Chromium and uranium isotopic exchange kinetics and isotope fractionation during oxidation of tetravalent uranium by dissolved oxygen

Chromium;equilibrium fractionation;isotope exchange rates;Uranium;equilibrium fractionation;isotope exchange rates;Uranium;oxidation;isotope fractionation

Wang, Xiangli

关键词: Chromium;    equilibrium fractionation;    isotope exchange rates;    Uranium;    equilibrium fractionation;    isotope exchange rates;    Uranium;    oxidation;    isotope fractionation;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/46900/Xiangli_Wang.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Chromium (Cr) and Uranium (U) isotopes (53Cr/52Cr and 238U/235U ratios) are very powerful new geochemical tools in environmental remediation and paleoredox reconstruction studies. Correctly interpreting Cr and U isotopic compositions in groundwater and sed- imentary rock samples requires thorough understanding of relevant isotope fractionation mechanisms, many of which are still poorly understood. Isotopic exchange and oxidation reactions are two types of them.High concentration, acidic-pH experiments were conducted to determine equilibrium fractionations between hexavalent Cr (Cr(VI)) and trivalent Cr (Cr(III)), and between hexavalent U (U(VI)) and tetravalent U (U(IV)). Low concentration, neutral-pH experiments (closer to natural settings) were conducted to determined Cr(III)-Cr(VI) and U(IV)-U(VI) isotopic exchange kinetics. Experiments were also conducted to investigate U isotope fractionation caused by oxidation of both dissolved and solid U(IV) by dissolved oxygen.In the high concentration, acidic-pH experiments to determine Cr isotope exchange rates, when at isotopic equilibrium, the 53Cr/52Cr of dissolved Cr(VI) was found to be 5.8±0.5‰ higher than that of dissolved Cr(III) in chloric acid media at 25 ◦C (pH 1.2). The isotopic exchange rate at pH 1.2 was found to be on time scales of decades, even with extremely high concentrations (0.2 M for both Cr(III) and Cr(VI)). In contrast, in the low concentration, neutral-pH experiments, significant isotopic exchange was found on time scales of months at pH 7, when Cr(III) is solid and Cr(VI) is dissolved, even with much lower concentrations comparedto the high concentration, acidic-pH experiments. Faster isotopic exchange is attributedto adsorption of Cr(VI) to Cr(III) particle surfaces, which keeps Cr(III) and Cr(VI), andintermediate species Cr(V) and Cr(IV), in close proximity long enough to allow multipleelectron transfers. The isotopic exchange rate at pH 7 was found to conform to the rate law:R = k · [Cr(V I)]adsorbed, in which R is the isotopic exchange rate (mol adsorbed Cr(VI)L−1 day−1); k is the rate constant determined to be 0.002 day−1; [CrO2−]adsorbed is the 4concentration of Cr(VI) adsorbed to solid Cr(III) (mol adsorbed Cr(VI) L−1). The impact of isotopic exchange on the 53Cr/52Cr ratio of the dissolved Cr(VI) depends on relative masses and 53Cr/52Cr ratios of the starting Cr(III) and Cr(VI), as well as the fraction of Cr(III) atoms exposed to solution. However, the time scale of the impact is very long (tens of years to around one hundred years) due to very small amount of Cr(VI) adsorbed onto Cr(III) solid surfaces in natural settings, where solid surfaces are dominated by other minerals other than solid Cr(III).Oxidation of dissolved U(IV) by dissolved oxygen in 0.1 M HCl caused the 238U/235U of the remaining U(IV) to increase as a function of the extent of oxidation, and the 238U/235U of the product U(VI) to closely follow the trend of U(IV), but 1.1±0.2‰ lower than U(IV). In contrast, oxidation of solid U(IV) by dissolved oxygen in 20 mM NaHCO3 caused only a weak fractionation (∼0.1‰). We suggest that isotope fractionation during oxidation of solid U(IV) is inhibited by a “rind effect”, where the surface layer of the solid U(IV) is completely oxidized before the inner layer is exposed to oxidant, and complete conversion of each layer limits isotopic effect. The weak isotopic shift is attributed to adsorption of some of the produced U(VI).In the high concentration, acidic-pH experiments to determine U isotope exchange rates, when at isotopic equilibrium, the 238U/235U of dissolved U(VI) was found to be 1.64±0.16‰ lower than that of dissolved U(IV) in chloric acid media at 25 ◦C (pH 0.2). With 0.032 M dissolved U(VI) and 0.035 M dissolved U(IV), the isotopic equilibrium was reached in about 19 days. In contrast, in the low concentration, neutral-pH experiments, the isotopic exchange between solid U(IV) and dissolved U(VI) was found to be on time scales of days. The isotopic exchange rate was found to conform to the rate law: R = k · [U(V I)]adsorbed, in which R is the isotopic exchange rate (mol adsorbed U(VI) L−1 day−1); k is the rate constant determined to be 0.199 day−1; [U(VI)]adsorbed is the concentration of U(VI) adsorbed to solid U(IV) (mol adsorbed U(VI) L−1). The impact of isotopic exchange on the 238U/235U ratio of the dissolved U(VI) depends on the relative masses of the starting U(IV) and U(VI), as well as the percentage of U(IV) atoms on the surface of U(IV) particles. The time scale of the impact is roughly a few years, due to very low abundances of U(VI) adsorbed to U(IV) solid surfaces in natural settings, where solid surfaces are dominated by other minerals other than solid U(IV).