Magnetic susceptibility measurement and heavy metal pollution at an automobile station in Ilorin, North-Central Nigeria

Magnetic susceptibility measurement was carried out on 26 top-soil samples randomly collected from the study area and 5 selected top-soil samples outside the station, using the Bartington MS meter linked to a computer operated using Multisus2 software. The Measurements was done at both low (0.47 kHz) and high (4.7 kHz) frequency susceptibilities which was further used to calculate the frequency dependent susceptibility (ΧFD). The values for low frequency mass magnetic susceptibility ranges between 96.6 × 10−5 m3 kg−1 and 146 × 10−5 m3 kg−1 with an average value of 117.35 × 10−5 m3 kg−1 and standard deviation of 12.22 × 10−5 m3 kg−1. The result reveal high magnetic susceptibility values at the station compared with the values observed outside the station which ranges between 53.0 × 10−5 m3 kg−1 and 72.3 × 10−5 m3 kg−1 with an average value of 63.2 × 10−5 m3 kg−1 and standard deviation of 7.01 × 10−5 m3 kg−1. This significant magnetic enhancement indicates high concentration of ferrimagnetic minerals in the soil and thus evidence of pollution due to the activities at the station which implies that the magnetic enhancement is of anthropogenic source than pedogenic and lithogenic. Analysis of the heavy metals also reveals higher values at the station. The correlation analysis between the mass specific magnetic susceptibility and the heavy metals concentrations (i.e. Cu (R = 0.92), Fe (R = 0.88), Cr (R = 0.85), Zn (R = 0.83), Cd (R = 0.79), Mg (R = 0.72), Mn (R = 0.60), Pb (R = 0.67)) which was conducted to further investigate the relationship between the soil magnetic susceptibility values and elemental variations, demonstrated magnetic susceptibility can be used as a proxy method for assessing the pollution of these heavy metals.


General Introduction
Magnetic susceptibility (X) is a dimensionless quantity given by the ratio of the total magnetization induced in a matter to the intensity of the magnetic field that produces the magnetization. It measures the concentration of magnetic crystals, grain size, shape and type of the magnetic minerals present in a sample (Mullins 1977, Dearing et al 1985, Beget et al 1990, Dearing 1999, Dearing et al 2001, Meglish et al 2008, Blundell et al 2009, Kanu et al 2013aand 2013b. These magnetic minerals present in soil may either be inherited from the parent rocks during the formation of the soil or because of anthropogenic activities (Ayoubi and Karami 2019, Ayoubi and Adman 2019). Pollutants released into the atmosphere by human activities eventually settle and accumulates in the soil. Accumulation of these anthropogenic particles originating from human activities such as the one taking place at the station (i.e. welding, painting, vehicular discharges and dusts, poor disposal of spare part, etc), results in significant enhancement of soil magnetic susceptibility There is a growing interest in using magnetic techniques for monitoring environmental pollution. Lecoanet et al 2003 efficiently use magnetic parameters to discriminate soil contamination sources, while Yang et al (2007), Morton-Bermea et al (2009) andMücella, (2010) and many others investigated the relationship between heavy metals contamination of soil and its magnetic susceptibility. Kanu et al (2013b) successfully investigated and applied the magnetic properties of top soil samples from parts of Jalingo, Taraba State, N-E Nigeria to assess the level of soil pollution and identify pollution hotspots using magnetic proxy parameters. Similarly, Spiteri et al (2005) studied the relationships between topsoil magnetic susceptibility and heavy metal distribution in the Lausitz region of eastern Germany and opined that magnetic susceptibility can be used as a proxy for soil heavy metals contaminations.
A good relationship between magnetic susceptibility and concentration of some heavy metals in top soils has been reported by several authors (Strzyszcz and Magiera 1998, Jordanova et al 2003, Hu et al 2007, Yang et al 2007, Morton-Bermea et al 2009, Mahamed et al 2011Brempong et al 2016). Since anthropogenic pollution usually have strong magnetic signature, this non-destructive magnetic technique looks promising in monitoring soil pollution (Mahamed et al 2011). Following the effectiveness of the integration of chemistry and magnetic properties in studies of the degree of pollution of the soil, dust, sediment and land systems, we have employed similar technique to successfully characterize and quantify the degree of pollution at an automobile station in Ilorin, Kwara State of Nigeria.

Study area
The study area 'Ilorin' is situated between latitudes 8º20' N and 8º50' N and Longitudes 4º25' E and 4º65'E (figure 1). Ilorin city is one of the fastest growing cities in Nigeria with a tropical wet and dry climate with mean annual rainfall of 1,200 mm (Olanrewaju, 2009). Its average annual temperature is 26.2°C; it peaks at about 30°C in March which marks the hottest month. Wet season is experienced from April to October and dry season from November to March.
The study area consists of Precambrian basement of south western Nigeria. The soils are formed from metamorphic and igneous rocks which are about 95%. The metamorphic rocks consist of quartzite augitegnesiss, granitic gnesiss, biotite gnesiss and banded gnesiss . The assortment of basement complex rocks brings about large number of ferruginous groups of soils. Therefore, ferrallitic soil type (generally deep red in colour with high clay content) is the major type of soil in Ilorin (Oyegun 1985  2.2. Sample collection, preparation and laboratory analysis A total of 26 samples of the top soil were collected randomly from strategic points within the study area. 5 topsoil samples were collected randomly outside the study area. The soil samples were collected in a fit rubber test container of about 10 cc each using a plastic spoon. These samples were sent to the laboratory where they were screened to remove macroscopic traces of stones, glass, rubber, hair, animal and plant matter to ensure that the materials to be analysed are free from such contaminants. The samples were air-dried at room temperature in the Laboratory for some days to reduce the mass contribution of water and to avoid any chemical reaction (Kanu et al 2013a(Kanu et al , 2013b. The samples were grinded using agate mortar and sieved through a 1 mm sieve mesh and stored in well labelled plastic containers for magnetic susceptibility measurements and further analysis. Magnetic susceptibility measurement was carried on each of the collected sample using the Bartington MS2B meter linked to computer operated with Multisus2 software. For all the measurements, the sensitivity was set at 1.0. Measurements were carried out three times; first air reading, sample reading and a second air reading before and after each series for drift correction (Kanu et al 2013b). The MS2B sensor is a handy laboratory sensor which makes use of 10 cm 3 samples in plastic containers. It has the ability of taking measurements at two different frequencies i.e. at 470 Hz (low frequency) and 4700 Hz (high frequency). When the 10 cm 3 cylindrical plastic bottles is in use, the accuracy of the MS2B meter is 1% (Dearing 1999). The susceptibility measurements were done at both low (470 Hz) and high (4700 Hz) frequencies which were further used to compute the frequency dependent susceptibility (Χ FD ).
For the geochemical analysis of the soil, we used Aqua Regia method for the digestion for trace metals in soil samples. 1 g of each soil sample was measured into a sanitized digestion flask. 3 ml of concentrated HCl and 9 ml of concentrated HNO 3 was added into the sample in the sanitized digestion flask (Orosun et al 2016b andUSEPA 1986). For more details on this method, see Orosun et al (2016). The AAS technique which measures the concentrations of elements in digested samples down to parts per million of a gram (mg kg -3 ) in a sample was carried out at ROTAS Soil-Lab in Ibadan using Buck Scientific Model 210 VGP Atomic Absorption Spectrophotometer.

Result of magnetic susceptibility measurements
The results of the magnetic susceptibility measurement are given in tables 1 and 2.
The mass specific magnetic susceptibility values for the top-soil samples collected randomly within the study area are given in table 1. The values for low frequency mass magnetic susceptibility range between 96.6×10 −5 m 3 kg −1 and 146×10 −5 m 3 kg −1 with an average value of 117.35×10 −5 m 3 kg −1 and standard deviation of 12.22×10 −5 m 3 kg −1 . These values of the magnetic susceptibility measurements at the station were much higher than the observed values outside the station. This significant magnetic enhancement indicates high concentration of ferrimagnetic minerals in the soil and thus increased pollution (Kanu et al 2013b, Strzyszcz and Magiera, 1998 For the selected top-soil samples collected outside the study area, magnetic susceptibility measurements were performed at both low and high frequency. The result is given in table 2. The value for low frequency mass magnetic susceptibility ranges between 53.0×10 −5 m 3 kg −1 and 72.3×10 −5 m 3 kg −1 with an average value of 63.2×10 −5 m 3 kg −1 and standard deviation of 7.01×10 −5 m 3 kg −1 . The value of high frequency mass magnetic susceptibility ranges between 50.6×10 −5 m 3 kg −1 and 65.7×10 −5 m 3 kg −1 with an average value of 59.1×10 −5 m 3 kg −1 and standard deviation of 5.47×10 −5 m 3 kg −1 . The frequency dependent mass magnetic susceptibility ranges between 3.40 % and 9.13 % with an average value of 6.71 % and standard deviation of 2.56 %. These values are very low compared with values observed at the station. This implies that the magnetic enhancement at the station is more of anthropogenic origin than lithogenic and pedogenic (Dearing, 1999).

Results of the geochemical analysis
The results of the geochemical analysis are given in tables 3 and 4.
Results of the heavy metals concentrations for the top-soil samples collected randomly within the station given in table 3, follows that, the mean concentration of the heavy metals of the selected top-soil decreases in the order Fe>Cr>As>Mg >Pb>Cu>Zn>Cd >Mn>Ag. The concentrations of Fe, Cr and As in the station were much higher than other selected heavy metals. This is believed to be due to gradual accumulation   Lu et al 2007;Mücella, 2010, Duan et al 2009Durza, 1999). Zn compounds are expansively utilized as antioxidants and also as agents for improving dispersant for motor oil (Lu et al 2007). We did similar analysis for the top-soil samples collected randomly outside the study area (presumed virgin/control soil) and the result given in table 4 shows that the mean concentration of the selected heavy metals are much lower than their respective concentrations at the station. This also reveals that the higher values observed at the station could be due to the anthropogenic inputs ( The results of the heavy metals analysis at the mechanic station despite higher than the measured values outside the station, are still lower than the values reported by Isinkaye (2018) where he measured the distribution and multivariate pollution risks assessment of heavy metals around abandoned iron-ore mines in North-central Nigeria. The calculation of coefficient of variation (CV) also reveals the variability in the distribution of the magnetic susceptibility and the heavy metals in the study area. CV20% indicates little variability, 20<CV50% implies moderate variability, while 50%<CV100% indicates high variability and CV value greater than 100% is regarded as exceptionally high variability (Isinkaye, 2018). From the results, the low frequency magnetic susceptibility (Χ LF ) and all the heavy metals (except Ag) show low variability in the soil.

Pearson correlation analysis
The Pearson correlation coefficients between the heavy metals concentrations and MS values and between different elements are presented in table 5.  The correlation analysis between the mass specific magnetic susceptibility and the heavy metals concentrations was conducted to further investigate the relationship between the MS values and elemental variations (see table 5 The correlation between magnetic susceptibility measurements and heavy metals content reveals a good relation between ferrimagnetic oxides and heavy metals in the studied station. This relationship is believed to be as a result of the fact that heavy metals are adsorbed onto surface of pre-present ferrimagnetics in the environments or are subsumed into the lattice structure of the ferrimagnetics during combustion process

Conclusion
Magnetic susceptibility measurement was carried out on 26 top-soil samples randomly collected from the study area and 5 selected top-soil samples outside the station, using the Bartington MS meter linked to a computer operated using Multisus2 software. The Measurements was done at both low (0.47 kHz) and high (4.7 kHz) frequency susceptibilities which was further used to calculate the frequency dependent susceptibility (X FD ). The results reveals high magnetic susceptibility values at the station compared with the values observed outside the station. This significant magnetic enhancement indicates high concentration of ferrimagnetic minerals in the soil and thus evidence of pollution due to the activities at the station. It also implies that the magnetic enhancement is of anthropogenic source than pedogenic and lithogenic. Analysis of the heavy metals also reveals higher values at the station.
So based on the results of this study it follows that the study area is polluted as a result of the activities at the station (i.e. welding, painting, vehicular discharges and dusts, poor disposal of spare part, etc) and the strong correlation observed between the heavy metals and magnetic susceptibility indicated a strong affinity of heavy metals to magnetic materials. Hence, since the MS method is cost effective, fast and can cover a very large area in a short time, it becomes very essential and should be utilized as a preliminary method/tool to identify polluted spots before applying the geochemical method that is time consuming and expensive.
Considering the significant magnetic enhancement and heavy metals pollution at the station, we recommend that the government of Nigeria via the environmental protection agency should monitor the location and activities of Automobile repair stations. If possible, the stations should be sited far away from residential areas or any drinking water source.