Sequential extraction and bioavailability of some heavy metals on dumpsite soils in a semi-urban community

Solid wastes are composed of organic and inorganic pollutants (heavy metals) that can contaminate soil, underground and surface water; resulting in serious health challenges to humans. The bioavailability of cadmium, chromium, copper, lead and nickel on dumpsite soils, were investigated during the wet and dry seasons of 2018 by a sequential extraction method (Tessier).

The soils were sampled from eight points within each of two dumpsites in Ilisan-Remo, Ogun State. Top soil of 0–15 cm depth was sampled, air-dried and sieved. The extracted soil samples were analysed for heavy metal concentrations using flame atomic absorption spectrometer.

In the dumpsite soils examined, cadmium, chromium, copper and lead were majorly bound to the residual fractions at the first and second dumpsites, respectively for both seasons with mean concentrations (mg/kg) of: Cd – 0.65 ± 0.12  and 1.20 ± 0.07, Cr – 36.83 ± 5.70  and 26.83 ± 3.57, Cu – 28.37 ± 3.69  and 8.04 ± 0.32, Pb – 12.40 ± 2.34 and 14.11 ± 2.44, but Ni was found mainly in the Fe-Mn oxide fraction during the wet season, with mean concentrations (mg/kg) of 6.22 ± 1.33 and 8.24 ± 0.78, for dumpsites A and B, respectively. The values obtained for these metals were higher during the dry season than a wet season and there were no metals in the mobile fractions for wet season. However, cadmium and lead were found in the carbonate fraction during the dry season, resulting in their bioavailability in the soils.

The speciation results revealed that heavy metals were more bioavailable in the residual fractions. The absence of mobile fractions in the soils indicated that metals are not bioavailable for plants uptake at present condition; hence, the metals obtained are of geogenic rather than anthropogenic origin.