Evaluation of Contamination and Ecological Risk of Heavy Metals Associated with Cement Production in Ewekoro, Southwest Nigeria

Background. Exposure to heavy metals emanating from cement production and other anthropogenic activities can pose ecological risks. Objectives. A detailed investigation was carried out to assess the contamination and ecological risk of heavy metals associated with dust released during cement production. Methods. Sixty samples, including 30 soils and 30 plants, were collected around Lafarge Cement Production Company. Control samples of soil and plants were collected in areas where human activities are limited. Samples were dried, sieved (for soil; 65 μm), packaged and analyzed using inductively coupled plasma mass spectrometry at Acme Laboratory in Canada. Results. The average concentration of heavy metals in soils of the area are: copper (Cu): 41.63 mg/kg; lead (Pb): 35.43 mg/kg; zinc (Zn): 213.64 mg/kg; chromium (Cr): 35.60 mg/kg; cobalt (Co): 3.84 mg/kg and nickel (Ni): 5.13 mg/kg. Concentrations of Cr in soils were above the recommended standards, while other metals were below recommended limits. The average concentrations of heavy metals in plants were: Cu: 26.32 mg/kg; Pb: 15.46 mg/kg; Zn: 213.94 mg/kg; Cr: 30.62 mg/kg; Co: 0.45 mg/kg and Ni: 3.77 mg/kg. Levels of heavy metals in plants were all above international limits. Geo-accumulation of metals in soils ranged between −0.15 and 6.32, while the contamination factor ranged between 0.53 and 119.59. Ecological risk index of heavy metals in soils ranged between 49.71 and 749. Discussion. All metals in soils of the study area except for Cr were below the allowable limits, while the levels of metals in plants were above the permissible limits. Levels of heavy metals reported in this study were higher than those from similar cement production areas. Soils around the Ewekoro cement production area were low to extremely contaminated by toxic metals. Cement production, processing, transportation in conjunction with the abandoned railway track in the area greatly contribute to the high degree of contamination observed in the area. Metal transfers from soil to plant are a common phenomenon. The metals pose low to considerable ecological risk. Conclusions. Anthropogenic sources, especially cement processing activities, release heavy metals which leads to progressive pollution of the environment and poses high ecological risk. Competing Interests. The authors declare no competing financial interests

Research oxygen in the soil, this consequently makes the presence of calcium silicate a major problem when ingested or even with dermal contact. 29 Cement dust also contains hexavalent chromium, a compound that is highly toxic in nature, and has major health impacts when bioaccumulation occurs up the food chain. [35][36][37] Little or no study has been carried out to assess the extent of contamination of heavy metals in soils and plants or evaluate the health and ecological risks associated with cement production in the study area. 38- 40 The research was therefore carried out to assess the impact of heavy metals found in cement dust on the soils and plants within and around Ewekoro Portland cement factory and also evaluates the health impacts.

Methods
Ewekoro is located within latitude N 6 o 53'00"-N 6 o 55'00" and longitude E 3 o 12'00"-E 3 o 13'00" in southwestern Nigeria (Figure 1). Ewekoro is found along the Sango-Ifo-Abeokuta Expressway of Ogun State, bordering Papalanto in the west and Abeokuta in the east. The town is 54 km from Lagos and 24 km from Abeokuta. Ewekoro is easily accessible and drained mainly by the River Ewekoro, which is seasonal in nature and has many tributaries. 41 The topography of Ewekoro is an immeasurable low land. The area experiences high levels of humidity and shrubbery is primarily located on tree plantations. The climate is significantly marked by two alternating wet and dry seasons. The average temperature in the area is 27.1 o C with the highest and lowest temperatures recorded in March and August, respectively. The mean annual rainfall in the area is 1305 mm with the highest rainfall observed in June and the lowest rainfall in Research January. 42 According to the United States Department of Agriculture classification scheme, the soils of the area are ferric, quartz and highly weathered clay minerals. 43, 44 Human activities in the area include mining, quarrying, farming and cement production, and has a population of 55,156. 45,46 The area is found within the Dahomey basin, which is one of the major sedimentary basins in Nigeria. Ewekoro is a type locality for limestone deposits in the country and soil type reflects the presence of clay minerals. 47,48 Sampling Thirty (30) topsoil samples weighing 1 kg each were collected at a depth of 20 cm in March 2015 using a hand auger. After each sample was collected, all instruments used for sample collection were washed using distilled water and dried before use at the next sampling point. 49 This method was repeated for of all of the collected samples. Three control samples were collected in areas with no observed anthropogenic activities. Likewise, 30 healthy plant samples, commonly consumed by locals were collected from four species of plants: Celosia argentea (soko), Corchorus olitorius (ewedu), Colocasiaesculenta (cocoyam), Musa sp.(banana) and Saccharum officinarum (sugar cane). The foliage was picked into a container and tagged. 50,51 Two (2) control samples of soils and plants were also collected in areas with no observed anthropogenic activities.

Chemical analysis
In the laboratory, all the soil and plant samples were dried at room temperature. The sods were pulverized and sieved using an impact electric sieve shaker. After sieving, clay-sized (63 µm) sediments were collected and packed into small zip-lock bags. The grains were divided into roots, stems, and leaves and pulverized to fineness (<0.002 mm) using a china clay mortar and pestle. Soil and plant samples were then digested before analysis.
One (1) gram of soil was weighed from each pulverized sample and dissolved with 15 ml nitric acid, 20.0 ml perchloric acid and 15.0 ml hydrofluoric acid, and heated for three hours and thereafter measured into a 100 ml flask with distilled water. 51 Plant samples were thoroughly washed in deionized water because they are more prone to battering of sediments, heated at 105 o C for 5 minutes, cooled at 70 o C for 48-72 hours to a stable mass, then turned into a powdery form in a 100 μm blender so they could be easily dissolved. 52-54 A homogenized measured mass of 0.25 g was put in a 100-ml dry Pyrex digestion tube and digested with 5 ml of concentrated nitric acid was measured with it for the metals analysis. Digested samples were diluted with ultrapure water using a 1:50 dilution factor. Digestates were sent for analysis at Acme Laboratory Canada. Metals analyzed in the samples included copper (Cu), lead (Pb), zinc (Zn), chromium (Cr), cobalt (Co) and nickel (Ni). Statistical analyses were carried out using the Statistical Package for the Social Sciences (SPSS) software program version 21. It was used to calculate mean, minimum, maximum, standard deviation and bivariate correlation.

Contamination and risk assessment
The equations described below were used to evaluate the contamination, ecological and health risks of heavy metals in samples. Heavy metal contamination in soils was calculated using the geo-accumulation index (Igeo), contamination factor (CF), contamination degree (CD) and pollution load index (PLI).

Geo-accumulation index
The Igeo was used to assess contamination of a specific metal in soils by evaluating metal enrichment above baseline or background values. Geo-accumulation index was calculated according to Equation 1. 55

Equation 1
I geo = log 2 x C n / (1.5×B n ) where, C n is the metal concentration in the sample; B n is the concentration of metal in the background sample, and the constant 1.5 is introduced to minimize the effect of possible variations in the background values which may be attributed to lithologic variations in the samples. The following interpretation for the Igeo was given by Loskaet al.: Igeo<0 = practically unpolluted, 0<Igeo<1 = unpolluted to moderated polluted, 1<Igeo<2 = moderately polluted, 2<Igeo<3 = moderately to strongly polluted, 3<Igeo<4=strongly polluted, 4<Igeo<5= strongly to extremely polluted and Igeo>5 = extremely polluted. 56

Contamination factor
The assessment of soil contamination was also carried out using the CF in Equation 2. The CF is the single element index, and all four classes are recognized. 57

Contamination degree
The sum of contamination factors for all examined elements represents the CF of the environment. 58 The CD is aimed at providing a measure of the degree of overall contamination in surface layers in a particular sampling site. The formula for calculating the CD is shown in Equation 3.

Equation 3
where, C d is the contamination degree and C f is the contamination factor. A CD<6 indicates a low degree of contamination, while 6<CD<12 implies a moderate degree of contamination. In addition, 12<CD<24 indicates a considerable degree of contamination, while CD> 24 reflects a high degree of contamination.

Pollution load index
The PLI is defined as the ratio of element concentration in the study to the background content of the abundance of chemical elements in the continental crust and is used to assess environment quality. 59

Contamination load index
Equation 5 was used to determine the rate of contamination of specific metals in the grain/plant.

CLI = Ccrop /MPC
Where, CLI is the contamination load index; C crop is the concentration of metal in a plant; MPC is the maximum permitted concentration of metals in crops, and a contamination load index >1 indicates contamination of grains by metals. 61

Bioaccumulation factor
Bioaccumulation factor is defined as the ratio of metal concentration in plant to that in the soil. 62 It is expressed using Equation 6.

BAF = Cp / Cso
Where, BAF is the bioaccumulation factor, and C p and C so are the metal concentration in aerial parts of the plant (mg/kg) and in soil (mg/kg), respectively. When the bioaccumulation factor>1 there is mobility of metal from soil to plant.

Ecological risk assessment for metals in soils
Ecological risks of metals were evaluated using the ecological risk index (ERI) (Equation 7) as presented by Mamut et al. 63 Equation 7 Where, ERI is the potential ecological risk of a single element ; T R is the toxicresponse factor; and C F is the pollution of a single element factor, which is also the contamination factor. The toxicresponse factors for some metals used in the study were Zn = 1, Cr = 2, Cu = 5, Pb = 5, cadmium (Cd) = 30, Ni = 5. The results from Equation 7 help to produce the risk index (RI), which is the summation of the ecological risk assessment (Equation 8). 64 Equation 8

Results
The average concentrations of heavy metals in soils in the study area are presented in Table 1 Figure  2). For Ni, sample 20 had the highest concentration, while sample 30 had the lowest (Figure 2). The concentration of Zn was highest in sample 4 and lowest in sample 1 (Figure 3). The concentration of Cr was highest in sample 27 and lowest in sample 1 (Figure 3). The highest concentration of Cu was found in sample 27, while the lowest was found in sample 1 (Figure 4). The highest concentration of Pb was found around sample 23, while the least values were found in sample 1 (Figure 4). The study further showed that the concentrations of Cr in soils in the study area were above Research the United States Environmental Protection Agency (USEPA) standards, while the concentrations of Zn, Cu, Pb, Ni and Co were below this standard. 65 The average concentrations of the metals in soils were greater than those in the control samples ( Table 1).

Metal concentrations in plants
The decreasing order of metals in plants in the present study area are Zn>Cr>Cu>Pb>Ni>Co (

Contamination assessment of soils and plants
Results of the contamination assessment of heavy metals in soils in the study area are presented in  Table 3).

Bivariate correlation
The results of the significant values (> 0.01) (Table 3) revealed that all metals in plants and soils originated from the same source, with the exception of Zn, which showed a varied value in soils and plants, suggesting different sources for the two media. This was confirmed in the results of the bivariate Table 1

Bioaccumulation assessment of heavy metals in plants
The outcomes of the bioaccumulation assessment of heavy metals in plants in the study area are shown in Table  3. The minimum and maximum bioaccumulation factor for Cu was 0.04 and 1.84, respectively, and ranged between 0.01 and 5.09 for Pb. The average bioaccumulation factor for Zn, Cr, Co and Ni was 1.39, 0.86, 0.43 and 0.78, respectively.

Ecological risk
Results of the ecological risk assessment of heavy metals in soils in the study area are presented in Figure  5.

Discussion
Except for Cr in soils of Ewekoro, concentrations of heavy metals were below the recommended limits set by the USEPA. 65 However, they were above their corresponding measured Laniyan, Adewumi

Figure 4 -Spatial distribution of copper and lead in topsoil in the study area
concentrations in the background soils (Table 1). Relating the above results with similar studies done in Ashaka and Obajana cement production areas in Nigeria, the concentrations of Cu, Pb, Zn and Ni in soils of the study area were above those reported in the two areas. 55-58 However, concentrations of Co in soils were lower than those reported in Ashakasoils, and higher than those in Obajana soils. In addition, concentrations of potentially toxic metals in plants in this area were above the recommended limits. 60 The spatial distribution of Co and Ni in soil (Figure 2) revealed that Jagun had higher concentrations of Co than other parts of the study area. In addition, Zn was highly concentrated in the northeast and southwest of the study area (Figure 3). Concentrations of Cr were high in the northwest around Ewekoro, Lapeleko and Jagun ( Figure  3). Copper was well distributed in soils across the study area, but was highly concentrated around Ewekoro, Lapeleko, Jagun and Papalanto ( Figure  4). Concentrations of Pb were high in soils around Ewekoro and Lapeleko (Figure 4). The study revealed that aerial deposition of metal-laden soils might have contributed significantly to their concentration in the area. A study by Afolabi et al. showed that more than 70% of the inhabitants of the area live in houses about 2 km from the cement processing factory. 45 This indicates that toxic metals in soils of the area are highly concentrated in Laniyan, Adewumi

Table 2 -Geo-Accumulation Index, Contamination Factor, Contamination Degree and Pollution Load Index of Heavy Metals in Soils Table 3 -Bioaccumulation Factor and Contamination Load Index of Heavy Metals in Plants
Research areas where people live. In addition, concentrations of heavy metals in plants of the area were above the recommended limits set by CODEX. 61 In the study area, the Igeo revealed that soils are unpolluted to strongly polluted by Cu Pb and Cr (the classification has been clearly defined in Equation 1), while they are unpolluted to extremely polluted by Zn and Co (Table 2). In addition, soils are moderately to extremely polluted by Ni (Loska et al). 56 However, the results of the CF showed that the soils in the area ranged from lowly to very highly contaminated by Pb, Zn and Co, while Cu and Cr present moderate to very high contamination (the classification is defined in Equation  2). Nickel showed considerable to very high contamination in soils of this area. The CD showed that heavy metals in soils of the area pose a moderate to high degree of contamination (Table 2). This was affirmed by the PLI which also revealed a moderate Table 4

Laniyan, Adewumi
Research to high degree of pollution (Table 2). This showed that cement production, processing, and transportation coupled with the abandoned railway track in the area might have significantly contributed to the high degree of contamination recorded in the area. According to Afolabi et al., land and aerial pollution contributed 6.40% and 80.81% of the total pollution in the study area. 45 Oral ingestion, dermal contact and inhalation of contaminated soils and dusts might have contributed significantly to the spread of diseases in the area as reported by Afolabi et al. 45 In addition, the contamination load index revealed that plants in this area are highly contaminated by heavy metals. Consumption of contaminated vegetables and plants may also contribute to health issues in the study area.
The present study further revealed the presence of metals transfer from soil to plants across most of the study area (Table 3). Bivariate correlation (Table  4) revealed that Cu, Pb, Zn, Cr and Ni were mobilized from soils to plants, while Co was not. Although metals in plants might have originated from soils in the area, aerial deposition of contaminated dusts is another possible means of contamination, entering plants through their stomata.
Correlation analysis of metals in soils showed that the potentially toxic elements in soils of the area might have originated from common mixed anthropogenic and point sources. Major possible sources of metals in soils of the area are cement production, processing, processing, rail and vehicular transportation. It was observed that Cu, Pb, Zn and Cr in soils of the study area posed low to considerable ecological risk ( Figure 5), while Ni posed considerable to very high ecological risk (the classification is defined in Equation 8).

Conclusions
The present study was carried out to assess the extent of heavy metals contamination and their potential ecological risk in soils and plants of Ewekoro, southwest Nigeria. Concentrations of heavy metals in soils and plants in the area were above those in background samples and cement production areas across the country. Soils and plants in the area are contaminated by heavy metals which possibly originate from anthropogenic activities, especially from cement production and processing as well as rail and vehicular transportation. In addition, crops in the area are strong bioaccumulators of these heavy metals, although aerial deposition of contaminated dust is also a potential source of metals. Furthermore, the ecological risk potential of heavy metals in soils of the area ranged from low to considerably high. Further studies should be conducted on the extent of heavy metal bioaccumulation in this area and the potential health risk to local residents. Stricter rules should be introduced to regulate cement production activities to lower the emission rate of cement dust polluted with metals into the environment.