Levels and geochemical fractions of Cd, Pb and Zn in valley bottom soils of some urban cities in southwestern Nigeria

Abstract

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.