Leaching of Radon from Weathered Granite into Water
Jun SATO and Tsshihiro NAKAMURA Department of Industrial Chemistry, School of Science and
Technology, Meiji University
Higashi-mita, Tama-ku, Kawasaki-shi, Kanagawa-Pref. 214, Japan Received September 9, 1993
Leaching efficiency of radon from rock was observed with fresh and weathered granites. Portions of 500 g of samples were brought into contact with a 21 of water, respectively, for the period of 1 month or more. Radon was determined by the liquid scintillation counting The observed radon concentrations ranged from 0.3 to 1.0Bq/l. The leaching efficiencies from the fresh granite were around 2%, while those from the weathered around 6%, indicating that weathering accounts significantly for the enhancement of the leaching efficiency. Microscopic surface areas of the
weathered granite were larger than that of the fresh one, and a positive, but not adequate, correlation between the leaching efficiency of radon and the microscopic surface area was observed.
Depletion of 210Pb in the weathered granite was estimated to be attributed to the escape of radon into the atmosphere and the meteoric water from the outcrop during a long period of time, together with possible leaching of 210Pb itself
Key Words: leaching, radon, weathering, granite
Rocks contain small amounts of uranium with their decay products. Randon, one of the daugh-ter nuclides of uranium, is formed within the rock and a part of it escapes from the surface. Detect-able amounts of randon exist in underground waters and are considered to be supplied from the rocks which constitute the water-bearing layer.
Radon concentrations measured for hot-spring waters from Kakuto Basinl', one of the collapsed calderas in southern Kyushu, were classified into two groups of the low (<0.3 Bq/1) and the high (1- 7 Bq/1) concentration. Data for boring-core samples and the spatial distribution of the hot-springs in the basin indicated that the sources of the low concentration group were located within solid materials (Masaki propylite and Kakuto vol-canics) constituting the basement or margin of the caldera, whereas the sources of the high concen-tration group were all distributed within
un-consolidated, post-caldera sediments (Kyomachi and Ikemure formations2'). No siginficant differ-ence in the radium content was observed between the post-caldera sediments and the volcanic rocks from the margin''. It was suggested that the randon concentration of the hot-spring waters reflected the degree of consolidation of the mate-rials with which waters were in contact.
The outbreak of spontaneous landslide is thought to be closely related to an unusual change in the water circulating systems under the ground. Landslides often occur in the districts constituted by the strata weathered heavily. A continuous measurement of radon concentrations of waters
from the sliding zone and its surrounding areas of a large-scale landslide occurred in Narao dis-trict, Nagano-Pref., revealed that the radon con-centrations in waters from the moving zone was
slightly but consistently higher than those from the surrounding inactive areas, though both groups of concentrations were in the range of
those known in the generally occurring natural underground waters3'.
The present paper deals with the leaching effi-ciency of radon from weathered granite into the water, because leaching efficiency of radon from rocks may have some bearings on the variety of radon concentration in underground waters.
2. Experimental 2-1 Granite samples
As granite contains uranium generally as high as a few ppm which is higher than the content of rocks of other rock type, weathered granite sam-ples were considered to be preferable to estimate the degree of contribution of weathering to leach-ing of radon from the rock surface.
Weathered granite samples were collected from
several outcrops of a single intrusive granite body in Chugoku mountain range (sample identi-fication : JS 770819-1 4, JS 790820-1 '- 3) in Takahashi City, Okayama-Pref. Sampling locali-ties are shown in Fig. 1.
A fresh granite sample having a high uranium content (sample identification : A-144) from Abukuma mountains was kindly furnished by Dr. Kanaya, Geological Survey of Japan, to serve the controlled material.
2.2 Radium and calcium content of granite
The radium contents of these granite samples
were determined by a non-destructive y-ray
spec-trometry. Approximately 100 g of the crushed
gra-nite samples were enclosed in canisters of 79 mmƒÓb
X 13 mmH, and were placed on the front surface of a Ge(Li) detector. Samples were counted for (10 -18) X 104 s. Typi-cal y-ray spectrum is shown in Fig. 2. Counting efficiency for the 609-keV y-ray from 214Bi, one of the equilibrated daugh-ter nuclides of 226Ra, was estimated from the efficiency curve which was con-structed in the manner as has been de-scribed previously4'. Radium contents of the granite samples is given in Table 1 in Bq/g, together with the equivalent con-tents of uranium in ppm (pprUey).
One gram of each granite samples were decomposed with HN03, HC104 and HF by heating on a hot-plate. The product was dissolved in 6 M HCl and hot water followed by heating. After filtration, the filtrate was diluted to 100 ml with de-ionized water. The flame photometry was applied to the determination of calcium by using a Hitachi Model 208 atomic absorption spectrometer. Calcium con-tents are also included in Table 1. Fig. 1 Sampling localities of the weathered
granite samples (JS770819-1~4 and JS 790820-1~3).
Fig. 2 A typical y-ray spectrum of the granite sample.
Spectral lines are all assigned to the U- and Th-series nuclides and 40K. Ra content is deduced from the intensity of the 609 keV r-ray from 214Bi.
2.3 Measurement of radon concentration leached into water
The present method consisted of an extraction of radon into toluene containing scintillators, and a radioactivity measurement with the liquid scin-tillaton spectrometer. Scintillation cocktail was made from a 4 g of PPO and a 0.1 g of dimethyl-POPOP dissolved in a 11 of toluene. A Packard Tricarb Model 3380 liquid scintillation spectrom
eter was employed for the measurement.
The radon extracted into liquid scintillator decays in the counting vial and, after 5 nuclear transformations, reaches the long-lived daughter unclide, 210Pb. Three a-rays and 2 $ -rays are counted in a liquid scintillation spectrometer after the radioactive equilibrium with short-lived daugh-ters was established (approximately 3.2 h after the extraction5~). The counting efficiency for these 5 radiations are estimated to be almost 100% when the integral counting mode is applied.
Table 1 Radium and calcium contents of the granite samples
2.4 Leaching of radon and calcium from granite
The fresh granite from Abukuma mountains (A-144) was crushed by a jaw crusher and the product was classified into 3 grain-size groups (coarse, medium and fine) using a set of sieves. The ranges of the grain sizes are given in Table 2. The grain sizes of 7 weathered granite samples (JS770819-1^'4, JA790820-1^r3) lay in the range of medium and fine.
Two portions of 500 g of fresh granite of re-spective grain size, and portions of 500 g of each
Table 2 Classification of grain size of the crushed fresh granite (A-144)
weathered granite were soaked in a 21 of de-ionized water within a glass bottles respectively at the room temperature. Another 2 portions of 500 g fresh granite of coarse grain size were soaked in a 11 of water under the same experimental condi
tion for the controlled experiment.
After a period of 1 month or more, a 1.21 of respective sample water was introduced through a filter with an aid of compressed air into a 500 ml separatory funnel in order to minimize the free-volume of air, a 25 ml of scintillation cocktail having already been prepared therein. Radon in
the water was extracted into toluene by shaking the mixture for 5 min and the organic layer was recovered into the 24 ml counting vial. The second extraction was carried out with the residu-al aqueous layer, which served to have the extrac-tion efficiency in an individual extraction proce-dure.
The amount of radon extracted in the first
extraction process from the sample water, A,, is A, =rC, where r is the extraction efficiency and C is the radon concentration in the sample water. As-suming that the extraction efficiency r is not changed significantly from the first to the second extraction, the amount of radon extracted in the second extrac-tion, A2, is A2=r(1-r)C. The extrac-tion efficiency r is thus deduced as r =1- (A 2/A , ) for respective extrac-tion process. The mean extracextrac-tion effi-ciency in the present experimental pro-cedure is approximately 60%.
The water in which calcium was leached from granite together with radon was measured by the flame pho-tometry without any further treatment.
Fig. 3 X-Ray diffraction pattern of (A) one of the weathered granite samples (JS790820-1) and (B) the fresh granite sample (A-144).
Symbols stand for ; A : quartz, B : orthoclase, C : albite, D : anorthite and E : biotite. The diffraction line from biotite (20= '-'8 .5°) is absent for the weathered sample.
2.5 X-Ray diffraction pattern of rock samples
An X-ray diffraction powder pattern was observed for mineral compositions of fresh and weathered granites by use of a Shimadzu XD-3A, Powder X-ray Diffractometer. Typical diffraction pat-terns are shown in Figs. 3(A) and
Table 3 Grain size and microscopic surface areas observed for the powdered fresh and weathered granite samples
Table 4 Radon and calcium concentrations in water and thier leaching efficiencies from the granite samples, and microscopic surface areas of 60 -100 mesh grains
* C : coarse, M : medium, F : fine (see Table 2).
(B). Diffraction lines from the fresh granite (B) were assigned to quartz, plagioclase and biotite, while the line from biotite is absent for the weath-ered granite (A).
2.6 Microscopic surface area of samples The fresh granite (A-144) and one of the weath-ered granites (JS790820-1) were crushed and classified into 5 grain sizes using a set of sieves. The microscopic surface area of granite was mea-sured by the BET method. A Shimadzu Model
ACCUSORB 2100E was employed for the mea-surement and nitrogen gas was used for the absorp-tion gas. The range of the grain sizes and respec-tive microscopic surface areas are given in Table 3. Those of other weathered granites crushed to the grain size of 60- 100 mesh are given in Table 4. Weathered granites tend to have microscopic surface areas of one order of magnitude larger than fresh one, suggesting that weathering pro-duces a large amount of microcracks on the rock surface.
3. Results and Discussion 3.1 Leaching efficiency of radon
Radon concentrations in waters observed in the present experimental system are given in Table 4. Though the observed values scattered rather widely, the following profiles are featured.
3.1.1 Radon concentration in water
The concentrations in waters of 11 (A-144, 1 and 2) are twice as much as those in waters of 2 1 (A-144, 3 - 8), showing that the observed radon was entirely leached from the granite in the bottles. The radon concentrations in the 21 of waters in contact with 500 g of granite samples, fresh and weathered, are in the range of 0.3 -1.0 Bq/l. Katsura6' also observed leaching of radon of 1.1- 2.2 Bq/l in 21 of waters which were in contact with 500 g of crushed fresh and weath-ered granite. These values were in the same order of magnitude as was measured in the present observation.
3-1-2 Leaching efficiency
The "leaching efficiency", L, is expressed as L = VX C~aq (Bq/1)/WX CRar~k (Bq/g) where V and W represent the water volume (1 1 or 21) and the mass of granite (500 g), respec-tively, and V(1) X CRn~ (Bq/1) is the portion of radon leached from the surface of the granite particles and W(g) X CRer"k (Bq/g) is the total amount of radon in equilibrium with radium within the granite.
Leaching efficiencies thus calculated with each of samples are tabulated in Table 4. The amounts of radon leached from the granite into the water is less than 10% of those generated within the granite : radon atoms generated in the vicinity of the surface alone may be leached into the water.
The values of the leaching efficiency for the fresh granite range from 1 % to 3%, whereas those for the weathered granite range from 4% to 8 % ; the amounts of radon leached from weath-ered granite are significantly larger than those
from fresh ones, implying that weathering pro-cess may attack the rock structure and the rock-forming minerals, including biotite, to enhance the leaching efficiency.
3-1-3 Grain size of granite samples and leaching efficiency
Samples of smaller grain size have larger bulk specific surface area than those of larger grain size, so that samples of smaller grain size are estimated to give larger leaching efficiency than those of larger grain size.
The geometrical surface area of samples of each grain size can be calculated, assuming that the grains are in the form of spherule. The approxi-mate specific surface area of the "coarse",
medium" and "fine" samples are calculated and given in Table 2. The difference in the surface area between the "coarse" and the "fine" samples is one order of magnitude. The leaching efficiency with fresh granite given in Table 4, however, do not show any clear trend against the difference in grain size, indicating that the apparent difference in bulk grain size is not important for the leaching efficiency of radon.
3.1.4 Microscopic surface area and leaching efficiency
Microscopic surface areas obtained with weath-ered granite of the grain size ranging from 60 to
100 mesh are listed in Table 4. The weathered granites tend to show greater values than those for the fresh granite. These values are compared with the values of the leaching efficiency of radon in Fig. 4 ; higher leaching efficiencies of radon are observed with the weathered granite which have greater surface area, though the correlation is not adequate.
Observable amount of calcium was leached out from granite powders toghther with other major elements into water in a few days''. Calcium con-centrations in the waters which were in contact with them are given in Table 4. Leaching ef-ficiencies of calcium are calculated in the same
Fig. 4 Correlation of the leaching efficiencies of Rn with the surface area.
Symbols stand for ; • : fresh granite samples (A-144,1-8) and 0 : weathered granite samples (JS770819-1-4, JS 79O820-1-3).
manner as was made in the case of radon, and are compared with microscopic surface areas and with leaching efficiency of radon in Figs. 5(A) and (B).
The leaching efficiencies of calcium from the weathered granite were observed to be almost in the range of the fresh granite, indicating that weathering is not so efficient for the chemical leaching of elements as for leaching of radon the apparent contrast suggests that the recoil mechanism may be attributed to the contribution to effective radon release from the rock surface.
3.2 Instability of 210Pb in weathered rocks Figure 6(A) shows the low energy region of the 7-ray spectrum obtaind with the fresh granite (A-144) by an LEPS. Gamma-rays from 210Pb, 234Th
, 235Us 226Ras 228Ac and 212Pb, and X-rays from lead and bismuth are observed. Figure 6(B) is the 7-ray spectrum of the same energy region obtained with the weathered granite
(JS790820-1). A comparison of the two spectra shows that the 46 keV 7-ray from 210Pb is absent in Fig. 6(B), indicating that 210Pb depleted significantly in the weathered granite.
Emanating efficiency of radon into the atmo-sphere from soil particles of weathered granite was
Fig. 5 Correlation of the leaching efficiency
of Ca with (A) the surface area and
(B) the leaching efficiencies of Rn. Symbols stand for ; •œ : fresh granite samples (A-144,1-8) and •› : weathered
granite samples (JS770819-1-4, JS
Fig. 6 Low energy region of y-ray spectrum for (A) a fresh granite (A-144) and (B) a weathered granite (JS790820-1).
observed to be as high as 25%8) which is greater than the leaching efficiency into the water (4-8%) presently observed. The amount of radon escaping from the surface of the weathered gra-nite at the outcrops may be estimated to be of the same order of magnitude.
Large amounts of excess 210Pb in the lake-sediments9' indicates that a significant amounts of 210Pb is also being leached out of the rock surface by the meteoric water. Escape of a small but significant portion of radon generated within the weathered rocks into the atmosphere and into
the surface water, together with leaching of 210Pb itself into the meteoric water, may be responsible for the depletion of 210Pb within the weathered granite.
The authors are gratefully indebted to Mr. Y. Ohoka, Meiji University, for his co-operation in the present measurement. Liquid scintillation counting of radon and 7-ray measurement were carried out at Radioisotope Cnetre, The Univer-sity of Tokyo. The present work was financially supported by the 1993 grant-in-aid from Meiji University.
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