A new method for determination of uranium isotopes in water, vegetation and soil by alpha spectrometry

National Environmental Protection Agency, Via V. Brancati 48, 00144 Roma, Italy

Abstract

A new and accurate method for determination of uranium isotopes (²³⁸U, ²³⁴U and ²³⁵U) in environmental samples by alpha spectrometry has been developed. Uranium is preconcentrated from filtered water samples by coprecipitation with iron (III) hydroxide at pH 9-10 using ammonia solution and the precipitate is dissolved with HNO₃ and mineralised with H₂O₂ and HF; uranium in biological samples is ashed at 600 °C, leached out with Na₂CO₃ solution and mineralised with HNO₃, HF and H₂O₂; uranium in soil samples is melted with Na₂CO₃ and Na₂O₂ at 600 °C and leached out with HCl, HNO₃ and HF. The mineralised or leaching solution in 2 M HNO₃ is passed through a Microthene-TOPO (trioctyl- phosphine oxide) column; after washing uranium is directly eluted into a cell with ammonium oxalate solution, electrodeposited on a stainless steel disk and measured by alpha spectrometry. The lower limits of detection of the method are 0.37 Bq kg^-1 (soil) and 0.22 mBq l^-1 (water) for ²³⁸U and ²³⁴U and 0.038 Bq kg^-1 (soil) and 0.022 mBq l^-1 (water) for ²³⁵U if 0.5 g of soil and 1 litre of water are analysed. Five reference materials supplied by IAEA have been analysed and reliable results are obtained. The average uranium yields for waters, mosses, lichens and sediments are 74.5±9.0%, 80.5±8.3%, 77.8±4.9% and 89.4±9.7% respectively.