Environ. Sci. Technol. 2001, 35, 385-390
Uranium Precipitation in a Permeable Reactive Barrier by Progressive Irreversible Dissolution of Zerovalent Iron S T A N J . M O R R I S O N , * ,† DON R. METZLER,‡ AND CLAY E. CARPENTER† Environmental Sciences Laboratory,§ 2597 B3/4 Road, Grand Junction, Colorado, 81503, and U.S. Department of Energy Grand Junction Office, 2597 B3/4 Road, Grand Junction, Colorado, 81503
A permeable reactive barrier (PRB) containing zerovalent iron [Fe(0)] was installed at a former uranium milling site in Monticello, UT. A large-scale column experiment was conducted at the site to test the feasibility of Fe(0) to treat U prior to installing the PRB. Effluents from the field column experiment had pH values near 7.34, moderate decreases in C(IV) and Ca concentrations, and an elevated Fe concentration (27.1 mg/L). In contrast, groundwater exiting the PRB had a pH value of 9.82, decreases in C(IV) and Ca concentrations, and a low concentration of Fe (0.17 mg/L). A geochemical model was used to explain the chemical changes that occurred in both the field column experiment and the PRB. The model simulated the systems by the progressive irreversible dissolution of Fe(0). Modeling results indicated that a longer residence time in the PRB compared with the shorter residence time in the column contributed to the disparate effluent qualities. Prior to modeling, a controlled laboratory column experiment was conducted to help evaluate the dominant chemical mechanisms by which Fe(0) removes U from aqueous solutions. Results of the laboratory column experiment indicated that only a small amount of U could be adsorbed to ferric minerals, and, therefore, this mechanism was not considered in the model.
Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) at a former uranium-ore milling site near Monticello, UT. At many of these former ore processing sites, U has entered the groundwater system resulting in contamination of more than 10 billion gallons of groundwater (3). Uranium ore processing outside the United States, particularly in Australia, Canada, South Africa, and Europe, has also resulted in significant groundwater contamination by U. In addition to tailings sites, U has been reported in groundwater at 12 of 18 major DOE facilities because of contamination from the weapons production cycle (4). Cost-effective means of cleaning up groundwater contaminated by U are needed. Groundwater at some of the tailings sites is being extracted and treated ex situ, but costs are high and no site has yet been remediated to EPA’s prescribed standards. A permeable reactive barrier (PRB) is an engineered, subsurface zone of reactive material that treats contaminated groundwater flowing through it. PRBs are beginning to be used to treat groundwater contaminated by U, but few data are yet available to evaluate their performance. PRBs are currently being tested at four sites (Monticello, UT; Fry Canyon, UT; Durango, CO; and Oak Ridge National Laboratory Y-12 Plant, TN) to treat U as a low-cost alternative to pumping and treating groundwater. Zerovalent iron [Fe(0)], a scrap-metal product that is available from the automotive industry, is being used as a reactive material in the PRBs at these four sites. Sufficient contact with Fe(0) causes U concentrations in groundwater to decrease to nondetectable levels (less than 1 µg per L). Results of numerous laboratory experiments have confirmed the ability of Fe(0) to remove U from groundwater. Because of the promising results of laboratory and field studies, project managers at many DOE sites have expressed interest in using Fe(0) to treat U. Results of field projects are needed to determine the cost-effectiveness and reliability of such efforts. Research is also needed to understand the chemical mechanisms responsible for U uptake, and models are needed to optimize PRB designs and to make accurate predictions of longevity. The purpose of this study was 3-fold: (1) to present performance monitoring data from a full-scale PRB and a treatability field column experiment; (2) to better define the chemical mechanism for U uptake by Fe(0); and (3) to provide a model that explains observed water chemistry and is based on existing knowledge about the chemical mechanism.
Introduction
Experimental Section
More than 150 million tons of uranium mill tailings has been removed from 22 former uranium ore-processing sites in the United States. Remediation of groundwater at these sites is mandated by Congress and is being conducted by the U.S. Department of Energy (DOE) Uranium Mill Tailings Remedial Action (UMTRA) Ground Water Project (1). The U.S. Environmental Protection Agency (EPA) promulgated a groundwater concentration limit of 30 pCi/L (approximately 44 µg/ L) for U to ensure protection of human health and the environment near these sites (2); this U concentration is also being used as a groundwater cleanup goal under the
Monitoring of the Monticello PRB. Fifty-two monitoring wells in and adjacent to the Monticello PRB were sampled with a peristaltic pump (see Figure 1 for locations). At least three bore volumes were purged from each well before collection of samples. Samples were passed through a 0.45µm filter. Alkalinity values were measured at the well head in filtered samples. Field Column Experiment. The field column experiment was conducted at the Monticello field site before installation of the PRB. A clear acrylic column was set up in a mobile laboratory adjacent to a well located 15 m from the future location of the PRB. The column had a 10-cm inside diameter and a 1.2-m length of 16.7-kg Peerless Metal Powders & Abrasive (Peerless), Detroit, MI, -8 +18 mesh Fe(0). Fe(0) was lightly tamped during filling of the column; porosity was about 0.5. Flow was directed from bottom to top with a peristaltic pump at a rate ranging from 20 to 80 mL/min, resulting in residence times of 1-4 h. Flow rate was calculated
* Corresponding author phone: (970)248-6373; fax (970)248-7628; e-mail:
[email protected]. † Environmental Sciences Laboratory. ‡ U.S. Department of Energy. § Operated by MACTEC Environmental Restoration Services for the U.S. Department of Energy Grand Junction Office under DOE contract no. DE-AC13-96GJ87335. 10.1021/es001204i CCC: $20.00 Published on Web 11/30/2000
2001 American Chemical Society
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FIGURE 1. Uranium concentrations (µg/L) in and near the PRB, September 1999. Diameters of circles are proportional to concentrations. from outflow volume over time. Groundwater was pumped directly from the well into a holding tank adjacent to the mobile laboratory and then upward through the column. Air and groundwater temperatures ranged from 15 to 25 °C. Laboratory Column Experiment. The laboratory experiment was conducted in a glass column with an inside diameter of 15 mm containing 37 g of Peerless -8 +18 mesh Fe(0). The Fe(0) was lightly tamped in the column during filling; the flow length through the Fe(0) was 120 mm. The column was purged overnight with argon before starting the experiment. The influent solution was constantly purged of oxygen by bubbling with argon. Joints were wrapped with wax film to minimize air infiltration. A peristaltic pump was used to pump the solution through the column, from bottom to top, at 2 mL/min, resulting in a residence time of 6.5 min. One pore volume was equivalent to 13 mL, and the porosity was about 0.5. Influent solution was made by the addition of reagentgrade chemicals to milli-Q pure water. The composition was 1638 mg/L of NaHCO3, 100 mg/L of sodium azide (a bactericide), and 200 µL/L of a 10 000-mg/L U solution containing 3% HNO3. The pH of the solution was adjusted to 9.2 with the addition of 60 µL of 10 N NaOH. Alkalinity of the solution was 950 mg/L (as CaCO3). Effluent was collected in a plastic tank that was purged continually with argon. Effluent samples were analyzed under argon immediately after collection for pH, dissolved oxygen (DO), and oxidation-reduction potential (ORP). Alkalinity and conductivity were measured within 1 h of collection, and samples were preserved with HNO3 for Fe and U analyses. After completing the flow portion of the experiment, Fe(0) was dried by passing argon through the column for 2 days. After the column material was completely dry, the column was opened, and six samples were collected, each containing 20 mm of the spent Fe(0); one sample of original (unused) Fe(0) was also collected. Each of the seven samples was split into three portions. One portion of each sample set was 386
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embedded in epoxy and made into a polished thin section, one portion was digested for chemical analysis, and one portion was used for X-ray diffraction (XRD) analysis. The samples for XRD analysis were powdered in an agate mortar, placed in a randomly oriented mount, and analyzed using Cu KR radiation at 40 kV and a scan speed of 1° 2θ per minute. XRD, a semiquantitative technique, usually requires the presence of more than 1% of a mineral to make an identification. The intensity of the magnetite peak was calibrated with magnetite standards that provided an accuracy of (3%. Because Fe(0) is difficult to digest, two different processes were used: (1) a mixture of hot concentrated nitric, hydrofluoric, and perchloric acids and (2) microwave digestion with concentrated nitric acid (5). The measured concentrations in samples from both digestions were similar; data derived from the first digestion method were used because a larger proportion of the sample was digested. Chemical Analysis. Values of pH were determined with a silver/silver chloride glass combination electrode calibrated using pH buffer solutions. Values of ORP were determined using a platinum redox and a silver/silver chloride reference combination electrode. Eh values were computed by adding 200 mV (difference between the measured ORP value of a ZoBell solution and the potential of the ZoBell solution relative to the standard hydrogen electrode) to the measured ORP values. DO was measured with a semipermeable membrane method (YSI Company Model 55). Calibration was performed with water equilibrated with atmospheric oxygen. A zero oxygen check was made using a solution of 1 g of sodium sulfite and 1 mg of cobalt chloride. Measurements on the zero check solution indicated that the lower detection limit was about 0.1 mg/L of O2. Alkalinity was measured by titration with H2SO4. Concentrations of Fe and Mn in the digested samples were analyzed with an inductively coupled plasma (ICP) atomic emission spectrometer. Dissolved Fe concentrations in laboratory column effluents were measured with a flame atomic absorption spectrometer and in the field column effluents with the Ferrover method (Hach Co.). Concentrations of U in the digestates were measured with an ICP mass spectrometer (MS). Dissolved U concentrations in laboratory column effluents were measured with laser-induced kinetic phosphorescence analysis (Chemchek KPA-11) and in field column effluents with laser-induced fluorescence (Scintrex UA-3). The two laser methods respond only to U(VI). To confirm the accuracy of the analyses, splits of some samples were analyzed with ICP/MS, a technique that responds to all oxidation states of U. The ICP/MS results were the same as those measured with the laser techniques, confirming the assumption that all dissolved U was in the U(VI) state. The laser methods were able to detect concentrations of U less than 0.1 µg/L.
Description of the Monticello Field Site Groundwater contamination was caused by seepage of leachate from about 1.6 million cubic meters of uranium mill tailings that was formerly located on a 44-ha tract about 300 m upgradient of the PRB. The tailings were removed from the site in 1999. Contaminated groundwater flows through an alluvial valley that is about 125 m wide and is underlain by an aquiclude of Cretaceous gray shale. Hydraulic conductivity of the alluvium is about 10-2 cm/s based on pump test data. Groundwater flux is about 190 L/min based on a flow model developed before installation of the PRB. The PRB contains 170 m3 of -8 +20 mesh Peerless Fe(0) and is 30 m long and 1.2 m wide (in the direction of the groundwater flow). The bottom of the PRB was keyed into the aquiclude 4.5-6 m below ground surface. Gravel packs
TABLE 1. Compositions Used for Modeling (mg/L) field column constituent pH alkalinity (as CaCO3) Eh (mV) Ca Fe Na Mg K Cl SO4 U
influent
effluent
permeable reactive barrier influent
7.12 7.34 6.72 375 205 249 170 -156 211 330 210 339 0.07 27.1 0.009 231 231 347 80.7 80.2 88 11.1 11.7 21.8 151 153 134 925 923 1190 0.643
