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Levels and geochemical
fractions of Cd, Pb and Zn in valley bottom soils of some
urban cities in south-western Nigeria
G. A. Oluwatosin1*,
O. D. Adeyolanu1, T. O. Dauda1 and G.
E. Akinbola2
1Institute
of Agricultural Research and Training, Obafemi Awolowo
University, P. M. B. 5029, Ibadan, Nigeria.
2Department
of Agronomy, University of Ibadan, Nigeria.
*Corresponding author. E-mail:
gaboluwatosin@yahoo.com.
Accepted
8 May, 2008 |
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The valley bottom soils of urban cities are sink for
municipal and industrial solid and liquid wastes. These
valleys are used by urban farmers to grow vegetables for the
consumption of urban dwellers. Knowledge of total content of
heavy metals is not enough to fully assess the environmental
impact of these soils. Therefore the determination of metal
fractions in solution is very important to understand their
behaviour and their mobility capacity to the vegetables
grown on them. Sequential extraction procedure was used to
speciate Cd, Pb and Zn from eleven locations from the cities
of Lagos, Ibadan, Akure, Ikare, Ondo and Okitipupa in
southern western Nigeria into seven operational defined
geochemical species; available, carbonate, easily reducable,
organic, amorphous Fe-Mn oxide, crystalline Fe-Mn oxide, and
detrital (residual). All the metal investigated were
significantly associated with non-residual fractions. The
highest amounts of metals in non-residual fractions were
found in organic fractions while the lowest were found in
oxide species. The residual fractions contained on average
65 - 66% for Pb, 8 - 50% for Cd and 25 - 29% for Zn.
Assuming that mobility and bioavailability of these metals
are related to the solubility of the geochemical forms and
that the decrease in the order of extraction sequence, the
potential bioavailability of these metals were: Cd > Zn > Pb.
The mobility indexes of the metals correlated negatively and
significantly with total contents of the metals. Correlation
among species of each metal indicates a dynamic equilibrium
between labile and non-labile forms of the non-residual
fraction.
Key
words:
Geochemical factions, levels, valley bottom, soils, urban
cities. |
