Distributional Coefficients and Enrichment Studies of Potentially Toxic Heavy Metals in Soils Around Itakpe Iron-Ore Mine , North Central Nigeria

Soil samples were collected randomly but uniformly distributed around Itakpe iron-ore mines in both dry and wet seasons. Surface soils were collected from 0cm to 10cm using stainless steel augers and located using Global Positioning System (GPS). Soil samples were air-dried, sieved through 500um mesh and 1.0g digested, evaporated and analysed using Atomic Absorption Spectrometer (AAS).Five (5) geo-environmental indices were used to quantitatively evaluate the degree of soil contamination due to iron ore mining. The anthropogenic factor (AF) for both seasons revealed that all heavy metals have greater than 50% AF except for Cd in the dry season. The geo accumulation index (Igeo) for both seasons showed background concentration to unpolluted for Cu and Zn while Fe, Ni, Cdand Pb recorded moderately to very highly polluted. The pollution index (Er), showed tiny hazard level for all the heavy metals in dry season and in wet season, Cd and Ni recorded strong hazard level while tiny hazard level were observed for Cu and Pb. The ecological pollution index for the area is strong (RI=323.25). Dry and wet season enrichment factor (EF) revealed background concentration for all the heavy metals except Fe with EF> 40 (extremely high enrichment). While contamination factor (CF) was very high for Fe in both seasons, Cu and Ni recorded considerable to very high contamination in dry season. The wet season also revealed considerable contamination for Ni and Cd; moderate to considerable contamination for Cu, Zn and Pb. The sites in both seasons have experienced various degrees of deterioration but more significant in wet season. Based on these indices, the soils around Itakpe iron-ore area has suffered significant degrees of contaminations with respect to Fe, Ni, Cd and Pb.


Introduction
The Itakpe iron-ore deposit is mined from ferruginous quartzites.The ferruginous quartzites are metamorphosed iron-rich sediments that occur as bands and lenses within the Precambrian gneisses and migmatites (Olade, 1978 andOdigi, 2002).The Itakpe deposit contains more than 300 million tons of iron-ore, with an average of 40% Fe (Olade, 1978).Iron and steel are the backbone of human civilization and industrialization.Iron ore can be used as a measure of level of industrial development and living standards of nations.If this Itakpe iron-ore deposit is properly harnessed to its logical conclusion, Nigeria could experience rapid industrial and economic developments.
Pollution of the environment due to mining is ubiquitous because these metals are indestructible and most of them have toxic effects on living organisms and man.Heavy metals with potential hazards and occurrences in contaminated soils around iron ore mines include: Cd, Cr, Pb, Zn, Ni, Fe, Cu among others (Akoto et al. 2008).This study is to evaluate the spatial distribution of these heavy metal pollutants in soils around Itakpe iron-ore mines using locally determined background values (control values) for metal concentrations, employing in-depth heavy metal analysis using integrated approaches.
The objectives of the present work therefore,include: i) assessment of heavy metal contamination by Cu, Pb, Zn, esr.ccsenet.

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The soil sa wire.Sam  the Dahomeya derlain by cryst an) granitoids rejuvenated laces ferrugino st belts steepl Odigi, 2002) tramafic schist quartz gangue nd quartzites ( ding 200m alo to 25m in the ore (4% of ore (15%).Av ave shown that (Olade, 1978) 12) and wet se site but with ned at the surfa pled sites wer sieved throug qua regia (1:3 ea and devoid ent factor (EF) e) Anthropoge an shield whic talline rocks of s (Olade,197 crystalline ba ous (Figure 1 ly infolded in t, pelitic and s e.The Itakpe (Olade, 1978 Where the contamination factor, CF<1 refers to low contamination; 1<CF<3 means moderate contamination; 3<CF<6 indicates considerable contamination and CF>6 indicates very high contamination.Each sample location was evaluated for the extent of heavy metal pollution by employing the method of pollution load index (PLI) developed by Thomilson et al., 1980 as follows: Where n is the number of metals studied (six in this study) and CF is the contamination factor calculated (equation 2).The PLI provides simple but comparative means of assessing a site quality, where a value of PLI<1 denote perfection; PLI = 1 indicate that only baseline levels of pollutants are present and PLI > 1 indicate deterioration of site quality (Thomilson et al. 1980).
This method has been defined in many ways by different authors as numerical sum of eight factors (Hakanson, 1980).Ibrahim (2005) defined site quality as arithmetic mean of analyzing pollutants but in this study, it is appropriate to express the PLI as the geometric mean of the studied pollutants because this method reduces outliers which could bias reported results.
(iii) Geo-accumulation index (Igeo): Heavy metal enrichment above the control point values was evaluated using the geo-accumulation index (Igeo) method proposed by (Muller, 1979).This method assesses the heavy metal pollution in terms of seven (0 to 6) classes of enrichment, ranging from background concentration to very heavily polluted as follows: The factor 1.5 is introduced in this equation to minimize the effect of possible variations in the control values, C m control, which is attributed to lithogenic variations in soils.The proposed descriptive classes for Igeo values are in Table 6b (Muller, 1979).
(iv)Anthropogenic factor (AF): This was calculated for the top sediment samples.
The AF = C s /C c (5) where C s = concentration of heavy metal in sediments; C c = concentration of heavy metals in control values.This result indicates the extent of anthropogenic influence on heavy metals in top sediment samples.
(v) Ecological risk factor (Er i ): An ecological risk factor (Er i ) is used to quantitatively express the potential ecological risk of a given contaminant as suggested by Hakanson, (1980) as follows: Er i = Tr i x C i f (6) where Tr i is the toxic-response factor for a given substance (Table 1) and C i f. is the contamination factor.The following terminologies are used to describe the risk factor: Er i < 40, low potential ecological risk; 40<= Er i <80, moderate potential ecological risk; 80<= Er i < 160,considerable potential ecological risk; 160<= Er i < 320, high potential ecological risk; and Er i >= 320, very high ecological risk.The control values of all analytes were lower than their respective mean values except for K and Cd whose control values were 60.13mg/l and 1.68mg/l and their mean values were 56.27mg/l and 1.24mg/l respectively.The major and heavy metal concentrations order were Ca> K>Na>Mg for both seasons and Fe>Ni>Cd>Zn>Cu>Pb and Fe>Cu>Cd>Zn>Ni>Pb for dry and wet seasons respectively (Table 2).In dry season at P<0.01, Fe-Ca displayed strong correlations.At P<0.05, Pb-Fe-Ni-Ca-Cd-K also recorded significant correlations (Table 3).During rainy season, at P< 0.01, Mg-K, Pb-K showed positive and strong correlations (Table 4).Also at P<0.05, Cu-K revealed strong relationships.These strong relationships indicate same sources for these elements.The likely sources of these heavy metals from the study area are: mining and processing activities, fuels from automobiles and agricultural sources among others.The I geo = log2 [(C m )/ (1.5* Cv)]: Where C m = measured concentration; Cv = control values; 1.5 = a factor for possible variations in reference concentration due to lithologic differences.
Table 6b.Geo-accumulation indices of heavy metal classes (Muller, 1979) Igeo index Pollution intensity 0 background concentrations 0-1 unpolluted 1-2 moderately to unpolluted 2-3 moderately polluted 3-4 moderately to highly polluted 4-5 highly polluted >5 very highly polluted The dry season Igeo showed that Fe was highly polluted to very highly polluted in all locations while the wet season was very highly polluted for all locations.Except for few locations (which recorded moderately to unpolluted), Cu recorded unpolluted in all locations in both seasons.During both seasons, Zn recorded unpolluted in most locations.Moderately polluted was observed with respect to Pb in the dry season while in wet season, Pb recorded unpolluted in most locations while fewer locations experienced moderately to unpolluted.
During wet season, Ni revealed moderately to very highly polluted in most locations while in dry season, most locations recorded moderately to unpolluted.Cadmium in wet season was moderately to unpolluted in all locations while during the dry season, background concentrations were observed in all locations (Tables 6 & 7).The Igeo index order were Fe > Pb > Ni > Cu > Zn > Cd for dry season and thus Fe > Ni > Cd > Pb > Cu > Zn for wet season (Figure 3).Dry soil samples around Itakpe iron-ore showed tiny ecological pollution index (RI =81.37) and all the heavy metals also showed tiny hazard level.Wet season soil samples from the same area revealed strong ecological pollution hazard level (RI= 323.25), while Cd and Ni also showed strong hazard levels.Both Cu and Pb recorded tiny hazard levels (Tables 9 and 10).For both seasons, the enrichment factor for Fe was extremely high with raining season having higher degree of enrichment.For other heavy metals, background enrichment was recorded in both seasons.All heavy metal's EF were lower during rainy season than dry season (Tables 11and 12).On the average, order of heavy metal enrichment were Fe > Ni > Pb > Cu > Zn > Cd for dry season and Fe > Ni > Cd > Pb > Cu > Zn in wet season (Figure 4).5).Heavy metals such as Fe, Ni and Cd were relatively higher during wet season than dry season.While the samples were not exactly from same point in both seasons, dry season PLI were on average lower than in wet season.Both seasons have shown site deterioration but the pollution load for wet season was significantly higher when compared site by site with dry season (Tables 15).

Discussion
The soil samples around Itakpe showed tiny (RI = 81.37)ecological hazard risk level in dry season and strong ecological hazard risk level during the rainy season (RI = 323.25).The potential ecological risk was in the order Ni> Cd >Pb.All the heavy metals indicated tiny hazard level in dry season.While Cd and Ni showed strong hazard level, Cu and Pb showed tiny hazard level and Zn showed none in the rainy season.Sources of Ni in this study area include: both mining and fuel sources.Though Pb sulphides may be present in the area, automobile fuels which have historically contained Pb as additives for value lubrication, chemical fertilizers were the major sources of Pb.Another factor could also be the strongly hydrophobic nature of lead (Akoto, et al., 2008).Cadmium was also strongly enriched and its major sources were pesticides, fertilizer applications as well as mining activities (Akoto et al., 2008).
On the basis of the indices used, Pb was lower in wet season than dry season.This could be due to its immobility, possible dilution due to rainwater and flood water.It's also possible that Pb attached to the soil particles were removed from the soil surfaces and translocated elsewhere by the action of water and wind (Harrison et al., 1981).On the other hand, Fe, Ni and Cd were all higher in rainy season than in dry season.Iron (Fe) is generally present in the secondary oxides due to its mobility and dispersion which may contribute to its higher spatial content (Harrison et al., 1981).Over 75% of Cd is associated with mining ferrous and nonferrous materials.The absorption of Cd on particulate matter and bottom sediments are major factors affecting Cd concentration (Ibrahim, 2005).The hydrous iron and Mn oxides have a large capacity for sorption or co-precipitation with heavy metals such as Ni.This may suggest why Ni was higher in rainy season than in dry season.
These hydrous oxides exist as coatings on the particles, particularly clays and can transport sequestered metals to great distances (Ibrahim, 2005).The Ni may also have been incorporated into solid minerals by nonspecific and specific adsorption, co-precipitation and precipitation of discrete oxides and hydroxides (Ibrahim, 2005).Furthermore, Fe and Mn oxydroxides form surface coatings on other mineral surfaces such as clays, carbonates and grains of feldspars and quartz (Ibrahim, 2005).The Cd concentration was also higher during rainy season because of its relative mobility which enables its dispersion/discharge.Besides this, Cd may have been remobilized from commercial fertilizers, pesticides, animal and waste water discharges as a result of runoff arising from rainfall (Ibrahim, 2005).
High PLI suggests appreciable input from anthropogenic sources (Chakravarty and Patgiri, 2009).The higher PLI observed in rainy season can be traced to rainfall discharge and dispersion of metals under tropical conditions where soils are scarcely vegetated and the subsequent severe erosions due to runoff and landscape topography (Chakravarty and Patgiri, 2009). Fig

Table 3 .
Correlation matrix of dry season soil samples

Table 4 .
Correlation matrix of wet season soil samples

Table 5 .
(Ameh, et al., 2014) of heavy metals in dry and wet season soils From table 5, it is clear that dilution has reduced the effect of AF between the dry and wet season concentrations.Iron (Fe) was relatively higher during wet season as a result of precipitation and co precipitation of ox hydroxides from solution(Ameh, et al., 2014).The highest AF during dry season was iron (Fe) 99% and the least was Cd (42.47%) while in wet season, Fe recorded the highest AF value of 99.71% and Zincthe least AF of 59.12%.

Table 7 .
The Igeo of wet season soils

Table 9 .
Assessment of potential ecological risk of heavy metals in dry season soil

Table 10 .
Assessment of potential ecological risk of heavy metals in wet season soil

Table 11a .
Enrichment factor (EF) of heavy metals in Itakpe dry season soils

Table 13a .
The CF of heavy metals in dry season soilsThe contamination factor for Fe was generally lower at dry season than during rainy season.In both seasons, Fe had very high contaminations.The contamination factor for Cu and Zn were moderate for both seasons.Fewerlocations showed considerable contamination in dry season for Cu.Dry season CF for Pb revealed very high contamination in most locations while wet season recorded mostly considerable contamination.Nickel in wet season showed higher degree of contamination than in dry season.While Ni recorded very high contamination in wet season, considerable contamination was observed in most locations in dry season.Cadmium revealed considerable contamination in wet season while low to moderate contamination was recorded during dry season (Figure

Table 15a .
The PLI of dry and wet seasons