Current World Environment

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 -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 - 22 Norway,23 Malaysia,24 Czech Republic,25 India, 26-28 , 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 soil could 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 km2 (Wikipedia), Ado-Ekiti the capital city covers about 732 km2 of this area and lies between latitude 7o 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.
 

Figure 1: A map showing the position of the sampling locations. Click here to view Figure

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.

Table 1: The coordinates of the sampling area.

AJI = Ajilosun Road; ADY = Adebayo Road; EKSU = Ekiti State University Road; NNPC = Nigerian National Petroleum Corporation Road. 

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.

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 Na₂SO₄ (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 ^oC.

Clean-up column was developed by slurring about 10g of activated silica gel (about 100-200 mesh, activated at 200 ^oC for 16 h) with n-hexane into a chromatography column. About 1g of anhydrous Na₂SO₄ 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.

GC Operating Condition

Table 2: Operating conditions of GC.

Limit of Detection (LOD) and Limit of Quantification (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.

Statistical analysis

Statistical analysis used includes mean, corresponding standard deviation and coefficient of variation. Data obtained were subjected to Analysis of variance (ANOVA) using Statistical Software (SPSS Version 17).  

Results and Discussion

 Distribution of PAHs in Soil

The PAHs (µg/kg) level in soil from the four selected roadsides in Ado-Ekiti are depicted in Table 3. The average level of PAHs ranged from ND – 51.6 ± 46.7µg/kg with the highest recorded for naphthalene in Ajilosun. The PAHs concentration (µg/kg) ranged from 1.11±0.33 (acenaphthylene) - 51.6±46.7 (naphthalene) in Ajilosun. 1.82±0.30 (benzo(a)anthracene) - 46.6±12.7 (benzo(g,h,i)perylene) in Adebayo, 1.33±1.88 (pyrene) - 13.4±4.20 (benzo(a)pyrene) in EKSU and ND - 14.5±7.44 (benzo(g,h,i)perylene) in NNPC. The TPAHs ranged from 61.8±32.3 - 204±28.0 µg/kg with contamination pattern of ADY ? AJI ? EKSU ? NNPC. The presence of carcinogenic PAHs was highly observed in the samples. The seven carcinogenic PAHs were between 34.4 ± 17.9µg/kg (NNPC) and 101 ± 20.2µg/kg (ADY). The carcinogenic PAHs in the soil was in order: EKSU (60.4%) ? NNPC (55.7%) ? ADY (49.5%) ? AJI (30.8%). A high level of spatial variation was mostly recorded for most PAHs as revealed by the coefficient of variation. Only seven PAHs exhibited low (0.20 - 26.1%) spatial variation in Ajilosun.

Table 3: PAHs (µgkg^-1) concentrations of roadside soil from the study area.

Mean ± SD (CV%); ND = Not detected; *= Carcinogenic PAHs; ∑7C-PAHs = Total 7 carcinogenic PAHs; ∑HMW-PAHs = Total high molecular PAHs; ∑LMW-PAHs = Total molecular PAHs.

Five (Chrysene, fluorene, anthracene, fluoranthene and benzo(k)fluoranthene) in NNPC (2.73- 21.3%), three (acenaphthylene, phenanthrene and chrysene) in EKSU, while most PAHs in Adebayo exhibited moderate spatial variations. The level (0.51 – 51.6 µgkg^-1) were comparably lower than those reported for soil in Nigeria (5649 µgkg^-1)¹³, Delhi, India (41.1 – 9145 µgkg^-1)²⁸, USA (58680 µgkg^-1)¹⁶, Hong kong, China (2- 554 µgkg^-1)¹⁹, Malaysia (1450 µgkg^-1)²⁴, Shanghai, China (3780 µgkg^-1)²², Czech Republic (860 -10840 µgkg^-1)²⁵, Australia (3300 µgkg^-1)¹⁴, India (4694 ± 3028 µgkg^-1,²⁶; 6962 ± 4823 µgkg^-1)²⁷, Germany (16000µgkg^-1)¹⁵, Korea (15800 µgkg^-1)¹⁷, China (81820 ± 796200 µgkg^-1,²⁰ ; 3780 µgkg^-1)²² , while the soil in Brazil (96 µgkg^-1)¹⁸ is comparable to the present study. The concentration trends of the carcinogenic PAHs were in order: I₁₂₃P ? BaP ? BbF ? BaA ? BkF ? DahA ? Chry in Ajilosun;  I₁₂₃P ? BaP  ? BbF ? DahA ? Chry ? BkF ? BaF in Adebayo; while BaP ? I₁₂₃P ? BbF ? BaA ? DahA ? BkF ? Chry in EKSU and I₁₂₃P ? BaP ? BbF ? BkF ? BaA ? Chry in NNPC roadside. The percentage distribution of LMW in the study areas ranged from 17.3 – 54.2% with a concentration pattern of Ajilosun ? EKSU ? Adebayo ? NNPC, while the HMW-PAHs ranged from 46.0 – 82.7% with concentration a pattern of NNPC ? Adebayo ? EKSU ? Ajilosun. The percentage distribution of LMW-PAHs showed Ajilosun (54%), EKSU (24.3%), Adebayo (20.0%) and NNPC (17.3%), while the HMW-PAHs reflected NNPC (82.7%), Adebayo (80.0%), EKSU (75.3%) and Ajilosun (46.0%). Based on European classification system of soil contamination, ³² ∑16PAHs ? 200µg/kg showed no contamination, 200 - 600 µg/kg corresponds to weak, 600 - 1000 µg/kg revealed moderate, while ? 1000 µg/kg shows heavy contamination. All the sampling sites except Adebayo indicate no contamination, where its PAHs concentrations revealed ? 200 µg/kg and weak contamination.

One-way analysis of variance (ANOVA) revealed that the p-value = 0.005 (? 0.05 level of significant), this showed enough evidence that the parameters differs. Further analysis also revealed significant variations (p ? 0.05) in the levels of naphthalene, fluoranthene, pyrene, benzo(a)anthracene, indeno(1,2,3,cd)pyrene, benzo(b)fluoranthene,, benzo(a)pyrene, benzo(g,h,i)perylene and acenapthylene, while the other PAHs showed no significant variations among the sampling locations. The significant variations could be attributed to some factors such as physiochemical properties (soil pH, clay content and 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, NOAA³³ and Canadian Council of Ministers of Environment CCME³⁴ guidelines for the residential, community, and industrial soils.

Table 4: PAHs guidelines in soil (µgkg^-1) in comparison with the present study.

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

Table 5: Percentage distribution of PAHs composition in the sampling area.

Levels (%)

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 to BaP (TEQ_BaP) was used to establish the carcinogenic potency.

The total benzo(a)pyrene equivalent:   

TEQ_BaP= ∑(TEFi × Ci)

Where Ci = measured individual PAHs for (ith) compound, while TEFi = corresponding toxic equivalent factor.

Table 7: TEQ_BaP (µg/kg) of the soil from the study area.

The average sum of TEQ_BaP (µg/kg) were 11.8 (Ajilosun), 36.1 (Adebayo), 24.6 (EKSU) and 10.4 (NNPC). The PAHs toxicity level of NNPC is double and thrice of EKSU and Adebayo. The mean TEQ_BaP value were lower than 650 µgBaP_eq/kg, India⁴⁴; 542.8µgBaP_eq/kg, India²⁷; 124µgBaP_eq/kg, Spain⁴⁵ 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.

Acknowledgement

The authors acknowledge the technical assistance rendered by the Nigerian Institute of Oceanography and Marine Research (NIOMR) Central Laboratory, Victoria Island, Lagos.

Conflict of interest 

The author has no conflict of interest including any financial, personal, or other relationships with other people or organizations that can influence the work.

Funding Sources

The author received no financial support for the research, authorship, and/or publication of this article.

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