Geochemical Speciation of Toxic Metals in Paddy Soils along Ogbesse River, Southwest Nigeria

Abstract

The sole purpose of the present study was to investigate the geochemical speciation of toxic metals and to assess the association and bioavailability of certain metals (Zn, Cd, Cu, Pb, Fe and Mn) in paddy soils around Ogbesse River, Ondo State, Southwest Nigeria. For this study, a total of six paddy soils were collected across the study area (2 upstream; 2 mid-streams, 2 downstream). After collection, the samples were air dried in the laboratory and physical parameters such as pH, organic matter content (OMC), and particle size distribution (PSD) were measured. Trace metals were analyzed using Atomic Absorption Spectrophotometer (AAS). Geochemical fractionation was carried out using the Tessier method. All statistical analysis was carried out using SPSS version 21. The Tessier method of sequential extraction was selected for this study. Results showed that pH is between 5.82 and 6.69 while OMC was between 2.17 and 3.08. The concentration of Zn, Cd, Cu, Pb, Fe and Mn in the paddy soil samples were between 4.87 mg/kg to 6.91 mg/kg, 0.11 mg/kg to 0.24 mg/kg, 0.48 mg/kg to 1.09 mg/kg, 0.97 mg/kg to 2.02 mg/kg, 365.65 mg/kg to 609.16 mg/kg and 30.17 mg/kg and 42.23 mg/kg respectively. The fractionation of metals in the samples showed that major portions were associated with the residual fraction, followed by Fe–Mn oxide, organic matter, exchangeable, carbonate respectively. The significant positive correlation with pH in soils of the area indicates an increasing tendency of sorption with increasing pH. The higher concentration of Fe in the Fe–Mn oxides fractions indicates that Fe–Mn oxides act as a natural sink for the metal. Metal factor (MF) showed that metals in the soils have low mobility and may not pose any toxicological effects in humans.