Levels of polycyclic aromatic hydrocarbon in the soil around typical automobile repair workshops in Nigeria

Study area

The study area locations are shown in Table 1. In order to assess the implication of automobile repair workshops’ activities on the soil levels of PAHs, soil samples were taken from five different local automobile repair workshops (locations A-E) in Ado-Ekiti, Southwestern Nigeria. Ado-Ekiti is the capital of Ekiti state in Southwestern Nigeria and is a city on 7° 37′ 15.9996″ N and 5° 13′ 17.0004″ E. The samples were collected between September – December 2022.

Soil sampling of PAHs

The top layer of soil samples (0–15 cm) was collected around the automobile workshops using stainless hand held auger. The sample was collected in the afternoon during the rush hour at workshop. The samples were wrapped in foil papers after collection and transported to the laboratory for analysis. At the laboratory, the impurities were then removed by hand picking, and then sieved through a 2 mm sieve shaker. The samples were then oven dried using a universal drier at 80 ^oC to constant mass, and they were then kept in aluminum foil bags and stored at -20 °C in the freezer.

Sample pretreatment

The soil samples were initially spiked with recovery standards of PAHs to monitor the integrity of the treatment. The sample was then extracted with 30 ml dichloromethane (DCM) using a Soxhlet extractor at 30°C for 8 hrs to enable PAHs trapped on the PUF disk and soil to dissolve in DCM. A clean-up procedure was done to remove unwanted compounds in the matrix by using silica-alumina column chromatography. The column was prepared by adding about 10 to 15 mm plug of glass wool to a chromatograph column and stuffing it down using a glass rod, then alumina and silica were added (1:2). The column packing was partially deactivated using a methanol-DCM solution (1:3) for better recovery. The sample extract was decanted into the column and eluted with 40 mL 1:1, DCM: hexane (Adesina et al., 2018). The extracted samples were then concentrated using a rotary evaporator (Büchi^®), to gently remove the solvent from the extract in order to bring down the volume of extract to 5ml. The resultant extracts were later analyzed for PAHs.

PAHs analysis

Quantification of PAHs present in the sample was carried out using gas chromatography (GC) (Agilent 7890) with a mass detector (Agilent 5975) operated in a selected ion monitoring mode and using electron impact ionization (EI). The chromatographic column dimensions are 30 m × 0.25 mm, and the internal diameter of 0.25 μm film thickness. The temperature program for the analysis was set as 90°C (1.0 min), 30°C/min, 250°C, 4°C/min, and 330°C (5 min). Determination of the concentration of PAHs was done using the internal standard method. The internal standards are naphthalene-d₈, acenaphthene-d₁₀, phenanthrene-d₁₀, chrysene-d₁₂, and perylene-d₁₂, used to quantify the amount of PAHs in the extract.

Quality assurance/quality control (QA/QC)

Apart from normal samples, field and laboratory blanks were also taken and treated the same way as the samples to ensure high integrity of the data. Soil blank samples were taken at locations far from the workshops, that have not been contaminated by automobile repair activities. Determination of instrument detection limits (IDLs) and method detection limits (MDLs) followed Norlock et al. (2011) procedure. Standard solutions were prepared for PAHs and six surrogates which were analyzed four times in the SIM mode using GC-MS. The PAH concentrations in each calibration solution were recalculated from the regression equation obtained using all six calibration standards. The average of the four replicates with the lowest detectable concentration was taken as the IDL. MDLs were calculated by multiplying the standard deviation of the replicates by the one-side t statistic at the 99%. Before the extraction, 20 ng of phenanthrene d10 recovery standard (RS) was used to spike the sample and the recovery range of the PAH was between 80% and 90%. Concentrations of PAHs in the field blanks were below the detection limit for all targeted compounds and no blank correction was carried out.

Health implications

Toxic equivalent

The potential toxicity of the PAHs is calculated by multiplying the individual concentration, C, with the toxicity equivalence factor (TEF) (eq. i) (Van den Berg et al., 2006).

ILCR from ingestion, inhalation, and dermal contact with PAHs contaminated soil are calculated using eq. ii, iii, iv, respectively. Also, the non-carcinogenic associated risk is assessed using the hazard quotient index (HQ) which is the ratio of the estimated to the reference dose using eq. v (USEPA, 1991, 2011).

C is the PAH concentration in the soil solid residue (mg kg⁻¹). ${\mathrm{IR}}_{\mathrm{ing}}$ is the soil ingestion rate (100 mg d⁻¹ for adults and 200mg d ⁻¹for children). ${\mathrm{IR}}_{\mathrm{inh}}$ is the inhalation rate (20 m³/day was assumed for adults while 9.6 m³/day was assumed for children). Table 2 shows the exposure parameters and factors used for the study.

Distribution of PAHs based on aromatic rings and molecular weight

PAHs are also classified based on the number of aromatic rings in each compound. The following ring compounds were present in this analysis: 2-ring PAHs (naphthalene), 3-ring PAHs (acenaphthylene, acenaphthene, fluorene, anthracene, and phenanthrene), 4-ring PAHs (pyrene, fluoranthene, chrysene, and benzo [a]anthracene), 5-rings PAHs (benzo [b] fluoranthene, benzo [a] pyrene, benzo [e] pyrenes, dibenzo [a,h] anthracene, and benzo [k]fluoranthene) and 6-rings PAHs (indeno [1,2,3-cd]pyrene). Figure 1 shows the distribution of PAHs based on the number of rings. 5-ring PAHs have the highest percentage at 39.37%, followed by 4-ring PAHs at 23.77%. 3-ring PAHs have a percentage contribution of 23.23%, 6-ring PAHs have a percentage contribution of 11.67%, and 2-ring PAHs have the lowest contribution at 1.97%. PAHs classification can also be by molecular weight: low molecular weight (LMW), consisting of 2- and 3-ring PAHs, middle molecular weight (MMW) consisting of 4-ring PAHs, and high molecular weight (HMW), consisting of 5- and 6-ring PAHs. Based on the results in Figure 2, LMW accounts for 25.2% of the total PAHs, MMW accounts for 23.77% and HMW accounts for the highest percentage of 51.03%.

Figure 1. Distribution of PAHs based on the number of aromatic rings.

Figure 2. Distribution of PAHs based on molecular weight.

Health implications of PAHs in the soil sample

The TEQ approach is used to determine the toxic potency of complex mixtures, the ILCR is used to determine the probability of developing cancer as the result of exposure to a specific carcinogen, and the hazard quotient is the ratio of the potential exposure to a substance and the level at which no adverse effects are expected. Table 3 shows the TEQ of the PAHs in all the soil samples analyzed. DhA has the highest toxicity with a mean value of 6.68 μg/g. BaP, which is usually used as an indicator of PAHs contamination, has a toxicity of 5.4 μg/g. Other compounds with high TEQ are BbF and BkF with mean values of 1.73 and 1.75 μg/g, respectively.

The ILCR is used to assess the probability of developing cancer by having contact with soil from the automobile workshop. Table 4 shows the ILCR and HQ values obtained from the study. The ILCR values from accidental ingestion of soil from these workshops are $3.9\times {10}^{-4}$ and $4.4\times {10}^{-4}$ for adults and children respectively, while ILCR values for dermal contact with the soil are $6.9\times {10}^{-4}$ and $5.4\times {10}^{-4}.$ The value is higher than the permissible limit of ${10}^{-6}$ stipulated by the World Health Organization. This implies that, with exposure to this soil for a particular period, there is a probability of developing cancer. However, the ILCR values for accidental inhalation of the soil are $3.2\times {10}^{-8}$ and $8\times {10}^{-9}$, for adults and children respectively. This indicates the chances of developing cancer from inhalation of this soil are slim. The combination of ILCR from accidental ingestion, inhalation and dermal contact gives $1\times {10}^{-3}$ and $9.8\times {10}^{-5}$, for adults and children respectively. These values are higher than the permissible limit of 1 × 10⁻⁶.

Table 4 also shows the hazard quotient, which is several folds higher than 1, the permissible limit, which implies that there is great non-carcinogenic risk associated with exposure to this soil.