William S. Richardson S. Cohen and Associates/Auburn University Montgomery. Charles R. Phillips and John Mauro S. Cohen & Associates

Transcription

1 STUDY ON THE FEASIBILITY OF REMOVING 137 Cs FROM BIKINI ISLAND SOILS ABSTRACT William S. Richardson S. Cohen and Associates/Auburn University Montgomery Charles R. Phillips and John Mauro S. Cohen & Associates Russell S. Yost University of Hawaii Bikini Island soils are contaminated with 137 Cs from fallout during weapons tests. It was introduced into the soil either by direct deposition or as a result of rainfall-associated washout and deposition and presents a significant radiation exposure through plant uptake and subsequent appearance in the food chain. This study was to provide insight into the adsorption processes of cesium on the surface soils in order to assess the potential for reducing the availability of 137 Cs to plants grown in Bikini Island soils. Specifically, the goal was to reduce the concentration of 137 Cs by thirty-fold, equivalent to removing 97 percent of its activity from the soils. The study plan consisted of four experiments: (1) determination of the fraction of 137 Cs, associated with soil organic carbon, that is removable by acid pretreatment, typically used to remove cations that interfere with organic carbon extraction by base; (2) determination of extraction efficiency of acids and salts representing an acidic medium and/or ion-exchange potential; (3) determination of the extraction efficiency of bases and salts that provide a basic medium and/or exchange potential and might effectively remove 137 Cs associated with organic material without acid pretreatment; and (4) determination of 137 Cs activity by particle-size distribution, which might reveal an approach to remediation by particle-size separation. The following conclusions are drawn from this feasibility study: (1) no extracting solutions selected for Experiments 2 or 3 (acid, base, salt, or a combination of acid or base with salt) were successful in achieving a thirty-fold decrease in 137 Cs concentration (97 percent removal efficiency), even after a treatment period of 21 days. The maximum removal was 47 percent using 0.5M hydrochloric solution. It is not likely that any of the solutions would approach the extraction goal, even under optimal conditions and particularly under those conditions encountered in in-situ extraction technology (soil flushing); (2) The results of Experiment 2 also illustrates the exceptional reactivity of soil material with acid extractants -- addition of acid solutions readily dissolved the carbonate soil matrix; (3) During Experiment 1, selected to extract organic material with sodium hydroxide, most of the soil matrix was dissolved by the required addition of acid. The remaining solids represented less than nine percent of the original soil masses treated; the two aliquots examined had a 137 Cs concentration of 476 and 554 pci/g, both from soil aliquots with less than 70 pci/g. Very little 137 Cs was extracted by the acid or base solution: less than 15 percent of the total activity was found in the acid supernatant, and less than six percent was found in the base supernatant; (4) Most of the soil particles in the sample, approximately 72 percent, were collected in the +200mesh-size fractions during the performance of Experiment 4. The fraction containing the highest concentration of 137 Cs is the -200 fraction, 345 pci/g. It represents about 17 percent of the soil by weight but over 75 percent of the total cesium activity in the soil. Since the -8/+50 fraction represents 44 percent of the soil sample but the 137 Cs concentration is only 5 pci/g, removal of the remaining 66 percent could

2 leave a soil fraction with a concentration of only 5 pci/g. Mixing the remaining 44 percent with clean soil (soil with background concentrations of 137 Cs and with complement nutrient and particle characteristics) would dilute the effect of the 137 Cs concentration and might bring the concentration to less than 2.5 pci/g; (5) Particle-size separation indicated that most of the 137 Cs activity is associated with a fine black soil fraction, however, most is not extractable although acid solution dissolves over 90 percent of the soil matrix. Like the particle-size separation procedure, the procedure designed to extract organic material from soils left a fine black solid containing much of the radionuclide activity. INTRODUCTION The soils of Bikini Island are contaminated with several radionuclides that present a significant radiation exposure to the residents of the island. The major radionuclide of concern is 137 Cs because of its uptake by local plants and subsequent appearance in the food chain. Short-lived radionuclides from the fallout, such as 60 Co, have practically disappeared (1). Plutonium and americium radionuclides are largely trapped in lagoon sediments, and uptake into seafood is very low. Strontium-90 should be associated with calcium minerals such as calcium carbonate in the coral material of the soil. As a dipositive cation, any significant amount of strontium competing with monopositive cesium for binding sites in the soil should be preferentially bound to the sites, making the cesium relatively more soluble in extracting solutions than it would be in the absence of strontium. The concentration of strontium is so small, however, that the likelihood of direct electrostatic interactions with concomitant influence on the extraction efficacy are minimal or nonexistent. Several remediation strategies are being reviewed, therefore, to assess their potential for removal of 137 Cs from the surface soils or from the plant root zone. Chemical soil flushing or physical processes are options being evaluated. Cesium is one of the most electropositive elements in natural abundance, and the radioisotope 137 Cs, a fission product, is one of the by-products of early nuclear weapons testing. Its properties are consistent with those of the alkali metal group to which it belongs. These properties are critical to understanding the processes involved in the strong sorption of 137 Cs to soils and sediments (2). Among the alkali metals, cesium has the lowest charge-to-radius ratio and, correspondingly, the lowest hydration energy. Therefore, it is the most strongly adsorbed to soils (3). This relatively strong sorption seems to be correlated both with cation exchange capacity and a strong fixation component, apparently related to 137 Cs penetration into the crystal lattice. Ultimately, removal of 137 Cs from contaminated soils depends on knowledge of where and how the 137 Cs is sorbed (2,4). Developing an understanding of the physical and chemical form of 137 Cs in the Bikini Island soils is challenging because of the conditions under which it was initially deposited. Some of the 137 Cs found in the contaminated areas of the Marshall Islands likely became part of the soil matrix near the actual nuclear detonation site as a result of high temperature and pressure reactions. In this case, it might be associated with fundamental soil particles and thus be an integral part of the crystal structures of the soil. For areas outside the immediate detonation site, materials such as 137 Cs apparently were constituents of fallout and were introduced either by direct deposition or as a result of rainfall-associated washout and deposition. The ultimate type of association between 137 Cs and soils may be quite different, depending on which of these mechanisms led to the radionuclide accumulation (5). In the case of Bikini Island, the main accumulation of 137 Cs in surface soils apparently is a result of fallout from weapons tests. The sorption mechanism that retains 137 Cs in soils, but provides it in radiologically significant quantities for plant absorption and uptake, is not well understood. One purpose of this

3 feasibility study was to provide additional insight into the processes affecting the adsorption of Cs on the surface soils of Bikini Island in order to assess the potential for reducing the availability of 137 Cs to plants grown in Bikini Island soils. Associated with this objective is an evaluation of a subsequent method of cleanup that might be effective in reducing the food plant absorption of 137 Cs and, thereby, reducing the entry of 137 Cs into the human food chain. Specifically, the goal was to reduce the concentration of the radionuclide by thirty fold, which is equivalent to removing 97 percent of 137 Cs activity. The study plan consisted of four experiments conceived to evaluate the adsorption processes and assess the potential for treating the surface soils to remove 137 Cs: Experiment 1. Determination of the fraction of 137 Cs associated with soil organic carbon and removable by acid pretreatment used to remove cations that interfere with extraction and subsequent dissolution of the organic material with base. The hypothesis for experiment 1 is that the soil organic matter, because of its predominant role as the source of negative charge and its cation exchange capacity in atoll soils, will likely be the site of most 137 Cs sorption that takes place by exchange processes. If this hypothesis is true, sodium hydroxide extraction of the soil organic matter (humic and fulvic acid removal) will remove more than its weight proportion of 137 Cs activity. This vigorous extraction process would not likely be feasible on a large scale, but it might provide information about the sorption site location of 137 Cs. Experiment 2. Determination of extraction efficiency of acids and salts representing an acidic medium and/or ion-exchange potential. This experiment is based on the hypothesis that extractants that contain an acidic medium and/or ion-exchange potential will likely be the most efficient in extracting 137 Cs because the acidic medium will neutralize anionic (basic) soil sites and excessive concentrations of cations in the salts will tend to displace the cesium ion. Extractants were chosen following a review of the available literature on 137 Cs retention and removal from soil and on the nature of the Bikini Island soils. Only those extractants that were believed to be efficacious for 137 Cs removal and had other characteristics, which allow it to be used in a practical treatment scheme, were proposed. These characteristics include; (1) cost effectiveness, (2) nonhazardous nature, (3) capable of being applied in a manner consistent with soil flushing methodology, and (4) not rendering the soil incapable of supporting plant growth. Experiment 3. Determination of the extraction efficiency of bases and salts. The hypothesis for experiment 3 is that extractants providing a basic medium or combination of basic medium and exchange potential may be efficient in removing organic material and 137 Cs without pretreatment with acid used in experiment 1, which has the potential to dissolve a significant portion of the Bikini Island soil matrix. Experiment 4. Determination of 137 Cs activity by particle-size distribution. The hypothesis for experiment 4 is that the distribution of 137 Cs in a specific particle size range might reveal an approach to remediation of the Bikini island soil by particle-size separation. EXPERIMENTAL PROCEDURES Gamma Spectrometry Whole soil samples, particle-size fractions, and solutions were analyzed for gamma-emitting radionuclides using a high-purity germanium detector (6,7). Cesium-137 is measured from the gamma emitted from 137m Ba, the daughter of 137 Cs.

4 Soil Sample Preparation Soil samples were collected on Bikini Island at depths of 0-5 cm, 5-15 cm, and cm. The samples from each depth were thoroughly mixed and passed through a No. 8 sieve (2.38 mm). Two 400-mL aliquots were taken from each of the three depths. These aliquots were dried overnight at 60 C and gamma counted. The results of the gamma counts from a representative sample are listed below in Table I. Table I. Concentration of 137 Cs and Soil Organic Carbon in Bikini Island Soils Depth 137 Cs (pci/g-dry) Soil Organic Carbon (%) 0-5 cm 5-15 cm 15-25cm The experiments in this study were conducted on the three samples presented in Table I. The replicate was conducted on the 0-5 cm sample. Experiment 1. Determination of the Fraction of 137 Cs Associated with Soil Organic Carbon Although a considerable amount of the soil matrix was expected to be dissolved in the acid treatment in Experiment 1, the experiment was performed in an attempt to liberate the 137 Cs activity with extraction of the organic material present in the soil. The procedure was performed according to Swift (8) with modifications because of the exceptional reactivity of the sample with acid solutions: A 40-mL aliquot of soil was gamma counted for 137 Cs activity. Hydrochloric acid solutions were added to each aliquot until bubbling ceased. Initially, 1M HCl solutions were added, but when the large quantity of acid required became apparent, concentrated HCl was added to minimize the total volume of acid required for complete reaction. The final ph was less than one. The mixture was added to a 1-gal container, brought to approximately 600 ml with 0.1M HCl, and shaken for one hour at 350 rpm (9). At the end of the shaking period, the mixture was transferred to four 250-mL glass centrifuge bottles with a minimum amount of water. The bottles were balanced with water and centrifuged for ten minutes at approximately 1500 rpm (10). The supernatant was decanted and gamma counted. The solid was washed into a container with water and titrated using a ph meter to ph greater than 7 with 0.1M NaOH. One-tenth M NaOH solution was added to a total volume of approximately 600 ml. The mixture was shaken intermittently at 350 rpm for four hours (ten minutes after every 30 minutes and finally for 10 minutes) and allowed to stand overnight. The contents of the mixture were separated by centrifugation as described above, the supernatant was collected by decanting for gamma counting, and the solid was washed with water into containers for drying at 60 O C for 48hr. The dried solid was weighed, partially crushed to form small particles, and gamma counted. Experiment 2. Determination of the Extraction Efficiency of Acids and Salts Soil samples from Bikini Island were subjected to six different extractants (water, 1M KCl, 1M CaCl 2,

5 0.1M HCl, 0.5M HCl, and 0.1M HCl/1MKCl) for four periods of time (one-day, three-day, seven-day, and 21-day) and the 137 Cs removal was evaluated. The procedure employed in the extractant scheme is described below. The procedure is adapted from standard operating procedures of Sanford Cohen & Associates (9,10): A 50-gram aliquot of soil from one of the previously counted, 400 ml sample containers was placed in a plastic counting vial and gamma counted. Once the count was completed and verified, the soil was placed into a 1-gal wide-mouth container and 500 ml of the appropriate extractant was added. The sample was shaken at 350 rpm for 30 minutes (9). After the intermediate standing time period (one-day, three-day, seven-day, and 21-day), the sample was shaken for 5 minutes at 350 rpm and then transferred to four 250-mL centrifuge bottles with a minimum amount of fresh extractant solution. For standing periods longer than one day, the sample was shaken each day for 30 minutes at 350 rpm. The bottles were balanced with fresh extractant and centrifuged for ten minutes at approximately 1500 rpm (10). The supernatant was decanted and the solid was washed from each bottle with 30-mL of extraction solution and centrifuged again. The solid was transferred into the original extracting container, using fresh extractant to wash it from the bottle. Fresh extractant was added to the container to make the volume up to 500 ml, and the mixture was shaken for 30 minutes at 350 rpm. The spent extractant was counted in a 1-L Marinelli beaker on the gamma detector for 30 minutes. Experiment 3. Determination of the Extraction Efficiency of Bases and Salts Soil samples from Bikini Island were subjected to two different extractants (0.5M NaOH and 0.5M NaOH/1M KCl) for one day, and the 137 Cs removal was evaluated. The general procedure employed in the extractant scheme is described above in experiment 2, except that the soil samples were extracted only one time, and the supernatant and solid in the extracting mixture were collected like a 21-day sample described in Experiment 3. Experiment 4. Determination of 137 Cs Activity by Particle-Size Distribution A particle-size/ 137 Cs distribution was performed on a 0-5 cm soil-sample aliquot. All soils used in the experimental procedures represented -8-mesh (a) (<2.38 mm) material. The sample was vigorously washed before sieving. The vigorous washing process liberates small soil particles from large particles without generating excessive fines. The following procedure was used to washing and sieving: A 250-g aliquot of soil was placed in a 1-gal container and vigorously washed in water for 30 minutes at a rotational velocity of 350 rpm with a liquid-to-solid ratio of 4 ml/1g (9). After vigorous washing, the sample was fractionated by size using wet sieving with American Standard Testing of Materials (ASTM) standard sieves (11). The soil was separated into particle-size fractions at 50 mesh (0.297 mm), 100 mesh (0.149 mm), and 200 mesh, (0.074 mm). The resulting fractions were dried at 60 C for 48 hr, weighed, and analyzed for 137 Cs content by gamma spectrometry. Water from the vigorous wash and separation procedures for each sample was collected and a Percol 788N flocculant added to settle suspended material (11). The water was then

6 filtered through a 25-µ pore filter paper to separate suspended solids from the wash water. The material retained on the filter was analyzed by gamma spectrometry and represents the -200 activity. RESULTS AND DISCUSSION Three Bikini Island soil samples, representing depths of 0-5 cm, 5-15 cm, and cm, first were treated in Experiment 2 at room temperature with various extraction solutions to evaluate the potential for site remediation by in-situ extraction technology (soil flushing). The solutions contained an acid to lower ph and/or a salt to establish an ion-exchange potential. The goal was to reduce the 137 Cs activity by thirty fold, which would be achieved by removing 97 percent of the activity. The extractants were selected to provide protonating power to neutralize cationic binding sites (anionic groups) in the soil matrix and/or provide a high concentration of cation similar to Cs +1 to displace exchangeable cesium. Duplicate aliquots of the 0-5 cm soil sample, and one aliquot each of the 5-15 cm and cm soil samples were treated individually with either water, 1M potassium chloride, 1M calcium chloride, 0.1M hydrochloric acid, 0.5M hydrochloric acid, or a mixture of 0.1M hydrochloric acid and 1M potassium chloride. The extracting solutions were changed at one-day, three-day, and seven-day intervals and removed after 21 days.. The results of the experiment are summarized in Table II. The percent removal of 137 Cs activity after 21 days, averaged over all samples extracted by any solution, ranged from a low value of about 11 percent, using 1M calcium chloride as an extractant, to 47 percent with 0.5M hydrochloric acid. Data from samples extracted with water were based on three soil samples only, because one of the 0-5 cm samples was lost during the initial extraction procedure. The percent removed with water was 15 percent overall. One-molar potassium chloride extractions resulted in an average percent removal of 19 percent, producing a final concentration of about 72 pci/g for the 0-5 cm soil sample and its duplicate. One-molar calcium chloride removed an average of only 11 percent of the total activity of 137 Cs, but the average is that high because the percentage of extraction of the cm sample (21 percent) inconsistent with the other values. The average of the remaining three samples extracted with 1M calcium chloride is only 8 percent. One-tenth molar hydrochloric acid extracted about 19 percent of the activity of the soils; the final concentration of the 0-5 cm sample was 74 pci/g. The most successful extraction occurred with 0.5M hydrochloric acid. Overall, an average of 47 percent of the 137 Cs activity was eliminated from the four soil samples, but the final specific activities of the 0-5 cm sample and its duplicate were 55 and 50 pci/g, corresponding to 40 and 46 percent removal, respectively. The most successful extraction in this group was accomplished by the 0.5M hydrochloric acid solution on the 5-15-cm soil sample. Sixty-two percent of 137 Cs activity was removed, leaving a radionuclide concentration of 25 pci/g. The removal of activity by 0.5M hydrochloric acid was accompanied, however, with considerable dissolution of the soil sample as the coral component reacted with the strong acid solution, producing bubbles formed by carbon dioxide. The second most effective extractant was the mixture of 0.1M hydrochloric acid and 1M potassium chloride; approximately 32 percent of the activity was removed. The final 137 Cs concentration of the 0-5 cm sample was 54 pci/g. No specific trends were observed relative to the degree of removal as a function of contact time with the extractant. Although there were several exceptions for all the extractants for some time periods, the one-day extractions generally removed more activity than later extractions, but each subsequent contact period (3, 7, and 21 days) was almost as effective at extracting 137 Cs.

7 Table II. 137 Cs Removal Efficiency Using Acid and/or Salt Extractants Extractant Soil Depth (cm) Beginning 137 Cs (pci/g) Final 137 Cs (pci/g) % 137 Cs Removal (Range) Average % 137 Cs Removal Tap Water 1M KCl 1M CaCl 2 0.1M HCl 0.5M HCl 0-5 Dup (a) (-4-32) (b) (0-32) (-8-31) (2-25) 0-5 Dup (2-37) (-3-16) (-12-29) (-11-24) 0-5 Dup (-9-24) (-4-38) (5-23) 0-5 Dup (8-40) (3-36) (-8-32) (31-48) 0-5 Dup (30-59) (48-73) (17-59) M HCl (27-55) + 1M KCl 0-5 Dup (5-36) (9-44) (16-52) (a) The 0-5 cm sample was lost during the initial extraction. (b) Negative values indicate that the range of concentrations of 137 Cs within the 2-sigma error from gamma analysis produces a high value for the final concentration of 137 Cs greater than the low value of the initial concentration.

8 The results of Experiment 2 also illustrate the exceptional reactivity of soil material in the presence of acid extractants as well as the inefficiency of this class of extracting solutions for removing 137 Cs activity. On average, none of the solutions removed more than 50 percent of the radionuclide, but addition of acid solutions readily dissolved the solid material as the carbonate matrix reacted, producing carbon dioxide generated bubbles. Salt solutions exclusively, containing high concentrations of cations that would be expected to displace exchangeable cesium, were only slightly more effective extractants than water, indicating that only a small percentage of cesium was exchangeable in 21 days of extraction, considerably more time than typically required for exchange to take place in soils. Solutions containing acids, which could neutralize exchange sites (inorganic sites as well as acidic organic material) were more effective. Indeed, 0.5M hydrochloric acid was more effective than 0.1M acid, and the combined effect of 0.1M hydrochloric with 1M potassium chloride, providing both neutralizing and exchange potential, was almost twice as effective as 0.1M hydrochloric acid alone. The results with these extractants suggest the presence of sites that were rendered less active for binding cesium by neutralization. Both inorganic and organic soil components are candidates for these sites. Since there is no silicate clay material in the Bikini Island soil (12) but extraction of cesium is partially successful with acid solutions that can neutralize anionic groups, one likely candidate for cesium binding is organic material containing acidic functional groups. A greater fraction of these groups can be converted to a more water-soluble anionic form in the presence of bases at higher ph than is normally found in soil. Extraction of these organic constituents with a base solution by the standard method for removing humic substances form soils (8) might release cesium. Experiment 1 was proposed to extract the organic material with a sodium hydroxide solution, but pretreatment with acid to a very low ph (1-2) is required in this standard procedure. Considering the results of the acid-extraction experiments, dissolution of a considerable amount of the soil matrix was anticipated using the acid prescribed by this method. For that reason, Experiment 3 was performed to attempt the extraction directly with sodium hydroxide without acid pretreatment. In Experiment 3, two soil samples representing the 0-5 cm depth were extracted for one day with either 0.5M sodium hydroxide or a mixture of 0.5M sodium hydroxide and 1M potassium chloride. The results of these extractions are presented in Table III. For the 0-5 cm soil samples, treated with 0.5M sodium hydroxide, 26 percent of the 137 Cs activity was extracted, leaving a concentration of 51 pci/g. One-half molar sodium hydroxide with 1M potassium chloride removed 22 percent, leaving 54 pci/g. These results are coupled with the observation that soil samples treated with sodium hydroxide solutions changed from a gray-black color to an off-white color. Apparently, the base solution either dissolved organic material that imparted a black color to the extracting solution, dissolved some other black material, or a combination of both were dissolved to leave mainly a coral-appearing material. In any case, only about 25 percent of the cesium present was extracted, less than that removed by acid solutions.

9 Table III. Extractant 137 Cs Removal Efficiency Using Base or Base/Salt Extractants Extractant Soil Depth (cm) Beginning 137 Cs (pci/g) Final 137 Cs (pci/g) % 137 Cs Removal (Range) 0.5M NaOH 0.5M NaOH + 1M KCl (10-40) (15-36) Overall, no extracting solutions selected for Experiments 3 and 4 were successful in achieving a thirty-fold decrease in 137 Cs concentration (97 percent removal of the activity), even after treatment periods of 21 days. It does not appear likely that any of the solutions examined C acid, base, salt, or a combination of acid or base with salts C would approach the extraction goal, even under the optimal conditions of intimate contact with the soil matrix for extended periods of time employed in this experiment. Conditions encountered for site remediation by in-situ extraction technology (soil flushing) would likely be less successful than the laboratory experiments and can not be recommended for extraction using the solutions examined in this study. Treatment of the soil with more aggressive extractants using heap leaching or batch technology (ex-situ) to remove significant 137 Cs activity from the soil might be possible. However, these could render the soil unusable for agricultural purposes, and these treatment technologies are usually very elaborate, expensive, time consuming, and disruptive to normal activities. In many radionuclide-contaminated soils, the majority of radioactivity from metals is found in the smallsized particles, typically less than 200-mesh size [0.074 mm or 74 micron (µ)], that contain anionic sites for cation binding and provide a relatively large high sorption area because of a large surface area-to-mass ratio. In Experiment 4, a soil sample [-8-mesh-size (2.38 mm) after sample preparation] was wet sieved after vigorously washing the sample to remove loosely bound soil particles. Table IV presents the results of the experiment. The majority of the soil particles in the sample, approximately 72 percent, were collected in the +200 fraction (0.074 mm). These fractions have a cream-brown color with some black particles in the -100/+200 material ( mm/ mm). About 17 percent and 12 percent are in the fraction. Note that the fraction containing the highest concentration of 137 Cs is the -200 fraction, with a concentration of 345 pci/g. The 137 Cs in this fraction represents over 75 percent of the total activity of the radionuclide in the soil. Considering the 2-sigma counting uncertainty from the gamma analysis for 137 Cs, this fraction might represent almost 92 percent of the activity of the soil sample. Since the -8/+50 fraction (-2.38 mm/ mm) represents 44 percent of the soil sample, by weight, removal of the remaining 66 percent by volume-reduction separation processed could leave a soil fraction with a 137 Cs concentration of only 5 pci/g. However, 5 pci/g represents 93 percent removal of cesium (less than 97 percent) relative to the concentration of the radionuclide in the whole soil. Considering the specific activities and their associated uncertainty of the soil sample and the -8/+50 fraction (76 6 and 5 1, respectively), separation of the -8/+50 fraction represents 137 Cs removal ranging between 88 percent and 100 percent. Mixing the 44 percent remaining soil after volume reduction with clean soil (soil with background concentrations of 137 Cs and with complement nutrient and particle characteristics) would dilute the concentration of 137 Cs and might bring the concentration to less than 2.5 pci/g [(0.44 x 5 pci/g) + (0.66 x 0.1 pci/g) = 2.3 pci/g]. This process could represent a reasonable remediation step if, as reported

10 by Robinson and Stone (12), subsequent treatment of the soil with potassium salts suppresses cesium uptake by plants by a factor of ten. Evaluation of the soil to determine the precise percentage that could be recovered in a volume reduction process and the actual concentration of 137 Cs in the recovered fraction and the reconstituted soil would require more extensive testing. Volume reduction can be economically beneficial, however, if a competing remediation option includes disposal of the total soil, even when the disposal option is accompanied by correspondingly modest disposal and replacement costs (13). Since a major portion of the 137 Cs activity was found to be in a specific, small-sized particle fraction and base extraction to remove soil organic material by the procedure of Swift (8) was still an option to release, an extraction to remove organic material was performed on two aliquots of a soil sample. The experiment was preformed disregarding the potential for soil-matrix dissolution by the acid pretreatment. Addition of acid to bring the ph to between one and two, as prescribed in the procedure, consumed an unexpected volume of dilute hydrochloric acid solutions, so concentrated acid was added until the sample ceased to bubble. The ph of the mixture was then less than one. After acid addition was completed, only a small amount of solid material remained. It was collected by centrifugation, and the acid supernatant was counted. The remaining solid was a black, oily-appearing material. It was extracted with base, and the resulting supernatant liquid and solid were collected by centrifugation. The solid was dried, producing a very fine, somewhat hard, black material. An account of the 137 Cs activity recovered in the extracted fractions is summarized in Table V. Footnotes (c) and (d) in the table record that less than 9 percent of the mass of the original samples remained after the extraction procedure (less than four grams each from aliquots of more than 40 grams). Most of the soil matrix was dissolved, therefore, by the addition of acid. Although an average of more than 80 percent of the total activity from the soil aliquots was recovered in the experiment, the total activity errors range from about 65 percent to over 100 percent. The solids remaining after both extractions had a 137 Cs concentration of 476 and 554 pci/g, both from soil aliquots with less than 70 pci/g 137 Cs. The activity in the solids remaining after complete extraction represented approximately 65 percent of the total activity of the soil aliquots. Since the total recovered activity ranged from about 80 to 85 percent, 15 to 20 percent of the activity could have been lost from the soil aliquots during the extraction procedure, but very little was extracted into the acid solution during dissolution of most of the soil matrix or into the base solution during the sodium hydroxide extraction. Less than 15 percent of the total activity was found in the acid supernatant and less than six percent was found in the base supernatant.

11 Fraction (a) Weight (g) Table IV. Soil Particle-Size and 137 Cs Distribution Weight % 137 Cs Specific Activity (pci/g) 137 Cs Activity (pci) (Range) % Activity (b) (Range) -8/ ( ) -50/ ( ) -100/ ( ) 3.0 ( ) 2.8 ( ) 1.8 ( ) Subtotal (-8/+200) ( ) 7.6 ( ) -200/+25µ ( ) 75.4 ( ) Subtotal (-8/+25µ) ( ) 83.0 ( ) -25µ 28.8 (c) 11.6 (c) 3300 (c) 17.0 (c) ( ) (c) ( ) (c,d) Total (-8) (a) Mesh size, unless otherwise noted: 8 = 2.38 mm, 50 = mm, 100 = mm, and 200 = mm. A negative number indicates that the soil particles due less than the indicated mesh size; a positive value indicates that they are larger. (b) Total activity of soil sample = g x (76 6 pci/g) = pci ( pci) (c) By difference (d) Negative values indicate that the range of concentrations of 137 Cs within the 2-sigma error from gamma analysis produces a high value for the final concentration of 137 Cs greater than the low value of the initial concentration. Reviewing the results of the experiments in this study, it appears that most of the 137 Cs activity is associated with a fine, black soil fraction, as indicated by particle-size separation, but most is not extracted into either acid or base solutions, although acid solution dissolved over 90 percent of the soil matrix. Like the particle-size separation procedure, the procedure designed to extract organic material from soils left a fine black solid containing much of the radionuclide activity. As shown in Table I, soil organic carbon (O.C.) is very high in the surface soil, ranging from 5.35 to 8.19 percent O.C. These results compare very closely with those of Robison and Stone (12), who reported organic matter contents ranges from percent, which corresponds to 5.8 to 8.3 percent O.C. Table I also illustrates that cesium-137 activity is related to the organic carbon content of the soil and the depth of the soil.

12 Table V. Total 137 Cs Activity of Soil Aliquots Compared to 137 Cs Activity of Recovered Components During Extraction of Organic Material Acid Supernatant Base Supernatant Soil Aliquot A Total Activity = 2900 pci (Range = ) (a) Activity (pci) (Range) ( ) ( ) % Activity (Range) 13.0 ( ) 6.7 ( ) Solid 1760 ( ) (c) 60.7 ( ) Total Recovered 2330 ( ) (a) 42.9 g x (68 6 pci/g) = 2900 pci ( ) (b) 45.7 g x (67 6 pci/g) = 3100 pci ( ) (c) 3.7 g x ( pci/g) = 1760 ( ) (d) 3.9 g x ( pci/g) = 2160 ( ) CONCLUSIONS 80.3 ( ) Soil Aliquot B Total Activity = 3100 pci (Range = ) (b) Activity (pci) (Range) ( ) ( ) % Activity (Range) 12.6 ( ) 4.6 ( ) ( ) (d) ( ) 2690 ( ) 86.9 ( ) The following general conclusions about removing 137 Cs from Bikini Island soil are drawn from the results of this feasibility study: (1) Overall, no extracting solutions selected for Experiments 2 or 3 (acid, base, salt, or a combination of acid or base with salt) were successful in achieving a thirty-fold decrease in 137 Cs concentration (97 percent removal of the activity), even after a treatment period of 21 days. The maximum amount of removal was 47 percent with 0.5M hydrochloric solution. It does not appear likely that any of the solutions would approach the extraction goal, even under the optimal conditions of intimate contact with the soil matrix for the extended periods of time employed in this experiment. Less ideal conditions employed for site remediation by in-situ extraction technology (soil flushing) would likely be less successful than the laboratory experiments and are not recommended for extraction using the solutions examined in this study. (2) The results of Experiment 2 also illustrates the exceptional reactivity of soil material in the presence of acid extractants; addition of acid solutions readily dissolved the solid material as the carbonate matrix reacted. (3) During Experiment 1, selected to extract organic material with sodium hydroxide, most of the soil matrix was dissolved by the required addition of acid. The solids remaining after extraction represented less than nine percent of the original soil aliquots treated; they had a 137 Cs concentration of 476 and 554 pci/g, both from soil aliquots with less than 70 pci/g 137 Cs. Very little 137 Cs was extracted into the acid solution during dissolution of most of the soil matrix or into the base solution during the sodium hydroxide extraction. Less than 15 percent of the total activity

13 was found in the acid supernatant and less than six percent was found in the base supernatant. (4) The majority of the soil particles in the sample, approximately 72 percent, were collected in the +200mesh-size fractions during the performance of Experiment 4. The fraction containing the highest concentration of 137 Cs is the -200 fraction with a concentration of 345 pci/g; it represents about 17 percent of the soil by weight, but over 75 percent of the total activity of the radionuclide in the soil. Since the -8/+50 fraction represents 44 percent of the soil sample, but the concentration of 137 Cs is only 5 pci/g, removal of the remaining 66 percent could leave a soil fraction with a concentration of only 5 pci/g. Mixing the 44 percent remaining soil with clean soil (soil with background concentrations of 137 Cs and with complement nutrient and particle characteristics) would dilute the effect of the 137 Cs concentration and might bring the concentration to less than 2.5 pci/g. Evaluation of the soil to determine the precise percentage that could be recovered in a volume reduction process and the actual concentration of 137 Cs in the recovered fraction and the reconstituted soil would require more extensive testing. (5) It appears that most of the 137 Cs activity is associated with a fine, black soil fraction as indicated by particle-size separation, but most is not extracted into acid or base solutions, although acid solution dissolves over 90 percent of the soil matrix. Like the particle-size separation procedure, the procedure designed to extract organic material from soils left a fine black solid containing much of the radionuclide activity. (6) Although the 137 Cs activity is associated with soil organic carbon (O.C.) in the whole soil samples, the procedure designed to extract organic material from the soil samples removed only a small percentage of the total activity and left a fine, black solid containing much of the radionuclide activity. FOOTNOTES (a) A negative number preceding a mesh size indicates that the soil particles are less than the indicated size; a positive number indicates that they are larger. REFERENCES 1. Proc. Int. Symp. Measures to Strengthen International Co-operation in Nuclear Radiation and Safety, Vienna, Austria, Sept , GC(40)/INF/5, Part C, Annex C-6, International Atomic Energy agency (1996) 2. D.H. OUGHTON, P. BORRETZEN, and B. SALBU, "Tracer Studies on Sediment-Water Interaction Kinetics of Sr-90 and Cs-137," Proceedings Nordic Society for Radiation Protection, Reykjavík, (August 26-29, 1996) 3. P.A. HELMKE and D.L. SPARKS, 1996, "Lithium, Sodium, Potassium, Rubidium, and Cesium," Chapter 19 in Methods of Soil Analysis, Part 3, Chemical Methods, D.L. SPARKS, editor, Soil Science Society of America, Inc. and Americal Society of Agronomy, Inc., Madison, Wisconsin (1996)

14 4. K. ROSEN, I. OBORN, and H. LONSJO, "Migration of Radiocaesium in Swedish Soil Profiles after the Chernobyl Accident, ," Proceedings Nordic Society for Radiation Protection, Reykjavík, (August 26-29,1996) 5. R.C. JONES, personal communication (1996) 6. ARadiochemical Determination of Gamma-Emitting Radionuclides in Water by EPA Mehtod SCA SOP No. 337, Sanford Cohen & Associates, Inc., Southeastern Environmental Laboratory, Montgomery, Alabama (1999) 7. AOperation and Calibration of the Gamma Spectrometry SCA SOP No. 402, Sanford Cohen & Associates, Inc., Southeastern Environmental Laboratory, Montgomery, Alabama (1997) 8. R.S. SWIFT, "Organic Matter Characterization," Chapter 35 in Methods of Soil Analysis, Part 3, Chemical Methods, D.L. SPARKS, editor, Soil Science Society of America, Inc. and Americal Society of Agronomy, Inc., Madison, Wisconsin (1996) 9. AChemical SCA SOP No. 408, Alternate Methods, Sanford Cohen & Associates, Inc., Southeastern Environmental Laboratory, Montgomery, Alabama (1996) 10. ASoil Washing -- Orbital SCA SOP No. 502, Sanford Cohen & Associates, Inc., Southeastern Environmental Laboratory, Montgomery, Alabama (1995) 11. ASize Fractionation -- Wet Sieving of Radioactively Contaminated SCA SOP No. 503, Sanford Cohen & Associates, Inc., Southeastern Environmental Laboratory, Montgomery, Alabama (1995) 12. W.I. ROBISON and E.L. STONE, "The Effect of Potassium on the Uptake of 137 Cs in Food Crops Grown on Coral Soils: Coconut at Bikini Atoll," Health Physics, Vol. 62, No. 6, pp (1992) 13. W.S. RICHARDSON, III, C.R. PHILLIPS, J. LUTTRELL, R. HICKS, and C. COX, "Application of Remedy Studies to the Development of a Soil Washing Pilot Plant That Uses Mineral Processing Technology: a Practical Experience," Journal of Hazardous Material, Vol. 66, pp (1999)