Environ. Sci. Technol. 2004, 38, 240-247
Methods for Estimating Adsorbed Uranium(VI) and Distribution Coefficients of Contaminated Sediments M A T T H I A S K O H L E R , * ,† G A R Y P . C U R T I S , ‡ DAVID E. MEECE,‡ AND JAMES A. DAVIS‡ Environmental Science and Engineering Division, Colorado School of Mines, Golden, Colorado 80401, and U.S. Geological Survey, Menlo Park, California 94025
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.
Introduction A critical aspect of risk assessment and remediation studies at many uranium-contaminated sites is estimating the migration of U(VI) in groundwaters (e.g. refs 1-3). In model simulations, retardation of U(VI) is often estimated based on a distribution coefficient, KD, or a range of KD values that is meant to describe the partitioning of U(VI) between the solid and aqueous phases. The distribution coefficient, KD, is defined by the concentration of sorbed U divided by the concentration of dissolved U, regardless of the solid and * Corresponding author phone: (650)329-4464; fax: (650)329-4545; e-mail:
[email protected]. † Colorado School of Mines. ‡ U.S. Geological Survey. 240
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 1, 2004
solution compositions. In practice, KD values are generally chosen based on laboratory batch experiments with materials from contaminated sites, single mineral phases, mineral mixtures, crushed rock materials, or on studies/observations at other U-contaminated sites. A significant problem is that many batch experiments performed in the laboratory for KD values are equilibrated in air and thus do not account for the fact that the partial pressures of carbon dioxide gas are generally greater in aquifers and may undergo seasonal variations (4, 5). KD values, especially those for U(VI), are dependent on the geochemistry of an aquifer, which can vary temporally and spatially. U(VI) is relatively weakly adsorbed compared to many metal contaminants, and it forms strong aqueous complexes with carbonate. Numerous batch adsorption studies on natural and synthetic materials have demonstrated the dependence of U(VI) adsorption on pH and carbonate (and other ligands) concentrations in aqueous solutions (6-12), and variability in KD values has been the subject of several studies (8, 10, 13-18). Simulations with a constant KD can, therefore, introduce considerable uncertainty into risk assessment or evaluation of remediation alternatives. Methods are needed to estimate KD values in the field (“in-situ”) for validation of surface complexation models (SCM) to be used in risk assessment transport models and to constrain the initial conditions in transport modeling. Lienert et al. (4) pointed out that great care should be taken when laboratory KD values are used to estimate in-situ KD’s. Based on the retarded appearance of a uranium pulse in a well at a distance of 5 m from a river they estimated a KD of (7 ( 2.5) mL/g. They emphasized that their value was in general much smaller than KD’s obtained in laboratory experiments on mineralogical constituents of the aquifer material (quartz, calcite, K-feldspars, illite, and chlorite). Over the last two decades many researchers have pointed out the limitations and/or inadequacies of a constant KD approach for transport modeling (19-21). More recent transport simulations take advantage of multicomponent reactive transport models, which can account for variable chemical speciation (e.g. refs 22-25). SCMs have provided an approach to describe adsorption processes (through welldefined stoichiometric reactions of surface and aqueous species) rather than the mere distribution between aqueous and solid phases (26). This process-oriented approach to adsorption and transport modeling better reflects the dynamic behavior of a complex aquifer system (27-29). Thus, in addition to hydrogeological parameters, a detailed knowledge of geochemical conditions at a contaminated site is required to make accurate transport simulations in groundwater. An important initial condition that must be specified for U(VI) transport simulations at U-contaminated sites is the mass and oxidation state of U associated with the contaminated sediments. Several experimental studies have been conducted to evaluate the extent to which U(VI) is associated with natural soils (e.g. ref 30). Payne and Waite (31) showed that exchangeable U of rock samples determined by isotope exchange techniques was comparable to the amount of U extracted by Tamm’s acid oxalate (TAO) and determined in-situ KD values at the Koongarra U deposit in Northern Australia. Mason et al. (32) found that a 0.5 M sodium bicarbonate solution was an efficient extractant for Ucontaminated soils from the Fernald Environmental Management Project (FEMP). The amount of dissolved U corresponded approximately to the amount of U(VI) present 10.1021/es0341236 CCC: $27.50
2004 American Chemical Society Published on Web 11/21/2003
FIGURE 1. Locality map of Naturita, Colorado, U.S.A. in the soil. Gadelle et al. (33) tested the efficiency of surfactants and bicarbonate (among other extractants) for the extraction of U(VI) adsorbed under laboratory conditions onto Oak Ridge soils. Bicarbonate was as efficient as the surfactants as long as the pH of the extracting solutions remained high. None of the studies above was focused on developing experimental methods for quantifying initial adsorbed U(VI) for reactive solute transport modeling. The overall objective of this study is to evaluate various methods to determine the labile fraction of U(VI) sorbed on U-contaminated sediments collected at the Naturita Uranium Mill Tailings Remediation Act (UMTRA) site (a list of abbreviations is given in the Supporting Information). Specific objectives are as follows: (1) to assess total exchangeable U by isotopic exchange and to compare the results to the quantities of U extracted from the contaminated sediments by various extractants in order to determine if any of the extraction methods provide a good estimate for sorbed U(VI) on the U-contaminated sediments and (2) to estimate in-situ KD values for the sediments and compare them with KD values obtained from a surface complexation model developed to describe U(VI) adsorption by uncontaminated sediments from the Naturita site.
Methods and Materials The Naturita Field Site. The Naturita UMTRA site is located in southwestern Colorado in Montrose County, about 3 km northwest of the town of Naturita, and on the west bank of the San Miguel River (Figure 1). At the Naturita site uranium(U) and vanadium(V) ores were processed at the site between 1939 and the early 1980s (34). Stockpiled tailings were removed from the Naturita site in 1979. Surface remediation of the site began in 1994 and was completed in 1998 (35). U and V were extracted from the ores by salt roasting, followed by carbonate leaching in percolation tanks. Carbonate leach residues were then sent to a second stage of sulfuric acid percolation leaching. Leach tails were slurried onto the land surface at the site, in an area downgradient of the mill yard between State Highway 141 and the river. The climate of the area is semiarid; annual precipitation is approximately 33 cm. Off-site disposal of U-V tailings from the site was completed during 1977-1979. Contaminated soils and vadose
zone materials were removed from the site between 1996 and 1998, and the excavated areas were filled in with clean backfill. U(VI) contamination in groundwater at the site has been observed primarily within an unconfined, shallow alluvial aquifer composed of sand, gravel, pebbles, and cobbles. The San Miguel River recharges the aquifer. The saturated thickness of the alluvial aquifer is about 3-4 m. Materials. Experiments were conducted on 16 samples collected from U-contaminated alluvial sediments and with a sample of uncontaminated sediments from the aquifer (referred to as Naturita Aquifer Background Sediment or NABS). The NABS sample was collected in July 1998, using a backhoe to collect sediments from beneath the water table in an area upgradient of wells DOE-547 and NAT-20, -21, and -22 (Figure 1). Cobbles (>64 mm) were removed with a coarse mesh in the field. A total of 734 kg of wet material was placed in sealed plastic buckets and transferred to the laboratory for detailed processing and characterization. U-contaminated sediments were collected from auger flights during the installation of monitoring wells in October 1998 or with a hand auger at a few selected locations during 20002001. Sample NAT-25B was collected in September 2000 with a hand auger at a location about 2 m from the monitoring well NAT-25. The collected sediments were drained and collected into sealed plastic buckets. The sediments were air-dried in the laboratory, dry sieved through 3 mm nylon mesh, and stored at 4 °C in the dark. Sample NAT-25J was collected in September 2001 with a hand auger at a location about 3 m from NAT-25 and 4 m from site NAT-25B. Sample NAT-25J was neither air-dried nor sieved and was used immediately in glovebag experiments in the field as described below. All other experiments described in this study were carried out with the air-dried
