An investigation of the distribution, mobility and bioaccessibility of potentially toxic elements in urban soils of Lagos, Nigeria

Lagos is one of the fastest-growing metropolises in the world, with a population of 21 million, increasing by 600,000 or 2.8% annually. The impact of this rapid development and industrialisation on levels of potentially toxic elements (PTE) in the city’s soils is of concern, and requires investigation, because of possible effects on human health.
Soil samples were collected from different land-use types and socioeconomic areas in Lagos State and returned to the UK. They were dried, sieved, and subjected to microwave-assisted aqua regia digestion, the revised BCR sequential extraction, and the simplified bioaccessibility extraction test (SBET). The analytes Cr, Cu, Fe, Mn, Ni, Pb and Zn were determined in soil extracts and digests by ICP-MS.
Mean pseudototal PTE concentrations at locations remote from point sources of pollution were generally low, but soils taken from locations close to industrial activities such as foundries, dumpsites, and e-waste recycling sites were often highly contaminated. Sequential extraction indicated that Cr, Fe and Ni were mainly of geogenic origin, whilst the ‘urban metals’ Cu, Pb and Zn were in more labile forms of greater environmental concern. The SBET indicated that PTE uptake by a hypothetical 10 kg child with pica tendencies (consuming 10 g of soil per day) would greatly exceed tolerable daily intake (TDI) values for Cu, Pb and Zn at many locations.
The study demonstrated that, although general urban background PTE levels in Lagos soils are currently low, there is an urgent need for introduction of appropriate environmental and human health protection legislation.