Chemical speciation and bioavailability index of cadmium for selected tropical soils in Kenya

Determination of various chemical forms of a metal in soils is important to evaluate its mobility and bioavailability. Little is known on the chemistry of Cd species of variable charge soils in the tropics which account for a large proportion of the world's arable land. A study was conducted to investigate the chemical speciation of particulate-bound Cd and its availability index of selected Kenyan soils varying widely in physicochemical properties. Surface and subsurface soils were collected from the main agricultural areas in Kenya. Cadmium in the surface soils was present mainly in the metal-organic complex-bound form which accounted for 25.0 to 45.8% with an average of 37.1% of the total Cd present in the soils. The average amount of each particulate-buund Cd species in the surface soils followed the order: metal-organic complex-bound (0.026 mg kg(-1)) > residual (0.021 mg kg(-1)) > crystalline Fe oxide-bound (0.011 mg kg(-1)) > organic-bound (0.007 mg kg(-1)) > amorphous mineral colloid-bound (0.003 mg kg(-1)) > easily reducible metal oxide-bound (0.002 mg kg(-1)). In the subsurface soils, Cd was present mainly in the residual form which accounted for 20.0 to 48.7% with an average of 43.4% in the 40-70 cm depth and 23.1 to 50.0% with an average of 37.5% in the 70-100 cm depth of the total Cd present in the respective subsoils. Cadmium was neither present in exchangeable nor in carbonate-bound forms in the soils studied. Both the total Cd and the Cd availability index generally decreased from the topsoil to the subsoil and varied with soil type. Cadmium availability index was taken as ammonium acetate-acetic acid-ethylene diamine tetraacetic acid (AAAc-EDTA) extractable Cd. Statistical analysis of the Cd availability index with different particulate-bound Cd species of the soil profiles showed that the Cd availability index was most significantly correlated with the metal-organic complex bound-Cd (r = 0.960, p = 1.53 x 10(-8)). The information obtained from this study is fundamental in establishing a soil Cd database and should be of significance in interpreting the importance of Cd speciation in Cd bioavailability in tropical soils. (C) 1999 Elsevier Science B.V. All rights reserved.