Distribution of ⁹⁰Sr Activities in the Environmental Radiation Samples of Jeju Island, Korea

1. Materials and apparatus

All chemicals used are of analytical grade. The carrier solution Sr(NO₃)₂ is prepared by weighing from the corresponding salt. The soil and moss samples were selected from a collection of samples gathered from the Jeju Island. Reagents used for the digestion of the moss samples were 37% analytical grade HCl (Merck, Darmstadt, Germany) and 65% analytical grade HNO₃ (Merck, Darmstadt, Germany). Sr-Spec resin (particle size: 100 μm to 150 μm) was obtained from Eichrom Industries, Inc. (Lisle, IL). A standard solution of ⁹⁰Sr/⁹⁰Y, provided by Korea Research Institute of Standards and Science (KRISS), was used for calibrating the counting efficiency of ⁹⁰Sr and ⁹⁰Y as a function of quenching. Separation of ⁹⁰Sr and ⁹⁰Y was carried out with extraction chromatography utilizing a Sr-Spec column. Ultima Gold LLT (Perkin Elmer, Waltham, MA) scintillation cocktail was mixed with the sample solution in a 20 mL low-potassium glass vial. Quantulus 1220 (Wallac, Turku, Finland; now Perkin Elmer, Waltham, MA) was used for the measurement of radiostrontium by liquid scintillation counting (LSC).

2. Sampling

This study took place at Jeju Island, ca. 1,400 km southeast of the Fukushima Daiichi Nuclear Power Plant. Annual rainfall of Jeju Island ranges between 1,000 mm and 1,800 mm, with an average of 1,460 mm. The altitude of Mt. Halla (center place of Jeju Island) is 1,950 m. Figure 1 shows the geographic distribution of the sampling locations in Jeju Island.

Fig. 1

Distribution of the sampling locations in Jeju Island.

The soil samples were collected from 4 locations in 2013 (Figure 1, Table 1). Sampling area and locations are presented in Table 1. The sampling locations were selected based on the following preferences: the land should be exposed to open air without any growing tree except for small grasses, and it should be flat and stable with very little disturbance induced by natural and human actions. Our soil samples stem from the eastern part (Sangumburi, 424 m), northern part (Gwaneumsa, 548 m), the western part (Dolohreum, 692 m) and the southern part (Miaksan, 472 m) of the Mt. Halla in Jeju island. At each location, the soil sample was collected from a square area of 50×50 cm after removal of organic matters such as leaves or twig pieces lying on the ground. A rectangular soil core (15×10 cm, 20 cm depth) was taken with the help of a steel coring tool. The cored sample was carefully divided into 10 depth subsamples in every 2 cm. In order to remove moisture, the soil sample was dried for 24 h in an oven kept at 105°C. The dried soil was then passed through a sieve with 2 mm holes, and pulverized for a homogeneous granularity.

Table 1

Geographical Information of the Soil Sampling Locations

The moss samples were collected from 5 locations in 2011 and 2012 (Figure 1). Sampling area and locations are presented in Table 2. The moss sample was collected by scraping it off stones or rocks existing nearby. The raw soil sample collected was weighed after removal of rubble sand organic matters to ensure that similar amounts of samples had been collected at all locations. Each moss sample was carefully cleaned from soil and other debris in the laboratory. The samples were rinsed in water twice to remove any remaining debris or soil particles. All samples were dried at 105°C for 24 hours. The samples were homogenized and powdered in the laboratory.

Table 2

Geographical Information of the Moss Sampling Locations

3. Strontium separation of soil

To determine the typical radiometric and gravimetric yields, carriers were spiked into the samples. The samples were then digested with HCl. The analysis flowchart is shown in Figure 2. The 100 g dried soil samples were ashed at 550°C in an electric furnace. Soil ashes were dissolved in 9 M HCl (500 mL), and 10.0 mg·mL⁻¹ Sr carrier (1.0 mL) was added. After stirring for 5 h, the sample solution was filtered; the beaker and filter were washed with 9 M HCl (500 mL) and a few milliliters of hot water. Iron precipitate was removed from the alkaline solution (>pH 10) by conc. NH₄OH. A few milliliters of HCl (<pH 1) and oxalic acid (40 g) were added to the solution. Strontium oxalate was precipitated by conc. NH₄OH (ca. pH 4.2). The precipitate was filtered through a GF/C filter paper. The precipitate was dissolved in 8 M HNO₃ (ca. 30 mL). Subsequently ⁹⁰Sr was purified on Sr-Spec resin. After washing with of distilled water (50 mL), the Sr-spec column was preconditioned with 8.0 M HNO₃ (20 mL). Then the sample solution was loaded onto the column. After rinsing with 20 mL of 8.0 M HNO₃ solution, strontium was stripped with 20.0 mL of distilled water (or 0.05 M HNO₃) and collected in a glass vial. The flow rate for all procedures was kept at 1 mL·min⁻¹.

Fig. 2

The flowchart of procedure for isotopic analysis of ⁹⁰Sr in soil and moss samples.

4. Strontium separation of moss

The whole of each sample was ashed at 550°C in an electric furnace. The mass of ashes from the samples was in the range 5 to 20 g depending on sample size. Moss ashes were dissolved in 250 mL of 9 M HCl and 0.1 mL of 10.0 mg·mL⁻¹ Sr carrier was added. After stirring for 5 hours, the sample solution was filtered; the beaker and filter were washed with a few milliliters of 9 M HCl. 20 g of oxalic acid was added to the solution. Strontium oxalate was precipitated by conc. NH₄OH (ca. pH 4.2). The precipitate was filtered through a GF/C filter paper.

The precipitate was dissolved in 8 M HNO₃ (ca. 30 mL). Subsequently ⁹⁰Sr was purified on Sr-Spec resin. After washing with of distilled water (50 mL), the Sr-spec column was preconditioned with 8.0 M HNO₃ (20 mL). Then the sample solution was loaded onto the column. After rinsing with 20 mL of 8.0 M HNO₃ solution, strontium was stripped with 20.0 mL of distilled water (or 0.05 M HNO₃) and collected in a glass vial. The flow rate for all procedures was kept at 1 mL·min⁻¹.

5. Radioactivity determination

A saturation Na₂CO₃ (5 mL) was added to the stripped solution of Sr-Spec resin in the alkaline solution (>pH 10), and then allowed to stand at 50°C until a white precipitate (SrCO₃) separated. This was collected by centrifugation, and thoroughly dried in vacuo. The Sr yield of the respective soil sample was measured by gravimetric method. The dried sample was dissolved in 8.0 mL of 0.05 M HNO₃. Then 8.0 mL of this solution transferred to 20 mL low-potassium glass vial and 12 mL of Ultima Gold LLT scintillation cocktail; to make a total volume of 20 mL. The vial was shaken vigorously and immediately counted by LSC on a Quantulus1220. The lower limit of detection (LLD) of ⁹⁰Sr was calculated according to [18]. It was 0.1 Bq·kg⁻¹ for soil samples (counting times and chemical recovery as with ash samples, sample mass: 100 g of dried soil).

To determine the activity of radiostrontium of a sample, the counting efficiency of the radiostrontium isotope with a quench level must be measured. This was done by counting a quench series of reference standards for each isotope. Five milliliter of ⁹⁰Sr (27.96 Bq·mL⁻¹) standard solution was added in a teflon vessel and evaporated to near dryness by heating on a hot plate at 150°C. The dried standard was dissolved in 10.0 mL of 8.0 M HNO₃, then loaded onto a Sr-spec column prepared as for the samples. After washing with 30 mL of 8.0 M HNO₃ solution, strontium was stripped with 5×4 mL of distilled water (or 0.05 M HNO₃) and collected in a glass vial. The flow rate for all procedures was kept at 1 mL·min⁻¹. The solution evaporated by heating on a hot plate at 100°C. The dried sample was dissolved in 10.0 mL of 0.05 M HNO₃. Then 2.0 mL of this solution transferred to five 20 mL low-potassium glass vial and 18 mL of Ultima Gold scintillation cocktail and nitromethane (0 μL, 150 μL, 300 μL, 450 μL, 600 μL) of quenching agent were added, to make a total volume of 20 mL. The vial was shaken vigorously and immediately counted by LSC.

1. ⁹⁰Sr activity concentrations of soil

The ⁹⁰Sr activity concentrations of soil samples are shown in Table 3. The ⁹⁰Sr vertical concentrations in the investigated soil samples were 18.24±0.42–2.77±0.22 Bq·kg⁻¹ in eastern part (S1; Sangumburi, 424 m), 18.27±0.28–1.69±0.11 Bq·kg⁻¹ in northern part (S2; Gwaneumsa, 548 m), 45.27±2.60–3.76±0.19 Bq·kg⁻¹ in the western part (S3; Dolohreum, 692 m) and 8.70±0.59–1.09±0.14 Bq·kg⁻¹ in the southern part (S4; Miaksan, 472 m) of the Mt. Halla in Jeju island, respectively.

Table 3

Activity Concentrations of ⁹⁰Sr in Jeju Island Soil Samples with Depth

Activities of ⁹⁰Sr show the highest value at the surface soil and decrease with depth (Figure 3).

Fig. 3

Vertical distribution of ⁹⁰Sr activities in the soils from Jeju island: (A) S1, (B) S2, (C) S3, and (D) S4 sites.

2. ⁹⁰Sr activity concentrations of moss

Moss samples were measured immediately after extraction, one week later and finally after two weeks. Samples were spiked prior to digestion to correct for losses of analytes during sample preparation procedure. The Sr concentrations in the investigated mosses samples were 363±3.0 Bq·kg⁻¹, 97.7±1.0 Bq·kg⁻¹, 91.7±0.7 Bq·kg⁻¹, 79.6±0.8 Bq·kg⁻¹, and 225±2 Bq·kg⁻¹ (-dry weight), respectively. The chemical yield and the ⁹⁰Sr activity concentration of each sample are shown in Table 4. The measured ⁹⁰Sr concentration is higher at high-altitude locations.

Table 4

⁹⁰Sr Activity Measured with Liquid Scintillation Counting in Moss Samples from Jeju Island