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–diethylenetriaminepentaacetic acid (ABDTPA)-extractable Cd, with different particulate-bound Cd species showed high correlation (r= 0.916, p= 6 × 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 and bioavailability of soil Cd merits in-depth research in explaining the toxicity and food chain contamination of Cd in the environment.