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

A sequential extraction method consisting of six operationally-defined frac tions has been developed for determining the geochemical partitioning of na tural (U, Th, K-40) and antropogenic (Pu, Am, Cs-137) radionuclides in a 40 -50 cm deep sediment sample collected in a Venice canal. Extraction chromatography with Microthene-TOPO (U, Th), Microthene-TNOA (Pu ) and Microthene-HDEHP (Am) column was used for the chemical separation of a single radionuclide; the final recoveries were calculated by adding U-236 , Th-229, Pu-242 and Am-243 as the yield tracers. After electrodeposition t he alpha spectrometry was carried out. Cs-137 and K-40 were measured by gam ma spectrometry. The total concentrations in the wet sample (Bq/kgd), obtained by a complete disgregation of the matrix by wet and dry treatment, were the following: P u239+240=1.03 +/- 0.07, Pu-238 = 0.022 +/- 0.005, Am-241 = 0.337 +/- 0.027, Cs-137 = 9.78 +/- 0.78, U-238 = 28.84 +/- 1.62, Th-232 = 21.42 +/- 1.93, K -40 = 376.05 +/- 12.78. The mean ratio Pu-238/Pu239+240 (0.02) shows a cont amination due essentially to fall-out and U and Th alpha spectra indicate t he natural origin of two elements. The absence of Cs-134 in the sample prov es that at 40 - 50 cm depth the sediment was not affected by the Chernobyl fall-out. As far as the speciation is concerned the following fractions were consider ed: water soluble, carbonates, Fe-Mn oxides, organic matter, acid soluble, residue. Pu (similar to 67%) and Am (similar to 95%) were present principal ly in the carbonate fraction; U was more distributed and about 30% and 45% appeared in the carbonate fraction and in the residue respectively; the maj ority of Th was present in the residue (similar to 60%); K-40 was totally p resent in the residue; finally 137Cs was found mostly in the acid soluble f raction (similar to 53%) and in the residue (similar to 47%). Some stable elements (Fe, Mn, Al, Ti, Ca, Pb, Ea) were also determined in t he different fractions to get more information about the chemical associati on of the single radionuclides.