fission product tin in sediments

J. Environ. Radioactivity 10 (1989) 201-111

Fission Product Tin in Sediments*

T. L. Patton & W. R. Penrose*

Center for Environmental Research, Biological, Environmental and Medical Research Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

(Received 7 December 1988; revised version received 20 March 1989; accepted 28 March 1989)

ABSTRACT

We describe a method for the determination of long-lived tin isotopes arising from nuclear fission. The isotopes are extracted from sediments with hydrochloric acid and separated by methylisobutylketone extraction, ion-exchange chromatography, precipitation of cesium hexachlorostannate and ferric hydroxide coprecipitation. The 100 O00-year 1265n was not detected but beta activity consistent with 12tmSn (55 year) was found in sediment samples from a location known to be contaminated with fission products.

I N T R O D U C T I O N

The products of nuclear fission are accumulating in temporary repositories around the world, while research continues to seek ways of reducing the risk of release and exposure of the biosphere to the resulting radiation. The most likely means of disposal of these wastes is en tombmen t in deep, inaccessible rock or salt formations. Even so, it is necessary to ensure that no reasonably predictable geological events will breach the conta inment in the future. Groundwa te r intrusion is the most likely, and the most feared, of possible breaches, because of its potential for carrying wastes for long distances in geologically brief times.

*Work supported by the US Department of Energy, Office of Health and Environmental Research, under contract W-31-109-Eng-38.

*To whom correspondence should be addressed at: Transducer Research Inc., c/o 526 West Franklin Avenue, Naperville, Illinois 60540, USA.

201 © 1989 US Government

202 T. L. Patton, W. R. Penrose

Two recent studies have called our attention to 126Sn as an important fission product from the standpoint of long-term storage. The National Research Council (1983) has authored a study using numerical models to predict subsurface movements of various components of high-level wastes. In several scenarios, these simulations predicted that 126Sn will be an important contributor to the overall radiation dose in the 100- to 10 000-year timescale. Examination of the model parameters selected to represent tin revealed that they were based on very skimpy evidence indeed. The three available measurements of solubility covered nine orders of magnitude (National Research Council, 1983, p. 203). The authors conservatively chose the highest solubility (2 × 10 -9 mol/liter or 0-1 parts per billion) for their efforts but also labeled their selection as based on shaky evidence. Another study (summarized in Chapman & McKinley, 1987) selected 1 x 10 -9 mol/liter as the solubility of inorganic tin.

It is our intention to clarify the environmental chemistry of tin, particularly as it applies to isotopes found in radioactive wastes. 126Sn is one of the more interesting of the long-lived waste products. Studies of its occurrence and environmental chemistry are scarce; they seem, in fact, to be limited to a single recent report by Koide & Goldberg (1985). Studies of the behavior of non-radioactive tin, moreover, have been limited. The inorganic metal is virtually non-toxic and therefore of little interest to environmental agencies.

The aqueous chemistry of tin is complex; it is potentially capable of existing in the environment in at least two oxidation states. It is capable of hydrolysis and many analysts have noted that tin-bearing samples have a 'history'; the behavior of apparently equivalent samples of tin may differ, depending on storage conditions and previous treatments. This is presumed to be due to the hydrolytic formation of metastannic acids (Nervik, 1960). Tin can also form organometallic compounds that are very stable in the environment (Chau, 1986).

This study is an attempt to fill the gap in the information base on tin isotopes arising from nuclear fission. Recently, Koide & Goldberg (1985) published a method for measuring 126Sn and 12h~Sn and claimed to detect tin isotopes in sediment samples. Their method consisted of a sodium peroxide fusion, coprecipitations with ferric and nickel hydroxides, sulfide precipitation and solvent extraction. In our hands, ferric hydroxide precipitation early in the procedure did not always recover all of the tin, although it worked on partially purified samples. In addition, the sulfide precipitation step generally produced copious amounts of elemental sul- fur, which complicated later steps. As a result, we have developed an alternate method which has to date yielded activity resembling 121mSn in

Fission product tin in sediments 203

one location. The development of this method was assisted by the use of the gamma-emi t te r ~3Sn.

M E T H O D S

Sediment samples

Samples were obtained from several locations, some of which were known to be contaminated with fission products. In order to gain access to some of the samples, we have been asked to maintain confidentiality; all but one source is therefore unnamed. The named source is the Irish Sea near the British Nuclear Fuels reprocessing plant at Sellafield, Cumbria, UK (54°22'48"N, 3°41'00"W). The unnamed sources are freshwater lakes or streams. Sediments were stored in a refrigerator or freezer after collec- tion, except for the Irish Sea sediment, which was stored dry.

Analysis of beta-emitting tin isotopes

While we were developing individual steps of the method, we added 1~3Sn (New England Nuclear, Boston, MA) to samples to de termine the recovery at each step. l l3Sn has a conveniently measured gamma ray which allowed us to locate it at any stage in a procedure . Because of the relatively high efficiency at which gamma rays were detected in our beta-counter , however , l l3Sn could not be used as an internal standard in the analysis. If sufficient sample was available, we often ran the determinat ions in dupli- cate, with a small quanti ty of 113Sn added to one of the samples to moni tor the overall recovery.

A sample of sediment (10 g) was mixed with an internal s tandard of 5.00 mg tin (as SnC12) and ashed for 2 h or more at 500°C to remove organic matter . The ash was digested with 6 M hydrochloric acid until the undissolved residue was nearly white. Bromine water was added to the sample to ensure that the tin was in the (IV) state, then iron was extracted from the sample with 3 × 20 ml isopropyl ether.

Tin was extracted from the acid phase with 2 x 20 ml methylisobutylketone and stripped into 2 x 20 ml 0.1 M HCI. The extract was evapora ted over low heat to about 5-10 ml. (Note: It is important not to let the sample go dry at this stage; otherwise considerable tin is lost.) Af ter cooling, the extract was made to 3 M in HBr and applied to a 0.4 cm diameter x 5 cm column of AG-1-X8 (Bio-Rad Laboratories, Richmond, CA) which was pre-equil ibrated with 3 M HBr. The column was washed with 25 ml 3 M HBr, and the adsorbed tin eluted with 25 ml 0-5 M HBr. I ron(I l l ) (5 mg) as


ferric chloride was then added and the ferric hydroxide precipitated with 1 M sodium bicarbonate. Because tin hydroxides may redissolve as stan- nate ion at high pH, phenolphthalein was usually added to prevent the sample from becoming too alkaline during addition of bicarbonate. Only enough bicarbonate was added to generate a faint color in the indicator.

After separation by centrifugation, the precipitate was dissolved in 10 ml concentrated HCI and diluted with 10 ml water. Tin was precipitated as the cesium hexachlorostannate by the addition of 4 ml 1 M CsCI2 in concentrated HCI; 5-10 min were needed for the precipitate to form. The precipitate was separated by centrifugation and dissolved in 10 ml water.

The sample was prepared for beta-counting by adding 2.0 mg iron(Ill) as ferric chloride and one drop of phenolphthalein solution. Ferric hydroxide and tin hydroxide were precipitated by the dropwise addition of sodium bicarbonate. The suspension was filtered onto a 2-4 cm glass fiber filter which was dried, mounted on a 3.5 cm stainless-steel planchet and covered with 6.3/xm thick, Mylar X-ray film. Beta radiation was counted during several 100 min periods in a gas-flow counting system with a background of 0.5 cpm or less.

After counting, the precipitate was dissolved from the filter with 4 M nitric acid. The yield of total tin was measured by atomic absorption spectrometry. Appropriate corrections for background, radiochemical yield and counting efficiency were applied to the resulting data.

When we measured recoveries of total radioactivity from a sample, an aliquot of the sample at each step was evaporated to dryness and transfer- red to a cup-shaped stainless-steel planchet and covered with Mylar film for counting. Afterwards, the material was dissolved from the planchet and pooled with the sample.

Reagent blanks were found to be necessary when we used reagents for the first time.

Plutonium analysis

Analyses of plutonium were carried out on separate samples of the sediment, using established techniques (Wahlgren & Orlandini, 1982).

Distribution test

At an HCI concentration of 1-75 M, tin was found to be partitioned more or less equally between the acid and methylisobutylketone. A purified sample, containing 'tin-like' beta activity derived from location A, was diluted into 1.75 M HCI and shaken with an equal volume of methylisobutylketone. The phases were separated and the tin stripped from the organic phase into 0-1 M HC1. Iron(IlI) (2 mg) was added to each HCI fraction and


the ferric hydroxide precipitated with sodium bicarbonate. Each fraction was counted for beta activity, then dissolved in 4 M HNO3 for atomic absorption analysis of total tin. Specific activities (cpm/mg Sn) were calculated for each fraction.

Estimation of beta energy by aluminum absorption

Absorbers were cut from sheets of aluminum. Their thickness was deter- mined from their weights and areas and expressed in terms of mg/cm 2, as is customary. These absorbers were selected, singly and in groups, and placed between the sample and the detector aperture in a Tennelec LB4000 low-background beta-counter. Energy estimates were made according to the methods outlined by Glendenin (1948).

RESULTS

Beta and alpha activity in sediments

Measurements of beta activity and plutonium for the sediment samples are listed in Table 1. Although several samples contained plutonium, only samples from location A contained significant levels of beta activity attributable to tin. When stable tin was used as a recovery monitor, yields from the entire procedure varied between 36 and 92% among samples from different sources.

The relative effectiveness of the early steps of the procedure is shown in Table 2. The methylisobutylketone extraction is the most efficient step, reducing sample bulk by 99%.

Recovery of radioactivity was also measured. Some of the samples contained substantial amounts of other fission products and it was necessary to demonstrate that these other activities were not 'leaking' through the procedure. In Table 3, the amount of uncorrected beta activity surviving each step in the procedure is shown. The anion column essential- ly completed the procedure, because activity did not decrease further in the subsequent steps. The ferric hydroxide precipitation and cesium hexachlorostannate steps have been left in the procedure, however, to ensure the complete separation of tin activity.

Solvent distribution tests

The beta activity in the most active available sample was used for a solvent distribution test for identity with tin. The specific activity in the organic phase was 0.30 + 0.15 cpm/mg Sn; that in the acid phase, 0.54 + 0-20 cpm/


TABLE 1 Radiotin and Plutonium Activities in Sediment Samples

121mSn 239.240pu Sample (mBq/g) (mBq/g)

Location A, May 1986 49 + 2 129 (moderately fresh 26 + 2 - - fission products; 48 + 2 - - replicates of one sample) 61 + 2 - -

Location A, October 1986 29 + 2 164 (two samples) 58 + 3 - -

Location A, July 1987 31 + 3 60 (four samples) 16 + 3 119

22 + 3 23 25 + 2 108

Irish Sea, October 1977 15 + 2 322 (nuclear fuel reprocessing wastes)

Location B, August 1983 3.3 + 1.1 0.1 (no known contamination)

Location C, December 1986 6.9 + 1.5 - - (10-year-old spill of 6-2 + 1.5 - - fission wastes; separate samples)

Location D, December 1984 11 + 1-5 - - (20-year-old spill of 13 + 1.5 - - fission wastes; separate samples)

Errors shown are propagated counting errors only.

TABLE 2 Recovery of Total Sample Weight After Early Steps of the Procedure

Incremental Overall Step recoverff (%) recoverff (%)

Ashing 90.2,89.7 90.2, 89.7 Digestion 19.9, 34.5 17-9, 31-0 Isopropyl ether 59.3,93-8 10-6, 29.1 Methylisobutylketone 1.07,0.58 0.113, 0.170 Ion-exchange <7-8, < 11.3 <0-02, <0.02

The numbers are for two independent samples of the blank (location B) sediment. "Incremental recovery is the sample weight remaining after an individual step. hOverall recovery was based on initial dry weight.


TABLE 3 Recovery of Total "Beta' Radioactivity (cpm/g dry wt) After Each Stepofthe Procedure

Sample

Step 1 2 3 4

Digestion 144 + 0.8 123 _+ 0.7 166 + 0-2 153 + 0-2 Methylisobutylketone 11.5+0-1 5.6+0-1 5-1+0.1 8 .0+0.1 Ion-exchange 0.50 + 0.03 0.68_+ 0.03 0.18 + 0-03 0.65 + 0.03 Fe(OH)3 precipitation 0.60 + 0-03 0.56 + 0-03 0.35 + 0-03" Cs2SnCI~ 0-72 + 0.05 0.94_ 0.05 0.64 + 0.04 0-79 + 0.05

Count rates were not corrected for contributions from gamma radiation. The samples were from location A.

mg Sn. Given the accumulation of propagated errors due to low sample activity, we regarded these data as supporting the association of the beta activity with tin.

Energy estimation by aluminum absorption

A sample prepared from pooled samples of location A sediment was used for energy estimation by the aluminum absorption method. A small percentage of the sample activity seemed to be due to photons, as was evidenced by the horizontal tail on the uncorrected curve (Fig. 1). Because of the low count rate, as well as the limitations of the available instru- mentation, it was necessary to place the sample and absorber in close proximity; the usual configuration for measurements of this type involves considerable distance between the sample and absorbers and between absorbers and detector (Feather, 1938; Glendenin, 1948). The activity tail could therefore have been due to bremsstrahlung from the action of the beta particles on the stainless-steel planchet and the aluminum absorber. The photons could be reconverted to electrons in the detector. (We have found similar tails in absorber measurements of 9°Sr and l lYmCd in the same apparatus.) On the basis of this observation, we subtracted the 13% tail from each data point before further analysis.

Using the fourth-power relationship of beta absorption to absorber thickness suggested by Glendenin (1948), we obtained a curve that suggested an energy very close to the reported value for 121mSn (Fig. 2). The Feather analysis method, as described by Glendenin, gave extremely poor results because of its dependence on the high-absorption (low count-rate) tail of the plot.


4

3 ,

0 o 50 I00

T h i c k n e s s - m g / c m 2

i 0 - i

150 300

Fig. 1. Absorpt ion of beta radiation from a tin-containing isolate from location A sediment. The background-corrected raw data are displayed in the upper curve; the 0.51 cpm tail has

been subtracted to obtain the lower curve.

©

©

5 -

I

4 -

3 -

2-

2'0 40 60 80 100 120 140

T h i c k n e s s - r a g / e r a a

Fig. 2. The corrected data of Fig. 1 are replotted as the fourth root of the count rate against the thickness of the absorber. The one-sigma error band, based on counting error only, is shown by the dot ted lines. The intercept of the solid line with the abscissa is the estimate of the range

of the beta particles. The arrow points to the expected range of 121mSn.

Fission product tin in sedirnents 209

The beta absorption data suggested that the measured activity was 12~'Sn. Since the samples from location A contained traces of fission products less than 2-3 years old, some contributions from 123Sn (~ 129-day) may have been present. This isotope has a 1.42 MeV beta emission, whose effect would be to bend the tail of the activity curve toward the horizontal.

DISCUSSION

Fission gives rise to several tin isotopes (Z = 117m, 119, 121,121m, 123, 123m, 125,125m and 126) with yields of 0.015-0.057% (Mathews, 1977; General Electric Co., 1983); of these, 121mSn and 126Sn have half-lives of 55 and 100 000 years, respectively, long enough to be of concern for waste management. Because their fission yields are within an order of magnitude of each other (Mathews, 1977), the shorter-lived 121mSn will be more active and therefore easier to detect. Both isotopes are relatively weak beta- emitters but 126Sn decays to t26Sb and 126m Sb, which have energetic beta and gamma emissions (see Koide & Goldberg, 1985). 123Sn ( -129 days, 1-42 MeV beta and 1-0 MeV gamma) might also be found in suitably fresh samples.

Identification of such low levels of beta-emitting nuclides is inherently difficult. Because of the distribution of energies of the beta radiation from a given nuclide, identification cannot be made on the basis of discrete lines in an energy spectrum. Most methods for identifying beta-emitters are exclusionary, in that they eliminate potential interferences; they do not confirm the presence of the target nuclide. As a result, our policy has been to include as many unique steps as possible, to maximize the selectivity of the procedure. We have done this without drastically reducing the yields. The method presented here can be carried out on a small number of samples by one operator in one working day.

The most serious potential interference was expected from antimony, which has several beta-emitting isotopes of intermediate half-life that are also fission products. The most important is ~2sSb, which has a half-life of 2.76 years and decays by the emission of 0.30 MeV beta radiation.

The solvent extraction steps employing isopropyl ether and methylisobutylketone (MIBK) were suggested by the method of Koide & Goldberg (1985). Using ~3Sn, we demonstrated that the ether extraction removed nearly all of the iron and none of the tin from solution. According to Minczewski et al. (1982), Mn, Bi, Pb, Ti and AI should also be removed. The MIBK extraction is not particularly selective (Minczewski etal . , 1982) but is useful in that it substantially reduces the bulk of the sample for the succeeding ion-exchange step.


The anion-exchange procedure in hydrobromic acid was taken directly from Andersen & Knutsen (1962). They reported complete separation of germanium and antimony from tin. After this step, ferric hydroxide coprecipitation reliably collected all of the tin in the sample. The precipitation of cesium hexachlorostannate was modified from Browne et al. (1951) and has been reported to give good separation from antimony (Erdal & Wahl, 1968).

Significant amounts of beta activity corresponding to tin isotopes were found only in the samples recovered in 1986 from location A, which is known to be slightly contaminated by recent fission wastes. The following year, the tin activity concentration had decreased substantially; the plutonium activity was relatively unchanged.

The absence of tin isotopes in the Irish Sea sediments is not easily explained. To a first approximation, tin and plutonium would be expected to behave somewhat alike in the environment. Santschi and colleagues measured the binding of several trace metals onto a variety of sediment types in seawater and found tin to be very tightly bound, with Ko values over 105 liters/kg (Li etal., 1984; Nyffeler etal., 1984; Santschi etal., 1986). A possible explanation is that the nature of the waste treatment process at Sellafield prevented the escape of tin. Another possibility is that, because tin is known to be biochemically active, it may form mobile organotin compounds under environmental conditions; no analogous property is known for plutonium.

ACKNOWLEDGEMENTS

We wish to thank Kent Orlandini and Wu Tao for assistance in the measurement of plutonium in the sediment fractions. Wu Tao, a radiochemist with the Institute for Radiation Protection in Tiayuan, Peo- ple's Republic of China, had visited Argonne National Laboratory as a Fellow of the International Atomic Energy Agency. The Irish Sea sediment sample was a gift from P. J. Kershaw of the Fisheries Radiobiology Laboratory, UK Ministry of Agriculture, Fisheries and Food, Lowestoft, UK. Assistance in obtaining samples was provided by B. R. Harvey and M. B. Lovett, also of the Fisheries Radiobiology Laboratory; M. W. Findlay of Transducer Research Inc., Naperville, IL; J. Bowling of Procter and Gamble Co., Cincinnati, OH; and K. A. Orlandini of Argonne National Laboratory. D. M. Nelson and G. T. Tisue provided useful discussion and assistance in the determination of beta-energy by absorption. Low-background aluminum foils for the absorber experiments were provided by A. Stein of Argonne's Chemistry Division. The project itself was suggested by T. M. Beasley.


R E F E R E N C E S

Andersen, T. & Knutsen, A. B. (1962). Anion-exchange study: I. Adsorption of some elements in HBr-solutions. Acta Chem. Scand., 16,849-54.

Browne, C. I., Craig, R. P. & Davidson, N. (1951). Spectrophotometric and radiochemical investigation of the interaction between tin(II) and -(IV) in hydrochloric acid solutions. J. Am. Chem. Soc., 73, 1946-51.

Chapman, N. A. & McKinley, I. G. (1987). The Geological Disposal of Nuclear Waste. John Wiley & Sons, Chichester, UK, 280 pp.

Chau, Y. K. (1986). Occurrence and speciation of organometailic compounds in freshwater systems. Sci. Total Environ., 49, 305-23.

Erdal, B. R. & Wahl, A. C. (1968). 121Sn, 1235n, 125Sn, and lZSSn from fission-- purification, half-lives, and128Sn gamma-rayenergies,J. Inorg. Nucl. Chem., 30, 1985-93.

Feather, N. (1938). Further possibilities for the absorption method of investigat- ing the primary beta-particles from radioactive substances. Proc. Cambridge Phil. Soc., 34, 599-611.

General Electric Company (1983). Chartofthe Nuclides. GEC, San Jose, CA., 59 PP.

Glendenin, L. E. (1948). Determination of the energy of beta particles and photons by absorption. Nucleonics, 2, 12-32.

Koide, M. & Goldberg, E. D. (1985). Determination of 99Tc, 63Ni, and 121m+126Sn in the marine environment. J. Environ. Radioactivity, 2,261-82.

Li, Y.-H., Burkhardt, L., Buchholtz, M., O'Hara, P. & Santschi, P. H. (1984). Partition of radiotracers between suspended particles and seawater. Geochim. Cosmochim. Acta, 48, 2011-19.

Minczewski, J., Chwastowska, J. & Dybczynski, R. (1982). Separation and Preconcentration Methods in Inorganic Trace Analysis. Ellis Horwood Ltd, Chichester, UK, 543 pp.

National Research Council (1983). A Study of the Isolation System for Geological Disposal of Radioactive Wastes. National Academy Press, Washington. DC, 345 pp.

Nervik, W. E. (1960). The Radiochemistry of Tin. National Academy of Sciences, Washington, DC, p. 38.

Nyffeler, U. P., Li, Y.-H. & Santschi, P. H. (1984). A kinetic approach to describe trace-element distribution between particles and solution in natural aquatic systems. Geochim. Cosmochim. Acta, 48, 1513-22.

Santschi, P. H., Nyffeler, U. P., Anderson, R. F., Schiff, S. L., O'Hara, P. & Hesslein, R. H. (1986). Response of radioactive trace metals to acid-base titrations in controlled experimental ecosystems: evaluation of transport parameters for application to whole-lake radiotracer experiments. Can. J. Fish. Aquat. Sci., 43, 60-77.

Wahlgren, M. A. & Orlandini, K. A. (1982). Comparison of the geochemical behavior of plutonium, thorium, and uranium in selected North American lakes. In Environmental Migration of Long-Lived Radionuclides. International Atomic Energy Agency, pp. 757-74.

fission product tin in sediments

Documents