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Concentration of Vehicular and Generators Soot in Lafia Metropolis, Nasarawa State, Nigeria

The concentrations of Pb, Cd, Cr, Al and Fe in Heavy Duty Trucks (HDT), Power Generating Plants (PGP), Motor Vehicles (MV) and Small Generator (SG) Soot derived from the College of Agriculture, Bukan Sidi and Mararaba Akunza in Lafia City, Nigeria, were assessed for the level of contamination using Atomic Absorption Spectrophotometer (AAS). The results indicated that the ranges of heavy metal concentrations in soot (mg/kg) were: Pb (0.0911±0.001 to 0.069±0.008) for the HDTs, (0.775±0.008 to 0.002±0.000) for the PGPs, (0.456±0.000 to 0.091±0.000) for the MVs and (0.837±0.000 to 0.027±0.000) for the SGs. The concentrations of Cd and Cr in the HDTs and PGPs were below the detection limits of the AAS. More so, Cd and Cr in MVs were below the detection limits of the AAS except for one of the MVs which gave a highly objectionable Cr concentration of 4.336±0.004 above WHO permissible limits for emissions into air and water. Al concentrations across the soot samples ranges from (0.126±0.002 to 0.093±0.001) for HDT, (0.170±0.001 to 0.097±0.000) for PGPs, (0.150±0.000 to 0.057±0.001) for MVs; and (1.348±0.000 to 0.518±0.000) for SGs. Iron concentrations ranges from (65.885±0.006 to 27.834±0.009) for HDTs, (62.663±0.004 to 11.422±0.019) for PGPs, (52.448±0.003 to 2.418±0.001) for MV, and (129.433±0.008 to 9.129±0.002) for the SGs. The results revealed that the concentration of Pb, Al and Fe in the HDTs, PGPs, MVs and SGs are above the WHO permissible limits for emissions into air and water. Cd, Cr, Al and Fe concentrations were higher in concentrations in SGs. The presence of heavy metals in the sampled soot portends their transfer to the food chain, especially as crop and vegetable cultivation in and around Lafia city.


I. INTRODUCTION
Soot formation is the conversion of a hydrocarbon fuel molecule containing few carbon atoms into carbonaceous agglomerate containing some millions of carbon atoms in an extremely complicated process [1], [2]. The process is gaseous-solid phase transition where the solid phase exhibits no unique chemical and physical structure [2]. Soot is known as an unwanted byproducts derived from incomplete combustion of carbon containing materials [3]. Heavy metals from vehicular emissions possess threats to the humans and the environment due to their adverse effects on the environment leading to the contamination of air, water, and soil [4]. Heavy metal refers to any metal and metalloid element that has a relatively high density ranging from 3.5 -7.9 gcm -3 and is toxic or poisonous at low concentrations [5]. They are elements with atomic number 21 or higher, for example, scandium and above [6]. They are discharged into the environment in the form of air, water, or soil contaminants through several activities like agriculture, power generation, automobile emission, industrial discharge, seepage of municipal landfills, septic tank effluents, and extensive use of agrochemicals, etc. [6], [7]. In Nigeria, Heavy Duty Trucks (HDT), automobiles, power plants, and generator sets are the largest industrial and domestic sources of air pollution [8].
These pollutants vary between different internal combustion engines and depend on variables like ignition timing, load, speed, and air/fuel ratio [9].
In gasoline exhaust emission, the pollutants of major concern are carbon monoxide (CO), hydrocarbon (HC), carbon dioxide (CO2) and polycyclic aromatic hydrocarbons (PAHs); in diesel exhaust emission, the pollutants of major concern are nitrogen oxides (NOx) and particulate matter (PM) [10].
Lafia Metropolis depends on Heavy Duty Trucks (HDT), for transportation of goods and animals, and Generators for both households and commercial power supply. Heavy metals from vehicular emissions possess threats to the humans and the environment due to their adverse effects on the environment leading to the contamination of air, water, and soil [4]. Lead (Pb) is used as anti-knock additive in conventional gasoline fuels. Leaded gasoline exhaust not only infects the air we breathe, but also responsible for adding Pb to the environment [10]. It is observed that Lafia Metropolis is hot, and the roofs of houses within the metropolis are corroded and rusty due to soot effects. This study is aimed at determining the concentration of some heavy and trace elements such as Pb, Cd, Cr, Al, and Fe in the soot of heavy-duty trucks, motor vehicles, and generators.

II. STUDY AREA
Lafia town is the capital city of Nasarawa State. It lies within latitude 8⁰29ˈ30"N and longitude 8⁰31ˈ00"E. It is home to both Federal and State Higher institutions of learning. There are so many agricultural and mining, business and educational activities going on in the city and its environs which involve the use of heavy-duty trucks, motor vehicles, and generators.
III. SAMPLE/ SAMPLING PROTOCOL Soot samples were collected in Lafia Metropolis from three locations; Bukan Sidi, College of Agric and Mararaba Akunza. Thirty six samples were collected. Sampling location was based on the population density, areas of industrial activities, and anthropogenic events. Soot samples from the exhaust of Heavy Duty Trucks (HDT), Motor Vehicles (MV), Power Generating Plants (PGP), and Small Generators (SG) were randomly collected. Plastic containers and spatula were used to collect the soot samples for digestion and metal analysis. The containers for the sample collection were thoroughly washed with detergent and dried to eliminate adsorption as described by [6], [11], [12]. The sampling was done twice per day for a period of three months and treated as bulk.

IV. HEAVY METAL ANALYSIS
Analysis of heavy metals in soot samples was done using Atomic Absorption Spectrophotometer (MRC Instruments, Model PG 990 and AA500) equipped with Perking Elmer HGA 850 Graphite Furnace auto sampler with a computer interface for operation and readings display, Varian Spectra AAS.

V. QUALITY CONTROL AND STATISTICAL TREATMENT OF DATA
The data under different amendments were subjected to ANOVA test for assessing the significance of differences in heavy metal concentrations in soot derived from vehicular and generators. All the statistical tests were performed using SPSS software (SPSS Ins., version 12) [13]. Whereas precision and accuracy of analysis were assured through repeated analysis of samples against National Institute of water resources Kaduna, Nigeria Standard Reference Material (SRM 1410) for all the heavy metals. The results were found within ± 2 % of the certified value [14].

VII. DISCUSSION
From the results obtained in Tables I-IV, Pb concentration in the studied Soot samples varied in the range of HDT>SG>PGP>MV with values above the [15] permissible limits for emissions into air and water. In this study, the highest peak for Pb was observed in HDT (0.911±0.001 mg/kg) followed by SG (0.837±0.000 mg/kg) and PGP (0.775±0.008 mg/kg). MV showed the least representation of 0.456±0.000 mg/kg. Similar research in Ghana by [12] revealed the value obtained for Pb to range from 0.09 to 0.16mg/kg; and it compares well with this study. According to [16] in the impact of automobile exhaust fumes on concentration levels of lead on bread in Port Harcourt City, Nigeria, there is a correlation in the concentration of lead in bus terminals and the volume of vehicular counts. A study by [17] in the determination of Heavy Metals in Soot from Petroleum Vehicles Exhaust Tailpipes in Kaduna Metropolis reported an average concentration value for Pb in gasoline engine as 14.097±0.644 mg/kg and 11.278±0.028 mg/kg for diesel engine with the gasoline engine having the highest value. The values are higher when compared with this study.
From the Tables I-IV results obtained for Cd in HDT, PGP, and MV were below the detection limit of the AAS. Hence highest value for SGs is 0.258+0.0001 mg/kg while the lowest is 0.054+0.0008 mg/kg. In their study, [18] reported cadmium was not detected in engine load of diesel engine exhaust and 2.8µg/g was reported for diesel fuel, and it agrees with this study. A similar study by [8] in Assessment of some selected Heavy Metals in Soot from the Exhaust of Heavy Duty Trucks and Power Generating Plants in Idah Local Government Area of Kogi State, Nigeria revealed that Cd has an average discharge in the soot samples as 0.3964ppm.
More so, Cr from the result showed MVI have a higher concentration value of 4.336 +0.004 mg/kg. While the HDTs and PGPs have concentration values below the detection limit of the AAS and below [15] limits for emissions into air and water. The SGs have Cr values above [15] limits with the exception of SG4 which is below the limits. In a related study by [19] in Heavy Metals and Black Carbon Assessment of PM10 Particulates along Accra-Tema Highway in Ghana, chromium mean concentration was reported as 0.115±0.016 µg/m 3 , and it agrees with the values obtained in SGs in this study. Cr was found to be below the detection limit of the Spectrophotometer in a similar study by [12], and this compares well with the values obtained for all HDTs, PGPs, and MVs in this study. Tables I-IV above Al showed a wide range of differences across the parameters. The highest Al peaks across the soot samples is 1.348±0.000 mg/kg for SGs, 0.170±0.000 mg/kg for PGPs, 0.150±0.000 mg/kg for MVs and 0.126±0.000 mg/kg for HDTs. A similar study by [20] revealed the presence of Al and other metals in irregular combinations. In their research [21] suggested a relationship between aluminum exposure and neurodegenerative diseases, including dialysis encephalopathy, amyotrophic lateral sclerosis, and Parkinsonism dementia in Kii Peninsula and Guam. Alzheimer disease due to aluminum exposure has also being suggested in their study.

As presented in
The Fe concentration recorded higher values of 129.433±0.008 mg/kg for SGs, 65.885±0.006 mg/kg for the HDTs, 62.663±0.004 mg/kg for the PGPs, and 52.448±0.003 mg/kg for the MVs. Concentration values obtained in all the soot samples are above the [15] permissible limits for emission into air and water. A similar study by [17] in Determination of Heavy Metals in Soots from Petroleum Vehicles Exhaust Tailpipes, Kaduna Nigeria, reported iron emission as the most significant with 1153.560±0.361 mg/kg and 796.816±0.522 mg/kg as mean concentrations for gasoline engine vehicle and diesel engine vehicle. Their report agreed with this study. In a Toxicological Mechanisms of Environmental soot study, [3] reported that iron among other heavy metals in their study contributes to the reactive oxygen species (ROS)-mediated DNA damage.
Using a multivariate test, the Levene's test was calculated for all the heavy metal. In the case of Fe, Cd, and Al, the null hypotheses were rejected since their significant values are below 0.05 being 0.004, 0.009, and 0.007, respectively. However, the error variance of the dependent variables is not equal across groups. Pb and Cr on the other hand accept the null hypothesis since their significance value is greater than 0.05. A 3(Direction: College of Agric vs Bukan Sdi vs Mararaba Akunza) ×4(1 km vs 2 km vs 4 km) between subject ANOVA was conducted to study concentration differences in 5 heavy metals which are Lead, Chromium, Iron, Cadmium and Aluminium.

VIII. CONCLUSION
From the results obtained, soot samples showed that the concentration of heavy metals from the sampling locations exceeded permissible limits set by W.H.O respectively. Variations in heavy metal concentrations as derived from Tables I-IV tend to be a consequence of a wide range of vehicular, generators usage and human activities in the Lafia Metropolis. A comparison of the diesel engine exhaust and the gasoline engine exhaust in the study showed that diesel engine exhaust has the highest concentration of Pb compared to gasoline engine exhaust. However, cadmium, chromium, aluminum, and iron concentrations were found to be higher in the gasoline engine exhaust compared to the diesel engine exhaust. Based on the results obtained in this study, it is concluded that Lafia Metropolis is suffering the soot pollution effects from Heavy Duty Trucks (used in transportation of animals, agricultural produce, and petroleum products, etc.), Power Generating Plants (used in Banks, Telecommunication Base Transmission Stations (BTS), Petrol Stations, and other industrial activities), Motor Vehicles (used for personal and commercial purposes); and the Small Generator (used in homes, offices, and businesses due to epileptic power supply within the metropolis). The emitted heavy metals in the soot find their ways into the water bodies, plant vegetations, and infect the air within the metropolis. Soot pollution as a major contributor to climate change can be felted in the hotness of Lafia Metropolis. Much precaution has to be taken especially the choice of area of residence. Above all, proper awareness must be in place on the effects of soot.