Patent Number: 056407017
Section: summary

BACKGROUND OF THE INVENTION This invention relates to a method for remediating soil contaminated with radioactive species. This invention further relates to a method for remediating uranium and radium contaminated soil by selectively removing the radioactive contamination without removing desirable organic material which enriches the soil for subsequent plant growth. The method of the invention results in a leachate solution which is amenable to further treatment and reuse. The contamination of soils, ores or other materials with radioactive species, such as uranium, radium, and thorium, is a common environmental problem. In a great number of cases, the radioactive contamination is distributed throughout various fractions routinely found in soil, including gravel, sands, clays and silt, vegetation, organic matter, and groundwater. Radioactive contaminants may be found in mining sites, radioactive processing facilities, and even residential areas. In all cases it is necessary to remove or clean up the soil or other material. Two approaches typically are used to treat contaminated soil. The first approach involves methods for preventing or restricting the dispersion of the contamination to the immediate surroundings. However, the application of such methods is problematic, since any breakdown is likely to result in widespread leakage of contaminants to adjacent land and water areas by groundwater, and can have a deleterious effect on flora, fauna and humans. The second approach involves methods for removing or destroying contamination from soil, also referred to as "cleaning" or "remediating" the soil. One suitable remediation technique involves excavating the contaminated soil and reburying it in another area. But, the practice of digging the soil up in one area only to bury it in another, is expensive and becoming less environmentally acceptable. Other techniques have been developed for remediation on-site or elsewhere. For example, U.S. Pat. No. 4,783,253, issued to Ayres et al., describes a method for treating water insoluble contamination which uses a concurrent flow of water to float away lighter uncontaminated particles from heavier contaminated particles. The slurry of particles is dewatered using a spiral classifier, centrifuge, filter or the like. Heavy metal contaminants, including radioactive species, can also be removed from soil by size separation techniques, such as tiltable tables, or concurrent flow size separation in a mineral jig. Size separation and leaching techniques are described in U.S. Pat. No. 5,045,240, issued to Skriba et al., U.S. Pat. No. 5,128,068, issued to Lahoda et al., and U.S. patent application Ser. No. 07/722,458, filed Jun. 27, 1991, in the name of Grant, et al. Leaching methods and soil washing methods are also used to remove radioactive species from soils. The use of aqueous extracting solutions to treat contaminated soils is described in "Extractive Methods For Soil Decontamination; A General Survey and Review of Operational Treatment Installations," J. W. Assink, in Contaminated Soil, Edited by J. W. Assink, and W. J. Van Den Brink, pp. 655-667, 1986, Martinus Nijhoff Publishers. Such solutions usually contain only water. Alternatively, they can contain bases like sodium carbonate or sodium hydroxide, or strong mineral acids like hydrochloric acid and nitric acid, to aid in the extraction process. After soil treatment, the contaminated extracting fluids can be cleaned with any known physical, chemical or biological purification methods. For instance, coagulation, flocculation followed by sedimentation, or flotation may be used. Other suitable techniques include ion exchange, aerobic and anaerobic biological purification, electrolysis, and membrane filtration. There are several limitations associated with the above mentioned techniques. Each of the methods described above strips the soil of valuable organic material like humus which is crucial for subsequent plant growth. Moreover, depending upon the treatment process used, the presence of soluble organics in the extracting solution may interfere with the subsequent removal of the radioactive contaminant from solution. In particular, the presence of organic matter reduces the capacity of ion exchange resins which typically are used to remove radioactive species from the extracting solution. In addition, such methods do not address treatment of fine vegetation, such as root hairs, which also are likely to be contaminated. Another disadvantage of the above mentioned remediation methods is that they are not suitable for all types of soil. Loamy soil, clay, and peat sites are generally difficult to clean by extraction. Humus-like components, silt, and clay particles readily form relatively stable suspensions with the extraction liquid. This is especially true for aqueous extracting solutions having a high pH. If the contaminants are present in the extracting solution as separate small particles, it is often impossible to separate relatively clean soil particles from the contaminated particles and effluent. What is needed is a simplified method of treating large volumes of materials of different types, and containing soluble portions of hazardous or radioactive waste, so as to segregate the clean from the contaminated material and to concentrate the contaminated material. In particular, the method should successfully remove the radioactive contaminants from different types of soil without removing desirable materials. There is a further need for a system that can effectively recover the contaminants once they have been removed from the soil, requiring a minimal amount of equipment, chemicals, and which further allows for the processing of recovered contaminants, such as metals, or other salable minerals. SUMMARY OF THE INVENTION Accordingly, the present invention resides in a method for treating material contaminated with soluble radioactive species comprising the steps of: a. providing contaminated soil; PA1 b. introducing into said soil an aqueous extracting solution, said extracting solution having a pH greater than or equal to about 7.5; PA1 c. segregating fine vegetation from said soil and extracting solution; PA1 d. introducing into said soil an acid in an amount sufficient to lower the pH of the extracting solution; PA1 e. separating said soil from said extracting solution; and PA1 f. removing said radioactive species from said extracting solution. Generally, to meet the required remediation levels for radioactive contaminants like uranium and radium, it is necessary that the optimum extraction conditions be used. As FIG. 1 illustrates, increasing the pH of the extracting solution results in increased solubility of radioactive contaminants, in this case uranium, into the extracting solution (which is desirable). Thus, to maximize contaminant removal from contaminated soil, the pH of the extractant must be maintained relatively high. However, as illustrated in FIG. 2, as the pH of the extracting solution is increased to enhance contaminant removal, the solubilization and mobilization of natural organic (i.e., carbon based) materials like humus also increases (which is not desirable). The novel method of the present invention selectively targets harmful, radioactive species for solubilization, dispersion, mobilization and removal from the soil, without concomitant removal of desirable organic material, such as humus and the like. Moreover, by using the novel methods of the invention, removal of contaminant from the extracting solution by ion exchange means occurs relatively unimpeded, that is, without any interference from humus and other organic materials, thereby permitting the extracting solution to be recycled. This goal is achieved by the methods of the invention which are characterized by introducing into the contaminated soil an aqueous extracting solution, wherein the extracting solution has a pH greater than or equal to about 7.5, and is added in an amount sufficient to solubilize, mobilize, or disperse the radioactive species into solution. This step is followed by introducing into the soil an acid in an amount sufficient to lower the pH of the extracting solution, and to remove substantially all organic material from the extracting solution. The addition of acid results in the coagulation and/or precipitation of desirable organic material, including humus, thereby removing it from the extracting solution without precipitating the radioactive species, which remain substantially in solution. In carrying out the method of the invention, it is essential that the reduction in pH of the extracting solution be carried out prior to separation of the cleansed fine soil components (such as clays and silts) from the contaminated extractant. It has been found that the soil particles provide necessary adsorption sites which aid in the coagulation/precipitation of the organic matter. In a preferred embodiment, this method uses a unique aqueous extracting solution comprising both sodium carbonate and potassium carbonate, or alternatively ammonium carbonate, which results in soil that readily settles, thereby overcoming the dispersion problems associated with other processes. Thus, using the method of the present invention, relatively clean soil particles are easily separated from the contaminated extraction solution. In addition, the use of ammonium carbonate results in added, readily available, nitrogen to the soil which in turn enhances the soil's capability to support new plant growth. Another novel aspect of the invention is the treatment of contaminated soil in the presence of fine vegetation, like root hairs, which can contain unacceptable levels of contamination. Such fine vegetation contains contamination which is not readily solubilized or mobilized by an extracting solution. The method of the invention provides a step for separation and removal of the fine vegetation from the soil. Thus, the methods of the invention provide systems which can be applied on-site or off-site, and which are capable of handling major amounts of soil or other material containing portions of hazardous or radioactive species, in an economical and energy efficient fashion. The methods of the invention effectively remediate contaminated soil in the presence of clay and silt particles, organic matter, and fine vegetation. Moreover, the methods of the invention result in enrichment of the cleansed soil to promote future plant growth, and the cleansing and recycling of extractant solutions.