SPECIATION OF PARTICULATE-BOUND CADMIUM OF SOILS AND ITS BIOAVAILABILITY

A modified sequential chemical extraction procedure was developed for partitioning particulate Cd into eight fractions: exchangeable, carbonate-bound, metal-organic complex-bound, easily reducible metal oxide-bound, organic-bound, amorphous mineral colloid-bound, crystalline Fe oxide-bound, and residual. Results of experimental data on 16 surface soils of Saskatchewan, widely varying in physico-chemical properties, indicate the presence of little exchangeable Cd. Cadmium in these soils was predominantly in the form metal-organic complex-bound, accounting for 31-55%, with an average of 40%, of the total Cd present in the soils, The average relative abundance of the different forms of Cd present in these soils is in the order: metal-organic complex-bound (0.107 mg kg(-1)) > carbonate-bound (0.052 mg kg(-1)) > residual (0.042 mg kg(-1)) > organic-bound (0.035 mg kg(-1)) > crystalline Fe oxide-bound (0.016 mg kg(-1)) > easily reducible metal oxide-bound (0.010 mg kg(-1)) > amorphous mineral colloid-bound (0.002 mg kg(-1)), Statistical treatment of the Cd availability index, measured as ammonium hydrogencarbonate-diethylenetriaminepentaacetoc acid (ABDTPA)-extractable Cd, with different particulate-bound Cd species showed high correlation (r = 0.916, p = 6 x 10(-7)) of the Cd availability index with the metal-organic complex-bound Cd. The beta coefficients obtained from the multiple regression analyses have given an insight into the importance of Al-organic complex-bound Cd species in estimating the bioavailability of Cd in these soils, The relationship of the metal-organic complex-bound Cd and the mobility acid bioavailability of soil Cd merits in-depth research in explaining the toxicity and food chain contamination of Cd in the environment.