Patent Number: 056132387
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention relates to improved methods for separating from soil unwanted nuclear waste material and hazardous metals, particularly radionuclides and ionized forms of potentially toxic nonradioactive metals and metalloids, such as arsenic, antimony and selenium, by concentrating in some instances in very small particles or fines of soil or clay. The concentrated radionuclide and nonradioactive metallic ion-containing fines, for example, are thus in a state which permits more efficient disposal, such as by storage, or for further treatment to modify the hazardous substances to less toxic and more environmentally benign substances. The methods are based on the observation that liquid ammonia possesses the unique ability to break up soils into very fine particles. It was also found that suspensions of what appear to be extremely fine particles of these can be prepared by mixing with ammonia. Radio-nuclide-contaminated soils and soils contaminated with ions of hazardous nonradioactive metals and metalloids, or soils contaminated with both are mixed, preferably with anhydrous liquid ammonia, to form finely-dispersed suspensions or slurries. Because of the lower density of ammonia relative to water, significantly smaller soil particles remain suspended in the liquid, and particles which would otherwise be suspended in water readily precipitate from the dispersion because of the lower density and viscosity of ammonia. The greater bulk fraction of the soil consisting of larger precipitated particles are sufficiently free of the radionuclide or ions of the hazardous nonradioactive metal or metalloid contaminants as to permit reclaiming large volumes of the treated soil. It was observed, for instance, that washing soils in ammoniacal solutions, and particularly anhydrous liquid ammonia results in significant reduction in concentrations of certain metal ions even when no particulates are visible in the ammonia following treatment. Accordingly, liquid ammonia was found to be effective in both physically and chemically enhancing decontamination in breaking down even tightly bound clays into fine slurries of platelets coupled with a metal transporting mechanism for maximizing extraction and exposure of metal contaminants, while also performing as a ligand in binding the contaminating metals in complexing or chelating type reactions. The ammoniacal liquid is preferably anhydrous liquid ammonia, but solutions of at least 50 percent-by-weight of ammonia in water can also be employed when using ammonia exclusively. Soils contaminated with ions of hazardous nonradioactive metals and metalloids, such as arsenic and chromium (VI), or soils contaminated with mixed wastes, such as radioactive isotopic metals, like uranium, plutonium and thorium along with hazardous nonradioactive metal ions can also be effectively treated by forming dispersions or slurries with anhydrous liquid ammonia, which in-turn can be treated with solvated electrons by contacting the ammonia-soil slurry with a reactive metal, particularly a more electropositive metal, like sodium, potassium, barium and calcium. When a metal like sodium dissolves in the liquid ammonia, it becomes a cation by losing its valence electron as illustrated in the following equation: EQU Na.apprxeq.Na.sup.+ +e.sup.- The ammonia molecules then solvate these ions and electrons reversibly according to the equations: EQU Na.sup.+ +xNH.sub.3 .apprxeq.Na(NH.sub.3).sub.x.sup.+ EQU e.sup.- +yNH.sub.3 .apprxeq.e.sup.- (NH.sub.3).sub.y The "ammoniated electron" is responsible for the strong reducing properties exhibited by such solutions. In this regard, the methods of the invention are suited for treating soils contaminated with hazardous chromium (VI) wherein the solvated electrons reduce the ions from the hazardous higher oxidation state to the less hazardous chromium (III). The methods as described herein are especially well suited for the selective removal of lead from soils, particularly when treated with solvated electrons. Solvated electrons are also useful in decontaminating soils having mixed waste, like ions of a hazardous non-radioactive metal or metalloids along with polyhalogenated organic compounds, such as polychlorinated biphenyls (PCBs), as well as dioxins, e.g., 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin, and any of the several other members of the family of chlorinated dioxins, and various pesticides. The term "pesticide" is intended to denote any substance, organic or inorganic, used to destroy or inhibit the action of plant or animal pests. Thus, pesticides would include insecticides, herbicides, rodenticides, miticides, etc. Accordingly, this aspect of the invention is particularly effective in treating soils contaminated with mixed wastes by separating and complexing hazardous metal ions through the action of the ammonia, forming dispersions while simultaneously reducing halogenated compounds to compounds of lesser toxicity and impact on the environment. Methods for the destruction of halogenated organic compounds are disclosed in U.S. Pat. No. 5,110,364. A general method of decontaminating soils containing ions of hazardous nonradioactive metals or metalloids with ammoniacal solutions according to the present invention is illustrated by FIG. 1. Soil can be first added to the mixing vessel of FIG. 1. Anhydrous liquid ammonia is circulated from the ammonia holding tank and used to fluidize the soil resulting in the formation of a fine slurry of soil suspended in the ammonia. Agitation of the suspension can be provided by circulating the ammonia (A) between the mixing vessel and ammonia holding tank by pump means, although other mixing methods may be used. After the slurry has been sufficiently mixed the liquid phase ammonia can be separated from the soil by decanting, pressure filtration (B), or by other known methods. The ammonia in the holding tank contains metal ions which are recovered by evaporating the ammonia through the vent, where it is captured for reuse by conventional ammonia collection methods. Methods of the invention may also be performed on soils which are predominantly sand, and which are practically free of clay and organic constituents. In this embodiment, clays possessing ion exchange properties, such as atapulgite, montmorillonite, kaolinite are added to the ammoniacal reaction mixture wherein the hazardous ions are adsorbed by the clay, and the clay-metal dispersion decanted from the sand. As a further embodiment, the ammoniacal solutions can also employ known complexing/chelating agents, such as EDTA, NTA (nitrilotriacetic acid), 8-hydroxyquinoline, cyanide ions, and so on, which enhance solubility of the metal ions in the solvent for removing hazardous metals from the soil slurries. The following specific examples demonstrate the invention, however, it is to be understood they are for illustrative purposes only and do not purport to be wholly definitive as to conditions and scope. EXAMPLE I Methods of the invention can be carried out by means of a system, such as that illustrated by FIG. 2. A closed reactor 10 is utilized as a mixing vessel for nuclear waste contaminated soil 14 positioned at the bottom of the vessel. The term "soil" is intended to have its ordinary understood meaning, and includes one or more components in varying proportions, such as of clay, stone, disintegrated rock particles or sand, organic matter, along with varying amounts of water and the like. Obviously, soil compositions will vary widely depending on source and location. For instance, soils from desert or other arid locations are mainly sandy compositions with little organic material or clay components. One representative soil from the State of Ohio known as Ohio Loam was found to have an analysis of 35% sand, 32% silt, 33% clay and 4.1% organic matter and have a pH 7.7. By contrast soil from Oak Ridge, Tenn. was found to contain only 1% sand, 26% silt, 73% clay, no organic matter, and have a pH of 5.2. In sum, the term "soil" for purposes of this invention is intended to have a broad compositional makeup, including varying ranges of clay, disintegrated rock/sand particulates, organic matter, silt-fines, moisture and so on. This would include soils which are mainly clay or sand. Anhydrous liquid ammonia 16 or a solution of liquid ammonia containing up to a small amount of water is introduced to closed reactor 10 from ammonia storage vessel 18. Once filled, liquid ammonia is withdrawn from reactor 10 from below the surface of the liquid by circulating pump 20 positioned in outlet line 22. The flow of ammonia is directed by means of 3-way diverter valves 24-25 to either by-pass line 26 or to solvator 28 containing a bed of reactive metal 30, such as alkali or alkaline earth metals or mixtures of the same. Suitable representative metals include sodium, potassium, lithium, calcium and magnesium. Aluminum would also a suitable reactive metal. By circulating ammonia 16 through a bed of metal in reactor 28 solvated electrons are formed in-line. This avoids the problems associated with injecting metal rods or other metal sources directly to reaction vessel 10. Accordingly, methods of the present invention contemplate the option of enhanced particle size demarcation and separation of radioactive components in fines of soil and clay with ammonia and electrons solvated in the ammonia. Whether ammonia circulates through by-pass line 26 or through solvator 28 the solution is recirculated to the bottom of reactor 10 through valve 32, setting up a fluidized flow pattern in the reactor. This produces a mixing action of the soil and ammonia solution and/or solvated electrons to form a slurry. Once the soil has been uniformly dispersed in the ammonia, pump 20 is deactivated to allow the dispersion to undergo phase separation, i.e. a lower solid phase and an upper liquid-solid phase. Large particulates of the dispersion precipitate out as solid phase 34 in the bottom of reactor 10, and are sufficiently free of radionuclide contaminants, the latter being concentrated in a smaller soil fraction consisting of fines or silt dispersed in the ammonia solution as upper liquid-solid phase 36. The slurry of suspended particle fines forming the upper liquid-solid phase 36 is withdrawn from reactor vessel 10 to evaporator tank 38 via line 40 by opening valve 42. Ammonia 43 is evaporated to separate it from radioactive fines 44. Optionally, the ammonia can be transferred via line 48 to compressor 46 for reliquification if it is desired to recycle the ammonia for further use in the decontamination process. The liquefied ammonia is then transferred to ammonia storage tank 18 through line 50. EXAMPLE II PART A The decontamination of soil with ammoniacal liquid was demonstrated by the following experiment: A two-kilo batch of common Ohio loam was doped with low levels of cobalt nitrate. The doped soil was analyzed and found to contain 4.5 ppm cobalt. A 10 gram sample of the doped soil was mixed with approximately 80 grams of anhydrous liquid ammonia and shaken until well mixed. The soil was then filtered from the ammonia and sent for analysis. The ammonia was allowed to evaporate from the residue. Analysis of the soil revealed the cobalt content had dropped from 4.5 ppm to 1.1 ppm. PART B In order to improve on the removal of Co.sup.+2 ions from soil which removal is not as efficient as Co.sup.+3, two methods may be employed: In the first method, 1.50 equivalents ethylenediaminetetraacetic acid (EDTA) per Co.sup.+2 ion is mixed with soil and anhydrous liquid ammonia. The soluble Co.EDTA complex is easily filtered from the soil matrix to lower the Co.sup.+2 concentration in the soil to an acceptable level. In a second method, ammonium nitrate (10 grams/100 grams of soil) is added to a soil sample and the mixture is agitated with anhydrous liquid ammonia. The solubilized Co.sup.+2 ions are removed along with the ammonia solvent upon filtration. The toxic impurity and excess ammonium nitrate is isolated by evaporating the solvent and disposed of by methods known in the art. EXAMPLE III A 150 gram sample of soil contaminated both with 150 ppm Sr.sup.90 and 500 ppm polychlorinated biphenyls (PCBs) is placed in reactor 10 (FIG. 2). The reactor is then charged with 1.5 L of liquid ammonia (anhydrous) and pumped through the recirculation loop described in Example I for agitating the soil. After a suitable period, the ammonia is allowed to flow through solvator 28 to generate a solution of solvated electrons by contact and dissolution of 10 grams of calcium metal 30. Solvated electron generation can be a one-time event in which the metal is completely consumed in a continuous flow of ammonia. Alternatively, bypass 26 can be employed at intervals to interrupt the flow of solvated electron solution, and thus cause the introduction of reactant to be a sequence of pulses. When a sufficient quantity of reactant has been added, the ammonia circulation pump 20 is stopped and the soil slurry allowed to settle briefly to delineate a bottom phase of larger soil particles and a supernatant suspension of ammonia/soil fines/metal particles. This suspension is transferred to tank 38 from whence the ammonia may be separated by vaporization leaving the greatly reduced volume of soil fines/metal material for final disposition in accordance with established local, state and Federal Regulations. The bulk of the original soil sample charged to the reactor remains therein. The concentration of both radionuclide and PCBs is low enough to permit the treated soil to be returned as landfill as permitted by accepted practices in the remediation field. EXAMPLE IV A soil having a higher clay content than that used in Example I, or a soil having a clay fraction with a higher cation exchange capacity than that used in Example I is doped with an arsenic compound. The soil is treated with ammonia as in Example I except no solvated electron solution is introduced. After agitation and separation of the soil fines, the larger particles soil fraction is sufficiently free from the toxic metalloid as to permit its return to a suitable landfill or to the original site of excavation. The clay fines containing the arsenic impurity are of greatly reduced volume and can be stored in less volume of space than otherwise required. EXAMPLE V A soil contaminated with hazardous chromium VI ions is mixed with a liquid ammonia in a closed reactor and agitated to thoroughly disperse the soil particles. About 0.5 gallon of liquid ammonia per pound of soil is employed. A solution of solvated electrons is formed from the reaction of the liquid ammonia with calcium metal introduced to the reactor. The addition of the metal may be in a one-time injection or by serial mode of addition. When the blue color typical of solvated electrons persists, the addition of further metal is terminated. After a few minutes to assure complete reaction, the solvated electron solution is quenched. The ammonia is allowed to evaporate and is recovered for further use. The soil has all the chromium ions now with an oxidation number of less than VI, e.g., chromium III ions, is in suitable form for reclamation without further remediation. EXAMPLE VI A 500 gram sample of sand contaminated with plutonium compounds is slurried with 1.0 l of anhydrous liquid ammonia in a reaction apparatus such as that illustrated in FIG. 1. Agitation is stopped and the sand quickly settles to reveal a clear ammonia layer due to the lack of any fined-sized particles. The ammonia is decanted and is found to contain very little dissolved plutonium compounds, demonstrating little change has occurred in the level of plutonium compounds in the sand. For purposes of comparison, 25 gram of montmorillonite clay is added to the reactor and the clay and sand mixture is resuspended in 1.0 l of anhydrous liquid ammonia. Agitation is continued for a time previously shown to permit the clay to adsorb the plutonium ions. Agitation is stopped and the sand quickly settles leaving a clay/ammonia suspension above it. The suspension is removed by decantation. Since some of the clay/ammonia suspension remains in the reactor, additional ammonia is added and the sequence of agitation, settling and decantation is repeated until the amount of plutonium laden clay is lowered to the desired extent. The treated sand is removed for appropriate disposal. The clay is freed from the ammonia by vaporizing the liquid. The clay solids are disposed of in a manner prescribed for plutonium contaminated materials. Because of the reduced volume of waste, handling and disposition are more efficient. EXAMPLE VII 150 grams of soil containing cadmium salts (144 ppm Cd.sup.+2) is treated with 1.5 liters of anhydrous liquid ammonia in a 3 liter pressure bomb. 8.5 grams of sodium cyanide is added and the mixture stirred for 1 or 2 hours at room temperature. The mixture is filtered. The largest bulk portion of the soil remains on the filter and the small soil fines pass through. Both soil batches are freed from ammonia by evaporation in open vessels. The larger soil particles on the filter (19.5 gram filter cake)is found to have only 38 ppm Cd.sup.+2 ions. The soil fines passing through the filter contain 116 ppm Cd.sup.+2 ions. This accounts for 90 percent of the original amount of cadmium in the soil. This example demonstrates the ability of an ammoniacal liquid and cyanide ion in removing and concentrating hazardous metal ions in small particle fractions of soil. Thus, the methods of the disclosed invention provide the advantages of separating nuclear waste and/or ions of hazardous nonradioactive metals or metalloids by means of smaller particles than relied on using aqueous based systems; permits recycling of ammonia not otherwise achieved with systems relying on more costly scrubbing chemicals; provides means for readily separating fines from liquid ammonia; eliminates transport and storage of water to desert locations, and provides additional means for controlling particle sizes within a predetermined range with solvated electrons. While the invention has been described in conjunction with various embodiments, they are illustrative only. Accordingly, many alternatives, modifications and variations will be apparent to persons skilled in the art in light of the foregoing detailed description, and it is therefore intended to embrace all such alternatives and variations as to fall within the spirit and broad scope of the appended claims.