ИСТИНА

We measured the quantity of Pu occurring in solutions in ionic and colloid forms. 239Pu was added as mark to 100 ml of Barents Sea water sample. The sample was mixed, allowed to stand for 24 h, and then filtered. The filtrate was subjected to ultracentrifugation (18000 revo/min). The 239Pu content was then measured in each fraction. The obtained data on the Pu distribution shows that about 38% of it is sorbed on suspended particles, and 20.9% is in the ionic form. The remaining Pu is either in the colloid state, or sorbed on other colloid particles to form pseudocolloids. Therefore, the flocculation precipitation of colloids and suspended particles on freshly precipitated chitosan could be an effective method to concentrate plutonium. The obtained results on the Pu and U coprecipitation on low-molecular chitosan (LMC) in salt solutions allowed us to consider the possibility of using LMC for Pu preconcentration in sea water with its simultaneous separation from U. Using LMC already forming a bulk precipitate at pH 6 excluded the possibility of its simultaneous coprecipitation with Ca2+ and Mg2+ occurring in sea water in microquantities (0.39 and 1.23 g/l, respectively). To develop the method, we studied the influence of the LMC content in the 0-1 g/l concentration range on the Pu and U coprecipitation degree in the Barents Sea water. The data on the kinetics of the Pu and U coprecipitation from the Barents Sea water show that the kinetic equilibrium on LMC was reached over 20-30 min for either element. Also it was found that coprecipitation degree for Pu was 92% at [LMC] = 0.2 g/l, and it increased insignificantly (95%) in the 0.2-0.7 g/l concentration range. Increasing the chitosan concentration further on had hardly any effect on the Pu coprecipitation efficiency. In contrast to Pu, the U coprecipitation degree increased monotonically over the entire LMC concentration range studied. Thus, coprecipitation degree for U was 50% at [LMC] = 0.2 g/l, whereas at [LMC] = 1.0 g/l, it increased to 70%; i.e., virtually 1.5-fold. Therefore, the investigations were carried out at [LMC] = 0.2 g/l, at which the difference in coprecipitation degree for U and Pu was the highest. Based on the obtained results, we proposed a method of sea water analysis for Pu concentration. The method was tested in expeditions in the Kara and Barents seas. The water samples were taken at different sea levels. The sample volumes were 60 to 100 l. Two liters of an LMC solution ([LMC] = 10 g/l and рН 3) was added to the sample. The coprecipitation of Pu on LMC was carried out by adding an ammonium solution during vigorous mixing and varying рН to 6. The resultant chitosan precipitate obtained after settling (12 h) and decanting was filtered and dried at 95оС in the air. The LMC precipitate containing Pu and U was burnt in a mixture of H2O2 and concentrated HNO3 taken at a volume ratio of 1:5. The obtained small mineral residue was dissolved in 20 ml of 7 М HNO3 solution. The radiochemical separation of Pu and U was carried out using extraction with trioctylamine (TOA) followed by reducing re-extraction with a solution of hydrochloric-acid hydroxylamine. The dry solid after evaporation was dissolved in 5 ml of a 7 M HNO3 solution. The obtained solution was used for making the target for alpha-spectrometry. The experiments carried out using the proposed method showed that the chemical yield of Pu from the samples taken in the Stepovoi Bay of the Novaya Zemlya Archipelago was as high as 84% versus 43% obtained by the traditional methods using Fe(OH)3 as a coprecipitator. The alpha-spectra of these samples show the presence of 238Pu, 239Pu, and 240Pu isotopes.