Methods for Estimating Adsorbed Uranium(VI) and Distribution Coefficients of Contaminated Sediments

Abstract

Assessing the quantity of U(VI) that participates in sorption/desorption processes in a contaminated aquifer is an important task when investigating U migration behavior. U-contaminated aquifer sediments were obtained from 16 different locations at a former U mill tailings site at Naturita, CO (U.S.A.) and were extracted with an artificial groundwater, a high pH sodium bicarbonate solution, hydroxylamine hydrochloride solution, and concentrated nitric acid. With an isotopic exchange method, both a KD value for the specific experimental conditions as well as the total exchangeable mass of U(VI) was determined. Except for one sample, KD values determined by isotopic exchange with U-contaminated sediments that were in equilibrium with atmospheric CO2 agreed within a factor of 2 with KD values predicted from a nonelectrostatic surface complexation model (NEM) developed from U(VI) adsorption experiments with uncontaminated sediments. The labile fraction of U(VI) and U extracted by the bicarbonate solution were highly correlated (r2 = 0.997), with a slope of 0.96 +/- 0.01. The proximity of the slope to one suggests that both methods likely access the same reservoir of U(VI) associated with the sediments. The results indicate that the bicarbonate extraction method is useful for estimating the mass of labile U(VI) in sediments that do not contain U(IV). In-situ KD values calculated from the measured labile U(VI) and the dissolved U(VI) in the Naturita alluvial aquifer agreed within a factor of 3 with in-situ KD values predicted with the NEM and groundwater chemistry at each well.

Use of Polyphosphate to Decrease Uranium Leaching in Hanford 300 Area Smear Zone Sediments

“…Extractant solutions used after the adsorbed uranium extraction are operationally defined, dissolving one or more uranium minerals: a) 1 mol/L sodium acetate (pH 5. 1 h); b) acetic acid (pH 2.3, 5 days, Kohler et al 2004); c) 0.1 mol/L ammonium oxalate, 0.1 mol/L oxalic acid (1 h); and d) 8 mol/L HNO 3 , 95°C, 2 h. Uranium associated with carbonates is dissolved in the two acetate extractions. Na-boltwoodite is predominantly dissolved in the acetic acid extraction (85%, Szecsody et al 2010).…”

Characterization of 200-UP-1 Aquifer Sediments and Results of Sorption-Desorption Tests Using Spiked Uncontaminated Groundwater

“…The solid to solution ratio was kept constant at 50 g/L by adding fresh reagent to replace the small aliquot (1 mL) removed at each sampling time. This procedure is described in Kohler et al (2004).…”

“…Based on work performed by Kohler et al (2004), labile U(VI) is assumed to refer to the adsorbed U(VI) fraction that can be easily leached out of sediments using the described (bi) carbonate extractant. The labile uranium fraction differs from the total uranium in the sediment, which also includes tightly bound uranium that is part of the crystal structure of minerals.…”

Multi-Scale Mass Transfer Processes Controlling Natural Attenuation and Engineered Remediation: An IFRC Focused on Hanford?s 300 Area Uranium Plume January 2010 to January 2011

“…Throughout the one-year period, significant changes in U(VI) concentration and other chemical compositional parameters were observed. Adsorbed U(VI) concentration on each composite was assayed using high-pH, high dissolved carbonate solutions (Kohler et al, 2004;Bond et al, 2008).…”