Chemical speciation and bioavailability index of cadmium for selected tropical soils in Kenya
Date
1998-12-10Author
Huang, P M
Onyatta, J O
Type
ArticleLanguage
enMetadata
Show full item recordAbstract
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-bound Cd species in the surface soils followed the order: metal-organic complex-bound
(0.026 mg kg-I»residual (0.021 mg kg-I»crystalline Fe oxide-bound (0.011 mg kg-I»
organic-bound (0.007 mg kg - I) > amorphous mineral colloid-bound (0.003 mg kg - I) > easily
reducible metal oxide-bound (0.002 mg kg - I). 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
Citation
Geodenna 91 (1999) 87-101 ChemicalPublisher
Department of Chemestry, School of Physical Sciences, University of Nairobi