An Urban Soil Study: Atomic Absorption Spectrophotometric Determination of Some Heavy Metals in Selected Towns in Niger State, Nigeria

ABSTRACT

Soil samples were collected from cultivated farmlands, roadside topsoil, industrial sites, and local brass industrial site located in three major cities (Minna, Bida and Suleja) of Niger State. United States Environmental Protection Agency (USEPA) Method 3050 was followed to digest heavy metals from the samples, and subsequently, the concentrations of Cu, Zn, Pb and Ni were detem1ined using Atomic Absorption Spectrometry (AAS) technique. Generally, the mean concentrations .Vere 14±18mgCuki 1 and I 1±13mgZnkg-1 for cultivated farmlands; 15±5.0mgCukg-1 • 53±32mgZnkg- 1 , 53±33mgPbkg· 1 , and 0.56:L-0.52mgNikg1 for roadside topsoil; 15±5.8mgCukg-1 , 28±22mgZnkg-1, 18±7.5mgP!ikg-1 , and l.9±0.96mgNiki 1 for industrial vicinities; 467±455 mgCukg· 1 , '181 ±44mgZnkg1 , and 181±20 I mgPbkg- 1 for local brass industrial site. These results indicate that Cu, Zn and Pb were accounted for mainly by anthropogenic activities, whereas, the accumulation of Ni was not readily apparent in the soils. Average metal contents in all the sites (except for Zn in cultivated farmland) were also substantially higher than in control soils; the Pb and Ni concentrations in background levels were however, too low to be dctccteble. Soil properties were also analyzed, and their results were of the following ranges: ~pH (5.0 -- 7.5), OC (0.36 -- 2.5)%, sand (46.4 -- 94.2)%, silt (2.3 - 20.0)% and clay (1.8 -- 33.6)% for cultivated farmland; pH (5.4 - 8.1), OC (0.3 -2.0)%, sand (77.4-90.8)%, silt (2.4- 19.8)% and clay (2.6 -- 14.6)% for roadside topsoil; pH (4.6 - 7.6), OC (0.42 - 2.7)%, sand (69. l -· 87.7)%, silt (5.7 - 20.5)% and clay (7 .6 -- 11.2)% for industrial sites; pH (7 .2 -- 9. 1 ), OC (0.4 -1.4 )%, sand (68. l - 85. 1)%, silt (6.7 -- 23.7)% and clay (7.2 -- 10.2)% for local brass industrial site. vi Metal contents were significantly correlated among themselves especially Cu, Zn and Pb, ... but exhibited little con-elation with soil pH and OC, while the correlation between metals and clay content was generally poor. Sequential extraction scheme was used to partition Cu, Zn and Pb in some selected contaminated soils of local brass industrial site into five operationally defined fractions namely; exchangeable, carbonate, oxide, organic and residual. Significant concentrations of these heavy metals were found in non-residual (potentially available) fractions with mean percentage values of 95.1 %, 88.7% and 87.3% for Cu, Zn and Pb respectively, thus, reflecting that these heavy metals are controlled by anthropogenic sources. CIA was primarily associated with the organic fraction with a mean of 50.4%, followed by carbonates with an aver~1ge value of 31 .2%. Both Zn and Pb had the highest mean values of 41.7% and 50.8% respectively in the carbonate fraction, followed by the reducible fraction (30.4% for Zn and 24.1% for Pb). The mean values of Cu and Pb in local brass industrial soils exceeded the quality criteria set by several ' developed countries, while the average Pb level of the roadside topsoil exceeds the Switzerland and Norway guide values.