DETERMINATION OF BIOAVAILABLE CADMIUM IN PADDY FIELDS BY CHELATING RESIN MEMBRANE EMBEDDED IN SOILS

Diffusion is an important mechanism in determining the bioavailability of heavy metals in cropland soils because it controls the rate of mov ement of heavy metal ions in soil solution to the root surface. Method s currently used for estimating the bioavailability do not take this p rocess into account. In this study a chelating resin membrane was embe dded in saturated soils for determining bioavailability of cadmium in situ. Nine Taiwanese soils, including five major agricultural soils wh ich were treated with various levels of Cd and four heavy metal contam inated soils, were used in this study. One piece of Ca-Chelex resin me mbrane (4 cm x 4 cm) was embedded horizontally in saturated soil paste s and recovered after a fixed time interval. I;rom the amounts of Cd a ccumulated on the resin membrane, the Cd bioavailability index ((CD)-D -2) was calculated, where C is the labile Cd concentration and D is th e effective diffusion coefficient. This index provides an integrated m easure of Cd ion diffusion, sorption-desorption rates, and solution co ncentration. The results showed that the index was approximately const ant and did not vary with the length of time embedded in soil (from 24 to 96 hrs). The index decreased as soil pH increased. When soils were flooded for 7, 15, and 30 days respectively before the resin membrane was embedded in soil for 1 day, the resulting index decreased as the length of flooding time increased. This suggests that the bioavailabil ity index reflects the dynamic change of Cd bioavailability in saturat ed soils. The indices were then correlated with total uptake of Cd by rice seedlings. The Cd bioavailability index correlated significantly with plant uptake of Cd (p <0.1%) for all the flooding time period tre atments. The above results indicated that the embedded chelating resin membrane method is suitable for determining bioavailable Cd of satura ted soils across a wide range of soil properties and Cd concentrations . In addition, the method is nondestructive, and therefore, feasible f or in situ determination of the bioavailable Cd of paddy soils.