Aqueous Systems

“…One challenge is that dispersion and diffusion may affect the concentration profiles and level out the degradation-induced gradients of the isotope ratios 5 while another complicating factor may be that the diffusion coefficients of different isotopologues differ, thus potentially causing reaction-independent isotope fractionation by the process of diffusion itself. 13 …”

“… 29 , 30 An even weaker dependence has been observed for ions (e.g., Li + , Na + , Cl – , and Br – ) with β < 0.07. 13 , 31 , 32 Also for organic compounds at natural isotopic abundance, observed mass-dependent isotope fractionation was weak with β < 0.1 for trichloroethene (TCE), 1,2-dichloroethane (1,2-DCA), and cis -dichloroethene ( cis -DCE). 6 , 7 In contrast, a much stronger mass dependence has been observed for the diffusion-induced isotope fractionation of labeled organic compounds (e.g., deuterated benzene and toluene).…”

“…One challenge is that dispersion and diffusion may affect the concentration profiles and level out the degradation-induced gradients of the isotope ratios 5 while another complicating factor may be that the diffusion coefficients of different isotopologues differ, thus potentially causing reaction-independent isotope fractionation by the process of diffusion itself. 13 Although such diffusion-induced isotope fractionation in the aqueous phase was repeatedly considered to be negligible, 9−12 significant diffusion-induced isotope fractionation has been reported in some studies with isotopically labeled organic compounds, 6,7,14−19 enrichment factors ε between −2.6‰ and −7.0‰), 14 benzene, toluene, and ethylbenzene (ε between −40‰ and 19‰). 7,15,19 By contrast, recent studies reported negligible isotope fractionation for labeled benzene, toluene, and cyclohexane.…”

“…7,20,21 Mode-Coupling Theory Analysis and Molecular Dynamic Simulations were brought forward to explain the weak mass dependence of diffusion-induced isotope effects observed for noble gases and ions. 13,31,33 These theories still conceptualize the molecules of the solutes and the surrounding water as rigid masses and neglect the influence of intramolecular vibrations and rotations. The Mode-Coupling Theory assumes that diffusion can be explained by frictions in series, 13 i.e., , in which the collision term 1/Friction collisions would show a squared power-law mass dependence and the hydrodynamic-motion term 1/Friction hydrodynamic shows no mass dependence at all, explaining a weaker dependence on the molecular mass, which is actually not even a power law.…”

Reactive Transport Modeling of Natural Gas Molecular and Isotopic Evolution During Diffusive Transport in the Subsurface

“…Although it is recognized that identification of the source for hydrocarbon gas can be complicated by microbially‐mediated molecular and isotopic fractionations (Humez, Mayer, Nightingale, et al., 2016; Loomer et al., 2020; McMahon et al., 2017; Sherwood et al., 2016), mixing between sources (Moritz et al., 2015; Sherwood et al., 2016) and, less frequently, fractionations due to variable gas solubility (Eymold et al., 2018), possible fractionations resulting from diffusive transport are commonly neglected (Alperin et al., 1988; Darrah et al., 2015; Humez et al., 2019; Schloemer et al., 2018; Schoell, 1984; Whiticar, 1999). Recently however, Wanner and Hunkeler (2018) noted the importance of diffusive isotopic fractionation for a wide range of chemical species. To date, no systematic evaluation of the possible effect of diffusive transport on hydrocarbon source discrimination has been made.…”