Geochemistry of radium in soils of the Eastern United States |
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| Institution: | 1. College of Chemistry and Chemical Engineering, College of Medical Science, Shaoxing University, Zhejiang 312000, PR China;2. Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, PR China;3. NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia;4. School of Electronic Engineering, Dongguan University of Technology, Guangdong 523808, PR China;1. University of Georgia, Department of Geography, Athens, GA 30602, USA;2. University of Georgia, Department of Anthropology, Athens, GA 30602, USA;3. Université de Toulouse Jean Jaurès, Toulouse, France;1. Helmholtz – Zentrum Dresden-Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, 01328 Dresden, Germany;2. Helmholtz – Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, Bautzner Landstraße 400, 01328 Dresden, Germany;3. Leibniz Institute of Plant Biochemistry, Weinberg 3, 06120 Halle (Saale), Germany |
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| Abstract: | The abundance and chemical/mineralogical form of 226Ra, 238U and 232Th were determined on samples of soil and associated vegetation at 12 sites in the eastern United States. Progressive, selective chemical extraction plus size fractionation determined the abundance and radiometric equilibrium condition of these nuclides in 6 operationally defined soil fractions: exchangeable cations, organic matter, “free” Fe-oxides, sand, silt, and clay.In soils, profile-averaged 226Ra/238U activity ratios (AR) are within 10% of unity for most sites, implying little fractionation of U and Ra when the entire soil profile is considered. However, 226Ra greatly exceeds 238U activity in most surface soil (AR up to 1.8, av 1.22), in vegetation (AR up to 65, av. 2.8), in the exchangeable+organic fraction (AR up to 30, av. 13), in some soil Fe oxides (AR up to 3.5, av. 0.83) and in the C horizons of deeply weathered soils (AR up to 1.5).A major factor in Ra behavior is uptake by vegetation, which concentrates Ra>U and moves Ra from deeper soil to surface soil. Vegetation is capable of creating the observed Ra excess in typical surface soil horizons (AR up to 1.8, av. 1.22) in about 1000 a. Of the total Ra in an average A horizon, 42% occurs as exchangeable ions and in organic matter, but only 6–8% of the parent U and Th occur in these soil forms. In contrast, U is slightly enriched relative to Ra in Fe-oxides of A horizons, implying rapid chemical partition of vegetation-cycled U and Ra.In deeper horizons, transfer by vegetation and/or direct chemical partitioning of Ra into organic and exchangeable forms provides a source for unsupported 226Ra in Ra-rich organic matter, and leaves all soil minerals Ra-poor (AR=0.73). Organic matter evidently has a strong affinity for Ra.The phenomena discussed above are relevant to evaluation of indoor Rn hazard, and behavior of Ra at sites affected by radioactive waste disposal, phosphate tailings, Ra-rich brine, and uraniferous fertilizer. |
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