HR: 10:20h
AN: V11H-01    [PDF]
TI: High Precision Measurements of 235U/238U Isotopic Fractionations Resulting From Uranium Reduction Induced by Zero Valent Iron
AU: * Rademacher, L
EM: lkr@uiuc.edu
AF: Department of Geology, UIUC, 1301 W. Green St., 245 NHB, Urbana, IL 61801 United States
AU: Lundstrom, C
EM: lundstro@uiuc.edu
AF: Department of Geology, UIUC, 1301 W. Green St., 245 NHB, Urbana, IL 61801 United States
AU: Johnson, T
EM: tmjohnsn@uiuc.edu
AF: Department of Geology, UIUC, 1301 W. Green St., 245 NHB, Urbana, IL 61801 United States
AB: Uranium is a widespread natural and anthropogenic contaminant in surface and subsurface waters. Like several other inorganic contaminants, uranium is mobile under oxidizing conditions but may be immobilized by chemical reduction. U(VI) moves with groundwater as (UO2)2+ and as soluble complexes with carbonate, phosphate, and fluoride. In many groundwater systems, uranium undergoes chemical reduction to U(IV), which is insoluble and immobile. Therefore, understanding the extent of reduction is essential for predicting the mobility of uranium in groundwater. Mass dependent isotopic fractionations of redox sensitive contaminants frequently found in groundwater (including chromate, selenate, and nitrate) have proven exceptionally useful for estimating the rate and extent of reduction and immobilization. Until recently, however, analytical limitations have prevented these techniques from being applied to heavier redox sensitive elements, such as uranium. The advent of highly sensitive multi-collector inductively coupled plasma mass spectrometers (MC-ICP-MS) enables high precision measurements of previously undetected variations in many elements. Laboratory reduction experiments with zero valent iron (ZVI) were performed in a controlled environment to test the hypothesis that uranium isotopes, specifically 235U/238U, behave similarly to other redox sensitive contaminants and produce a mass dependent fractionation during the transformation between valence states. Because of the large abundance differences between 235U and 238U, initial experiments used U500, an enriched uranium standard with approximately equal parts 235U and 238U. Results suggest that the highly sensitive MC-ICP-MS distinguishes 235U/238Uvariations to approximately + 0.02per mil. Measured isotopic fractionations between the 235U/238U of the initial and final experimental solutions (~70% reduced) are approximately 1.1 per mil, and increase with decreasing concentration. Measured variations in 235U/238U suggest that uranium isotopic ratios could also prove to be valuable indicators of contaminant immobilization and paleoenvironmental conditions.
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1094 Instruments and techniques
DE: 1832 Groundwater transport
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting