Mobility and accumulation of selenium and its relationship with other heavy metals in the system rocks/soils–crops in areas covered by black shale in Korea

The purpose of this study is to investigate the enrichment level and dispersion patterns of Se and associated elements in rocks, soils and crops which were collected in the Dukpyung and the Chubu areas covered with black shales of the Okchon Group in Korea. Rock and soil samples were analyzed for Se including multiple elements by ICP, ICP-MS and INAA, and crop samples by INAA. Soil pH and loss-on-ignition (LOI) were also measured.Selenium concentration in black shales and slates of the study area is relatively higher than that in the crust and shales, but lower than that in average black shales. Soil pH values range from 5 to 6, and LOI is in the range of 3 to 7%. The mean concentrations of Se in mountain, farmland and paddy soils overlying black shales and slates are 4.4 mg/kg, 1.8 mg/kg and 1.5 mg/kg, respectively. In particular, the maximum level of Se is up to 72.6 mg/kg in mountain soils, which is much higher than the world average concentration of Se. A significant level of Se was found in soils collected from the previous exploration sites for uranium-bearing black shales. Selenium concentration in soils derived from black shales and slates is enriched more than 2 times compared with that from nearby limestones and phyllites. Distribution patterns of Se also show a very similar trend with those of uranium. Scatter diagrams for Se and other elements in soils overlying black shales and slates show that Se has a significant correlation with Ag, As, Mo, U, V, and Zn, particularly with Ag (r = 0.71), As (r = 0.68), Mo (r = 0.66) and V (r = 0.67) at p < 0.001.The mean concentrations of Se in rice grains, rice stalks and Chinese cabbage from the Dukpyung area are 0.4 mg/kg, 2.2 mg/kg and 5.0 mg/kg, respectively, and these levels are much higher than those of food plants. In particular, the maximum level of Se is 6.9 mg/kg in Chinese cabbage, which is much higher than the world average content of Se in vegetables.

Research highlights►The maximum Se concentration is 72.6 mg/kg in mountain soils derived from black shales. ►The distribution pattern of Se shows a very similar trend with that of U. ►The BAC of Se, As, and Mo in crop plants decreases in the order of Chinese cabbage > rice stalk > rice grains. ►This indicates that leafy plants accumulate these elements from soil to a great degree than cereal plants.