Assessment of Pb contamination of soils, sediments and road dusts of the City of Lagos, Nigeria

Abstract

Lead (Pb) is of considerable concern among other heavy metals because of its toxic impact on the environment and human health. The geochemical nature and source-transport-fate of Pb in Nigeria's environmental media are relatively unknown. This study was undertaken to quantify and characterize Pb in soils, sediments and road dusts from Lagos, Nigeria. A total of forty-eight soil samples, seventeen sediment samples and thirteen roadside soils were collected from within Lagos metropolis. The samples were air-dried, screened, pulverized and sieved through 63-µm nylon mesh. Mineralogical constituent of samples were determined using X-ray diffraction. The sieved fractions and pulverized rock samples were digested and analyzed for elemental Pb components using inductively coupled plasma mass spectrometry (ICP-MS). The chemical fractionations of Pb in the samples were determined by sequential analysis. Lead isotope compositions of the samples were determined using quadrupole-based ICP-MS. Lead isotopic ratios and geo-accumulation indexes were used for discriminating and evaluating Pb sources. Quartz, microcline, K-feldspar, plagioclase and kaolinite were the preponderant minerals in powdered rocks, soils and sediments. The concentration (mg/kg) of elemental Pb in sediments and dusts for Lagos was 21-1963; 23-830 and 22-830, respectively. The plots (²⁰⁸Pb/²⁰⁷Pb)/(²⁰⁶Pb/²⁰⁷Pb) revealed that the Pb in the environmental media was derived from anthropogenic sources. The Pb isotopic ratios for soils, sediments and dusts were characterized by lower ²⁰⁸Pb/²⁰⁶Pb ratios, compared to bedrock. The Pb contents are held principally in the organic sulfide and reducible and carbonate phases. The Pb is held in very reactive geochemical phases, and these portend great risk to the environment and public health.

Keywords: Pb isotopic ratio; Reducible and organic sulfide phases; Sequential analysis.