Low temperature equilibrium isotope fractionation and isotope exchange kinetics between U(IV) and U(VI)

Abstract

Measurements of the uranium (U) isotope ratio 238U/235U provide an emerging redox proxy in environmental and paleoredox studies, but many key parameters concerning U isotope fractionation are still poorly constrained. Here we report the equilibrium isotopic fractionation between dissolved U(IV) and dissolved U(VI), and rates of isotope exchange between solid-phase U(IV) and dissolved U(VI). We conducted one experiment at high concentration [35mM U(IV) and 32mM U(VI)] and low pH (0.2) in hydrochloric acid media at room temperature to determine the equilibrium isotopic fractionation between dissolved U(IV) and dissolved U(VI). Isotopic equilibrium was reached in about 19days under such experimental conditions. The equilibrium isotope fractionation was determined to be 1.64±0.16‰, with U(IV) being enriched in 238U relative to U(VI). Applicability of the determined equilibrium fractionation is discussed.We also conducted a set of experiments to determine isotopic exchange rates between dissolved U(VI) and nanouraninite U(IV) under conditions closer to those in natural system, with lower concentrations and neutral pH. The exchange rate was found to conform to the rate law R=k[U(VI)]adsorbed, in which R is the isotopic exchange rate (μMday−1); k is the rate constant determined to be 0.21day−1; and [U(VI)]adsorbed is the concentration of U(VI) adsorbed to nanouraninite (μM).Our results, combined with consideration of the variables controlling U(VI)–U(IV) contact in natural settings, indicate that the timescale for significant isotope equilibration varies depending on environmental conditions, mostly uranium concentrations. In natural uncontaminated sediments with low uranium concentrations, equilibration is expected to occur on a timescale of hundreds to thousands of years. In contrast, in U-contaminated aquifers with high U concentrations, significant equilibration could occur on timescales of weeks to years.