1 “Uranium and Thorium: Radioactive Refugees Or Simply Irresistible

“Uranium and Thorium: Radioactive Refugees or Simply Irresistible?”

Kathryn Lee Department of Physics Miami University, Oxford Ohio

Research made possible by National Institution of Standards in Technology Summer Undergraduate Research Fellowship 2002

Abstract

Seven common resistate minerals (allanite, apatite, monazite, pyrochlore, titanite, xenotime and zircon) were analyzed to determine their ability to retain radioactive isotopes of uranium and thorium as well as their chemical and physical changes after chemical treatment. Crushed samples of the minerals were heated with concentrated nitric and hydrofluoric acids for up to fourteen days. At three different points during the fourteen days, samples were removed and analyzed using alpha spectrometry, microscopy, and X-Ray diffraction. Results concluded that even after three days of acid treatment, most of the minerals still had a higher radioactivity than average soil. This data can be used to help evaluate the efficacy of radioanalytical methods and improve the certification of standards.

Introduction:

Resistates are minerals associated with soils and sediments that tend to be resistant to chemical weathering. Some common and widespread resistates such as zircon and monazite are known to contain radioactive isotopes of uranium and thorium.

Uranium and thorium can substitute into the minerals due to similar ionic radii or crystalline structure. This usually occurs when the lava that originally forms the minerals is being cooled. As a result of this substitution, these resistates are radioactive in addition to being hard to dissolve and therefore the radioactive isotopes are not washed away by normal weathering processes. This can be problematic in environmental remediation.

The most common and inexpensive form of clean-up for radioactive sites is to remove layers of soil until the top layer meets safety standards. The soil is usually tested for the uranium and thorium content through a standard acid leeching process. This process consists of placing the soil in a beaker with a uranium and/or thorium tracer, adding perchloric and boric acids, heating it until dryness, adding HCl acid, heating the solution until dryness again, and then purifying the solutions with ion exchange columns.

Once the solution has been purified, it is heated until dryness and the residue is electrodeposited and counted by alpha detectors. Alpha detectors counted the samples for fourteen days to determine the amount of U-238 and Th-232 activity in comparison to the known tracers. The ratio of the alpha peaks to the known tracer peaks reveals the uranium and thorium content.

If the soil contains resistates, the sample might not completely dissolve by normal methods, resulting in inaccurate data. An example of this is the soil from the Department of Energy’s Rocky Flats Plant. In the certification of this soil 10 years ago, there were 3

discrepancies of 15% about the content of uranium and thorium in the soil (Inn). After

analyzing this soil, scientists at NIST (National Institute of Standards in Technology)

found suggestive evidence that the discrepancies were due to a small amount (about 0.3%

of the mass) of HF-nitric acid resistant residual material. Upon further analysis of this residual material, a zircon x-ray fluorescence signature was revealed, and an x-ray diffraction spectrum of the sample displayed many of the major zircon lines. Through further research, we found that zircon could contain considerable amounts of uranium and thorium.

This distortion of the uranium and thorium measurements can lead to insufficient environmental remediation, potentially exposing the public to more radiation than anticipated, as well as costing the public more money for a second clean-up of the site.

In order to learn more about resistates, we treated seven common and widespread resistates (allanite, apatite, monazite, pyrochlore, titanite, xenotime, and zircon) with nitric and hydrofluoric acids and then analyzed the samples to see how much of the original sample dissolved and how much uranium and thorium was released during acid treatment. We also analyzed the residual materials for physical and chemical changes with polarizing microscopy and x-ray diffraction. The information obtained from this research will lead to a better understanding about how to dissolve soils and sediments to obtain accurate measurements of uranium and thorium for environmental clean-up work, and for careful certification of standard reference soil and sediment materials that NIST customers use to evaluate the effectiveness of their methodologies.

Experimental Procedure:

The minerals were ordered from Ward Geology Supply of Rochester, NY. After

samples of the minerals were obtained, they were scanned with radiation survey meters to

determine if they were radioactive. The seven samples were considered safe to work

with, but monazite had by far the highest detected radiation rate, 50,000 counts/min.

The minerals were crushed to a powder using a microionizing mill and sieved in a

lab that was deemed acceptable for radioactive materials. Each mineral was then measured into three sub-samples of three to four grams in Teflon beakers. A 1:1

(volume:volume) mixture of concentrated HF and HNO3 was added to the beakers and

they were placed uncovered on hotplates. The samples were kept near boiling, until they

were heated to dryness. Then another aliquot of acid solution was added and the beakers

were covered with watch glasses on the hotplates with the temperature set at about 60o C.

Twenty-four hours later, one sample of each type of mineral was removed from the hotplate and allowed to cool. The cooled samples were each centrifuged with 30 mL

HNO3 for 5 minutes, and the acid solution was discarded. Water was added to the

residue, swirled, centrifuged and the supernatant was discarded. This process was

repeated one more time. The residue was then dried in an empty, preweighed specimen

cup and weighed to determine the amount of material that survived the acid treatment.

Meanwhile, the remaining samples were left on the hotplates and received a new

portion of acid whenever they were near dryness. On the third day, the second sample of

each type of mineral was removed from the hotplate. These samples had been spiked

with 10 disintegrations per minute (dpm) of 232U and 229Th for alpha spectroscopy. The

same centrifuging process was followed and they were dried and weighed. Instead of 5

discarding the nitric acid extract before the first centrifuge, this solution was used for the

alpha spectrometry. After being evaporated to dryness the nitric acid extract samples

were treated with 5 mL HCLO4 and 0.5 mL of saturated H3BO3, followed by heating to dryness with 10 mL of HCl. MP1 anion exchange columns were preconditioned with a series of acid washes for U and Th purification, then loaded with the HCl dissolved extract samples. This was followed by a series of acid washes of HNO3 designed to

extract the thorium. This acid washing process was repeated with HCl acid to extract the

uranium. The resulting purified U and Th solutions were heated to dryness and the

residue was electrodeposited on stainless steel plates that were then alpha counted for two

weeks.

Meanwhile, the fourteen day samples remained on the hotplates at near boiling.

After two weeks, these samples were removed, cooled, centrifuged, dried and weighed.

The residual crystals of the dried one-day, three-day, fourteen-day, as well as the

original untreated samples were analyzed for structural and color changes under a

microscope. The X-Ray diffraction spectrum of each of these samples was also analyzed

for chemical changes from the different stages of the heated acid treatment.

Results and Discussion:

The mass measurements of the three samples after different days of acid treatment

were averaged for each individual mineral because there was no apparent difference in

mass between the 1, 3 and 14 days of acid treatment (Fig. 1). Titanite and pyrochlore

seemed the most readily dissolvable, while allanite actually gained mass, and xenotime

was the hardest to dissolve. Monazite, apatite and zircon dissolved in relatively equal 6

ratios. None of the minerals completely dissolved, even after fourteen days of concentrated acid treatment.

% Dissolved

80.00 60.00 Xenotime

40.00 Monazite Apatite 20.00

% Zircon 0.00 Pyrochlore -20.00 Titanite -40.00 Allanite -60.00 mineral

Fig. 1: Average % Dissolved of Each Resistate.

We noticed that the minerals that dissolved the most in the acid treatment are phosphates, whereas the minerals that dissolved in the smallest amounts belong to the

silicate family (except for pyrochlore, which is an oxide). There is a possibility that the

calcium contained in titanite and allanite could be part of the reason that these minerals

did not readily dissolve. It is believed that allanite might not have dissolved due to the

silicate structure, then re-precipitated combined with some of the elements of the acids.

This would account for the gain in mass. More tests are currently being done with boric acid to see if some of the insoluble residual materials were fluorides. 7

Alpha spectrometry showed that none of the minerals seemed to have a significant amount of thorium content, however most of the minerals retained a higher than normal soil radioactivity (0.03 Bequerel/g) due to the uranium activity (Fig. 2 and 3). Monazite had specifically notable uranium activity even after the acid treatment compared to the other minerals.

Uranium-238 Activity

100.0000

10.0000

1.0000 Log Bq/g 0.1000

0.0100 Minerals

Fig. 2: Uranium activity of the samples.

Thorium-232 Activity

0.1

0.01 Log Bq/ g

0.001

0.0001 Minerals

Fig. 3: Thorium activity of the samples. 8

Linda Selvig, an Albert Einstein Distinguished Educator Fellowship recipient,

continued this research after I left the project. She found that after the standard HF-nitric

acid dissolution method that I used, she could actually obtain even more U and Th from

the precipitate by using a sulfuric acid to redissolve the precipitate. The U-238 counts

after redissolving the precipitate increased by 98% for Xenotime, 42% for Monazite, 80%

for Apatite, 54% for Zircon, 38% for Pyrochlore, and 20% for Allanite. The Th-232 data

is not available yet.

The X-ray diffraction spectra showed that most of the changes within the peaks of

the spectra occurred within the minerals that were the least resistant to the acid treatment and vice versa (see Appendix A). For instance, the peaks for the titanite samples with

varying degrees of acid treatment do not seem to match up at all, whereas all of the

spectra for the xenotime samples are almost identical. The allanite spectra leads us to

believe that the mineral crystal structure became destroyed in some way when it is being

prepared for X-Ray diffraction. This process has since been repeated with allanite using different types of acid, but the X-ray spectra is still illegible. The X-ray spectras are being

further analyzed with an expert to determine if the residue that remained after the acid

treatment can chemically still be considered the same mineral.

Optical microscopy showed the majority of the changes in appearance of the individual crystals occurring within the first day of acid treatment. The crystals generally became much smaller in size and at times changed in color.

Conclusions:

The seven resistates tested did in fact resist prolonged HF-nitric acid treatment, and some can contain considerable amounts of uranium and thorium. A large fraction of the U and Th precipitated using the standard HF-nitric acid leaching process, and therefore wasn’t in the solution that was used for the radiochemical analysis. This implies that a lot of the labs using the standard HF-nitric acid dissolution process are getting low results, and therefore underestimating the radioactivity left in their samples.

This in turn would signify that the radioactive sites are not being cleaned up adequately.

This means that different methods for dissolving soils with evidence of resistates should be developed for more accurate assessment of the uranium and thorium content for environmental clean-up work. The research began with these tests will also lead to more careful certification of NIST standard reference soil and sediment materials.

Works Consulted:

Bates, Robert L., Jackson, Julia A., 1984. Dictionary of Geological Terms, 3rd edn. The American Geological Institute, Random House, Inc, New York.

Chao, T.T, Sanzolone R.F., 1992. "Decomposition Techniques." Journal of Geochemical Exploration, Vol. 44, pgs 65-106, Elsevier Science Publishers B.V., Amsterdam

Deer, W. A., Howie, R.A., Zussman, J. 1966. An Introduction to the Rock-Forming Minerals, Halstead Press, John Wiley & Sons, New York.

Friedlander, G., Kennedy, J.W., Miller, J. M.1955. Nuclear and Radiochemistry, 2nd edn. John Wiley & Sons, Inc. New York

Gaines, R.V., Skinner, C.H., and Others 1997. Dana's New Mineralogy, 8th edn. John Wiley & Sons, Inc. New York.

Handbook of Chemistry and Physics, 50th edn., Weast, Robert C. editor, The Chemical Rubber Co. Cleveland, OH

Inn, Dr. Kenneth. “NIST private communication.”

Jenkins, R., De Vries, J.L. 1973. Practical X-ray Spectrometry,, Springer-Verlag New York Inc.

Matyi, R.J., Baboian, R., "An X-Ray Diffraction Analysis of the Patina of the Statue of Liberty", Powder Diffraction, Vol. 1, No. 4, December 1986

Mineral Database, http://www.webmineral.com, Barthelmy, David, 10/05/02

O'Keeffe, M., Hyde, B.G., 1996. Crystal Structures, 1. Patterns and Symmetry, Mineralogical Society of America Monograph, BookCrafters, Inc., Chelsea, MI.

Selvig, Linda. “NIST private communication.”

Sulcek, Z., Povondra, P., 1989. Methods of Decomposition in Inorganic Analysis, CRC Press, Inc., Boca Raton, FL