Spatially Resolved U(VI) Partitioning and Speciation: Implications for Plume Scale Behavior of Conta

时间：2009-04-18  阅读：    我要评论:

Jiamin Wan*^†, Yongman Kim^†, Tetsu K. Tokunaga^†, Zheming Wang^‡, Suvasis Dixit^†, Carl I. Steefel^†, Eduardo Saiz^§, Martin Kunz and Nobumichi Tamura Earth Sciences Division, Lawrence Berkeley National Laboratory, Pacific Northwest National Laboratory, Richland, WA, Material Science Division, Lawrence Berkeley National Laboratory, and Advanced Light Source (ALS), Lawrence Berkeley National LaboratoryEnviron. Sci. Technol., 2009, 43 (7), pp 2247–2253DOI: 10.1021/es802801bPublication Date (Web): February 18, 2009Copyright © 2009 American Chemical Society * Corresponding author phone: (510) 486-6004; email: jwan@lbl.gov., †

Earth Sciences Division.

, ‡

Pacific Northwest National Laboratory.

, §

Material Science Division.

,

Advanced Light Source (ALS).

Abstract

A saline−alkaline brine containing high concentration of U(VI) was accidentally spilled at the Hanford Site in 1951, introducing 10 tons of U into sediments under storage tank BX-102. U concentrations in the deep vadose zone and groundwater plumes increase with time, yet how the U has been migrating is not fully understood. We simulated the spill event in laboratory soil columns, followed by aging, and obtained spatially resolved U partitioning and speciation along simulated plumes. We found after aging, at apparent steady state, that the pore aqueous phase U concentrations remained surprisingly high (up to 0.022 M), in close agreement with the recently reported high U concentrations (up to 0.027 M) in the vadose zone plume (1). The pH values of aged pore liquids varying from 10 to 7, consistent with the measured pH of the field borehole sediments varying from 9.5 to 7.4 (2), from near the plume source to the plume front. The direct measurements of aged pore liquids together with thermodynamic calculations using a Pitzer approach revealed that UO₂(CO₃)₃⁴⁻ is the dominant aqueous U species within the plume body (pH 8−10), whereas Ca₂UO₂(CO₃)₃ and CaUO₂(CO₃)₃²⁻ are also significant in the plume front vicinity (pH 7−8), consistent with that measured from field borehole pore-waters (3). U solid phase speciation varies at different locations along the plume flow path and even within single sediment grains, because of location dependent pore and micropore solution chemistry. Our results suggest that continuous gravity-driven migration of the highly stable UO₂(CO₃)₃⁴⁻ in the residual carbonate and sodium rich tank waste solution is likely responsible for the detected growing U concentrations in the vadose zone and groundwater.

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