Patent Number: 056407017
Section: description

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to methods for treating various types of particulate materials, and especially soil, which are contaminated with soluble radioactive species. While this process will be described primarily for removal of radioactive material, such as uranium, radium, cesium, cobalt, strontium, americium, thorium, plutonium, cerium, rubidium, and mixtures thereof, and the like, it also encompasses removal of other hazardous species such as copper, lead, or mercury in soluble form. This method can also be used to treat sludge, sediments, scrap yard dust, and the like. As used herein, the term "soil" includes all forms of particulate matter to which contaminates may adhere, such as, for example, gravel, sands, clay, fines, sand, rock, humus, etc. As used herein, the phrase "desirable organic material" includes all forms of organic matter which provides nutrients to the soil to promote plant growth, such as, for example, humus, humic acid, etc. It is common for radioactive contamination to be present in a particular fraction or fractions of the soil in soluble form. For example, soluble cationic contaminants will exchange onto the negatively charged clay fraction of the soils. The soluble contamination is also likely to adsorb onto the humus fraction of the soil, and to be present in fine vegetation. Although the method of the invention may be applied to soil particles of any size and having any composition, the method of the invention ideally is applied to pretreated portions of the contaminated soil, or contaminated process streams (i.e., extraction solutions used in removing contaminants from contaminated soil) containing small to fine soil particles (say less than about 1000 micrometers, preferably less than about 100 micrometers), clay and silt particles, organic matter like humus, and fine vegetation including root hairs and the like. In a first embodiment of the invention, the material to be treated is excavated soil. Initially, the excavated soil is processed to remove large objects such as pieces of wood, vegetation, concrete, rocks and other debris, having diameters larger than about 150 mm (about 6 inches). Large objects may be removed by filtering the excavated soil through a sieve or a screen. These larger objects can be checked for contamination, and if necessary, washed with the contaminant extracting solution, rinsed with water, checked for residual contaminants, and returned to the site as a portion of the recovered soil. Alternatively, the large objects may be crushed and added to the smaller sized, contaminated soil. The soil then may be processed in a mechanical size separator, such as for instance a rotating drum or vibrating screen device, to sort and prewash the feed soil with a contaminant extracting solution. The intermediate to smaller soil particles and contaminated effluent can then be treated/separated in any number of ways. For example, the intermediate particles may be separated from the smaller particles and the fines using a screen, or sieve, or other size separation techniques. The intermediate pieces of soil then may be washed with the contaminant extracting solution, rinsed with water, checked for residual contaminants, and returned to the site as recovered soil. Alternatively, the intermediate to smaller soil particles and effluent can be processed in a countercurrent flow size separator such as a mineral jig, abraded in an attrition scrubber which dislodges mineral slime or fines from them, and then rinsed in a second concurrent flow size separator. Suitable soil pretreatment methods are described in U.S. Pat. Nos. 5,045,240, issued Sep. 3, 1991 in the name of Skriba et al., 5,128,068, issued Jul. 7, 1992 in the name of Lahoda et al., U.S. patent application Ser. No. 648,673, filed Jan. 31, 1991, in the name of Lahoda et al., and U.S. patent application Ser. No. 722,458, filed Jun. 27, 1991, in the name of Grant et al., the disclosures of which are incorporated herein in their entirety. Next, the soil cleansed by the pretreatment process (preferably containing intermediate to small particles) and the contaminated effluent are separated. The cleansed soil undergoes subsequent washing with clean extracting agent and/or water to remove as much of the contaminated extraction fluid as possible, and then may be checked again for contamination. The radioactive contaminants, smaller soil particles (say less than about 100 micrometers) and fines, clay and silt particles, fine vegetation, and the soluble components of the soil are generally carried off with the effluent, and will be treated using the novel methods of the invention. The soil (typically a slurry mixture as described above) is mixed with an aqueous extracting solution which will transfer the radioactive contaminants to the extracting solution, either as particles or as a solute. The solution used to wash the soil will be dependent upon the contamination to be removed. For soluble contaminants, the solution will contain an extracting (i.e., leaching) agent. Many suitable extracting agents are known and common extracting agents suitable for leaching radioactive compounds include, for example, potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate, sodium chloride, acetic acid, sodium hypochloride, ammonium carbonate, ammonium bicarbonate, and others. One preferred extracting solution of the invention comprises a mixture of potassium carbonate and sodium carbonate. Another preferred extracting solution comprises ammonium carbonate. Depending upon the pH of the extracting solution, suitable carbonate extracting agents exist in bicarbonate form. Accordingly, as used herein, the term "carbonate" includes bicarbonate forms of the extracting agents. Carbonates of sodium and potassium are preferred over ammonium when the introduction of an unnatural cation (i.e., a cation that is not native to the soil) like ammonium may not be permitted. Aqueous solutions of the preferred compositions effectively remove uranium and surprisingly, even radium, to environmentally acceptable levels. For example, radium levels of between 5 and 15 picocuries per gram of soil may be achieved, depending upon the depth of the soil. The extracting solution should have a pH and be added in an amount sufficient to solubilize, disperse, and/or mobilize at least about 10%, preferably at least about 20%, more preferably at least about 30%, even more preferably at least about 40%, and most preferably at least about 50% by weight, of said contaminate into solution. Accordingly, depending upon the properties and make-up of the soil to be treated, the extracting solution should have a pH greater than or equal to about 7.5, preferably greater than or equal to about 8, more preferably greater than or equal to about 8.5, even more preferably greater than or equal to about 9, and most preferably greater than or equal to about 9.5. The extracting solution also can have a pH greater than or equal to about 10. As indicated in FIG. 1, the pH of the extracting solution can be adjusted to achieve the desired amount of contaminant removal. For solutions of sodium and potassium carbonate, or ammonium carbonate, the concentration of the solution should be about 0.001M or greater, preferably between about 0.01 and 0.02M. Fine vegetation, and especially root hairs, adsorb unacceptable levels of contamination which is not readily solubilized, dispersed and/or mobilized by extracting solution. The methods of the invention require separating this contaminated fraction from the contaminated soil and effluent after treatment with the extracting solution. Separation may be accomplished using any method known in the art. In one preferred embodiment, the fine vegetation is floated and/or fluidized from the soil, and then gathered using any suitable means, such as, for example, a vibrating screen. Once the radioactive species are sufficiently solubilized or dispersed into solution, the pH of the extraction solution then is lowered by the introduction of an acid. The acid is added in an amount sufficient to lower the pH of the extracting solution, and preferably to remove substantially all organic material from the extracting solution. It has been found that by lowering the pH of the extracting solution to less than or equal to about 10, preferably less than or equal to about 9, more preferably less than or equal to about 8, and even more preferably less than or equal to about 7, organic matter in general, and humus in particular, are substantially removed from the extracting solution by precipitation and/or coagulation, without substantially precipitating the contaminant. For reasons explained above, it is essential that the pH of the extracting solution be reduced prior to separation of the extracting solution from the washed soil. It has been found that by using the methods of the invention less than 500 ppm of total organic carbon remains in the extracting solution, preferably less than 350, more preferably less than 250, even more preferably less than 150 ppm, and most preferably less than 100 ppm. Accordingly, the acid can be added in an amount to remove substantially all organic material from the extracting solution. Acceptable acids include one or more mineral acids selected from the group consisting of hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, perchloric acid, carbonic acid, and mixtures thereof. Hydrochloric acid is especially preferred. Suitable acid concentrations can be readily determined by one skilled in the art. Highly concentrated acids are preferred. In the next step of the invention, the soil is separated from the extracting solution using any suitable method known in the art. Preferably, in this step the soil is treated with a flocculent and/or a coagulant to precipitate or coagulate substantially all of the desirable organic material and the soil particles. Suitable flocculents and coagulants include for example, MAGNIFLOC 950N, supplied by American Cyanamid, of Wayne, N.J. Then the soil, including any precipitate and coagulant, are separated from the extracting solution using any appropriate means, such as, for example, filtration. The extracted, washed soil should be rinsed with clean water to remove substantially all residual traces of contaminant. For the soil remediation process to be cost effective, the solubilized, dispersed contaminant must be removed from the severely contaminated extracting solution to allow the extraction solution to be recycled. Accordingly, in the next step the contaminated extracting solution is cleaned, whereupon part or all of it is re-used. Where the contaminants include radioactive compounds or heavy metals, the severely contaminated solution can be passed through an ion exchange bed to remove the soluble metals. This type of procedure is well known. Ion exchange beads or the like, usually synthetic organic polymers or natural zeolite particles, having diameters over about 300 micrometers (30 U.S. Screen No. Sieve Size), well known to attract the contaminants present, would attract and remove most of the solubilized radioactive contaminants. As FIG. 3 shows, the presence of organics generated at a pH of 9.5 reduces the capacity of the ion exchange resin to remove radioactive contaminants from the extraction solution, while at pH 8 the organics are sufficiently insoluble so as to not adversely affect the ion exchange process. Depending upon the extraction solution and the form of contaminant, an anionic or cationic material may be used. Useful ion exchange materials include a strong acid cationic resin containing sulfonic functional groups with a styrene copolymer, and the like, for radium; and a strong base anionic resin containing quaternary ammonium functional groups with a styrene or styrene divinylbenzene copolymer, and the like, for uranium and thorium. In place of an ion exchange column, a precipitator could be used as an ion removal apparatus. For example, the solution could be mixed with ferric hydroxide, barium sulfate, or the like, to precipitate or co-precipitate radium or thorium, or with hydroxide to precipitate thorium or uranium, or with peroxide to precipitate uranium. Other ion exchange or precipitation materials could be used depending on the hazardous or radioactive material involved. For example, other adsorption media such as zeolites or treated clays may also be used to remove the contaminants. The ability to accomplish soil remediation using the methods of the invention is demonstrated in the following example. EXAMPLE Soil containing unacceptable levels of uranium and radium was washed using a 0.2M ammonium bicarbonate solution at a pH between 8.5 and 9.5. The excavated soil, which contained up to 40 weight percent clay, was contacted with the extractant for up to 1 minute. Up to 60 weight percent of the contamination was removed by the extractant. Root hairs, which were found to contain high levels of insoluble contamination (up to 400 ppm uranium and 20 pCi/g radium), were segregated from the soil/extracting solution mixture. The pH of the solution was then lowered to less than pH 8 using concentrated HCl. The clean soil was separated from the contaminated extractant by settling, filtration, and rinsing. The extractant solution was then successfully treated by ion exchange, and reused. The clean soil was capable of supporting plant growth. From the above, it can be seen that the invention provides a simple, yet highly effective method for treating soil contaminated with radioactive species. The method of the invention can be carried out on-site or off-site using any of the soil cleaning methods described herein. The invention having now been fully described, it should be understood that it may be embodied in other specific forms or variations without departing from its spirit or essential characteristics. Accordingly, the embodiments described above are to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.