Sequential Extraction, Mobility, and Pollution Loading of Heavy Metals in Soils from the Ikole–Itapaji Area, Southwestern Nigeria

This study investigates the sequential extraction, mobility, and pollution loading of heavy metals in soils from the Ikole–Itapaji area, Southwestern Nigeria. Soil samples collected from seventeen locations were analyzed using a modified seven-step Tessier sequential extraction procedure to partition metals into water-soluble, exchangeable, carbonate-bound, Fe–Mn oxide-bound, organic matter-bound, sulfide/strongly bound, and residual fractions. The objective was to evaluate metal speciation, mobility, and environmental risk using Contamination Factor (CF) and Pollution Load Index (PLI) as complementary pollution indices. Results show that most heavy metals are predominantly concentrated in the residual and Fe–Mn oxide fractions, indicating strong lithogenic control and low overall mobility within the soil system. The carbonate-bound, organic matter-bound, and sulfide fractions also contribute moderately to total metal retention, while the water-soluble and exchangeable phases contain relatively low concentrations, suggesting limited immediate bioavailability under present environmental conditions. However, elevated CF values for Cd and Pb indicate significant enrichment, with Cd reaching severe contamination levels at several sampling locations. The Pollution Load Index reveals that most sites have PLI values greater than 1, indicating overall soil quality deterioration, with pronounced contamination hotspots at SQ3, SQ4, SQ14, and SQ15. These spatial patterns reflect a combination of natural geological background variation and localized anthropogenic inputs. The study concludes that although the geochemical framework of the area is largely dominated by stable lithogenic sources, Cd and Pb represent key environmental concerns. The results highlight the effectiveness of sequential extraction in assessing metal mobility, pollution status, and ecological risk in basement complex terrains.