ABSTRACT
The present study investigated selected soil physicochemical characteristics and heavy metal speciation of Pb, Cd, Cr, Ni, Cu, Zn, Mn, Fe and V in a 50,000 ha land area of wetland soils (Bayelsa State) of the Niger delta. Also investigated were the heavy metal levels in the leaves and fruits of oil palms of ages: 2 – 10, 15-32 and greater than 60 years of age seen around the soil profiles. Two newly introduced steps and a modification of an earlier step were used in the selective sequential extraction scheme to fractionate the nine metals into nine forms bound to the soil matrices. The metals were determined by atomic absorption spectrophotometry. Statistical analyses which include, linear correlation analysis, determination of means, maxima, minima, standard deviations, linear plots and principal component analysis were employed to assess the mobility, bioavailability, distribution, dynamics and bioaccumulation of these metals in the wetland soils. The physicochemical characteristics showed that the top soils were sandy, while the sub soils had greater amounts of silt and clay. The soils were very strongly [pH = 2.98 – 5.00 (KCl) to slightly acidic (pH 3.80 – 5.16 (H2O)]. There were significant variations in the organic carbon content (0.15-2.80%), phosphorus (3.00 – 18.94 mg/kg), while Ca dominated the cation exchange capacity. Although, there was no consistent trend in the distribution of the metals in the individual soil horizons, the mean, maxima, minima and standard deviations of the soil matrices (representing 817 sets of data) as well as the amount of the metals bound to the various binding sites showed varied results. The plant available forms provided significant sites and contributions for metal binding (10.28%). The significantly high mean (42.87%) of the soil matrices for the crystalline iron oxide fraction showed that water soluble Cd correlated significantly with Cr (0.63), Ni (0.66) and Cu (0.77) all in the same fraction. Pb in the exchangeable fraction correlated significantly with Cu (0.68) and Ni (0.93), while Fe correlated well with Cd (0.63) all in the same fraction, while Mn and Cd (0.78); Mn and Cr (0.67) were significantly correlated in the manganese oxide and amorphous crystalline forms respectively. The transfer rate plots showed that there were significant amounts of bioaccmulated Cu, Zn, Mn and Fe in the leaves and fruits of the palms irrespective of the age of the palm. Zn was predominant in the older palms (> 60 years of age) with the presence of trace amount of Ni in these palms. The present study has shown that the heavy metal speciation sheme provided better understanding concerning the mobility, bioavailability, distribution and possible redefining of analytical criteria for critical level determination of the metals in the wetland soils. The introduction of the plant available fraction was able to provide a useful correlation basis between bioavailability and bioaccumulation of the metals in the oil palms.
