Reload Index (ZRiChK UMCS)

IMPROVEMENT OF SEQUENTIAL EXTRACTION METHOD OF 238Pu, 239,240Pu, 237Np AND 241Am IN ENVIRONMENTAL SAMPLES

 

Andrzej KOMOSA, Stanisław CHIBOWSKI, Jolanta ORZEŁ

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 aim of this paper is to improve the known sequential extraction method for the phase separation first introduced by Tessier [1] and modified by Cook at al. [2]. Our modified method consists in extraction of six various fractions: (i) water soluble, (ii) easily available, isolated by leaching with 1M MgCl2 solution, (iii) carbonate bound fraction, extracted with 1M acetic buffer, (iv) sesquioxide Fe/Mn bound fraction, isolated with 0.04M NH2OH·HCl dissolved in 25% acetic acid, (v) organic bound fraction, extracted with H2O2 and next with 3.2M ammonium acetate, and (vi) residual fraction, which was leached with 6M HCl solution. The 238Pu, 239,240Pu, 237Np and 241Am were analyzed in aliquot of every extracted fraction using appropriate radiochemical method. The plutonium was purified by co-precipitation with Fe(OH)3 and calcium oxalate followed by separation on Dowex 1x8 with HNO3, HCl and HCl/NH4I [3]. The 237Np was determined from plutonium spectrum using 242Pu tracer. The 241Am was analyzed by further radiochemical treatment of the eluate from column using anionic exchange method from mixed HNO3/methanol medium. Finally, these elements were electroplated onto stainless steel discs and counted by high-resolution alpha spectrometry (Canberra Alpha Spectrometer with the PIPS detector and the Genie-2000 software for quantitative isotope determination).

The subject of our study was a mud sample taken from a nuclear waste storage, which was provided by our Spanish collaborator C. Gascó (CIEMAT, Madrid).

The results of total concentration of isotopes of interest determined in the mud sample are presented in Table 1.

 

Table 1. Total concentration of transuranium isotopes in the analyzed mud sample.

 

Isotope

[Bq/g of sample]

238Pu

0.076 ± 0.008

239,240Pu

3.45 ± 0.19

237Np

0.785 ± 0.076

241Am

2.739 ± 0.166

 

The distribution of plutonium between separated phases of soil samples is presented in Figure 1.

 

 

Figure 1. Plutonium distribution between geochemical phases of the mud sample.

 

         As it is seen sequential extraction technique is good tool for predicting the association of transuranics to the sediment or soil geochemical phases. Plutonium is associated mainly with organic/oxides fractions. The soluble fraction of plutonium is negligible. Neptunium seems to be the more soluble and americium shows a tendency to be associated to oxides/carbonates fraction.

 

References:

 

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

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

[3] A. Komosa, Fizykochemiczne problemy oznaczania i zachowanie się izotopów plutonu w środowisku z uwzględnieniem beta-promieniotwórczego 241Pu, Wyd. UMCS, Lublin 2003, str.178+XII.