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STUDY ON GEOCHEMICAL ASSOCIATION OF PLUTONIUM IN SOIL USING SEQUENTIAL EXTRACTION TECHNIQUE

 

ANDRZEJ KOMOSA

DEPARTMENT OF RADIOCHEMISTRY AND COLLOID CHEMISTRY

 

 

Plutonium isotopes, which can be found in the environment, come from nuclear weapon testing and the Chernobyl incident. At present the majority of plutonium exists in the surface layer of soil, bound with various soil components. From this inventory plutonium migrates vertically downwards a soil profile and is taken by plants. Mobility of plutonium in the environment is controlled by many factors such as soil composition, its physicochemical properties, meteorological conditions and other. The most important factor, which controls the transport of plutonium, is its chemical speciation in soil. Determination of this parameter is difficult because of a very small amount of plutonium in environmental samples. More easily is to establish plutonium concentration in several geochemical phases, which can be isolated from soil by means of suitable extractants. These phases differ in chemical composition and properties. Plutonium present in the phase as trace element reveals the same properties as a bulk of the phase. In this way, although a mechanism of radionuclide binding inside the phase is not known, is possible to correlate the properties of the phase with its availability by plants or migration rate in soil.

The extraction method introduced by Tessier [1] is largely applied in soil science for the phase separation. This method utilizes acids, chelating agents and inorganic (buffered or not) salt solutions as single or sequential extractants. Most often the following fractions are isolated: exchangeable or adsorbed ion fraction (which are released by neutral salt, such as MgCl2, CaCl2 or NaNO3), carbonate bound fraction (extractable with acetic acid or acetic buffer), organically bound fraction (separated by oxidizing destruction with H2O2 or complexing with alkaline solution of sodium pyrophosphate), fraction bound to hydrous oxides of Fe and Mn (released by means of ammonium oxalate or by reduction with NH2OH·HCl), and residual fraction. Various modifications of the Tessier’s method are present in the literature [2,3]

In presented study two sequential extraction procedures were used with the aim to isolate the geochemical phases of soil samples. There were soil samples collected from the area lying near Chernobyl (named „BR”) and in two other places in SouthEast („WR”) and NorthWest („BS”) Poland. Five geochemical phases were separated from every sample and submitted to plutonium isotope determination. Plutonium analysis was performed by radiochemical separation of this element using hydrochloric acid leaching, coprecipitation with ferric hydroxide and calcium oxalate, anion exchange, electroplating of plutonium and alpha spectrometric measurement. The details of the procedure was described elsewhere [4].


The distribution of plutonium between separated phases of soil samples is presented in Fig. 1-3. As it is seen in the case of BS samples the greater part of plutonium is organically bound and connected with sesquioxides. This is the case also with BR samples. Different behaviour of plutonium is observed with WR samples - its distribution between extracted phases is almost uniform. Such difference can be connected with various origin of the soil. The BS and BR samples were coming from uncultivated meadow areas, sample WR from river valley, frequently flooded by periodical rise of water level. It is worthy of notice that in BS and BR samples the amount of plutonium bound with easily available phase, which is responsible for uptake by plants, is negligible.


Fig. 1. Plutonium distribution between geochemical phases of the BS soil sample.


Fig. 2. Plutonium distribution between geochemical phases of the BR soil sample.

Fig. 3. Plutonium distribution between geochemical phases of the WR soil sample.

 

References:

[1] A. Tessier, P.G.C. Campbell, M. Bisson, Anal. Chem. 51 (1979) 844.

[2] M.A. Haque, T. Nakanishi, J. Radioanal. Nucl. Chem. 239 (1999) 565.

[3] G.T. Cook, M.S. Baxter, H.J. Duncan, J. Toole, R. Malcolmson, Nucl. Instrum. Methods Phys. Res. 223 (1984) 517.

[4] A. Komosa, Polish J. Environm. Studies 7 (1998) 89.