Chapter 20
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Treatment and Disposal Options for a Heavy Metals Waste Containing Soluble Technetium-99 W. D. Bostick, J. L. Shoemaker, P. E. Osborne, and B. Evans-Brown Oak Ridge Gaseous Diffusion Plant, Martin Marietta Energy Systems, Inc., P.O. Box 2003, Oak Ridge, TN 37831-7272
Various equipment decontamination and uranium recovery operations at the Portsmouth Gaseous Diffusion Plant generate a "raffinate" waste stream characterized by toxic heavy metals, high concentrations of nitric acid, and low levels of radioactive nuclides(235Uand99Tc).Dilution and adjustment of solution pH to a value of 8.2 to 8.5 precipitates heavy metals that can be hydrolyzed. The precipitant is concentrated by paper filtration to yield afiltercake heavy metals sludge (HMS) and HMS filtrate. The HMS fraction may be incorporated into cement-based grout containing ground blast furnace slag to reduce the mobility of its toxic and radioactive components. Sorption of soluble mercury, pertechnetate, and nitrate anions from the HMSfiltratewas tested using organic resins and inorganic sorbents. Removal of Hg and99Tcby ironfilingsis efficient and economical, generating a small volume of spent sorbent amenable to co-disposal with HMS in a grout waste form, but is slow. For more rapid sorption, poly-4-vinylpyridine resin is very effective for the removal of soluble99Tcwith little uptake of interfering anions at near-neutral influent pH values. Technetium-99 ("Tc) undergoes radioactive decay by low-energy (0.29 MeV) beta emission, with a half-life of 213,000 years and a specific activity of 0.0171 Ci/g. It is formed with a high yield (-6%) in the thermal neutronfissionof ^ U . Typical lowenrichment light-water nuclear power reactors operated at optimum fuel exposure may yield 27.5 kg (467 Ci) of "Tc per 1000 MW-year of electric energy produced (1). Technetium from the nuclear fuel cycle may enter the environment as a result of disposal of aqueous wastes or during the separation and recovery of spent nuclear fuels (2). The predominant form of Tc under oxic conditions is the pertechnetate anion (TcO^) (3), which is highly soluble in water and readily mobile in the environment (1,2). Certain reduced forms of Tc [e.g., hydrolyzed Tc(IV) oxides (4)] have limited solubility. Technetium-99 is of particular concern because of its persistence and relative mobility (2^). 0097-6156/90/0422-0345$06.75/0 © 1990 American Chemical Society In Emerging Technologies in Hazardous Waste Management; Tedder, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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EMERGING TECHNOLOGIES IN HAZARDOUS WASTE MANAGEMENT
Technetium entered the U.S. gaseous diffusion uranium enrichment complexes as a volatile fluorination product impurity in reprocessed uranium from plutoniumproduction reactors that was fed to the Paducah Gaseous Diffusion Plant (PGDP) (6). This U F product, with ^Tc impurity, was subsequently fed to the Oak Ridge and Portsmouth enrichment cascades. Most of the technetium remains within the cascade adsorbed on surfaces. However, some "Tc is periodically removed from these surfaces by wet-chemical decontamination for personnel protection when equipment is removed for maintenance or repair. At the X-705 facility of the Portsmouth (Ohio) Gaseous Diffusion Plant (PORTS), decontamination is achieved with nitric acid, which solubilizes uranium residue and "Tc. Uranium is recovered by solvent extraction, with the great majority of the technetium remaining in the aqueous raffinate waste. Composition of the raffinate waste stream varies, but it is generally characterized by toxic heavy metals, high concentrations of nitric acid, and relatively low levels of radioactivity (Table I). The treatment of the raffinate stream generally consists of (1) dilution with an equal volume of water and pH adjustment to about 8.5 to precipitate the hydrolyzable heavy metals, (2)filtrationof the precipitation slurry to yield a wetfiltercake designated as heavy metals sludge (HMS) and an HMS filtrate, (3) processing of the HMSfiltratewith a strong-base anion exchange resin to remove the soluble pertechetnate ion, (4) biodenitrification, and (5) sewage disposal (7). This operation lowers the concentration of most heavy metals to levels below regulatory concern in the HMSfiltrate(Table I). However, some nonhydrolyzable anionic species—such as pertechnetate, nitrate, and complexed mercury ions—are not removed by this treatment and remain in the HMS filtrate and HMS sludge as a result of incomplete dewatering (8,9). Nitrate ion concentration in the HMS filtrate can be greatly reduced by biodenitrification, but toxic metals and radionuclides must first be removed to avoid formation of a voluminous contaminated biosludge that would not quality for economical disposal by simple landfill. Anion exchange with a type 1 (quaternary amine) strong-base resin (Dowex SRB-OH) has been used at PORTS to remove residual technetium activity. However, this resin has demonstrated a modest technetium loading capacity because of competition by other anions, thus generating a relatively large volume of spent sorbent for storage or disposal. In addition, an amine-type resin in the presence of sorbed oxidant (e.g., nitrate ion) is thermodynamically unstable and may present a potential hazard upon prolonged storage of spent resin (10). Inexpensive inorganic sorbents or reductants (ferrous sulfide and iron metal filings) are very efficient in removing technetium and certain other metal ions of regulatory concern, while generating a relatively small bulk of spent sorbent that is amenable to disposal in a grout waste form. However, the reduction of Tc(VII) to Tc(IV) is slow (batch equilibration times of several hours are recommended) (8). For more rapid contaminant removal (e.g., in treatment of relatively large volumes of slightly contaminated groundwater), resin materials may be advantageous. Therefore, we investigated the ability of several sorbents to remove soluble "Tc in the HMS filtrate. Cementfixationof chemically toxic heavy metals is often regarded as the "best available technology" for reducing their potential for environmental impact. The sludge filter cake from the raffinate waste treatment may be stabilized in a cement-based grout waste form for final disposal. Use of granulated blast furnace slag (BFS) or other reductive admixtures in the grout greatly decreases the leachability of "Tc from the waste form (9).
Downloaded by UNIV OF TEXAS EL PASO on November 8, 2014 | http://pubs.acs.org Publication Date: March 8, 1990 | doi: 10.1021/bk-1990-0422.ch020
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In Emerging Technologies in Hazardous Waste Management; Tedder, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Downloaded by UNIV OF TEXAS EL PASO on November 8, 2014 | http://pubs.acs.org Publication Date: March 8, 1990 | doi: 10.1021/bk-1990-0422.ch020
BOSTICK ET AL.
Treatment and Disposal for a Heavy Metals Waste
Table L Analysis o f P O R T S raffinate and filtrate from H M S precipitation o f raffinate Concentration (mg/L) Constituent or characteristic Aluminum Barium Cadmium Chromium Copper Iron Lead Manganese Nickel Uranium Zinc Mercury Nitrate Sulfate Technetium Gross alpha, pCi/L pH Specific gravity
Raw Typical range*
15 to 360 700 to 7,100
80 to 620 2 to 1,450 15 to 210 250,000 to 400,000 0.1 to 120
