The significance of (radio-) labile cadmium pools in soil

The radio-labile fraction of cadmium, in a wide range of contaminated soils, was studied in relation to fractions of cadmium defined by traditional extraction procedures. The proportion of labile Cd which was exchangeable was highly pH-dependent, but above pH 5 only about 5% of labile Cd was extractable with 1 m KNO(3). By contrast, EDTA released more than the labile Cd pool into solution, suggesting that significant dissolution of metal adsorbents may occur when chelating agents are utilised as trace metal extractants. The effect of time in immobilising Cd added to soil was also illustrated by following changes in the radio-labile Cd concentration. In calcareous soils, an instantaneous 'fixation' reaction as well as a time-dependent transfer to non-labile pools was identified. It is suggested that radio-labile Cd represents a relatively unambiguous assessment of the chemical and biological reactivity of Cd in soil compared with the fractions isolated by chemical extractants.