From the viewpoint of nuclear waste management, environmental mobility of tin (Sn) is important because ¹²⁶Sn (half life: 10⁵ y) and ^121mSn (half life: 55.5 y) are fission products of ²³⁵U and are found in nuclear wastes. In soil environments, Sn mobility can be affected by its sorption onto the soil solid phase. Thus, in this study, we studied sorption behavior of Sn in Japanese agricultural soils using radiotracer experiments with ¹¹³Sn tracer. Soil-soil solution distribution coefficient (K_d) of Sn (K_d-Sn) was measured for 110 soil samples from 4 Japanese soil types as an index of the sorption levels. K_d is defined as the concentration of an element in or on the soil solid phase divided by the concentration of the element in the soil solution. Since the Sn solubility is highly pH dependent, the pH effect on Sn mobility in soil-solution system was also investigated. Moreover, we evaluated the soil-sorbed Sn fractions in each type pf soil colloidusing selective extraction because metal sorption in soil is generally controlled by the soil colloids such as clay minerals and soil organic matters.

Materials and methods

Soil samples were collected from throughout Japan in order to include all major soil types in Japanese agricultural field. The collected 110 soil samples were classified into four soil groups, Andosols, Fluvisols, Cambisols, and Regosols. Three grams of each soil sample were placed in a polypropylene bottle (50 mL) and 30 mL of deionized water were added. The suspension was preconditioned by shaking for 24 h at 23°C. About 30 kBq of ¹¹³Sn including about 0.3 µg of stable Sn carrier were then added as SnCl₄. After shaking for 7 days, the solution phase was separated by centrifugation and filtration (0.45 µm). The activity concentration of ¹¹³Sn in the filtrate was measured to calculate the K_d value. In order to determine the pH effect on the Sn sorption behavior, the K_d-Sn values were measured under different pH levels (pH = 4-7) for two soil types (Andosol and Fluvisol). For the determination of soil-sorbed Sn fractions, selective extractions were carried out. The added ¹¹³Sn in the K_d measurement was extracted with four reagents. Sequential extraction with 0.11 M acetic acid and 0.1 M hydroxylammonium chloride solution was carried out for 12 selected soil samples to determine the acid-soluble and Fe/Mn-oxides bound Sn, respectively. For another set of 16 soil samples, extraction with 0.1 M sodium pyrophosphate (pH 10) was used to extract the Sn bound to soil organic matter. Then extraction with 0.2 M ammonium oxalate/oxalic acid (acid-oxalate, pH 3) was used to extract the Sn bound to Al/Fe-(hydr)oxides, such as non-crystalline or short-range-ordered Al or Fe minerals. Since the acid-oxalate solution also can dissolve the metals associated with organic matter, the Al/Fe-(hydr)oxide-bound Sn amount was calculated by subtracting the pyrophosphate extractable Sn from the acid-oxalate extractable Sn.

Results and discussion

For Japanese agricultural soils, the K_d-Sn values were in a wide range; K_ds of Sn ranged between 128-1590000 (L/kg), and the geometric mean value was 15900 (L/kg). These high K_d values indicated that Sn mobility was very low in the tested soil types. Among them, the values for Andosols tended to be higher than the other soil groups. The geometric mean of K_d values for Andosols was 38300 (L/kg). The pH effect on Sn mobility was observed clearly. The K_d measurement under different pH levels showed that the K_d-Sn increased with decreasing pH. The K_ds of Andosols increased from 904 to 20600 (original value: 2920 L/kg) on decreasing pH from 7.0 to 5.0, and the K_ds of Fluvisols increased from 188 to 13700 (original value: 1120 L/kg) on decreasing pH from 7.1 to 3.8. Since Sn solubility increased with decreasing pH, this result suggested that the low pH condition enhanced the sorption reaction of Sn in soils. The sequential extraction showed that the acid-soluble and Fe/Mn-oxide-bound Sn fractions included less than 1% of the soil-sorbed Sn. On the other hand, 18-72% (average: 53%) of the sorbed Sn was extracted by pyro-phosphate solution, and 24-88% (average: 71%) of the sorbed Sn was extracted by acid-oxalate solution. The Al/Fe-(hydr)oxide-bound Sn was found to be 4-30% (average: 18%) of the sorbed Sn. Therefore, most of the soil-sorbed Sn was as organic matter-bound and Al/Fe-(hydr)oxide-bound forms. The high K_d values indicated the ¹¹³Sn added to soils was strongly bound to soil organic matter or Al/Fe-(hydr)oxides, and the soil-sorbed Sn could not be dissolved with pH decrease. Specifically, the high K_d values for Andosols should be due to their high organic matter content.

Acknowledgement

This work has been partially supported by the Agency for Natural Resources and Energy, the Ministry of Economy, Trade and Industry (METI), Japan.

Keywords: Tin, Soil, Distribution coefficient, Sorption, pH, Selective extraction