DISTRIBUTION AND BIOAVAILABILITY OF HEAVY METALS OF DUMPSITE SOILS IN ILISAN REMO, NIGERIA

Abstract

Management of solid waste has been a major environmental concern and a fast growing issue for developing cities and countries in the world, including Nigeria. This study was carried out to determine the physicochemical, distribution and bioavailability of some heavy metals (Copper (Cu), Chromium (Cr), Cadmium (Cd), Lead (Pb) and Nickel (Ni)) during wet and dry seasons of year 2018 at two major dumpsites in Ilisan Remo, Ogun State, Nigeria. A total of eight composite soil samples were collected at a depth of 0-15 cm at both seasons. The soil texture was found to be loamy sandy for both seasons on the study sites. The samples were air-dried, sieved and analyzed. The physicochemical parameters (pH, total nitrogen (% TN), organic carbon (% OC), organic matter (% OM) and soil particle size distribution) were carried out using standard methods. Total metal concentrations were determined after wet digestion by aqua regia (HCl : HNO3 – 3:1) and the sequential extraction was done using Tessier’s method. The metal concentrations were determined using Flame Atomic Absorption Spectrophotometer (FAAS). The mean pH values ranged from 5.95 ± 0.21 to 7.60 ± 0.14 and 7.35 ± 0.21 to 7.60 ± 0.14 for wet and dry seasons respectively. The % TN of the study site soils in wet and dry seasons ranged from 0.24 to 0.42 % and 0.08 to 0.29 %, respectively. The % OM of the study site soils also ranged from 2.80 to 3.32 % and 3.97 to 4.72 % for wet and dry seasons, respectively. During wet season, the mean of total heavy metals concentration (mg/kg) of the soils ranged from 9.98±1.03 to 13.40±1.15, 39.29±4.40 to 41.38±3.99, 3.81±0.66 to 7.82±0.81, 29.18±2.66 to 75.76±9.12 and 6.77±1.28 to 11.68±1.06 for Cu, Cr, Cd, Pb and Ni respectively. On the other hand, the mean ranges of the heavy metals of soils collected during dry season were 21.86±3.94 to 55.33±1.91 (Cu), 38.14±4.56 to 60.10±3.61 (Cr), 0.00±0.00 to 4.26±1.53 (Cd), 38.03±4.12 to 106.45±8.51 (Pb) and 4.45±0.93 to 8.06±0.59 (Ni). Considering the total metal contents, the mean levels (mg/kg) of Cu and Pb obtained from the soils collected during the dry season were significantly (p<0.05) higher than those obtained during the wet season, whereas those of Cd and Ni were significantly (p<0.05) higher in soils collected during wet season. Higher amounts of Cu, Cr, Cd and Pb were found to be bound to Fe-Mn oxide, organic matter and residual fractions. The percentage bioavailability of metals in the two dumpsite soils were higher than the one from the control site and they were in the order Pb >Cr> Cu > Ni>Cd and Pb >Cr> Cu >Cd >Ni for wet and dry seasons respectively. Pb and Cd were above permissible limits when compared with international standards. However, they are of minimal concern because of their distribution being mainly within the residual fractions, that is, of lithogenic or geogenic origin.