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 180m, 125m, 90m and 65m
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 65m 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
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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.
