Environ. Sci. Technol. 1994, 28, 1116-1 123
Volatilization of Cesium-I 37 from Soil with Chloride Amendments during Heating and Vitrification+ Brian P. Spalding'
Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 3783 1-6036 During vitrification of soil and soi1:limestone mixtures, significant volatilization (>lo 7% ) of the radioisotope 137Cs occurred particularly in the presence of small amounts ( C 5 % ) of chloride-containing species including sodium chloride, calcium chloride, hydrochloric acid, and poly(vinyl chloride). Sodium chloride was found to be the most effective, resulting in volatilization of >99% of the 137Csby repeated amendment and treatment at 1000 "C. Amendment with sodium borate and subsequent heating to 1200 "C also stimulated significant volatilization of 137Cs.However, amendments up to 10%of other chemicals including carbonates, nitrates, phosphates, sulfates, fluorides, polystyrene, graphite, stainless steels, iron, zinc oxide, and antimony oxide did not increase 137Csvolatilization compared to unamended samples. The majority of the chloride-induced volatilization occurred between 800 and 1000 "C for sodium chloride-amended samples of both soil and soi1:limestone mixtures. Thus, an effective and potentially efficient soil decontamination technique for 137Cshas been identified.
Introduction During a recent field test of in situ vitrification (ISV) for the stabilization of radioactive contaminated soil at Oak Ridge National Laboratory (I), significantly more volatilization of one radionuclide, 137Cs,was experienced than was anticipated. Although the volatilized 137Cswas completely removed from the ISV off-gas by a specifically designed filtration system, its degree of volatilization (2.4 7% of the total activity) was considerably greater than the 0.12% volatilization that had been determined in a previous field test, without radioisotopes, using additions of Cs2CO3 (2). The objective of ISV is to produce a glass waste form which incorporates all or most of a soil's contaminating radionuclides. It thus becomes important to identify the causes of the relatively large degree of 137Cs volatilization. There were significant differences between the first field test in 1987 and the second one in 1991 which conceivably could have contributed to the enhanced degree of 137Csvolatilization in the 1991 radioactive test. Among the more salient differences were the use of a more dolomitic or magnesium-rich limestone in 1991; the use of radionuclides in 1991 rather than chemicals; the lower bulk density of soil materials in the 1991 test and, thus, the greater ground surface subsidence after melting in 1991;the apparently wetter soil conditions during the 1991 test; and the inability to retrieve a poly(viny1 chloride) (PVC) pipe in the melt zone, which had been used to emplace the radioactive sludge sample at a precise depth in the 1991 test configuration. Because of this large number of potentially interacting factors that may have contributed to the degree of 137Csvolatilization, the systematic assessment of some of these potential influences t Publication No. 4237, Environmental Sciences Division, ORNL.
* E-mail address:
[email protected].
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seemed amenable to a laboratory-scale investigation where relatively large numbers of samples could be examined facilely. A number of factors, which contribute to the volatilization of 137Csand other radionuclides during vitrification, have been identified in the many DOE-sponsored studies supporting vitrified waste forms as a nuclear waste processing technology (3-7). Included in these known factors are chloride and borate contents of the waste, temperature and its duration, melt turbulence, and offgas flow; like the problem of volatility during ISV, these factors were identified as part of an effort to minimize volatility during vitrification and, thus, keep potentially volatile radionuclides within the vitrified product. However, under certain environmental restoration conditions, it may be more desirable to volatilize most of the 137Cs leaving a decontaminated glass or residue. The contamination of soil with I37Cs has posed a previously insurmountable challenge for decontamination efforts because the specific fixation of l37Cs by soil minerals makes the resulting contamination extremely difficult to extract or remove (8-10); aggressive reagents, which dissolve most of the aluminosilicate soil minerals, are generally necessary to remove significant fractions of the 137Cscontamination (11-13). A recent assessment of the soil contamination with 137Cs following the Chernobyl nuclear accident concluded that soil decontamination was not a feasible alternative (14). Therefore, the maximization of 137Cs volatilization during vitrification or subvitrification heating of soil might have potential utility as a soil decontamination technique if a significant fraction of the 137Cs could be volatilized.
Methods and Materials Materials. The soil used for most of these 137Cs volatilization studies was collected from the floodplain of White Oak Creek a t Oak Ridge National Laboratory. Details of site description, the preparation of soil, and chemical analyses of a bulk sample of this soil have been described previously (15,2). Its elemental composition is listed in Table 1along with the compositions of other soils and carbonate rocks used in these studies. The contamination of the bulk sample of this soil is dominated by 137Cs a t 7.33 FCiikg (16 280 dpm/g) with much smaller activities of 90Srand 6oCo(384 and 32 dpm/g, respectively). Samples of uncontaminated soil and limestones, used in the 1987 and 1991 field ISV tests, were also used in some of the present work. All materials were ground and sieved to 90% of the l37Cs during the sequential thermal treatments. Thus, quite aside from the undesirable influence of chloride on the volatilization of 137Csduring ISV, it became apparent that modest chloride amendments of soi1:limestone mixtures could affect a major decontamination of 137Csthrough volatilization during heating. When the major fraction of soil 137Cs was being volatilized with increasing NaCl amendment, it was desired to determine whether an excess of chloride was forming a volatile species with 137Csor whether the 13Tswas simply behaving as a tracer for the NaCl that was distilling from the samples. The observation of sample weights during these sequential heating tests provided a simple mechanism to make this determination (Figure 4). As discussed above, heating samples of 7:3 soi1:limestone beyond 850 "C produced no further sample weight reduction. Figure 4 shows that both mineral dehydration weight loses (between 110 and 700 "C) and decarbonation (between 700 and 800 "C) were completed at temperatures below those required for 137Cs volatilization. The difference in sample weights between 800 and 1100 "C was approximately equal to the amount of NaCl added. Sodium chloride boils at 1413 OC and melts at 804 "C and, thus, would exhibit a significant vapor pressure above 800 "C; crucibles containing only NaC1, when heated to 1000 "C, returned to their tare weights indicating complete volatilization of the NaC1. After samples containing up to 10% NaCl had been heated to 1100 "C, sample weights were nearly equivalent to those of unamended soil: limestone mixtures. This indicated that NaCl was being volatilized from the mixtures at the same temperatures at which volatilization of 137Cs had occurred. Thus, it
appeared that NaCl was carrying the 137Cs from the mixtures rather than excess chloride inducing an independent and lower temperature volatilization of CsCl (boiling point 1290 and melting point 646 "C). Although the testing NaCl amendments on a 7:3 soil: limestone mixture provided much information on l37Cs volatilization during ISV of materials with that range of composition, an obvious extension of such findings for a decontamination process was to test similar NaCl amendments on soil alone. Although soil alone required a 200 "C higher temperature (i.e., 1400 vs 1200 "C) to produce an apparently homogeneous vitreous phase, two major differences in the behavior of the resulting 137Cs volatilization were apparent (Figure 5). First, a considerably lower maximum volatilization of 137Csoccurred (Le., 70 vs 90% ) with soil alone than with soi1:limestone mixtures (Figure 3). Second, considerably more NaCl amendment (i.e., 20 vs 10%) was required to achieve maximum volatilization of the 137Cswith soil alone. Significant volatilization at any temperature was not observed for soil at NaCl amendments below 6% whereas the same amendment rate was sufficient to produce maximum 137Cs volatilization in the 7:3 soi1:limestone (Figure 3). Notably, volatilization of l37Cs did occur in the same temperature range (between 800 and 1000 "C) in these soil samples as in the soi1:limestone mixtures. Although it did not affect temperatures a t which 137Csvolatilized, it would appear that the inclusion of limestone did affect the degree of volatilization. As in the soi1:limestone mixture investigation, the retention of sample weights at the various temperatures shed some light on the behavior of NaCl in these soil samples (Figure 6). Treatment to the highest temperatures did not result in sample weights equivalent to those of unamended samples; this indicated that NaCl was not completely volatilizing from soil as had been observed in the soi1:limestone mixtures (Figure 4). Significant amounts, e.g., 7 of the 20% maximum NaCl added, were retained in the sample even when heated sequentially to 1300 "C. A linear regression of the percent
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NaCl a d d e d (%) Flgure 8. Percent weight retention of soil samples amended with NaCl when heated to various incremental temperatures. Sequential heating to: ( 0 )700 O C (unmelted),(H)800 "C (unmelted),(A)900 OC (unmelted), (+) 1000 OC (unmelted), (V)1100 "C (unmelted), ( - - - - ) 1200 OC (unmelted), (V)1300 "C (fused).
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of 137Cs retained (Y) showed a highly significant linear correlation with the calculated percent of NaCl retained (X): Y = -17.5 1.27X ( r = 0.98, n = 35). This correlation implies that 137Cs was behaving as the NaCl during soil heating. Without the 30 % inclusion of limestone, soil samples tended to retain a significant fraction of added NaCl and, thus, a significant fraction of the 137Cs, From the point of view of performing ISV with minimal volatilization of l37C9, the conclusion would be that lower levels of limestone would be desirable if the sludge or soil contained significant amounts of chloride. From the point of view of 137Cs-soil decontamination, limestone amendment, perhaps at some level below the 30% selected for this investigation, would be desirable because greater volatilization of 137Cscould be attained at lower rates of NaCl addition. Also by the addition of limestone, the apparently complete removal of NaCl can be effected, which would be desirable to avoid potentially soluble salts in the decontaminated residuals. Degree of l37Cs Volatilization after Repeated Thermal Treatments. As a soil decontamination process, it would seem important to determine if residual 137Csin a NaC1-amended and thermally treated sample was susceptible to further decontamination by repeated treatment. If the initial heating resulted in a residue in which the 137Cswas fixed and, thus, no longer susceptible to further volatilization, then processes would likely prove extremely complicated to control to achieve a maximum decontamination. To determine if previous heating affected the degree of volatilization, samples of 137Cs-soil:limestone, which had previously been heated to 850 and 950 "C without NaCl amendments, were subsequently and repeatedly amended with 5 % NaCl and heated to 1000 "C (Figure 7). Because treatment to 1000 "C did not melt these mixtures, the repeated additions of NaCl induced repeated volatilization of significant fractions of the residual 137Csuntil after 10 such treatments greater than 99% of the starting 137Cshad been removed. Thus, previous temperature experience or previous NaCl amend-
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Flgure 7. Retention of I3'Cs in 7:3 soi1:limestone mixtures during repeated addition of 5% NaCl and heating to 1000 OC. (A) 850 "C pretreatment; ( 0 )950
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ments did not impose a limitation of the degree of decontamination that could ultimately be attained. As discussed above, melting of a sample inhibits further volatilization of 137Cs due, presumably, to the decrease in surface area of the sample. If this inference is valid, then previously melted material when pulverized to reestablish its surface area should also be susceptible to C1-induced volatilization of 137Csto the same degree as previously unmelted materials. When the vitrified product from the 1991 field ISV test was pulverized and amended with various increments of NaCl and subjected to heating at the conditions described in the section on chemical amendments, the influence of NaCl was essentially identical to that observed with previously unmelted materials. The resulting linear regression had a slope of -9.95% 137Cs volatilized/% of NaCl added with a correlation coefficient of 0.977. This regression slope is equivalent to those depicted in Figure 2 for the various chloride amendments to soiklimestone mixtures. The melting of materials imposes no limitation of potential volatilization of l37Cs other than the reduction of sample surface area. From the point of view of a soil thermal decontamination process, it would be an obvious advantage to avoid melting materials to maintain their surface area and, thereby, maximize Cs volatilization. Volatilization of lr4Csduring Melting of Different Soils and Limestones. When combinations of soil and limestone materials from the 1987 and 1991ISV field tests (Table 1)were melted in various proportions, after spiking with 134Cs,no systematic differences among the various combinations could be detected (Figure 8). This finding precluded the need to examine chemical composition differences among the materials, such as ambient chloride content, which may have contributed to systematic differences in Cs volatility between the 1987 and 1991 field ISV tests. The major difference in materials between the 1991 and 1987 field tests was in the composition of the limestone gravel used to fill the test trench; in the 1991 test, a magnesium-rich or dolomitic limestone had been used while in the 1987 test a magnesium-poor or calcitic limestone was employed. Although this difference in Environ. Sci. Technol., Voi. 28, No. 6, 1994
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Fraction of Limestone in Melt Figure 8. Retention of 134Csby samples of soil and limestone materials used in the 1987 and 1991 ISV field tests after heating to 1500 "C.(0)1987 soil plus 1987 limestone; (+) 1987 soil plus 1991 limestone; (0) 1991 soil plus 1987 limestone: (A)1991 soil plus 1991 limestone.
limestones proved important in the crystallization kinetics and the final mineralogical composition of the ISV product ( I ) , it does not seem likely to have exerted an influence on the difference in 137Csvolatilization between the two tests. It is important to note, however, that increasing limestone content in a soil mixture in general led to increasing volatilization of 134Cs (Figure 8). Samples amended with limestone above about 25% showed quite large increases in the fraction of 134Csvolatilized. Previous work has shown that limestone amendments above about 50% can result in samples with almost complete 134Cs volatilization (16). A t higher limestone contents, samples increase significantly in melting temperatures (CaO melts at over 2500 "C) and cannot form any melted or fused phase in which to retain the volatile Cs at any treatment temperature below 1500 "C. Previous work ( I ) has indicated that the variation of bulk density between 0.9 and 2.2 g/cm3 of soi1:limestone mixtures in laboratory crucibles had no effect on the degree of volatilization of l37Cs from the samples when heated. However, a t field scale during ISV operations, the bulk density of the materials being melted may influence the degree of volatility of 137Cs by providing the porosity necessary to conduct escaping gases including steam and carbon dioxide. Discussion A recent review of the general problem of 137Cs contamination of soil after the Chernobyl nuclear accident could identify no feasible technology for 137Csdecontamination because of this strong soil fixation (14). 137Cstends to be strongly and irreversibly fixed by the illitic clay mineral fraction contained in most soils (11). It is, of course, potassium rather than cesium that is the naturally fixed cation in illite. Extremely aggressive reagents, which dissolve a significant portion of the aluminosilicate clay mineral structure, are required to extract a significant portion of the 137Csfrom contaminated soils and sediments (8,9,12). In a recent review of illite in soils, Fanning and 1122
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Keramidas ( I 7) examined the differential thermal analysis curves and found that the crystal structure of illite persists up to about 900 "C followed by destruction at higher temperatures with the formation of spinel and mullite. Thus, the present observations that 800-1000 "C was necessary to convert the illite-fixed 137Csto an accessible form are consistent with the hypothesis that illitic crystal structures are destroyed by such thermal treatment. Once the crystal structure specific for the 137Csfixation has been destroyed, chloride species are free to volatilize 1376sfrom the heated sample. The above explanation is consistent with the observations based only on the 7:3 soi1:limestone mixtures that were employed to approximate the mixture which resulted from the 1991 field ISV test. However, when soil alone was tested, both the maximum fraction of 137Csvolatilized was decreased and the amount of NaCl required to attain that maximum volatilization was increased as compared to soi1:limestone mixtures. Although the temperatures where 137Csvolatilization occurred were identical for both soil and soi1:limestone mixtures, more than a passive role for limestone must be hypothesized. Perhaps the excess CaO, resulting from decarbonation of the limestone, supplies an excess of preferred available cations to replace the K released from the illite during its thermal destruction. Sodium would not be as preferred as Cain balancing the lattice charge of aluminosilicate minerals formed at high temperature. Notably, in the presence of 30 7'o 1'ime, NaCl was nearly completely distilled from the mixtures whereas, in the absence of lime, considerable NaCl was retained in the thermally treated soil residue. If this mechanism of CaO enhancement of 137Csvolatilization were due to a preference for Ca versus Na to substitute for K when thermallyreleased from soil illite, then it would seem possible that KC1 would work better than NaCl because K would be ideal in substituting for itself and, thus, for 137Cs. Regardless, the addition of lime, as well as sodium chloride, could be performed at a fixed ratio if greater efficiency of soil decontamination were desired. It appears that a potential technique to decontaminate the soil of 137Cs has been identified. Although the conceived process would be neither simple nor inexpensive in that temperatures above 800 "C, chloride, and, perhaps, lime are required, it is at least now possible. An obvious extension of this work would suggest that the concomitant effects on other radionuclides be determined. Since radioactively contaminated soils often contain other radionuclides, l37Cs decontamination by itself even if quantitative, would be of dubious value if the residue remains contaminated with other radionuclides. Alternately, if one were to go to all the technical difficulty and expense to heat a soil to 1000 "C, it would seem logical to determine if and how the process could be adapted for the removal or volatilization of other radionuclides. Many transuranic isotopes form volatile chlorides. Likewise, 6OCo will likely be somewhat volatile during such heating because Co, Hg, Pb, Cd, Cu, and Zn have been found to be semivolatile trace elements in our recent ISV test (1). Although 90Srdecontamination would be highly desirable during such a process, this isotope will probably behave more like Ca or Mg and exhibit quantitative retention in the residue (16). Alternately, 90Srcould be extracted from soil with solutions of NaCl and/or HCl with residuals of such solutions serving as a source of chloride during subsequent heating to remove l3'Cs. Nonetheless, the
presently conceived soil decontamination process would seem to have some direct applications for 137Cs-contaminated soil and sediments. The process appears to be capable of nearly complete removal of 13'Cs in a heated gas phase which would need only to be cooled resulting in a '37Cs-contaminated solid NaC1. Perhaps the NaCl could be reused in the process bypassing it through soil, perhaps as a gas but more likely as a brine, and, ironically, using the unheated soil's specific fixation properties to remove the 137Csfrom the NaC1. Thus, starting with a large volume of contaminated soil, the conceived process could approach an ideal decontamination scenario resulting in a large volume of decontaminated residue, a small volume of much more highly contaminated soil, and some salt. Acknowledgments
This research was sponsored by the Office of Technology Development, US. Department of Energy, to the Oak Ridge National Laboratory which is managed by Martin Marietta Energy Systems, Inc., under Contract DE-AC05840R21400 with the U S . Department of Energy. Gratitude is extended to my colleagues, Drs. Gary Jacobs, Chet Francis, Nelia Dunbar, and Michael Naney, for their diligent efforts during the in situ vitrification project, which stimulated many of the ideas described in this article, and for their helpful comments and suggestions on this manuscript and during performance of these experiments. Literature Cited Spalding, B. P.; Jacobs, G. K.; Dunbar, N. W.; Naney, M. T.; Tixier, !J. S.;Powell, T. D. Tracer-level radioactivepilotscale test of in situ vitrification for stabilization of soil sites a t ORNL; Technical Report No. ORNL/TM-12201; Oak Ridge National Laboratory: Oak Ridge, T N , 1992. Spalding, B. P.; Jacobs, G. K. Evaluation of a n in situ vitrification field demonstration of a simulated radioactive liquid waste disposal trench; Technical Report No. ORNLI TM-10992; Oak Ridge National Laboratory: Oak Ridge, T N , 1989. Kelley, J. A. Evaluation ofglass as a matrix for solidification of Savannah River Plant waste; Technical Report No. DP-
1382; E. I. Du Pont de Nemours & Co.: Aiken, SC, 1975. (4) Wilds, G. W. Vaporization of semi-volatile components from Savannah River Plant waste glass; Technical Report No. DP-1504; E. I. Du Pont de Nemours and Co.: Aiken, SC, 1978. (5) Gray, W. J. Radioact. Waste Manage. 1980, 1, 147. (6) Burkholder, H. C.; Allen, R. C. LFCM vitrification technology quarterly progress report July-September 1986; Technical Report No. PNL-5904-4; Pacific Northwest Laboratory: Richland, WA, 1986. (7) Sill, C. W. Nucl. Chem. Waste Manage. 1988, 8, 97. (8) Lomenick, T. F.; Gardiner, D. A. Health Phys. 1965, 11, 567. (9) Spalding, B. P.; Ceding, T. E. Association of radionuclides with streambed sediments in White Oak Creek watershed; Technical Report No. ORNL/TM-6895; Oak Ridge National Laboratory: Oak Ridge, TN, 1979. (10) Ceding, T. E.; Spalding, B. P. Environ. Geol. 1982,4, 99. (11) Lomenick, T. F.; Tamura, T. Soil Sci. SOC.Am. Proc. 1965, 29, 383. (12) Kahn, B. Anal. Chem. 1956,28, 216. (13) Kiley, P. V.; Jackson, M. L. Soil Sci. SOC.Am. Proc. 1965, 29, 159. (14) Howorth, J. M.; Sandalls, F. J. Decontamination and reclamation of agricultural land following a nuclear accident-a literature review;TechnicalReport No. AERER12666;United Kingdom Atomic Energy Authority; Harwell Laboratory: Oxfordshire, United Kingdom, 1987. (15) Van Voris, P.; Dahlman, R. C. Floodplain data: Ecosystem characteristics a n d I37C.s concentrations in biota a n d soil; Technical Report No. ORNLiTM-5526; Oak Ridge National Laboratory: Oak Ridge, T N , 1976. (16) Spalding, B. P.; Jacobs, G. K.; Davis, E. C. Demonstrations of technology for remediation and closure of Oak Ridge National Laboratory waste disposalsites;Technical Report No. ORNLiTM-11286; Oak Ridge National Laboratory: Oak Ridge, TN, 1989. (17) Fanning, D. S.; Keramidas, V. Z. In Minerals in Soil Environments; Dixon, J. B., Weed, S. B., Eds.; Soil Science Society of America: Madison, WI, 1977; p 195. Received for review August 30, 1993. Revised manuscript received February 10, 1994. Accepted February 14, 1994.' @
Abstract published in Aduance ACS Abstracts, April 1, 1994.
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