Distribution coefficients of selenium in Japanese agricultural soils

Selenium (Se) is a toxic element, an it is a significant pollutant from mining and smelting sites. Among Se radioisotopes, 79Se (half life: 65000 y) is of interest because it is present in spent nuclear fuel and the wastes resulting from reprocessing this fuel. Therefore, knowledge about the environmental behavior of Se is important for the risk-assessment of Se contamination, and also for the safe disposal of radioactive wastes. Se sorption to soil is an important factor limiting Se mobility in the environment. Thus, in this study, we measured soil-soil solution distribution coefficient (Kd) of Se (Kd-Se) as an index of Se sorption level in order to evaluate the mobility of Se in Japanese agricultural soils. We also evaluated relationships between the Kd and soil properties. The Kd value is defined as 'concentration of radionuclide in or on the soil solid phase' divided by concentration of radionuclide in the soil solution'. Soil samples were collected from 110 sites throughout Japan. The Kd-Se was measured by the batch process using 75Se as a tracer. Since the primary sorption mechanism of Se has regarded to be ligand-exchange, which is the same adsorption mechanism as for phosphate, effect of phosphate concentration on the Se sorption onto soil was also determined. After the Kd-Se measurement, soil-sorbed 75Se was extracted by 1 M Na2HPO4 solution for 22 selected samples. Moreover, inhibition of Se sorption by phosphate ion was measured using the batch process Kd-Se measurement for two typical soils under elevated levels of phosphate (0.1-10 mM PO4). The measured Kd-Se ranged from 4 to 1616 L/kg and the geometric mean was 219 L/kg. Among the soil groups, volcanic ash soils (Andosols) had higher Kd values. Previously, it was reported that active-Al and active-Fe were considered to be the major adsorbents of Se in soil. The high Kd-Se values of Andosols may be due to the large amount of active-Al. The extraction experiment showed that the 1 M Na2HPO4 solution extracted 27-83% of soil-sorbed Se (averaged: 57%). This suggested that about 50% of soil-sorbed Se was phosphate exchangeable. In addition, inhibition of Se sorption due to phosphate ion was observed. Kd-Se values decreased clearly with increasing phosphate concentration in the soil solution. When the PO4 concentration was at the same level as that in phosphate fertilized agricultural field, significant inhibition was observed. These results suggested that the Se sorption in Japanese soils was affected by the phosphate concentration in the soil solution. From the 110 Kd-Se data, we produced geographic trends of Kd-Se using the mapping system MdSoiL (Mapping system and database of Soil distribution for elements and radionuclides). MdSoiL is a PC software program developed at NIRS, which can draw contour maps of soil data allowing visualization of the geographic distribution of the data. The contour maps showed that the Kd-Se was higher in Kyushu, Kanto and Tohoku districts than in Chugoku, Shikoku and Kansai districts. These trends corresponded with the geographic distribution of Andosols. Using MdSoiL, we should be able to estimate Se sorption levels in Japan without obtaining Kd-Se data for all types of Japanese soils. (author)