“…Uranium reduction in natural environments today appears to be predominately biotic (e.g., enzymatic activity of sulphate or other dissimilatory metal-reducing bacteria; Barnes and Cochran, 1993;Rademacher et al, 2006;Basu et al, 2014;Stylo et al, 2015), and the fractionation associated with this process is caused by the nuclear volume effect (NVE) (Bigeleisen, 1996;Nomura et al, 1996;Schauble, 2007), which favors the heavy 238U isotope in the reduced state. However, abiotic reduction also results in observable isotope fractionation (e.g., Stylo et al, 2015), which can be due to the NVE as well as more standard mass-dependent effects (Brown et al, 2018). The direction of isotope fractionation depends on the rate of U removal (i.e., of insoluble U(IV) from solution): fast removal is primarily controlled by kinetic effects (mass-dependent fractionation, fractionation factor α < 1, favoring the light isotope; however, we should note that kinetic fractionation is very small with α ~ 1 due to the large mass number of U), whereas slow removal leads to equilibrium (NVE, fractionation factor α> 1, favoring the heavy isotope) (Brown et al, 2018).…”