Prior to the analysis, the soil samples were air-dried in the
laboratory and sieved through the 250 μm mesh before they were
ground using a mortar and pestle. Heavy metal extraction from the
soil samples was undertaken using the sequential extraction
method of Badri (1984). The extraction process was divided into
four fractions namely easily leachable and ion exchange (ELFE),
acid reducible (AR), organic oxidation (OO) and resistant (RR)
fractions. The ELFE fraction normally contained metals that were
weakly bound to the clay surface area, secondary minerals and
organic materials. Ten gram soil samples were put into Kartell
bottles followed by the addition of 50 ml 1.0 M NH4CH3OO (pH 7).
The samples were then shaken for 1½ h, and centrifuged at 3000
rpm for 30 min before being filtering through 0.45 μm millipore filter
paper and made up to 50 ml with distilled water. Samples were
washed with 50 ml distilled water, followed by further shaking and
centrifugation as described earlier.
Consequently, a total of 50 ml NH2OH.HCl (pH 2) was added in
order to extract the metals from the acid reducible (RA) fraction
using the procedure earlier described. Metals extracted from this
fraction mainly came from those which were strongly bound to
secondary minerals. Metals in the organic oxidation (OO) fraction
were extracted by adding 15 ml of H2O2 to the sample placed in a
water bath for 1 to 1½ h, followed by the addition of 50 ml
NH4CH3OO (pH 3.5), representing the metal bound to the organic
matter. Samples were then digested using HNO3:HClO4 at 25:10
ratio in a sand bath at 100°C as in the RR extraction method. The
digestion process was repeated until the samples turned whitish.
The determination of heavy metal concentration was carried out
using AAS (atomic absorption spectrophotometry) with the Perkin
Elmer model (1100B). Organic carbon analysis and soil pH were
also carried out by using the Walkey-Black (1934) and Duddridge
and Wainright (1981) methods, respectively, while the