56. Use of a Combustion Apparatus for Lowlevel ⁹⁹Tc Separations from Soil Samples

Keiko Tagami and Shigeo Uchida

Keywords: ⁹⁹Tc, volatilization, ICP-MS, soil sample

In the present study, To volatilization behaviour was studied to improve our analytical method for ⁹⁹Tc (T_1/2=2.1 x 10⁵y) in environmental soil sampies by ICP-MS.

A soil contaminated with ⁹⁹Tc for 1 y (1-y-aged) was used. At first, the soil was heated in an electrical oven at 45OC for 8 h to decompose organic mat ter. Then 80 g of the incinerated soil, which had already been contaminated with ⁹⁹Tc, was mixed with ^99mTc 3 h before using in the test (freshly added). The sample contaminated with both ^99mTc and ⁹⁹Tc was heated from 500 to 1000C in 100C increments. The volatilization at 1000C was carried out twice. The temperature was raised to the next step during a 10 min period and held there for 50 min. The oven, which was set just before the traps, was kept at 1000C during the study to avoid Tc fixation on the inside of the outer quartz tube.

The trap solutions, each containing 150 ml of deionized water, were changed at the end of each step, then 100 mL of a trap solution was transferred into a plastic vessel (250 ml) . The activity of ^99mTc in the solution was measured with the Ge detector system. For ⁹⁹Tc measurement, the trap solution was passed through a TEVA resin column (Eichrom Industry Inc.) to separate and concentrate the nuclide. The ⁹⁹Tc concentration was measured by ICP IVYS (Yokogawa, PMS-2000)

The percentages volatilized for ^99mTc and ⁹⁹Tc were almost the same; these Tc isotopes showed the same volatility behaviour although their contaminated periods differed. Thus, the freshly added ^99mTc could be used as a yield monitor of originally included ⁹⁹Tc.

The volatility of ^99mTc from the Andosol soil sampie is shown in Fig. 21. The ^99mTc volatility increased with temperature. However, the result of continuous heating at 1000C showed no remarkable increase. Technetium in the soil sample might not be cornpletely volatilized even if the temperature of 1000 C was maintained for a longer time. In our To sepa ration scheme, therefore, we decided to heat the soil sample for 3 h at 1000C

From the results, procedures to improve the method were changed as follows: 1) To in soil (incinerated at 45OC) is volatilized in a combustion apparatus for 3 h at 1000C and trapped in deionized water. 2) The trap solution is passed through a TEVA resin to remove Ru and to concentrate Tc. 3) After the eluate with 8M HNO₃ containing To is evaporated to dryness, To is dissolved in a 2% HNO₃ solution. 4) ⁹⁹Tc in the solution is measured by ICP-MS.

Three environmental soil samples were treated by the method to determine their ⁹⁹Tc concentration. The recoveries varied from 62 to 73%. Although soil samples differed from one another in their land usages and soil types, we saw that the recoveries were almost the same. The ⁹⁹Tc concentrations in these soils were 5 30 mBq kg^-1Tc dry weight basis. The values were similar to previous reports.

Fig.21. Temperature effct on Tc volatilization from Andosol samplea. a:The soil sample was volatilized for 1 h at each temperature. b:1000A and 1000B: First and second volatilizations at 1000C

Publication:
Tagami, K. and Uehida, S.: Radioact. Radiochem.10 30-34, 1999.