MAGNETIC AND GEOCHEMICAL STUDIES OF HEAVY METAL POLLUTION IN SOILS AROUND METAL-SMELTING PLANTS IN OGIJO AND OTA AREAS, SOUTHWESTERN NIGERIA

Abstract

Metal recycling, smelting and mining activities are potential contributors of heavy metal in soils, as the soil is the recipient of stockpiled scraps and discharged effluents from the activities. Metal recycling and smelting plants have been established in several parts of the country to meet the need for steel required for industrialisation without adequate safeguards for environmental concerns. The effects of these industries on their immediate environment in Nigeria are under investigated due to very limited available documents on the subject. Therefore, this study was aimed at assessing the level of heavy metals in soils around metal smelting industries in Ogijo and Ota, south western Nigeria. Soil samples were purposively collected between October and December 2016 at 0cm, 10cm, 20cm and 30cm depths from 105 and 32 locations in Ogijo and Ota, respectively. The soils were dried, disaggregated and divided into two portions; one part retained as whole soil samples, and the other parts were sieved to 180m, 125m, 90m and 65m fractions. A total of 420 whole soil and 1680 fraction-sized samples were collected from Ogijo, while 128 whole soils and 512 fraction-sized samples were collected from Ota. Each sample was subjected to magnetic susceptibility () analysis in the high and low frequency modes using the Bartington MS2B suite. Samples with high, medium and low magnetic values were selected and subjected to magnetic versus temperature analysis to determine the class of the magnetic elements. Geochemical analysis to ascertain the constituents’ metals and their concentrations was also undertaken using the inductively coupled plasma mass spectrometry. Data were evaluated using geochemical method and descriptive statistics. The magnetic susceptibility readings for whole soils samples at depths 0, 10, 20 and 30 cm were from 51 to 6159.3, 19.2 to 4240.1, 12.1 to 3961.5 and 33.4 to 3870.2, respectively indicating a decrease in magnetic susceptibility with depths. In terms of grain-fractions, the 65m fractions had the highest (92.6-7541.6) magnetic susceptibility. Magnetic susceptibility versus temperature analysis of the soil samples revealed mostly super-paramagnetic signatures, indicating that the magnetism of constituent materials increased with temperature. Geochemical concentrations (ppm) of copper, lead and Zinc for Ogijo and Ota were from 16 to 834 and 1to 1308; 29 to 2381 and 30 to 161 and, 54 to 10000 and 29 to1400, respectively; as compared to NESREA limits of 72, 164 and 421 respectively. A comparison of the metal concentrations with iii their crustal abundances revealed elevated concentrations in most of the soil samples. Calculated index of geo-accumulation for copper, lead and zinc in Ogijo and Ota were: 1.44 and 1.22; 4.80 and 3.10 and, 4.44 and 2.17, respectively. This suggested enrichment of metals in the soils in addition to geogenic sourcing. High and low magnetic susceptibility () correlated with high and low metal concentrations in soils at both Ogijo and Ota, respectively. Soil around metal recycling and smelting industries in Ogijo and Ota are polluted with copper, lead and zinc and these are especially concentrated in the finest soil fractions. Similar study on vegetation, water and air is recommended.