Abstract:
The adsorption behaviour of DDT in a tropical
silt-clay soil from Kenya showed that addition of increasing
metal ion concentrations from 10 to 100 lg/g of soil,
reduced the % DDT adsorption in the soil by factors
ranging from 6.1% to 15.4% depending on the type of
metal ion. The inhibition of adsorption by metal ions was
most observed in the lower ranges of metal ion concentrations
in soil, i.e. up to 100 lg/g, beyond which additional
increase in metal ion concentration did not result in
any further increase in % adsorption. In the standard
adsorption test procedure with different soil samples saturated
with 100 lg/g of metal ions, the rate of adsorption of
the pesticide varied with the type of metal ion and the
equilibrium maximum adsorption of DDT was lowered
with addition of metal ions, with the % adsorption of
78.4% (control) being reduced to lower values in the range
of 67%–77.4%, depending on the type of metal ion. The
data for control and samples fitted well into the Freundlich
adsorption model and showed that addition of metal ions
lowered the Kf values compared with control. The results
suggested that in the natural soil–water environments
where there was no disturbance, aggregation of humic
substances-pesticide molecule complexes altered the
observed lowering of adsorption caused by inhibition by
metal ions, and enhanced binding to solid soil phase
occurred instead. In the leaching experiments, DDT was
found to leach more extensively in soils with lower %OC
