Environ. Sci. Technol. 2005, 39, 5015-5021
Decontamination of Uranium-Contaminated Steel Surfaces by Hydroxycarboxylic Acid with Uranium Recovery A . J . F R A N C I S , * ,† C . J . D O D G E , † J. A. MCDONALD,† AND G. P. HALADA‡ Environmental Sciences Department, Brookhaven National Laboratory, Upton, New York 11973, and Department of Materials Science and Engineering, Stony Brook University, Stony Brook, New York 11794
We developed a simple, safe method to remove uranium from contaminated metallic surfaces so that the materials can be recycled or disposed of as low-level radioactive or nonradioactive waste. Surface analysis of rusted uraniumcontaminated plain carbon-steel coupons by X-ray photoelectron spectroscopy and Rutherford backscattering spectroscopy showed that uranium was predominantly associated with ferrihydrite, lepidocrocite, and magnetite, or occluded in the matrix of the corrosion product as uranyl hydroxide and schoepite (UO3‚2H2O). Citric acid formulations, consisting of oxalic acid-hydrogen peroxidecitric acid (OPC) or citric acid-hydrogen peroxidecitric acid (CPC), were used to remove uranium from the coupons. The efficiency of uranium removal varied from 68% to 94% depending on the extent of corrosion, the association of uranium with the iron oxide matrix, and the accessibility of the occluded contaminant. Decontaminated coupons clearly showed evidence of the extensive removal of rust and uranium. The waste solutions containing uranium and iron from decontamination by OPC and CPC were treated first by subjecting them to biodegradation followed by photodegradation. Biodegradation of a CPC solution by Pseudomonas fluorescens resulted in the degradation of the citric acid with concomitant precipitation of Fe (>96%), whereas U that remained in solution was recovered (>99%) by photodegradation as schoepite. In contrast, in an OPC solution citric acid was biodegraded but not oxalic acid, and both Fe and U remained in solution. Photodegradation of this OPC solution resulted in the precipitation of iron as ferrihydrite and uranium as uranyl hydroxide.
Introduction A major environmental concern is the presence of radionuclides and toxic metals on metallic and other surfaces from nuclear contamination (1). Innovative and improved processes are needed for decontaminating metal surfaces and other materials to facilitate their final disposition. Present methods for surface decontamination and remediation include sand blasting, chemical extraction, and electro* Corresponding author phone: (631) 344-4534; fax: (631) 3447303; e-mail:
[email protected]. † Brookhaven National Laboratory. ‡ Stony Brook University. 10.1021/es048887c CCC: $30.25 Published on Web 05/19/2005
2005 American Chemical Society
chemical dissolution (2). Chemical extraction techniques use mineral acids, caustic compounds, and synthetic chelating agents such as EDTA and DTPA (3-5). These methods result in generation of secondary waste streams which may create additional disposal problems. The consequences of not developing environmentally friendly, green-chemistrybased technologies will result in the continued use of slow, labor-intensive decontamination methods that generate large quantities of secondary waste and entail high costs associated with waste management, surveillance, packaging, transportation, and disposal. We investigated the advantages of using citric and oxalic acids for removing uranium contamination. The work involved (i) removing the uranium from contaminated carbon-steel surfaces and (ii) volumetrically reducing the waste while minimizing secondary waste. Although the oxide phases formed on various types of steel have been well characterized, the interaction and the association of the radionuclides with the oxide and its subsequent removal are not well understood. Previously, we studied the association of uranium with the iron oxides lepidocrocite, magnetite, and ferrihydrite, which are commonly formed on corroding plain carbon-steel surfaces. Uranium was associated with ferrihydrite and lepidocrocite as a bidentate complex with iron [U(µ2-O)Fe], and as uranium (oxy)hydroxide [UO2(OH)2] in magnetite (6). Extended X-ray absorption fine structure (EXAFS) analysis of U(VI) sorption on ferrihydrite suggested a bidentate as opposed to monodentate adsorption onto the goethite surface (7). Uranium was present as an inner-sphere complex with goethite (8). Adsorption of U(VI) on crystalline goethite was pH-dependent (9). The ability of U(VI) to desorb from aged ferrihydrite was less than that from fresh ferrihydrite, indicating inclusion of the uranium in the oxide matrix, while continuous flow column experiments with granite-hematite mixtures have shown that uranium was reversibly adsorbed to the hematite (10). Recently, we investigated the association of uranium in contaminated metal coupons by spectroscopic techniques (11). X-ray photoelectron spectroscopy (XPS) analysis disclosed the presence of hexavalent uranium on the surface of the coupons along with ferrous ion and mixed-phase iron (hydr)oxides. Ruthorford backscattering spectroscopy (RBS) indicated the uranium was heterogeneously distributed throughout the upper 1.0 µm of the corrosion layer. The occlusion of significant amounts of uranium as crystalline schoepite (UO3‚2H2O) and uranyl hydroxide [UO2(OH)2] species within the corrosion product layer was clearly shown by synchrotron-based FTIR microspectroscopy. IR chemical mapping indicated that U(VI) is embedded within the coating, associating with layers of magnetite, maghemite, goethite, and lepidocrocite. The removal of radionuclides bound to the surface of steel using organic acids has been studied to a limited extent. Citric acid has been shown to form a soluble complex with transition metals and actinides which include bidentate, tridentate, binuclear, or polynuclear complex species (12, 13). It has been used to decontaminate components of nuclear reactors, extract plutonium from soils, and remove metals and radionuclides from contaminated soils, wastes, and incinerator ash (3, 14-16). Carbon-steel contaminated with 137Cs, 60Co, 58Co, 57Co, 54Mn, and 51Cr showed a decontamination factor (DF) of 200 using a solution of 0.1 M HCl and 0.1 M disodium citrate (17). Surface decontamination of nuclear power plant components containing U, Np, and Tc was accomplished using citric acid to complex VOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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the metals (3, 15). Oxalic acid, a dicarboxylic acid, is also known to form a complex with uranium (18). The type of complex formed between metals and citric acid plays an important role in determining its biodegradability (15). The bidentate complexes of iron(III), nickel, and zinc citrate were readily biodegraded, whereas the complexes involving the hydroxyl group of citric acid, the tridentate aluminum, cadmium, and copper citrate complexes, and the binuclear uranium citrate complex were recalcitrant. The presence of the free hydroxyl group of citric acid is the key determinant in affecting biodegradation of the metal citrate complex. While uranyl citrate is resistant to biodegradation, upon exposure to light complete photodegradation of the complex is observed; the citric acid is converted to carbon dioxide and water and uranium precipitated as schoepite (19). Although biotransformation of metal citrates by microorganisms has been extensively studied, little is known of the biodegradation of metal oxalates. Pure cultures of microorganisms, such as Pseudomonas alcaligenes and Alcaligens sp. have been reported to mineralize calcium, iron, magnesium, aluminum, and hydrogen oxalate to varying degrees (20). Quayle (21) and Dijkhuizen et al. (22) showed that Pseudomonas oxalaticus could grow on oxalate as the sole source of carbon and energy. The enzymes involved in the initial metabolism of oxalate are cytoplasmic, and therefore, oxalate can only be utilized after its translocation across the cell envelope. A specialized group of aerobic bacteria, Axalicibacterium, (23) and anaerobic gastrointestinal bacteria (24) also were found to utilize oxalate. In this study we systematically examined the removal of uranium from contaminated plain carbon-steel coupons by oxalic and citric acids. Aqueous wastes generated from the treatment processes were subjected to biodegradation followed by photodegradation, resulting in minimization of the secondary waste stream with concomitant recovery of uranium.
Materials and Methods Uranium Solutions. Uranyl nitrate stock solution (400 mM) was prepared by dissolving the appropriate amount of UO2(NO3)2‚6H2O (Analar, BDH Chemicals Ltd., Poole, England) in deionized water. Dilute solutions of uranyl nitrate containing 10 and 40 mM U were used to determine its association with the steel surface. The pHs of the solutions were 3.2 and 2.8, respectively. Preparation of Steel Coupons for Studies of Uranium Decontamination. Coupons (230 cm2) of low-carbon steel (1010), commonly used in industry as structural and loadbearing steel and, subsequently, one of the primary metals that will undergo decontamination, were prepared as described previously (11). The cleaned coupons were placed in an HEPA hood and vertically supported, and the entire surface of one side of each coupon was sprayed with approximately 5 mL of 10 or 40 mM uranyl nitrate solution using an atomizer consisting of a variable spray pump attached to polycarbonate bottle (Nalgene). The solution was allowed to dry for 10 min, sprayed again with a similar amount of uranyl nitrate, and dried. Samples were placed horizontally in a specially constructed cyclic (wet/dry) humidity chamber made of Plexiglas with an attached blower and humidifier to maintain a relative humidity of 90-99% at 23-25 °C. They were exposed to 12 hourly wet/dry cycles (1/2 h wet and 111/2 h dry); humidity and temperature were monitored continuously. Lightly Corroded Sample. The coupons were exposed to 10 mM uranyl nitrate and rusted in the humidity chamber for 4 days. Moderately Corroded Sample. The coupons were exposed to 40 mM uranyl nitrate solution and left to rust in a humidity chamber for 7 days. 5016
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Heavily Corroded Sample. The coupons were exposed to humidity to develop an initial layer of rust, then sprayed with 40 mM uranyl nitrate, and placed in the humidity chamber for 4 days. Then, they were sprayed a second time with 40 mM uranyl nitrate, and left in the chamber for 7 more days. Sections of the U-exposed corroded coupon then were cut to size for spectroscopic (XPS and RBS) analyses. Decontamination of Uranium-Contaminated Coupons. On the basis of our previous studies on the remediation of uranium- and toxic-metal-contaminated soils (5) and examination of the literature (25-28), we tested the ability of citric acid alone and in combination with hydrogen peroxide and oxalic acid to remove uranium from the coupons. Initial testing of the contaminated coupons using varying concentrations of citric acid (0.05-0.2 M) resulted in removal of 50-60% of the uranium. Treatment with hydrogen peroxide and oxalic acid improved the overall removal efficiency of uranium. Oxalic acid was used for its ability to dissolve the iron oxide (28). Hydrogen peroxide, an oxidizing agent, was incorporated into the treatment scheme to (i) oxidize any reduced uranium which may be present on the coupon to the readily extractable hexavalent form, (ii) loosen up the rust layers through its effervescent action during decomposition and for its polishing effect on the surface, and (iii) maintain a chemically active surface by enhancing the dissolution of iron and iron oxide films (19, 26). In addition, the hydrogen peroxide oxidizes ferrous oxalate to the more soluble ferric oxalate. On the basis of the initial results, we developed the following protocol which consisted of treatment of uranium-contaminated coupons with oxalic acid (0.02 M)-hydrogen peroxide (0.05 M)-citric acid (0.1 M) (OPC) or citric acid (0.1 M)-hydrogen peroxide (0.05 M)citric acid (0.1 M) (CPC). The uranium-contaminated coupon was first treated by spraying it with 25 mL of 0.02 M oxalic acid or 0.1 M citric acid from an atomizer at room temperature to dissolve the corrosion product and activate the surface. This was then followed by application of 25 mL of 0.05 M hydrogen peroxide. The dark surface of the steel immediately brightened. The coupons then were exposed to a similar volume of 0.1 M citric acid. The treatment solutions were allowed to dry on the coupon. At the end of the treatment process the coupons were rinsed with deionized water, the solutions were filtered through a 0.22 µm filter, acidified with nitric acid (Ultrex), and analyzed for Fe and U by ICP-AES to determine the efficiency of their removal. A mass balance for uranium on the coupons was obtained by acidifying their surfaces with 6 M HCl to remove any remaining uranium, and the residual solids were solubilized by digestion of the acid extract. Citric acid was determined using an Aminex HPX-87H organic acid column (Bio Rad, California) connected to a Shimadzu HPLC with an LC-10AS UV-vis detector set at 210 nm. Oxalic acid was quantified using an enzyme assay procedure, no. 591 (Sigma, Missouri). Spectroscopic Analyses of U Association on the Coupons’ Surface before and after Decontamination. X-ray diffraction (XRD) analyses were performed with the oxides scraped from the corroded coupon surface to identify the types of iron oxides formed (6). Mineralogical analyses were performed using a Philips model XRG 3100 powder X-ray diffractometer with a 40 kV Cu KR X-ray source and a current of 30 mA. Scans were collected every 0.02° at a rate of 2 s/step. The lattice-spacing values were compared with information from the literature and the JCPDS-International Centre for Diffraction Data. XPS was used to determine the chemical state of U and Fe on the surface. RBS was carried out to determine whether the uranium was occluded in the oxide matrix. The contaminated corroded steel coupons were analyzed before and after they were cleaned with citric acid. The spectroscopic
techniques and the parameters used to determine the extent and the nature of the association of uranium with the contaminated surface before and after treatment by citric acid formulations are described in detail elsewhere (11, 29). Biodegradation of Oxalic and Citric Acids in Decontaminant Waste Solutions. The CPC or OPC decontamination solutions containing uranium and iron were biodegraded by Pseudomonas fluorescens biovar II (ATCC 55241) under aerobic conditions (15, 30). The bacterium was grown in a modified Simmon’s citrate medium containing the following (g): citric acid, 2; MgSO4, 0.2; NH4Cl, 1; glycerol phosphate, 1.5; NaCl, 5; deionized water to 1 L. The pH of the medium was adjusted to 6.1 with NaOH, and 100 mL aliquots were transferred to 250 mL Erlenmeyer flasks. The medium was autoclaved, cooled, and inoculated with 4 mL of a 24 h old culture. The cultures were incubated overnight at 26 ( 1 °C on a rotary shaker. The decontaminant waste solutions from each set were combined, diluted to 500 mL with deionized water, and filtered through a sterile 0.45 µm glass filter. The following ingredients were added (g): MgSO4, 0.1; NH4Cl, 0.5; glycerol phosphate, 0.75; NaCl, 2.5. The pH was adjusted to 6.1. A 150 mL aliquot of the solution was aseptically transferred to each of three 500 mL sterile Erlenmeyer flasks, and the flasks were inoculated with 5 mL of log phase (24 h, OD ) 1.4) P. fluorescens culture prepared as described above. Uninoculated samples served as controls. The samples were incubated in the dark on a rotary shaker at 100 rpm and 28 ( 1 °C. All samples were prepared under low light to minimize photochemical reactions (31). Periodically, a 4 mL aliquot was aseptically removed from the flasks, the optical density was measured, the sample was filtered through a 0.45 µm filter, and the pH was determined. The sample was acidified with 0.05 mL of concentrated HCl (Ultrex) and analyzed for citric acid and oxalic acid by HPLC, and Fe and U by ICP-AES. At the end of the experiment, the samples were centrifuged at 7000g to remove the precipitate and bacterial cells. The solids consisting of biomass/precipitate were dried at 60 °C overnight and weighed. They were then digested in 5 mL of concentrated HNO3, and total Fe and U were determined. The control samples were analyzed only at the start and end of the experiment. Photodegradation of Oxalic and Citric Acid in Decontaminant Waste Solutions. The supernatant from the samples after biodegradation was filtered through a 0.45 µm filter, and the pH was adjusted to 3.5 with HCl, as previously described (19, 31). Aliquots (100 mL) of each were transferred to three separate 250 mL acid-washed Erlenmeyer flasks fitted with cotton plugs. The flasks were placed in a growth chamber equipped with seven 60 W high-output growth lamps, and the temperature was maintained at 26 ( 1 °C. The lamps displayed a broad spectral curve in the visible region from approximately 400 to 700 nm. The light intensity was calibrated with a Biosphere Instruments QSL-100 analyzer, and the total intensity at the sample was 0.18 meinstein‚m-2‚s-1. A fourth sample flask was placed in the dark as a control. At periodic intervals, a 3 mL aliquot was removed, filtered through a 0.45 µm filter, and analyzed for pH, citric acid, oxalic acid, U, and Fe. Control samples kept in the dark were sampled at the start and end of the experiment.
Results Decontamination of Uranium-Contaminated Coupons. Decontaminated coupons clearly showed evidence of extensive removal of rust from the coupon (Figure 1). Table 1 shows the efficiency of U removal for the two treatments. The extent of uranium removed was similar for both the OPC and CPC treatments. The lightly corroded samples showed the greatest removal efficiency. Approximately 15% more iron was removed by OPC treatment of the coupons
FIGURE 1. Lightly, moderately, and heavily corroded uraniumcontaminated carbon-steel coupons before and after decontamination using CPC treatment.
TABLE 1. Efficiency of Uranium Removal from Contaminated Steel Coupons amt of uranium removed (%) treatment
lightly corroded
moderately corroded
heavily corroded
oxalate-peroxide-citrate 94.2 ( 1.5a ndb 67.6 ( 3.2 citrate-peroxide-citrate 90.3 ( 2.6 72.8 ( 2.4 71.5 ( 1.0 a
(standard error of the mean (SEM).
b
nd ) not determined;
compared to the CPC treatment. Solids were found in the effluent wash only after citric acid treatment. The amount of uranium removed depended on the extent of corrosion on the surface of the coupon. Surface Analysis of Coupons after Decontamination. X-ray diffraction analysis showed that lepidocrocite and magnetite were the major iron oxides found on each of the coupons (Figure 2). The rust scrapings from lightly, moderately, and heavily corroded coupons contaminated with uranium all contained 11-12 mmol/g Fe and 0.03-0.05 mmol/g U. XPS wide scan survey spectra (0-1000 eV) from contaminated lightly and heavily corroded samples, before and after decontamination (CPC), are shown in Figure 3. In general, XPS provides data from the outermost surface of VOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. XRD spectrum of rust scraped from a corroding coupon. Rust scraped from a corroded low carbon-steel coupon was predominantly lepidocrocite and magnetite.
FIGURE 3. XPS wide scan survey spectra (0-1000 eV) from contaminated lightly and heavily corroded samples, before and after decontamination (CPC). samples, due to the shallow photoelectron escape depth of no more than 30 Å. The area exposed to X-rays was approximately a circular region 0.5-0.7 cm in diameter. The prominent features from the nondecontaminated samples include photoelectron peaks associated with the C1s, O1s, U4f, and Fe2p core states. Broad features to the high binding energy side of the O1s peak are Auger peaks associated with Fe. The results show that the surface uranium contamination is much greater for the heavily corroded coupon. Spectra to the right are from the same samples following CPC treatment. It is clear that in these samples there is a loss of the observable peak associated with uranium (the U4f feature is missing), indicating removal of surface-bound uranium from the coupon. Figure 4 shows the RBS spectra from two locations on the surface of a heavily corroded and exposed steel surface before (left) and after (right) decontamination. The data indicate incorporation of U throughout the upper micrometer of the corrosion layer prior to decontamination, with varying degrees of removal following decontamination. In location A, almost all of the uranium is removed (with a very small amount retained, likely due to incompletely rinsed complexed U species), while, at location B, a percentage of U is retained by the corrosion layer, primarily below the outermost surface. RBS analysis of the heavily corroded coupons before and after treatment also suggested that there is an average ratio of 300 Fe atoms to 1 U atom in the top micrometer of the corrosion product before treatment, with an increase to an average of nearly 3000 Fe atoms to 1 U atom in the same range of depth after cleaning. 5018
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The locations of uranium association with the heavily corroded coupon before and after CPC treatment are schematically summarized in Figure 5. The iron oxyhydroxides associated with the heavily corroded sample following treatment showed a high degree of crystallization. Uranium is strongly associated with these corrosion products in occluded, heavily corroded regions as amorphous uranyl hydroxides or as crystalline schoepite. In addition, the spatially resolved nature of the other techniques shows that this retention, for all cases and treatments, occurs primarily in occluded, heavily corroded regions. Treatment of Decontaminated Waste Solutions. Table 2 gives the concentrations of iron and uranium in the OPCand CPC-decontaminated waste solutions after treatment of the heavily corroded low-carbon steel coupon. CPC treatment was marginally more effective in uranium removal than the OPC treatment. However, the OPC treatment removed more iron than the CPC treatment. OPC Solution. Biodegradation of the OPC decontaminated solution by P. fluorescens showed an increase in optical density from 0.02 to 1.39 ( 0.06 due to the growth of the bacteria. The pH of the medium rose from 6.30 ( 0.02 to 8.60 ( 0.03. Citric acid was rapidly metabolized, but oxalic acid was not metabolized by the bacterium, and it remained in solution (Figure 6A). There was no significant change in the concentration of iron or uranium in solution during biodegradation. However, photodegradation of the supernatant after biodegradation caused a decrease in the concentrations of uranium, iron, oxalic acid, and citric acid (Figure 6B). Acidification of the supernatant from pH 8.60 ( 0.03 to pH 3.50 ( 0.02 prior to photodegradation resulted in the dissolution of colloidal iron formed during biodegradation of the decontamination solution. The iron concentration in solution increased from 490 to 700 µM upon acidification. Photodegradation of both citric and oxalic acids was rapid, and they were completely degraded in less than 55 h. Precipitation of uranium from solution occurred more rapidly than that of iron during photodegradation. CPC Solution. Biodegradation of CPC solutions resulted in the degradation of free and iron-associated citric acid. The optical density of the medium increased from 0.02 to 1.32 ( 0.08 and the pH from 6.40 ( 0.02 to 8.60 ( 0.01. Iron in solution started to precipitate rapidly only after 34 h. This is related to the increase in the pH of the medium as a result of citric acid metabolism. Uranium remained in solution (Figure 7A). Photodegradation of the supernatant after biodegradation resulted in the precipitation of uranium from solution as UO3‚2H2O (Figure 7B). These results are consistent with the results reported previously (19, 31).
Discussion The efficiency of uranium removal from corroded steel coupons depended upon the nature of uranium association with the iron oxide on the coupon surface. This includes both chemical association with mineral phases (ferrihydrite, lepidocrocite, schoepite) and physical association (surfacebound, occluded). The effectiveness of decontamination of U-contaminated steel coupons by oxalate-H2O2-citrate or citrate-H2O2-citrate varied from 63% to 94% depending on the extent of corrosion and the composition of the treatment solution. The reduction in removal efficiency was due to occlusion of uranium in the iron oxide matrix, as shown by comparing the amount of uranium detected using highly surface sensitive XPS with the bulk-sensitive technique RBS (11, 29). RBS studies of coupons before and after treatment showed the occlusion of U by iron oxides and oxyhydroxide corrosion products having a complex topography. XPS revealed the presence of both Fe(III) and Fe(II) species, as well as some ferrihydrite. XPS results demonstrated that hexavalent ura-
FIGURE 4. Rutherford backscattering spectra from two locations on the surface of a heavily corroded and exposed steel surface before (left) and after (right) decontamination. The data indicate the incorporation of U throughout the upper micrometer of the corrosion layer before decontamination, with varying degrees of removal afterward. In location A, almost all is removed (with a very small amount of retention, likely due to incompletely rinsed complexed U species), while, at location B, a percentage of U is retained by the corrosion layer, primarily below the outer surface.
FIGURE 5. Nature of uranium association with a heavily corroded 1010 carbon-steel coupon before after citric acid-peroxide-citric acid treatment. Locations shown on the schematic of Rutherford backscattering (“RBS (Before)” and “RBS (After), (A), (B)”) refer to the data shown in Figure 4.
TABLE 2. Concentration of Iron and Uranium in Decontaminated Solutions from a Heavily Corroded Sample
a
treatment
citric acid concn (mM)
oxalic acid concn (mM)
iron concn (mM)
uranium concn (µM)
oxalate-peroxide-citrate citrate-peroxide-citrate
7.3 ( 0.2a 8.3 ( 0.4
0.39 ( 0.02 NAb
2.26 ( 0.33 1.81 ( 0.23
29.8 ( 2.7 34.1 ( 3.4
(SEM.
b
NA ) not applicable.
nium was present in all samples, but only in areas where it was associated with incompletely removed corrosion layers. These findings suggest that decontamination techniques which loosen surface corrosion-product scales attack the more crystallized forms of mixed iron-uranium oxyhydroxides (perhaps through ion-exchange processes), and which are better at penetrating occluded areas, will have greater success in removing radiological contamination (17, 29). The generation of large volumes of secondary waste and its treatment is of concern in any decommissioning and decontamination (D&D) operations. In this treatment process, biodegradation followed by photodegradation of waste solutions containing citric acid and oxalic acid destroyed the organic acids while precipitating uranium and iron. We observed that P. fluorescens degraded citric acid but not oxalic
acid as the sole carbon source. Discrimination by microorganisms among metal-organic complexes of citric acid and oxalic acid occurs in natural environments with different metal complexes being mineralized to different extents. Lack of biodegradation of metal-oxalic acid complexes may be attributed to the inability of the P. fluorescens to degrade oxalic acid or to the preferential use of citrate when both are present; alternatively, as with uranyl citrate, uranyl oxalate complexes may be recalcitrant (15). The removal of ferric ion from solution during biodegradation was more pronounced in the CPC decontaminant solution due to metabolism of the citric acid, whereas in the OPC solution the iron remained in solution as dissolved species. It has been previously reported that at pH above 7.5 in the presence of citric acid the FeIII(OH)2-citrate complex VOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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a complex set of photochemical reactions, resulting in the precipitation of uranium from solution as the mineral schoepite (5, 19) (reaction 2).
[(UO2)2(C6H5O7)2]2- + 2H+ f [U(C6H5O7)(OH)2]- +
UO22+ + C4H5O3- + 2CO2 (1)
[U(C6H5O7)(OH)2]- + 2H2O + 1/2O2 f
UO3‚2H2O(ppt) + C6H5O73- + 2H+ (2)
FIGURE 6. Biodegradation (A) and photodegradation (B) of oxalateH2O2-citrate-decontaminated solutions.
The photodegradation of uranyl oxalate may occur by one of several processes including (i) photochemical oxidation of oxalic acid by uranyl ion, (ii) uranyl-sensitized decomposition of oxalic acid, or (iii) sensitized autoxidation of oxalic acid (21, 32, 33). Although the mechanisms of photodegradation of mononuclear iron and uranium oxalate complexes are known, there is no information on complex mixtures containing iron, uranium, oxalic acid, and citric acid. It was demonstrated that in the presence of Fe and U, a mixed-metal complex with citric acid is formed (34). MINTEQA2 calculations of the photodegradation solution at pH 3.5 indicate the predominant species in solution is the mixed-metal citrate complex which binds 72% of the uranium and 41% of the iron. The remainder of the uranium is associated with oxalate (15%), and 44% of the iron is present as iron citrate complex. The thermodynamic constants used for the equilibrium reactions and listings of the minor species are presented as Supporting Information. Photodegradation of the mixed-metal citrate complex results in reduction of ferric ion to the ferrous form (reaction 3) with production of 3-oxoglutaric acid. Uranium is not photoreduced in the reaction, possibly due to the formation of a photochemically inactive mononuclear complex. Oxidation of the ferrous ion by photochemically generated hydrogen peroxide (photoFenton reaction) results in displacement of uranyl ion in the uranium citrate complex with formation of ferric citrate (reaction 4) and precipitation of iron and uranium as ferrihydrite and uranyl hydroxide, which were previously reported (31). The newly formed iron(III) citrate complex undergoes photodegradation with formation of ferrous ion, which is subsequently reoxidized to ferric ion with production of acetoacetic acid and carbon dioxide. The ferric ion then precipitates from solution as ferrihydrite (reaction 5). Photodegradation of the OPC decontaminant solution resulted in degradation of oxalic and citric acids and precipitation of uranium and iron.
[(UO2)(FeIII(C6H5O7)2]22- f [FeII(C6H5O7)]- + Fe2+ +
2[(UO2)(C6H5O7)]- + C5H5O5- + CO2 (3)
2Fe2+ + 3OH- + [(UO2)(C6H5O7)]- + H2O2 + O2 f [FeIII(C6H5O7)] + Fe(OH)3(ppt) + UO2(OH)2(ppt) + CO2 (4) [FeIII(C6H5O7)] + OH- f Fe(OH)3(ppt) + C4H5O3- + 2CO2 (5) FIGURE 7. Biodegradation (A) and photodegradation (B) of citrateH2O2-citrate-decontaminated solutions. is formed and is readily biodegraded by P. fluorescens with the precipitation of ferric hydroxide from solution (30). We have shown that in the presence of citric acid uranium forms a dimeric uranyl citrate complex. During photodegradation, the U(VI) is reduced to U(IV) and one of the citrates is oxidized to acetoacetic acid (reaction 1). The U(IV) in the complex reoxidizes to U(VI) in the presence of dioxygen by 5020
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These results show the use of organic acids to remove radionuclides from contaminated materials has the following advantages: (i) wet extraction, which reduces dust formation, (ii) dissolution of oxides from the surface layer to remove fixed contaminants including those coprecipitated with iron oxides, (iii) the ability to penetrate porous materials to remove contamination below the surface, (iv) biodegradation of the secondary aqueous waste stream destroys most of the organic acids to carbon dioxide and water, and (v) the subsequent
photodegradation of waste generated from biodegradation results in the precipitation and recovery of uranium in almost pure and concentrated form with volume reduction, which can be disposed of or recycled. In this study we observed that the OPC process yields a mixture consisting of uranyl hydroxide and ferrihydrite, whereas pure uranium as schoepite is obtained from the CPC process. These results also suggest that overall citrate-peroxide-citrate is an environmentally friendly green-chemistry process that uses all naturally occurring materials, citric acid, common soil bacteria, and sunlight.
Acknowledgments We thank J. B. Gillow for performing speciation calculations, C. W. Eng for her assistance throughout this study, and Drs. J. D. Demaree and J. Hirvonen, Army Research Laboratory, Aberdeen Proving Ground, MD, for their assistance in acquiring and analyzing the RBS data. This research was performed under the auspices of the Environmental Management Science Program (EMSP), Decontamination and Decommissioning Focus Area, Environmental Remediation Sciences Division, Office of Biological and Environmental Research, Office of Science, U.S. Department of Energy, under Contract No. DE-AC02-98CH10886. Partial support was provided by the Center for Environmental Molecular Science (CEMS), Stony Brook University, through NSF Grant CHE0221934 and the U.S. Department of Energy.
Supporting Information Available Speciation modeling of U(VI) species distribution in the photodegradation solution containing Fe(III), U(VI), oxalic acid, and citric acid. This material is available free of charge via Internet at http://pubs.acs.org.
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Received for review July 17, 2004. Revised manuscript received April 1, 2005. Accepted April 1, 2005. ES048887C
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