dc.description.abstract | The size fraction of soils is one of the important
factors that influence the retention of heavy metals. The
sorptive properties of soils for heavy metals are principally
associated with clay and silt-size fractions. Phosphate
fertilizers that are applied to highly weathered tropicalsoils
contain a wide concentration range of cadmium (Cd) as an
impurity. Tropical soils contain kaolinite and oxides of Al,
Fe, and Mn, which have the ability to sorb Cd. However,
the distribution of Cd in different size fractions and the
chemical speciation of particulate-bound Cd in the clay size
fractions when introduced to soil and allowed to incubate at
field moisture capacity merits attention. Cadmium was,
therefore, applied to selected surface Kenyan soils varying
widely in physicochemical properties to investigate its
distribution in different soil particle size fractions and the
speciation of particulate-bound species in clay size
fractions after incubation. The Cd content in different
particle fractions was analyzed by graphite furnace atomic
absorption technique after HF-HClO 4 digestion. The
particulate-bound Cd species were investigated using
chemical sequential extraction method. The study showed
that clay size fraction of the natural and the Idaho
monoammonium phosphate (MAP)-fertilizer or the Cd
perchlorate-added MAP chemical reagent-treated soils
contained the highest amount of the total Cd. However,
silt and sand fractions of the treated soils also retained
appreciable amounts of Cd. Speciation studies revealed
that metal-organic complex-bound Cd was the most
predominant compared to other particulate-bound Cd
species in the clay size fractions of the soils treated with
Idaho MAP fertilizer or the Cd perchlorate-added MAP
chemical reagent. The distribution of total Cd in the
different soil particle size fractions and the speciation of
particulate-bound Cd in the clay size fractions varied with
the soil type. The results indicate that clay size fractions
can retain Cd making it less available; however, the
influence of farming practices, which affect Cd mobility,
should not be overlooked. | en |