Patent Number: 056132387
Section: summary

TECHNICAL FIELD The present invention relates to methods of decontaminating soil, and more specifically, to the decontamination of nuclear waste-containing soils, soils contaminated with ions of hazardous nonradioactive metals or metalloids and soils contaminated with mixed wastes by methods which also permit the reclamation of residual soil products. BACKGROUND OF THE INVENTION As a result of military testing programs involving the detonation of nuclear devices, both in the United States and abroad, the environment, and particularly vast areas of soil in testing zones have become contaminated with nuclear waste materials. In some instances, for example, detonation of a nuclear device failed to achieve the needed critical mass of the radioactive components, resulting in substantial quantities of enriched uranium and plutonium being scattered over wide areas of desert testing grounds. In addition to nuclear testing programs, contamination of soil with radioactive materials has occurred at nuclear weapon manufacturing sites, such as at Hanford, Wash.; Rocky Flats, Colo.; Savannah River, Ga.; Oak Ridge, Tenn., and elsewhere through spills or releases into the environment. Efforts to successfully decontaminate these sites have proven difficult and extremely costly due to massive amounts of soil requiring treatment and/or storage. Cleanup has usually meant a slow and costly process where the contaminated soil is excavated and transferred to a different location for storage. Abandoned salt mines and mountain repositories have been proposed as storage facilities for nuclear wastes, but too often rejected later on for technical and/or political reasons. Because of a finite amount of space available for storage of nuclear waste materials progress in the reclamation of contaminated sites has been slow. In an effort to mitigate the nuclear waste storage crisis systems for reducing bulk quantities of contaminated soil requiring storage have been proposed wherein the radioactive components are concentrated in a soil fraction. One system, for example, employs an aqueous washing process requiring the use of soil scrubbing chemicals, multiple separation steps, water treatment, and so on. Although quite effective in concentrating radioactive components in silt and clay fractions of soil, capital and operating costs per ton of soil treated are viewed as economically unattractive. Consequently, most methods proposed for concentrating nuclear waste have not received wide acceptance. Like nuclear wastes, environmental contamination by metal ions, especially when present in soil or groundwater, poses serious threats to human, animal and plant life. Metals such as lead, chromium, cadmium and arsenic have been released into the environment in quantities which make large-scale remediation projects necessary in order to protect the health of the general public. Such metals represent some of the more difficult environmental contaminants to treat because they form salts or oxides, which in turn dissociate into ionized species facilitating their introduction into the food and biological chain. Accordingly, there is need for an innovative, cost-effective process for decontaminating soils containing nuclear waste materials, such as those generated at sites of nuclear weapon plants, nuclear testing sites, and wherever treatment calls for managing substantial volumes of soil contaminated with radioactive materials. The process should enable reduction of the space otherwise required for storage of untreated soils by concentrating in a small fraction of the soil while also permitting reclamation of these sites. Likewise, a cost effective process is needed for decontaminating soils containing ions of hazardous nonradioactive metals and metalloids, such as mercury, arsenic, selenium, chromium, lead, etc., and mixed wastes-containing such hazardous ions together with nuclear wastes like radionuclides of the actinide series, and/or organic compounds like PCBs. SUMMARY OF THE INVENTION It is therefore a principal object of the invention to provide improved, more economic methods for separating radioactive and non-radioactive components from contaminated soil wherein the treated soil is made sufficiently free of the potentially toxic components, i.e., metals and metalloids as to permit reclamation of the soil. The expression "sufficiently free" is intended to mean soil treated according to the present invention so it (I) is practically devoid of all unwanted radioisotopes (radionuclides), or (ii) contains residual amounts of low-level radioisotopes allowing treated soil to be reclaimed as is, or (iii) contains amounts of low-level radioisotopes which can be diluted sufficiently with an inert material to reduce its activity to an acceptable level. Expressions, such as "nuclear waste" and "radioactive waste" as recited in the specification and claims are intended to refer to soils contaminated with isotopic forms of elements having unstable nuclei which disintegrate and emit energy most commonly as alpha particles, beta particles and gamma rays. They include mainly products or by-products of nuclear fission or unreacted products of a nuclear device. Representative examples include such radionuclides as Cs.sup.137 ; Co.sup.60 ; K.sup.40 ; Pu.sup.236 ; U.sup.235 ; U.sup.238 ; Ru.sup.103 ; Te; Sr.sup.90 ; Rb; Y; Re; Rh; Pd; Tc; Np and Am. Methods of the invention provide for the recovery of nuclear waste materials in soil fractions, particularly in small, high surface area particles, such as soil fines and silt fractions of clay for subsequent storage or further treatment. By concentrating nuclear waste materials in soil fines and clay silt, for example, storage space requirements per ton of soil treated are significantly reduced, perhaps by as much as 90 percent over storage space requirements otherwise required for untreated soils. Methods of the invention comprise the steps of: (a) mixing a liquid ammonia or ammoniacal liquid with a soil contaminated with nuclear waste in a closed vessel to form an ammonia-nuclear waste containing soil dispersion or slurry; PA1 (b) allowing soil particles to selectively precipitate from the slurry or dispersion of step (a) to form a lower solid phase of soil particulates while forming an upper liquid-solid phase comprising soil fines dispersed in the liquid ammonia; PA1 (c) separating the upper liquid-solid phase from the lower solid phase of soil particulates, the fines of the upper liquid-solid phase having the majority of the radionuclide contaminant(s), or in other words, the lower solid phase is sufficiently free of the nuclear waste materials for reclamation of the soil particulates, and PA1 (d) separating the ammoniacal liquid from the soil fines containing the nuclear waste material for disposal or further treatment of the fines. PA1 (a) mixing a liquid ammonia or ammoniacal liquid with soil contaminated with nuclear waste in a closed vessel to form an ammonia-nuclear waste-containing soil dispersion or slurry; PA1 (b) treating the dispersion or slurry of step (a) with solvated electrons by contacting with a reactive metal; PA1 (c) allowing soil particles to selectively precipitate from the dispersion or slurry of step (b) to form a lower phase of soil particulates while forming an upper liquid-solid phase comprising soil fines suspended in the liquid ammonia; PA1 (d) separating the upper liquid-solid phase from the lower phase of soil particulates, the lower phase of soil particulates being sufficiently free of nuclear waste, and PA1 (e) separating the ammonia from the soil fines for disposal or further treatment of the fines. PA1 (a) mixing in a closed vessel an ammoniacal liquid with a soil contaminated with at least one ion of a hazardous metal or metalloid to form a dispersion or slurry; PA1 (b) separating an ammoniacal liquid-containing product from the dispersion or slurry of step (a) to yield a soil residue sufficiently free of ions of said hazardous metal or metalloid to permit reclamation, and PA1 (c) separating the ammoniacal liquid from the ammoniacal liquid-containing product of step (b) to yield a hazardous metal or metalloid-containing residue for disposal or further treatment. PA1 (a) mixing in a closed vessel an ammoniacal liquid with a soil contaminated with at least one ion of a hazardous metal or metalloid to form a dispersion or slurry; PA1 (b) allowing soil particles to selectively precipitate from the dispersion or slurry of step (a) to provide a lower phase comprising a precipitate of soil particulates while forming an upper liquid-solid phase comprising soil fines dispersed in said ammoniacal liquid; PA1 (c) separating the upper liquid-solid phase from the lower phase, the precipitate of soil particulates of the lower phase being sufficiently free of ions of the hazardous metal or metalloid to permit reclamation of said soil particulates, and PA1 (d) separating the ammoniacal liquid of the upper liquid-solid phase to yield a residue comprising the hazardous metal or metalloid for disposal or further treatment. PA1 (a) mixing in a closed vessel an ammoniacal liquid with a soil contaminated with at least one ion of a hazardous metal or metalloid to form a dispersion or slurry; PA1 (b) treating the dispersion or slurry of step (a) with solvated electrons by contacting with a reactive metal selected from the group consisting of an alkali metal, alkaline earth metal and aluminum; PA1 (c) separating an ammoniacal liquid-containing product from the dispersion or slurry of step (b) to yield a soil residue sufficiently free of ions of the hazardous metal or metalloid to permit reclamation of the soil, and PA1 (d) separating the ammoniacal liquid from the ammoniacal liquid-containing product of step (c) to yield a hazardous metal or metalloid-containing residue for disposal or further treatment. The term "disposal" is intended to include storage of the nuclear waste-containing soil fines. The expression "further treatment" is intended to include any procedure which will modify the potentially toxic properties of the radionuclide material to substances of reduced toxicity and impact on the environment, or to materials which can be recovered as useful by-products. It will be understood, methods of storage and further treatment of the concentrated nuclear waste material do not constitute part of this invention. Such methods are known by persons skilled in the art. Mazur et al in U.S. Pat. No. 5,110,364 disclose ammonia as a pretreatment in desorbing organic compounds, and particularly halogenated organic compounds like PCBs from soil, followed by chemical destruction of the compound by dehalogenation through a chemical reduction mechanism with solvated electrons. Mazur et al, however, fail to teach or suggest utilizing ammonia as a means of separating soil into fractions wherein the larger, lower surface area particulates are allowed to separate out from the less dense liquid ammonia-solid phase containing the smaller, higher surface area soil fines. In contradistinction, the methods of Mazur et al provide for treating "whole" soil in the reduction of the halogenated carbon compound contaminants without isolating soil particles or soil fractions from ammonia/soil slurries by allowing phase separation to occur and performing various separation steps. Serendipitously, it was found that radionuclides appear to have a preferential affinity for the smaller, higher surface area fines and silts of soils, clays and sand. Hence, by isolating the fines and silt particulates, especially the smaller particles having higher surface areas relative to the particles precipitating out of ammonia-soil dispersions one, in effect, is selectively concentrating the nuclear waste material in the smallest volume of natural solid carrier material to effectively lessen the tonnage volume of material requiring storage or further treatment. Accordingly, it is a primary objective of the invention to provide an improved more economic method for concentrating a substantial portion of the nuclear waste material in a reduced soil fraction for more efficient management of soil cleanup projects involving large volumes of soil, so as to permit reclamation of major volumes of previously contaminated soil. It is still a further object of the invention to optionally include the step of recovering and recycling for reuse in the foregoing process ammonia from step (d), the recovery and recycling being performed by methods already known in the art. For purposes of this invention, the expressions "liquid ammonia" and "ammoniacal liquid" as used herein are generally intended to include nitrogen-containing solvents, such as liquid ammonia. This would include anhydrous liquid ammonia and solutions of ammonia comprising small amounts of water. However, when used in dissolving metal reactions in forming solvated electrons, as will be discussed in greater detail below, the ammoniacal liquid is preferably non-aqueous. In addition to liquid ammonia, other nitrogen-containing solvents and co-solvents can be employed which are inert in the presence primary amines, secondary amines, tertiary amines, and of solvated electrons. Representative classes include mixtures of such amines. Examples of such amines include alkyl amines, like methyl amine, ethyl amine, dimethyl amine, triethyl amine, n-propyl amine, isopropyl amine, PYRROLIDINE and other nitrogen-containing solvents and co-solvents which are suitably inert in the presence of electrons. It is still a further object to provide an additional embodiment of the invention for decontaminating soil containing nuclear waste by the steps of: While it has been observed that ammonia has a unique ability to form very fine slurries when mixed with soils, it was observed that dispersions of soil appear to be further altered by some mechanism not fully understood, when in the presence of solvated electrons formed in dissolving metal reactions with ammonia. That is, by contacting the ammoniated soil dispersion with either an alkali or alkaline earth metal, solvated electrons are formed in the mixture, in-situ. The solvated electrons appear in some instances to optimize separation of smaller soil fines. In some instances where particle size cross-section is larger than desired, electrons solvated in liquid ammonia appear to provide more optimal demarcation and separation of the smaller fines containing nuclear waste materials from other particles of the slurry. As in the first embodiment of the invention, the foregoing second embodiment of the invention contemplates the step of recovering and recycling the ammonia from step (e) for reuse. Similarly, the precipitated residual solid soil particles of step (d) are "sufficiently free" of radioisotopes to permit reclamation of large bulk volumes of soil. In accordance with the invention, it was also discovered the foregoing process with ammoniacal solutions, etc., is also useful in decontaminating soils containing hazardous, but nonradioactive metals by the steps of: While not wishing to be held to any specific mechanism of action involved in separating ions of hazardous metals and metalloids from soils it has been observed the target material is frequently soluble in the ammoniacal liquid. In this regard, co-ordination compounds may form in the soil washing process with ammonia, and possibly form metal-ammonia ligand complexes. Representative metals of such coordination compounds and complexes may include those from the group of arsenic, antimony, selenium, cadmium, cobalt, mercury, chromium, lead and mixtures thereof. Co-ordination compounds can also be prepared by introducing other ligand complexing agents into the ammonia-soil slurry-containing hazardous metals. Such metals can be removed by forming, for example, ammonia soluble metal cyanide ligand complexes by adding a source of cyanide ions, e.g., sodium cyanide, ammonia cyanide, etc., to the slurry. Removal of the ammoniacal liquid results in the elimination of the hazardous metal from the soil fraction. As a further embodiment of the invention soils containing hazardous nonradioactive metals may be decontaminated by the steps of: It is yet a further aspect of the invention to provide a method of treating soils contaminated with mixed wastes, wherein the waste may be comprised of an ion of a hazardous non-radioactive metal or metalloid and a nuclear waste, for example. Typically, the nuclear waste is comprised of a radionuclide or radioactive isotopic metal. They are generally intended to include metals of the actinide series, such as uranium, plutonium, thorium and mixtures of the same. As a further embodiment of the invention, soil contaminated with hazardous nonradioactive metals may be decontaminated with ammoniacal liquids and solvated electrons by the steps of: The method of step (b) may be performed by circulating at least portion of the ammoniacal liquid through a by-pass containing the reactive metal. The solution of solvated electrons is recirculated back to the closed vessel for treating the contaminated soil. This aspect of the invention also contemplates treatment of soils contaminated with mixed wastes, i.e., an ion of a hazardous nonradioactive metal or metalloid and a nuclear waste, for example. Typically, the nuclear waste comprises a radionuclide or a radioactive isotopic metal of the actinide series, such as uranium, plutonium, thorium and mixtures of the same. The invention is also intended to include mixed wastes comprising an ion of a hazardous non-radioactive metal or metalloid and an organic compound, and more particularly, a halogenated organic compound, such as PCBs, dioxins and pesticides.