Empirical Analysis of Heavy Metal Speciation and Mobility in Soils Contaminated by Electronic Waste Recycling in Lagos State

Informal electronic waste recycling operations in Lagos State, South West Nigeria, release complex mixtures of heavy metals through open burning and acid stripping that enter surrounding soils in speciation forms with varying bioavailability and environmental mobility. This study conducts an empirical analysis of heavy metal speciation and mobility in soils from three informal e-waste recycling sites in Alaba International Market using the modified Tessier sequential extraction procedure. Soil samples were collected at depths of 0 to 10, 10 to 20, and 20 to 40 cm from contaminated and reference sites. Sequential extraction fractions analysed were exchangeable, carbonate-bound, iron and manganese oxide-bound, organic matter-bound, and residual fractions for lead, cadmium, copper, zinc, chromium, and nickel, measured by inductively coupled plasma optical emission spectrometry. Results demonstrated that lead and cadmium preferentially partitioned into the exchangeable and carbonate-bound fractions, accounting for 42 to 61% and 38 to 54% of total concentrations respectively, indicating high bioavailability and leaching potential. Copper and zinc were predominantly oxide-bound, and chromium was concentrated in the residual fraction. Depth profiling revealed significant downward migration of lead and cadmium, with elevated concentrations detected at 20 to 40 cm depth in two sites, indicating groundwater contamination risk. Speciation data were integrated into a geochemical mobility index to rank site remediation priority. The study fills a critical data gap regarding speciation-based risk assessment for e-waste-contaminated urban soils in South West Nigeria. Keywords: heavy metal speciation, sequential extraction, e-waste, Lagos State, soil contamination