DISTRIBUTION OF CD-109 AMONG DIFFERENT SOIL FRACTIONS STUDIED BY A SEQUENTIAL EXTRACTION TECHNIQUE

The association of Cd with different soil components and the rate of exchange between Cd-109 and stable Cd in the soils were studied by using a sequential extraction technique. The experiments were performed after different contact times between Cd-109 and the soils (5 min-221 days). According to the short term batch experiments, based on a loam soil (As, Eastern-Norway), no significant differences in the distribution of Cd-109 were observed after a contact time of about 1 hour. On average, the relative distribution of Cd-109 among exchangeable, easily reducible, oxidizable and acid-digestable fractions was about 25, 60, 10, and 2%, respectively. In the pot experiment, a loam soil (As) and a loamy sand (Birkenes, Southern-Norway) were adjusted to pH 5.5 and 6.5 and spiked with Cd-109 before storing them in pots at room temperature and at about 15% moisture content. Cd-109 was rapidly distributed, and the two soils showed only minor differences with respect to the distribution of Cd-109 among soil fractions with time (2-221 days). The amounts of Cd-109 in the more easily available fractions; NH4-acetate fraction (F1) and NH2OH . HCl fraction (F2), however, were higher in the soil from Birkenes (pH 5.0) which was originally more acidic than the soil from As (pH 6.1). The Cd-109/stable Cd ratio, calculated from the relative distribution of radioactive and stable Cd in the different fractions, did not vary significantly from 2 days to 221 days, but the ratios were higher in the more easily available fractions; i.e. NH4-acetate fraction (F1) and NH2OH . HCl fraction (F2). The fixation of Cd in soils appears to be a slow process as the relative distribution (%) of Cd-109/stable Cd in the strongly bound fraction; extracted with 7 M HNO3 at 80-degrees-C was only 0.1-0.2 after 221 days.