300 Area Uranium Leach and Adsorption Project

Abstract

The objective of the 300 Area K d /leach study was to perform controlled laboratory experiments to measure the leaching and adsorption characteristics of uranium in near-surface sediment samples collected from the 300 Area of the Hanford Site. The Environmental Restoration Contractor (ERC) will use the results of this research to model uranium mobility for the 300-FF-1 and 300-FF-2 operable unit (OU) waste sites.Eight near-surface sediment samples were collected by the ERC between December of 2000 and February 2001. The samples consisted of three uncontaminated background sediment samples (B11491, B11492, and B11493), two uranium-contaminated samples collected from the 300 Area North Process Pond (B11494 and B11495), and three uranium-contaminated samples collected in the vicinity of the 303-K building (B11BY4, B11BY5, and B11BY6).

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

“…Apparently, there are plenty of U(VI) favorable adsorption sites in the 200-UP-1 sediments given the high solid-to-solution ratio chosen (300 g/L), such that a linear isotherm was observed up to U(VI) concentration of 5 x 10 -6 M, which was the highest U(VI) concentration used. A similar linear adsorption isotherm was found for U(VI) adsorption onto coarse-grained Hanford formation sediments using two simulated 300 Area groundwaters (pH ranged from 7.5 to 8) and a solid-to-solution ratio of 100 g/L up to a U(VI) solution concentration of 3,000 μg/L (Serne et al 2002). In addition, because a linear isotherm was found to be valid up to 5 x 10 -6 M (~1 mg/L) of U(VI) initial concentration, and the existing groundwater plume contains dissolved U(VI) concentrations below 1 mg/L (Hartman et al 2005), a linear K d model can be applied to predict U(VI) transport in the 200-UP-1 groundwater condition.…”

“…The results of these more detailed studies (Task 3) of U(VI) adsorption and desorption suggest that the fate of uranium in the existing 200-UP-1 groundwater plume is quite complicated. These studies and numerous other studies (both laboratory and field) show that the K d for U(VI) is quite sensitive to groundwater alkalinity (total bicarbonate/carbonate), pH, dissolved U(VI) concentration, and time of reaction (kinetics) (see discussions both germane to Hanford and other sites in Serne et al (2002), Zachara et al (2005), Davis et al (2004)). …”

“…This discrepancy might result from high uncertainty in measuring either dissolved U(VI) concentration or adsorbed U(VI) concentration in the cores; however, because the sediments obtained did not contain total uranium concentrations above natural background values, they might not be relevant for determining in-situ desorption K d values for Hanford-contaminated sediments. Selective chemical extraction studies of natural and contaminated sediments from the 300 Area showed that contaminated sediments released much more uranium into "mild" chemical extractants, such as the alkaline sodium bicarbonate-carbonate solution used in this effort, than natural background sediments (Serne et al 2002).…”

“…These studies and numerous other studies (both laboratory and field) show that the K d values for U(VI) are quite sensitive to groundwater alkalinity (total bicarbonate/carbonate) and pH, and in some cases although not manifested for the 200-UP-1 existing plume, to dissolved U(VI) concentration. See discussions both germane to Hanford and other sites in Serne et al (2002), Zachara et al (2005), Davis et al (2004), and Bargar et al (2000). If remediation of the existing groundwater U(VI) plume at 200-UP-1/200-ZP-1 operable units is required and some form of pump and treat is chosen, it is recommended that the aquifer be treated with chemicals to increase pH and alkalinity and to decrease dissolved calcium and magnesium (to prevent the precipitation of additional calcite).…”

“…Because the sediment samples available are not significantly contaminated with Hanford-derived uranium, the K d values in Table 16 represent more the desorption of natural uranium out of the 200-UP-1 sediments rather than adsorption/desorption of uranium added to the environment by Hanford activities. Based on previous selective extraction studies on 300-Area sediments (see Serne et al 2002), natural uranium is bound much more strongly in sediment than uranium added by Hanford activities. Therefore, the desorption U K d values in Table 16 are likely not good proxies for Hanford-derived U contamination.…”