Determination and speciation of plutonium, americium, uranium, thorium, potassium and 137Cs in a Venice canal sediment sample

A sequential extraction method consisting of six operationally-defined fractions has been developed for determining the geochemical partitioning of natural (U, Th, 40K) and anthropogenic (Pu, Am, 137Cs) radionuclides in a 40-50 cm deep sediment sample collected in a Venice canal. Extraction chromatography was used for the chemical separation of a single radionuclide; the final recoveries were calculated by adding 232U, 228Th, 242Pu and 243Am as the yield tracers. After electrodeposition the alpha spectrometry was carried out. 137Cs and 40K were measured by gamma spectrometry. The total concentrations in the wet sample, obtained by a complete disgregation of the matrix by NaOH fusion, were the following: 239+240Pu = 610 ± 80 mBq/kg, 238Pu = 13 ± 5mBq/kg, 241Am 200 ± 30 mBq/kg, 137Cs = 6.9 ± 1.1 Bq/kg, U = 1.33 ± 0.14 ppm, Th = 2.69 ± 0.26 ppm, K = 0.72 ± 0.04 %. The mean ratio 238Pu/239+240Pu ( 0.02) shows a contamination due essentially to fall-out and U and Th alpha spectra indicate the natural origin of two elements. The absence of 134Cs proves that the sediment was not affected by the Chernobyl fall-out. For the speciation, the following fractions were considered: water soluble, carbonates, Fe-Mn oxides, organic matter, acid soluble, residue. Pu (about 65%) and Am (about 90%) were present predominantly in the carbonate fraction; U was more distributed and about 30% appeared both in the carbonate fraction and in the residue; the majority of Th was present in the residue (about 67%); 40K was totally present in the residue; and 137Cs was found mostly in the acid soluble fraction (about 46%) and in the residue (about 40%)