Determination of thorium isotopes in mineral and environmental water and soil samples by α-spectrometry and the fate of thorium in water

A method has been developed for determination of thorium isotopes in water and soil samples by α-spectrometry. After fusion with Na2CO3 and Na2O2 at 600 °C, soil samples were leached with HNO3 and HCl. Thorium in water sample or in soil leaching solution was coprecipitated together with iron (III) as hydroxides and/or carbonates at pH 9 with ammonia solution, separated from uranium and other α-emitters by a Microthene-TOPO (tri-octyl-phosphine oxide) chromatographic column, electrodeposited on a stainless steel disk, and measured by α-spectrometry. The method was checked with two certified reference materials supplied by the IAEA, and reliable results were obtained. The detection limits of the method for water (soil) samples are 0.44 μBq l−1 (0.070 Bq kg−1) for 232Th, 0.80 μBq l−1 (0.13 Bq kg−1) for 230Th and 1.0 μBq l−1 (0.16 Bq kg−1) for 228Th, respectively, if 100 l of water (0.50 g) for each sample are analysed. A variety of water or soil samples were analysed using this procedure and giving average thorium yields of 75.5±14.2% for water and 93.4±4.5% for soil. The obtained concentrations of thorium isotopes in water samples are in the range of 0.0007–0.0326 mBq l−1 for 232Th, ⩽0.0008–0.0258 mBq l−1 for 230Th and 0.0014–1.32 mBq l−1 for 228Th. The 230Th/232Th and 228Th/232Th ratios are in the range of ⩽0.57–3.9 and 1.06–717, respectively. The disequilibrium between 232Th and 228Th activity in water is observed and the fate of thorium isotopes in water was studied. The exposure impact due to intake of thorium in the analysed drinking water was evaluated, showing a negligible amount of dose contribution. The concentrations of 232Th, 230Th and 228Th in the analysed soil samples are in the range of 30.2–48.6, 32.5–60.5 and 31.0–53.0 Bq kg−1, respectively. The obtained mean ratio is 1.04±0.05 for 228Th/232Th and 1.20±0.41 for 230Th/232Th.