ENVIRONMENTAL GEOCHEMISTRY AND ASSESSMENT OF POLLUTION BY VANADIUM IN TOP SOIL OF KIRKUK, NORTHERN IRAQ

The vanadium geochemistry was estimated in the topsoil of Kirkuk, northern Iraq, and its potential sources are forecasted by using multivariate statistical analysis as useful tools in this field, in addition to assessing soil pollution with vanadium by using individual indices such as enrichment factor, geo-accumulation index (I geo ) and contamination factor (CF). Eighteen topsoil samples were collected with a depth ranges 0–20cm, and analyzed by using the inductively coupled plasma-mass spectrometer technique (ICP-MS). The results showed that the concentration of vanadium ranged from 37-51 mg/kg, with an average of 46 mg/kg, relatively it was high concentrations in the agricultural and residential areas compared to the industrial areas. Statistical analysis indicated the contribution of oil combustion and the use of phosphate fertilizers in increasing vanadium concentrations in topsoil. Pollution indices of enrichment factor were in the minimal enrichment category for all samples, geo accumulation index was non-contamination categories for all samples, while Contamination factor was low for all samples.


INTRODUCTION
The soil represents a sink for pollutants and provides a means for transporting them into the atmosphere, hydrosphere and biosphere (Alloway, 2012 andKabata-Pendias, 2010) and its contamination with heavy metal are the most serious problem due to the inability of these elements to decompose and accumulate in the human body which lead to health risks, as it enters the soil through various human activities such as industrial and household waste, cars exhaust gases, and emissions from factory chimneys (Awadh, 2015;Khan et al., 2015 andAl-Jaberi et al., 2016). Vanadium is one of these elements which represents an important pollutant in the environment at high levels due to many health risks to which humans are exposed (Schlesinger et al., 2017). Average contents of it in soils range from 10 to 500 mg/kg, and its DOI: 10.46717/igj.53.2E.6Ms-2020-11-28 world median content has been established at 90 mg/kg (Kabata-Pendias and Mukherjee, 2007), and it does not occur as a pure mineral in nature, but it found in about more than 65 different minerals as well as to it's found in carbon-containing deposits such as coal, crude oil, shale rocks and oil sands (Yassin and Al-Awadie, 2014). The combustion of hydrocarbon fuels is the most important anthropogenic source of vanadium, particularly V2O5 (Mejia et al., 2007).
Vanadium has four positive equivalents (II), V(III), V(IV) and V(V), and the two valence V (IV) and V(V) are most common in the natural environment, V(V) particularly is more mobile in the surface environment compared with V(IV) which is stable only in acidic conditions and turns gradually to V(V) when the pH increases to greater than 4 (Baken et al., 2012;Pyrzyñska, 2006 andWisawapipat andKretzschmar, 2017), in addition to that, V(V) is more toxic and impacted human health (Schiffer andLiber, 2017 andZou et al., 2019). Vanadium is mainly used as an additive element in steel alloys to increase its hardness and resistance to rust, in electronics and battery industries, also it's using to make fertilizers and some types of medicines (Imtiaz et al., 2015 andSchlesinger et al., 2017). Its many uses have led to an increase in demand for it, which has resulted in doubling its global production in the past fifteen years and thus an increase releases into the environment from industrial processes (Watt et al., 2018).
According to previous studies, Mohammed (2009) showed that the main source of heavy metals in Kirkuk is emissions from burning oil waste, vehicle exhausts and industrial activities. Awadh et al. (2015) used the pollution indices to assessment of soil pollution in al-Hawija (a part of Kirkuk) and concluded it was low contamination with vanadium. Ali (2007) found that based on the geographic accumulation index, the Al-Hawija soil was expressed as not contaminated to a slight contamination with vanadium. The importance of assessing soil contamination with vanadium is that it poses health risks to humans through direct exposure (ingestion, inhalation and dermal contact), or through the food chain (Zou et al., 2019), many studies have reported toxic and carcinogenic effects of vanadium at high concentrations (Wilk et al., 2017). The long-term oral exposure to it may lead to toxic effects to the digestive system include diarrhea, vomiting, low weight, and inflammation of the intestine (Jayawardana et al., 2014 andWilk et al., 2017). Inhaling particles rich in vanadium can cause many problems because the lung absorbs it well (Barceloux and Barceloux, 1999), that leads to chronic coughing, shortness of breath, bronchitis, lung fibrosis and may lead to lung cancer (IARC, 2006;Jaiswal and Kale, 2019). The objective of this study was to estimate vanadium concentrations and predict its potential sources in the topsoil of Kirkuk, to understand the vanadium geochemistry in it, and assessment the pollution levels of soil with vanadium by using 76 individual indices such as enrichment factor (EF), geo-accumulation (Igeo), and contamination factor (CF).

STUDY AREA
The study area represents Kirkuk, northern Iraq, it is 255 km northeast from the capital, Baghdad, it lies between longitude (44°16′30" -44°27′00" E) and latitude (35°21′30" -35°33′00" N). Its area is about 101.14 km 2 and its average height about 367 m above sea level surrounded by the Boor mountain from the north, northeast, and northwest. The seasonal Khassa river runs from north to south and divides it into left and right sides (Fig. 1).
The sediment cover of Kirkuk is made up of clastic rocks include claystone, siltstone, sandstone, and evaporitic rocks include gypsum, and limestone. It is represented by four formations in addition to quaternary deposits, and it is from oldest to newest, the Fatha  (Jassim and Goff, 2006).

Sampling
Eighteen topsoil samples were collected with a depth of 0-20 cm, distributed over the city's neighborhoods, representing residential, industrial and agricultural areas. Samples were collected by using manual (Auger) method. It was stored in polyethylene bags and transferred to a laboratory.

Laboratory Work
After the samples are transferred to the laboratory, they are left for a sufficient period of time to dry completely, then agglomerates samples were dismantled by a ceramic mortar, a portion of each sample was sieved with a 200 mesh sieve, and the weight of 20 gm from the passed parts through the sieve was taken and placed in plastic bags. It was then numbered and sent to ACME laboratories in Canada for the purpose of conducting the required chemical analyzes using inductively coupled plasma-mass spectrometer technique (ICP-MS). Perform a volumetric analysis of the samples by taking 40 gm from each sample, then the samples are putted in a cylinder, distilled water is added and mixed well, sand was separated from silt and clay, with wet sieving method using a sieve of 230 mesh. The non-passing portion of the sieve represents the sandy part, while the liquid passing through the sieve represents a mixture of clay and silt, and they were separated by using the pipette analysis method (Carver, 1971 andFolk, 1980).

Statistical Analysis
Some major, minor, and trace elements such as Al2O3, CaO,Cr2O3,Fe2O3,K2O,MgO,MnO,SiO2,TiO2,As,Cd,Cl,Co,Cu,Mo,Ni,P,Pb,Sc,U,Zn,), and some soil parameters (Organic material and Acidic function), were estimated and statistically processed by correlation coefficient and factor analysis using SPSS v.22 and Xlstat-2019 programs, respectively, to find out their geochemical behavior with vanadium.

Individual Pollution Indices
For purpose to assess the level of soil contamination by vanadium and to meet the objectives of this study, three individual indices were selected as follows:

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(Ci / Cr) Sample is concentration of the element studied in the sample (Ci) to the concentration of the reference element (Cr) in the sample, (Ci / Cr) Background is concentration of the studied element to the concentration of the reference element in the natural background sample, and it is divided according to the groups (Table 1). Extremely enrichment

Geoaccumulation Index (Igeo)
The Igeo is calculated from the following formula, and is divided according to the groups (Table   2) (Yaqin et al., 2008): Csample is concentration of the element in sample, Cbackground is geochemical background for same element, 1.5 as a constant introduced to minimize the effect of possible variations in the background values (Hussain and Al-Jaberi, 2020)

Contamination Factor (CF)
This factor can be calculated with the formula established by Hackanson (1980) as follows: Where Csample represents the concentration of the element studied in the sample, and Cbackground is the concentration of the same element in the natural background and is divided according to many categories (Table 3).

Vanadium Content in the Soil of the Study Area
According to the results which obtained from the chemical analysis of the samples, it was found that the vanadium concentration ranges between 37-51 mg/kg and the average is 46 mg/kg, (Table 4), it was found that its highest concentration was 51 mg/kg for the sites H2 and H6, the site H2 represents Khazraa neighborhood, which is characterized by the presence of places to trade cars, in addition to its location on the highway which is characterized by heavy traffic particularly trucks and tankers that leads to an increase in vanadium concentrations in the soil due to combustion emissions (Teng et al., 2011), as a high percentage of vanadium remains in oil derivatives even after the refining process and release when the fuel burning (Hope, 1997).
Whereas the site H6 represents farm on the edge of Khassa river in Adhar neighborhood, and increased vanadium content in this site may be due to the use of phosphate fertilizers which is usually rich in vanadium about 90-180 mg/kg (Akoumianaki-Ioannidou et al., 2016), and the using vanadium contaminated water for irrigation may increase its concentration in the soil (Naser et al., 2018).
Also, relatively high concentrations appeared at sites H5 and H7, as their concentration (50 mg/kg) was both, the H5 site represents Shoraw neighborhood which passes through the highway link Kirkuk and Erbil and is characterized by heavy traffic, which may contribute to increasing the concentration of vanadium, also its location is near the oil company and the  (Table 5). The relatively high concentration in agricultural areas may be due to the use of fertilizers and pesticides, and may be due to the presence of highly concentrated minerals in industrial areas such as zinc, cadmium, lead and phosphorous, which competes with vanadium for adsorption sites and displaces it based on several factors, including surface charge, soil pH, oxidation state, and soil properties (Nkwopara et al., 2016). Kirkuk soil is derived from formations that cover the city, which generally consist of sedimentary rocks (Jassim and Goff, 2006), and concentration of vanadium in these rocks ranges from 10 -20 mg/kg (Kabata-Pendias and Mukherjee, 2007), its occurrence at higher concentrations may indicate that it is influenced by other sources. Higher levels in agricultural and residential areas compared to industrial areas indicate that the concentrations may have been derived from anthropogenic sources rather than from the geological origin, such as emissions from oil installations and using phosphate fertilizers.

Vanadium Distribution in Size Fractions
Vanadium was analyzed in size fraction (sand, silt and clay) for five soil samples (Table 6), its concentration in sandy fraction ranged between 38-46 mg/kg with average 40.8 mg/kg and silty fraction ranges from 43-50 mg/kg with average of 45.6 mg/kg, in clay fraction ranges from 45-56 mg/kg with average of 49.6 mg/kg, and his concentration in size fraction was in order of Sand < Silt < Clay (Fig. 2), consistent with what mentioned by Sun et al. (2019) that the sandy soil is less retaining vanadium than clay soil. And also consistent with Kaplan et al. (1990), the vanadium is more related to fine soil than coarse soil. Sometimes its concentration in the fine 81 grains may exceed those found in the parent rocks (Kabata-Pendias and Pendias, 1985). In general, it is usually associated with the clay fraction and clay minerals of the soil and its abundance increases with increase of these minerals in the soil (Breit andWanty, 1991 andIvanov andKashin, 2010), and ion exchange and/or adsorption on these minerals represent the major mechanism of its accumulation in the soil (Ghani et al., 2012).

Effect of pH on the Valence State of Vanadium in Soil
Soil colloids, such as metal oxides, organic matter and clay minerals, are characterized by the presence of two types of charge, a permanent charge that is not affected by the pH, and a variable charge that depends on the pH, when the pH is greater than 7, soil has a negative net charge, and at lower values (pH <7) its net charge is positive (Karak et al., 2005 andSherene, 2010). Thus, the soil pH is a major factor controlling the heavy metal content in the soil, and adsorbed surfaces carry negative charges, but vanadium has different geochemical behavior , because of its ability to form cation and anion, depending on soil pH , tetravalence V(IV) cation type prevails at pH<4 and in reduction conditions, gradually turns to pentavalent V(V) in anion form when increasing pH values and or increasing oxygen, whereas the pentavalent prevails at pH values (pH> 8) in both the reducing and oxidizing conditions (Hu et al., 2018;Jiang et al., 2019;Wisawapipat andKretzschmar, 2017 andZhu et al., 2018).
In the current study, the values of pH in the Kirkuk soil ranged from 8-8.6 with an average of 8.2 (Table 7), which means that the soil of the city is alkaline and indicates the presence of pentavalent vanadium V(V) in its soil, which has the most health risks compared to V(IV), due to its solubility, mobility and bioavailability (Reijonen et al., 2016 andShaheen et al., 2014), this can be attributed to the competition between the hydroxyl ion (OH -) with vanadium anions on the available surface sites in the soil (Naeem et al., 2007), or to the electrostatic repulsion between the negative charges of both vanadium and the adsorbent material (Wällstedt et al., 2010).

Statistical Analysis
For the purpose of performing statistical analysis, the concentrations of some major, minor and trace elements as well as organic matter were estimated (Table 8).

Correlation coefficient
Correlation coefficient is an effective way to detect relationships between different elements, when there is strong positive correlation between two elements indicate that they may have the same source, and strong negative correlation indicate different sources (Micó et al., 2006). The correlation coefficient of vanadium was calculated with some major, minor, trace elements, and some soil properties, (Table 9). Vanadium showed a positive correlation with Al2O3, it may be due to accompanying in bauxite mineral (Ruyters et al., 2011), and attributed to direct adsorption on clay minerals (Ivanov and Kashin, 2010), or indirect by the negative charge reflection mechanism of the clay minerals surfaces, so Al +3 ions can adsorb to the surfaces of clay minerals, and change their charge to positive and lead to the adsorption of vanadium, potassium ions also act by the same mechanism and this is observed through the positive correlation with K2O (Huang and Yang, 2020). Vanadium showed a positive correlation with MgO with a value of r = 0.91 which may be due to their geochemistry accompanying in clay minerals and silicate minerals, or for the similarity of their sources as a result of weathering dolomite rocks containing about 30-120 mg/kg of magnesium (Gransee and Führs, 2013;Mayland and Wilkinson, 1989). It also has a positive relationship with SiO2 by value r = 0.8, which may be due to vanadium adsorption on silica, which is high in alkaline soils (Gan et al., 2020) and has a positive correlation with TiO2 with a value r = 0.61 which is due to the enrichment of titanium minerals with vanadium and can be attributed to its adsorption on TiO2 (Gäbler et al., 2009). It is associated in a positive correlation with Sc a value of (r = 0.69), as they are found together in many minerals, including llmenite, rutile, gauthite, hematite, aluminum and titanium minerals (Zhang et al., 2017).
Vanadium has a positive correlation with Ni may due to their presence in petroleum and petroleum products (Rzger et al., 2018) and it is positively correlated with Co which may be attributed to the similarities of their characteristics lithophilic in the earth's crust and sederophilic at the core of the earth (Pourret and Faucon, 2018), in addition to their presence in fossil fuels and phosphate fertilizers (Linhares et al., 2019). The similarity of the chemical structure of vanadium and phosphorous leads to the significant effect of phosphorous presence on vanadium interactions in the soil (Mandiwana and Panichev, 2004), this is obvious by the negative correlation between them, its value r=-0.67, which is due to the mechanism of competition between them unlike most elements that show a tendency to bind to phosphorous (Nalewajko et al., 1995), and the presence of phosphates may cause reduced vanadium adsorption on minerals in soil, or may lead to desorption of vanadium (Yang et al., 2014) also phosphorous can be replaced with vanadium in many minerals (Liu et al., 2018).
Vanadium has a negative correlation (r) with Pb, Cu, Zn, and Cd with values of -0.77, -0.73, -0.83 and -0.81, respectively, which may be due to their different sources, or this may be due to the fact that the soil in the study area is alkaline and has a negative charge so therefore the reactions of absorption and ion exchange with these cationic elements are greater than vanadium anions (Bolan et al., 1999). It was also negatively correlation with organic matter (r = -0.855), which may be attributed to the fact that the tetravalence vanadium, which is a cationic form and is tends to bind to the organic matter while pentavalent of it as in the study area are associated with soil oxides and minerals such as aluminum and titanium (Gustafsson, 2019; Al-Jaberi and Al-Jafar 2020), increasing organic matter in soil may increase its transport and bioavailability because the adsorption of the organic matter to the anions is much less than its adsorption to cations (Reijonen et al., 2016).

Factor analysis
Is one of the simplest and most widely used statistical methods in multivariate statistical analysis. It helps in understanding and interpreting the relationships between these variables by reducing the large number of variables to a smaller number. The first factor represents the best compatibility of the first basic components and the second factor represents the best compatibility of the second basic components that were not calculated in the first factor, thus the third and fourth factor. The first factor usually explains the highest percentage of variance (Muhammad and Ali, 2018). In the present study, four factors were relied upon after Varimax rotation, these factors explain 79.24% of the total variance, (Table 10). The communality value of vanadium is 0.920 and represents most of its variance, (Table 11). This factor explains 32.604% of the total variance, (Table 10), and vanadium appeared with a significant positive load value of 0.733 on this factor (Table 11), in addition to positive loading of Al2O3, MgO, K2O, SiO2, TiO2, Co, Ni, Sc, and As, Whereas, the significant negative loads were for Pb, Zn, Cd, Mo, CaO, and organic matter, The presence of a group of elements with a positive load and a group with negative load may indicate a different source for each group, When observing Table 12 for factor Score was found an important positive effect of H13 site in F1, (Fig. 3), this site is characterized by its proximity to the Jabal Boor station for crude oil production, it may be a source of vanadium emission, particularly Ni and Co were a positive load in this factor as the oil contains high concentrations of them (Awadh and Al-Ankaz, 2016). This is also noticeable from positive factor but to a lesser degree for the H4 and H5 sites, which represent the Arafa and Shoraw regions, respectively, possibly due to their proximity to north oil company. As for the sites H6, H7 and H8, they represent agricultural areas and they had a positive effect in the first factor score, which indicates the agricultural source of vanadium as chemical fertilizers and pesticides (Alloway, 1995). From the Biplot diagram (Fig. 4), it was found that the positive loading of vanadium and the rest elements were associated with positive effects of agricultural and residential sites on this factor and that the negative loads of the elements were associated with the industrial sites, indicating that they did not contribute to this factor.  This factor explains (7.416%) of the total variance (Table 10) (Table 11). Therefore, these convergent and dispersed distributions often indicate that this factor is not affected industrial and agricultural sources significantly and is likely to be from natural sources, (Table 12) and ( (Fig. 6).

Fig. 6. Factor score for F3 after rotation
Fourth factor (F4) This factor explains 7.416% of the total variance, (Table 10), and vanadium showed negative load (-0.204) (Table 11), which means that this factor is not explain potential sources of vanadium, and an important positive loading of arsenic only. From the observation of Table 12 and Fig. 7, it was found a relatively higher contribution of site H15 which may indicate the industrial source of arsenic in the study area, contributions to sites H14, H4, and H7 (residential sites) have also been observed which may indicate the contribution of natural sources also to this factor.

Enrichment factor (EF)
It is widely used to determine enrichment of an element in a sample due to human activities than natural abundance, and assess the presence and depth of anthropogenic pollutants in the soil, requires careful selection of the background value of the element under study and the reference component (Ahmad et al., 2019 andBern et al., 2019). The reference element is usually of low contrast in nature, such as Al, Fe, Mn, Ti and Sc. Iron is often used because it is Abundance in the earth's crust and has homogeneous distribution in it (Alekseenko and Alekseenko, 2014;Pragg and Mohammed, 2018). The EF enrichment factor for vanadium was calculated in soil samples for the study area (Table 13) and (Fig. 8), the results showed all samples were in the Minimal Enrichment Category (EF≤2).

Geo-accumulation index (Igeo)
This index is used to assess urban pollution by comparing current and pre-industrial concentrations, choosing the background has a huge impact, and a local background if available it will be more suitable (Johansson et al., 2009). The results obtained by calculating the geoaccumulation index, were less than zero for all samples Igeo ≤ 0 (Table 13) and (Fig. 8), and it was within the Uncontaminated Category.

Contamination factor (CF)
This factor provides an assessment of soil pollution and is calculated from the ratios of the heavy metal content in the sample to its ratio in the pre-industrial reference level (Hakanson, 1980). In this study the (CF) value was less than 1 for all samples (Table 13) and (Fig. 8), which mean it's of low contamination category. CONCLUSIONS 1. The current study showed that the concentration of vanadium was in the following order: Agricultural > Residential > Industrial areas, and its increase in agricultural areas is due to the use of chemical fertilizers, as for its increase in residential areas, it can be attributed to its proximity to places of burning oil and its derivatives and car emissions, which indicates that they are sources that contribute to enriching the surface soil with this element.
2. The highest concentration of vanadium was in the clay fractions compared to the silty and sandy fractions, due to its adsorption on the surfaces of clay minerals and the strength of the ion exchange and as an alkaline nature of Kirkuk soil likely that vanadium in its soil is an anionic form that is the most soluble and bioavailability.
3. Vanadium showed many correlations, including a positive correlation with Al2O3, MgO, K2O, and SiO2, which were attributed to adsorption on clay minerals. It also had a positive correlation with Ni and Co, which is due to the similarity of its sources from the combustion of oil and its derivatives and has an important negative correlation with phosphates, which is due to the similarity among them in the chemical structure, which enhances competition between them on the adsorption sites available in the soil.
4. The results of the factor analysis support that the emissions of burning crude oil and its derivatives and phosphate fertilizers contributed to increasing the concentration of vanadium in the soil of the study area. But despite the contribution of anthropogenic sources to increasing vanadium, individual pollution indices EF, Igeo and CF showed that the contamination of vanadium topsoil in Kirkuk was among the non-contamination categories and that the enrichment from anthropogenic sources was relatively low and not disturbing the environment