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

For the interpretation of stable isotope ratio trends in saturated geochemical systems, the magnitude of
aqueous phase diffusion-induced isotope fractionation needs to be known. This study reviews how five
diffusion models (Fick, Maxwell-Stefan, Einstein, Langevin, Mode-Coupling Theory Analysis (MCTA) of
diffusion) predict isotope fractionation due to aqueous phase diffusion and compares them with
experimental results. The reviewed diffusion models were not consistent regarding the prediction of the
mass (m) dependency of the aqueous phase diffusion coefficient (D). The predictions range from a square
root power law (D f m0.5) to an opposite mass dependency of D (D f mb). Experimental studies
exhibited consistently a weak power law mass dependency of the diffusion coefficient (D f mb with
b < 0.5) for the vast majority of dissolved species and a larger diffusion-induced isotope effect for low
weight noble gases (D f m0.5). The weak power law mass dependency of D for the species other than
low weight noble gases is consistent with the MCTA of diffusion. The MCTA suggests that the weak power
law mass dependency of D originates from interplays between strongly mass dependent short-term and
mass independent long-term solute-solvent interactions. The larger isotope fractionation for low weight
noble gases could be attributed to quantum isotope effects significantly magnifying the aqueous phase
diffusion-induced isotope fractionation. Our review shows, that except for low weight noble gases a weak
power law mass dependency of D is likely the most adequate assumption for aqueous phase diffusioninduced isotope fractionation in geochemical systems.