Heavy metal speciation in roadside surface soils and vegetation as a measure of environmental pollution in Ile-Ife, Osun State.
This study investigated the speciation, mobility and pollution indices of heavy metals in the roadside surface soils of Ile-Ife and the translocation factor in some of the associated plants. This was with a view to evaluating the heavy metal pollution status of the roadside surface soils of the study area. Ten roadside surface soil samples were collected for each of the wet and dry seasons. The soil samples were dried to constant weight and powdered using agate mortar and pestle. Sequential extraction procedure was used to fractionate seven heavy metals (Al, Cd, Cu, Mn, Ni, Pb and Zn) in the soil samples into five operationally defined geochemical fractions (exchangeable, bound to carbonates, bound to iron and manganese oxides, bound to organic matter and residual).The soil physico-chemical parameters such as pH, electrical conductivity, organic matter content and particle size distribution were also determined for each season. The concentrations of the metals studied were quantitatively determined using Atomic Absorption Spectrophotometre (AAS). Quality control measures included blank determination, recovery analysis and calibration of standards. Descriptive and inferential statistical methods were adopted for data analyses. Among the five chemical fractions, organic matter fraction had the highest pool for the seven metals studied for both seasons. Order of decreasing bioavailability of the metals followed the trend: Organic matter > residual >exchangeable> carbonates bound >bound to iron and manganese oxides for wet season while for the dry season, the trend was: Organic matter > residual > bound to iron and manganese oxides > carbonates bound > exchangeable fractions respectively. The mobility and bioavailability of Al, Cd, Cu, Mn, Ni, Pb and Zn in the soil samples for wet season followed the order: Al > Cu > Mn > Cd > Ni > Pb > Zn while for dry season, the order was Pb > Ni > Al > Mn > Cu > Cd > Zn. The total metal concentrations of roadside surface soils for wet season ranged from 1.85 µgg-1 Al to 5331.56 µgg-1Mn while for dry season, heavy metal concentrations ranged from 1.05 µgg-1 Al to 4945.55 µgg-1 Mn. Values of enrichment factor for the metals showed that the roadside soils of Ile-Ife was highly enriched with Cd, Mn, Zn and Cu. The values of translocation factor of metals in the plant samples ranged from 0.002 µgg-1 for the wet season to 0.713 µgg-1 for the dry season. The pH of the soil samples collected from roadside ranged from 2.90 at Toll Gate to 5.40 at Olonade site for wet season and dry season values ranged from 3.10 at OAU Sports Complex to 5.10 at Our Lady Junction site. The electrical conductivity (EC) for wet season of roadside surface soils ranged from 52.60 µSm-1 at Our Lady Junction to 65.20 µSm-1 at Toll Gate site, while the EC values for dry season ranged from 31.90 µSm-1 at OAU Sports Complex to 58.80 µSm-1 at Oduduwa College road, of Ile-Ife were less alkaline. The loamy sand and sandy loam textural classes predominated in the soil samples for both seasons and the percentage organic matter of the roadside surface soils of Ile-Ife for both seasons were low ranging from 0.06% to 0.13%. The study concluded that soils of Ile-Ife were polluted with respect to Cd, Mn, Zn and Cu and unpolluted with regard to Al, Ni and Pb in all the studied metals.