Polycyclic Aromatic Hydrocarbons in Ado-Ekiti Roadside Soil, Nigeria: Distribution, Source Identification, Composition Profiles, and Toxic Potential

The study considered the level, origin with toxic potentials of PAHs in soil roadside of Ado-Ekiti, Nigeria. Gas chromatographic technique was employed for the identification of PAHs after extraction and silica gel cleanup. Average concentration of the PAHs ranged from ND – 51.6 ± 46.7µg/kg, while total polycyclic aromatic hydrocarbons (TPAHs) ranged of 61.8±32.3 - 204±28.0 µg/kg with contamination pattern of ADY ˃ AJI ˃ EKSU ˃ NNPC. The percentage distribution of LMW ranged from 17.3 – 54.2%, while the HMW-PAHs showed 46.0 - 82.7%. The PAHs pattern was dominated majorly by 5- and 6- ring with distribution range of 15.1 and 43.2%. The overall diagnostic ratios of the soils suggested pyrogenic, coal/wood/grass/vehicular combustion in most cases. To evaluate the toxicity of the PAHs in the samples, PAHs toxic equivalence factors (TEFs) revealed that carcinogenic PAHs contributed majorly (over 95%) to toxicity of the soil


Introduction
Ubiquitous nature of PAHs in terrestrial and marine environment made them an important. 1 PAHs are concern due to persistent nature. The EU and USEPA have prioritized PAHs due to carcinogenic and mutagenic properties. 2 High and low molecular weight are the two class of PAHs. The 4 to 6 aromatic rings are HMW with less bio-degradable properties, while LMW-PAHs consists 2 to 3 rings with less carcinogen properties than HMW types. 3 US Environment Protection Agency have identified sixteen (16) PAHs as priority pollutants. 4 PAHs often contain elevated level of HMW and fewer LMW-PAHs. 5 PAHs could be transported through the atmosphere to remote areas with increase in general background level of soils [6][7] and lake sediments [8][9][10][11] where they accumulate due to their stability. Soils and river sediments often becomes environmental burden of these compounds. 12 In Nig, composeria, 13 and other countries, Australia, 14 Germany, 15 USA, 16 Korea, 17 Brazil, 18 China, [19][20][21][22] IBIGBAMI, Curr. World Environ., Vol. 17(1) 226-235 (2022) Norway, 23 Malaysia, 24 Czech Republic, 25 India, [26][27][28] Slovakia, 29 studies of PAHs in soil have been reported. Their toxicity and widespread distribution have made PAHs important globally. The number of PAHs in city soil and propinquity of the compounds in soilcould lead to life exposure. 30 The study, therefore, assesses the distribution, sources, make-up, and toxic potentials of PAHs in soil roadside soil of Ado-Ekiti in Nigeria.

Materials and Methods
The Study Area Soil were sampled from various locations in Ado-Ekiti, capital city of Ekiti State in Nigeria. The State covers an estimated land area of about 6,353 km 2 (Wikipedia), Ado-Ekiti the capital city covers about 732 km 2 of this area and lies between latitude 7 o 37'16'' N and longitude 5o 13'17'' E. The city is a major trade center for farm produce. The major means of transportation in Ado-Ekiti is by road. Figure 1, depicts the map of the area indicating the sampling spots

Sample Collection and Preparation
Samples of roadside soils were taken from various locations within Ado-Ekiti in June, 2017. Locations, where soil samples were collected include Adebayo (ADY), Ajilosun (AJI), Ekiti State University (EKSU) gate and Nigerian National Petroleum Corporation (NNPC) mega station roads. Table 1 showed the coordinates of the sampling sites.
Two composite samples were collected at each site with about 40 m between them. Composite soil samples were representative of four different spots within each area. These samples (four) were polled together to form a composite one. Soil samples were taken at 15cm depth. The soil samples collected from each area were placed appropriately in labeled glass sample bottles. Samples were later air-dried in the laboratory, ground in an agate mortar, and sieved with mesh size 2 mm. The sieved samples were later stored in glass bottles. IBIGBAMI, Curr. World Environ., Vol. 17(1) 226-235 (2022)

Extraction of PAHs in the sample with Clean-Up
Procedure Sample extraction and cleanup were done according to USEPA (method 3550C). 31 Twenty gramme each of dried soil sample and anhydrous Na 2 SO 4 (1:1 g/g) was ultrasonically extracted using 50 mL mixture of 1:1 v/v of n-hexane: acetone for 45 min. The supernatants were collected and extraction repeated three times with fresh 50 mL mixture. The supernatants were collected together and concentrated by a rotary evaporator to about 2 mL. The extracts was dissolved in n-hexane (5 mL) and concentrated in rotary evaporator to 2 mL at 40 o C.  with n-hexane into a chromatography column. About 1g of anhydrous Na 2 SO 4 was added and pre-eluted with 20 mL of n-hexane. The extract were moved quantitatively to the column and eluted using 25 mL of 2:3 v/v of n-hexane/dichloromethane. The hexane/dichloromethane extract was concentrated using rotary evaporator, adjusted to 2 mL with n-hexane for gas chromatographic (GC) analysis. Table 2 depicted the operating conditions of gas chromatography.

Limit of Detection (LOD)and Limit of Quantifica -tion (LOQ)
Four concentrations (standard) ranging between 0.2 and 1 mgL -1 were used to prepare calibration graph. Linear calibration curves was gotten in the tested concentration of PAHs standard. The LOD were based on a signal-to-noise ratio of 3 and the LOQ on the signal-to-noise ratio of 10.

Quality Assurance
Correlation coefficients were determined for analysed compounds. Values with ˂ 0.95 correlation coefficients were rejected while, those ˃ 0.95 were accepted. Values of correlation coefficient were between 0.9993 and 0.9999. They thus met the quality assurance standard and made the results acceptable.

Results and Discussion Distribution of PAHs in Soil
The PAHs (µg/kg) level in soil from the four selected roadsides in Ado-Ekiti are depictedin  OM) of the soil which could influence the dynamics and fate of both organic and inorganic pollutants in soil. Different accumulation and distribution pattern of some of these PAHs in soil could also contribute to the significant level. Table 4 showed the recommended guidelines of PAHs in soils. Polycyclic aromatic hydrocarbons level in the study areas was significantly lower than the National Oceanic Atmospheric Administration, NOAA 33 and Canadian Council of Ministers of Environment CCME 34 guidelines for the residential, community, and industrial soils.
The percentage distributions of PAHs compositions are depicted in Table 5. Adebayo, EKSU and NNPC exhibited a greater percentage of high molecular PAHs with a highest amount of 5-and 6-ringed PAHs. Ajilosun showed highest level of 2-and 6-ringed PAHs. Generally, the automobile smoke or exhaust is seen as the major fount or origin of high molecular PAHs. 35-36  The BaA/(BaA+Chry) of Adebayo and EKSU indicated grass, coal and wood combustion in most cases. IcdP/(IcdP+BghiP) were between 0.47 to 0.50, suggesting pyrogenic. 38 The overall diagnostic ratios suggested that the PAHs were predominately pyrogenic.

Assessment of Soil Toxicity
To assess the soil toxicity, benzo(a)pyrene toxicity equivalents were used. Investigators have established value for individuals PAHs Toxic Equivalence Factor. [42][43] It's also alluded to as benzo(a)pyrene toxicity. 42 Toxic equivalents (TEQs) were used to quantify PAHs toxic potencies of the soil. Total toxic equivalents of PAHs with respect toBaP(TEQ BaP ) was used to establish the carcinogenic potency.
The mean TEQ BaP value were lower than 650 µg BaP eq / kg, India 44 ; 542.8µgBaP eq /kg, India 27 ;124µgBaP eq / kg, Spain 45 and 428 µgBaP eq /kg, China. 22 The sum of carcinogenic toxic potency of the soils was in the order: Adebayo (35.9) ˃ EKSU (24.5) ˃ Ajilosun (11.6) ˃ NNPC (10.3). The result further revealed that the carcinogenic potency of the roadside soil was the highest in Adebayo and as such contribute majorly to toxicity of the soil.

Conclusion
The study revealed low PAHs contamination and high percentage distribution of HMW-PAHs with a percentage composition of 46.0 -82.7%. The PAHs composition was also characterized by a high 5-and 6-rings PAHs. The overall diagnostic ratio of PAHs suggested mixed pyrogenic sources. Carcinogenic PAHs level within the city soils could be toxic to human exposure. The research, therefore, suggests that PAHs pollution should be discouraged, most importantly traffic exhaust especially diesel exhaust.