3.1. PAHs concentrations in the soil of the tobacco agro-industrial area of Igboho
The concentrations of priority PAHs in the soil samples are shown in Table 2. The result showed that PAHs are ubiquitous in the soil samples of Igboho. The total concentration of the priority PAHs (∑PAHs) in the soil samples ranges from 136.70 ng.g− 1 to 889.30 ng.g− 1, with an average concentration of 569.78 ± 53.23 ng.g− 1. The soil samples in the vicinity of the tobacco curing site (TBS) accumulated the highest amounts of PAHs at a mean concentration of 889.30 ng.g− 1. The distribution pattern of PAHs shows that the PAHs concentration decreases with distance from the TBS. The PAHs levels were 792 ng.g− 1 at 20 m (FL1), 461 at 50 m (FL2), and 136.7 ng.g− 1 at 1.0 km away from TBS. The distribution pattern shows TBS's significance as the primary point source of PAHs pollution in the area. The average PAHs concentrations in the soil samples (569.78 ± 53.23 ng.g− 1) of the study area were higher than those of urban soil in Lagos, Nigeria (254 ng.g− 1, (Adetunde et al. 2014)), Agbabu, Nigeria (209.7 ng.g− 1,(Olajire et al. 2007)) and the concentration of soil samples in the agricultural soil of the Teskelewu community, Nigeria (236.40 ng.g− 1, (Enuneku 2019)), southern subtropical area of China (318.2 ng.g− 1, (HAO et al. 2007)). However, the average PAHs concentration in the soil was lower than that of the agricultural soil of Nanjing, China (3330 ng.g− 1,(Wang et al. 2015)) and 917 ng.g− 1 in agricultural soil of Changzhi, China (Liu et al. 2017). The result indicated that people should be cautious about environmental quality around the area's tobacco agricultural farms.
Table 1
Description of sampling stations in the study area.
Sampling Point
|
Coordinate
|
Description of activities
|
TBS
|
08051’910’’N, 003046’066’’E
|
Tobacco curing site
|
FL1
|
08051'904" N, 003046'097" E
|
Farmland 20 m from curing site
|
FL2
|
08052'003" N, 003046'089" E
|
Farmland 50 m from curing site
|
FL3
|
08052'403" N, 003046'518''E
|
Farmland 1 km from curing site.
|
Table 2
Level of PAHs in the tobacco agricultural area of Igboho (ng.g− 1)
PAHs
|
Sampling Point
|
TBS
|
FL1
|
FL2
|
FL3
|
Naphthalene
|
2.78
|
2.10
|
2.20
|
1.90
|
Acenaphthyl
|
2.19
|
ND
|
2.55
|
1.67
|
Acenathene
|
4.65
|
1.83
|
2.27
|
2.27
|
Fluorene
|
5.79
|
2.65
|
3.19
|
2.63
|
Phenanthren
|
1.58
|
5.97
|
4.92
|
7.47
|
Anthracene
|
7.74
|
4.17
|
3.95
|
ND
|
Fluoranthen
|
2.26
|
10.3
|
1.01
|
3.49
|
Pryrene
|
42.72
|
5.38
|
7.42
|
8.04
|
Benzo(a)pyr
|
3.69
|
5.07
|
153.0
|
7.10
|
Benzo(a ) anthracene
|
ND
|
43.78
|
ND
|
ND
|
Chrysene
|
241.5
|
46.94
|
10.73
|
19.41
|
Benzo(e )pyrene
|
85.54
|
53.08
|
86.90
|
39.14
|
Benzo(b)fluoranthene
|
5.74
|
4.77
|
7.22
|
5.54
|
Benzo(k)fluoranthene
|
21.00
|
ND
|
ND
|
ND
|
Benzo(j)fluoranthene
|
118.60
|
22.18
|
22.03
|
18.88
|
Benzo(a)pyrene
|
11.31
|
12.63
|
17.12
|
13.15
|
7,12-Dimethylb
|
60.54
|
242.6
|
74.34
|
ND
|
3 Methylcholanthrene
|
70.78
|
29.06
|
ND
|
ND
|
Indeno(123cd)pyrene
|
133.10
|
38.00
|
ND
|
ND
|
∑LMW PAHs
|
24.93
|
16.73
|
19.08
|
15.94
|
∑HMW PAHs
|
796.75
|
514.13
|
379.77
|
104.76
|
∑19 EPA PAHs
|
889.30
|
792.10
|
461.00
|
136.70
|
MEAN
|
46.81
|
41.69
|
24.26
|
7.19
|
SD
|
0.085
|
0.081
|
0.030
|
0.011
|
∑CPAHs
|
321.04
|
267.57
|
211.40
|
62.09
|
Figure 1 (a) shows the PAHs' composition profiles in the soil of the Tobacco agricultural area of Igboho based on the PAHs ring size. The PAHs are generally classified based on the number of aromatic rings as 2-rings, 3-rings, 4-rings, 5 rings, and 6-rings PAHs. The composition pattern is in the order of 6-rings ˃ 4-rings ˃5-rings ˃ 3-rings ˃ 2-rings. The six-ring sized PAHs have a relative abundance of 34.50%. The percentage abundance of the four rings PAHs, fluoranthene, Chrysene, benzo(a)anthracene, and pyrene was 32.46%, and the abundance of five-ring PAHs benzo(k)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene was 29%. The three rings sized PAHs; acenaphthene, fluorene, phenanthrene, anthracene, acenaphthylene, have a relative abundance of 3.53%, while two rings PAH has the least concentration in the soil samples of the area. To be more specific, as summarized in Table 2, the concentration of Chrysene (241.50 ng.g-1), a four-membered ring PAHs and indeno-(123cd)pyrene (133.10 ng.g-1), a six-membered ring PAHs were the highest at the curing site (TBS). Compared to the TBS, the concentrations of Chrysene and indeno-(123cd)pyrene were lower for the farmland samples. The five-membered ring, benzo(j)fluoranthene concentrations were also high with mean concentrations of 118.60 ng.g-1 at the curing site (TBS). The Benzo(j)fluoranthene in the farmland samples was 22.18 ng.g-1, 22.03 ng.g-1, and 18.88 ng.g-1, respectively. The concentration of all the detected PAHs was high at the curing site (TBS) and decreased across the farmlands. The PAHs ring-size distribution profile suggested that the incomplete combustion of the tobacco leaves and waste could be the main source of PAHs contamination in the soil sample. High molecular weight PAHs are found to be 89.6 %, 64.8 %, 82.4 %, and 76.7 % of the total PAHs concentrations at TBS, FL1, FL2, and FL3, respectively.
3.2. Level and compositions of PAHs in food crops
The total concentration of PAHs in the food crop samples from the agricultural farmland was also evaluated. Table 3 shows the level of PAHs in Zea mays, Dioscorea alata, and Manihot esculenta collected from the farmland around the curing sites' vicinity Igboho. The total concentration of PAHs in the Zea mays crop samples ranged between 2.16 ng.g− 1 to 126.00 ng.g− 1 with a mean concentration of 0.113 ± 0.05 ng.g− 1 and 6.63 ± 0.27 ng.g− 1. The highest concentration of PAHs (126.0 ng.g− 1) was detected in the Zea mays samples (FL1M) harvested from the farmland close to the tobacco curing site (TBS). Also, the highest level of carcinogenic PAHs like Benzo(b)fluoranthene (13.12 ng.g− 1) and Chrysene (11.64 ng.g− 1) were found in the Zea mays samples at FL1. The concentration of PAHs in the Zea mays samples at FL1M, FL2M, and FL3M was lower than other crop samples due to the phytovolatilization process that removed PAHs from the soils and groundwater and transferred into the vapor phase via plant leaves (Brady, 1990).
Table 3
Level of PAHs (ng.g− 1) in the crops of the Agro-tobacco farming area of Igboho, Nigeria
PAHs
|
Sampling point and food crop type
|
FL1M
|
FL2M
|
FL3M
|
FL1C
|
FL2C
|
FL3C
|
FL1Y
|
FL2Y
|
FL3Y
|
Naphthalene
|
2.16
|
2.18
|
ND
|
2.26
|
2.15
|
2.13
|
2.37
|
2.18
|
2.01
|
Acenaphthyl
|
I.70
|
ND
|
ND
|
ND
|
ND
|
ND
|
ND
|
ND
|
ND
|
Acenathene
|
2.08
|
ND
|
ND
|
ND
|
1.59
|
1.62
|
ND
|
1.80
|
1.80
|
Fluorene
|
2.77
|
ND
|
ND
|
3.24
|
3.36
|
3.21
|
3.39
|
3.43
|
2.65
|
Phenanthren
|
3.92
|
ND
|
ND
|
5.42
|
7.32
|
3.44
|
5.08
|
6.45
|
3.93
|
Anthracene
|
4.82
|
ND
|
ND
|
4.92
|
5.43
|
4.85
|
4.22
|
5.08
|
3.77
|
Fluoranthen
|
4.53
|
ND
|
ND
|
10.10
|
2.14
|
6.83
|
10.2
|
1.92
|
6.32
|
Pryrene
|
6.28
|
ND
|
ND
|
5.12
|
13.24
|
5.13
|
4.82
|
12.90
|
5.11
|
Benzo(a)pyrene
|
5.16
|
ND
|
ND
|
ND
|
38.12
|
8.34
|
37.88
|
8.97
|
ND
|
Benzo(a ) anthracene
|
ND
|
ND
|
ND
|
51.12
|
12.06
|
10.10
|
ND
|
ND
|
ND
|
Chrysene
|
11.64
|
ND
|
ND
|
53.14
|
33.26
|
2.85
|
52.48
|
33.80
|
1.99
|
Benzo(e )pyrene
|
ND
|
ND
|
ND
|
88.16
|
14.38
|
6.89
|
89.01
|
14.52
|
5.72
|
Benzo(b)fluoranthene
|
13.12
|
ND
|
ND
|
7.27
|
6.02
|
1.92
|
7.24
|
5.83
|
1.89
|
Benzo(k)fluoranthene
|
58.12
|
ND
|
ND
|
10.02
|
ND
|
ND
|
10.45
|
ND
|
ND
|
Benzo(j)fluoranthene
|
5.12
|
ND
|
ND
|
28.12
|
16.12
|
11.02
|
27.60
|
16.73
|
11.45
|
Benzo(a)pyrene
|
ND
|
ND
|
ND
|
51.12
|
12.06
|
10.10
|
51.01
|
12.86
|
10.14
|
7,12-Dimethylb
|
4.13
|
ND
|
ND
|
21.12
|
ND
|
ND
|
20.94
|
ND
|
ND
|
Methylcholanthrene
|
ND
|
ND
|
ND
|
ND
|
ND
|
ND
|
ND
|
ND
|
ND
|
Indeno(123cd)pyrene
|
8.70
|
ND
|
ND
|
ND
|
ND
|
ND
|
ND
|
ND
|
ND
|
∑LMW PAHs
|
17.28
|
2.16
|
ND
|
11.92
|
13.11
|
12.82
|
15.06
|
12.69
|
2.01
|
∑HMW PAHs
|
15.00
|
ND
|
ND
|
210.86
|
65.71
|
58.94
|
311.72
|
107.93
|
41.62
|
∑19EPA PAHs
|
126.0
|
2.16
|
ND
|
288.10
|
84.82
|
81.34
|
326.5
|
143.8
|
71.62
|
MEAN
|
7.88
|
0.13
|
ND
|
20.58
|
6.06
|
5.81
|
21.77
|
9.59
|
4.78
|
SD
|
0.01
|
0.003
|
ND
|
0.025
|
0.0091
|
0.0085
|
0.02
|
0.01
|
0.008
|
∑cPAHs
|
95.71
|
ND
|
ND
|
193.79
|
63.70
|
25.97
|
142.12
|
52.49
|
14.02
|
Tobacco curing site (TBS), Farmland 1 (FL1), Farmland 2 (FL2), Farmland 3(FL3), Maize 1(FL1M), Maize 2 (FL2M), Maize 3 (FL3M), Cassava 1 (FL1C), Cassava 2 (FL2C), Cassava 3 (FL3C), Yam 1 (FL1Y), Yam2 (FL3Y), Below detection limit (N.D.) |
As summarized in Table 3.0, PAHs' concentration in the Manihot esculenta decreased from the closest farmland FL1C to the distant farmland FL3C. The total EPA PAHs concentrations found in the Manihot esculenta ranged from 81.34 ng.g− 1 to 288.10 ng.g− 1 with a mean concentration of 15.17 ± 1.23 ng.g− 1. The highest PAHs concentration (288.10 ng.g− 1) was detected in FL1C samples around the tobacco curing site (TBS), and the lowest PAHs were recorded at the most distant farmland FL3. Benzo(e)pyrene (88.16 ng.g− 1), Benzo(c)pyrene (38.12 ng.g− 1) and Benzo(j)fluoranthene were found in highest concentration in FL1C, FL2C, and FL3C respectively.
The result shows that PAHs' concentration in the Dioscorea alata decreased from the closest farmland FL1Y to the distant farmland FL3Y. The total PAHs concentrations found in the Dioscorea alata samples ranged from 71.62 ng.g− 1 to 326.50 ng.g− 1, with a mean concentration of 23.32 ± 2.23 ng.g− 1. The highest total PAHs concentration (326.50 ng.g− 1) was detected in FL1Y samples around the tobacco curing site (TBS). Benzo(e)pyrene (89.01 ng.g− 1), Chrysene (52.48 ng.g− 1), Benzo(a)pyrene (51.01 ng.g− 1) have the highest concentration in the Dioscorea alata sample at FL1, and Chrysene (33.80 ng.g− 1) was the highest at FL2, while benzo(a) pyrene was the highest at FL3. The distribution of high molecular weight PAHs (ΣHMW) and low molecular weight PAHs (ΣLMW) in the soil samples showed that ΣHMW was dominant at the tobacco curing site (TBS). The total concentration of ΣHMW is higher than (ΣLMW) at most of the sampling points. The lower molecular weight PAHs with lower numbers of rings are volatile and readily biodegradable than the high molecular weight PAHs which are more persistent in the environment (Adedosu et al. 2013). The PAHs' relative concentration based on the numbers of rings to the total concentrations of PAHs in the curing site was 17.13 ng.g− 1. The concentration of PAHs in the Zea mays samples based on the ring size are in the order of 5-rings ˃ 4-rings ˃3-rings ˃ 6-rings, ˃ 2-rings. The five-rings sized PAHs, benzo(k) fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene have a relative abundance of 51.8 %. The percentage abundance of the four rings PAHs, fluoranthene, Chrysene, benzo(a)anthracene, and pyrene was 26.4 %, and the three rings sized PAHs; acenaphthylene, fluorene, phenanthrene, anthracene, acenaphthylene have the relative abundance of 11.60 %. The six-ring size and the two-ring size recorded the least relative abundance of 6.75 % and 3.45 %. The concentration of PAHs in the Manihot esculenta samples based on the ring size is in the order of 5-rings ˃ 4-rings ˃3-rings ˃ 6-ring ˃ 2-ring. The five-rings sized PAHs; benzo(k)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene have a relative abundance of 48.07 %. The percentage abundance of the four rings PAHs, fluoranthene, Chrysene, benzo(a)anthracene, and pyrene was 40.02 %, and the three rings sized PAHs; acenaphthylene, fluorene, phenanthrene, anthracene, acenaphthylene have the relative abundance of 8.60 %. The six-ring size and the two-ring size recorded the least relative abundance of 1.70 % and 1.60 %. The concentration of PAHs in the Dioscorea alata samples based on the ring size is in the order of 5-rings ˃ 4-rings ˃3-rings ˃ 6-rings, ˃ 2-rings. The five-rings sized PAHs benzo(k)fluoranthene, benzo(a)pyrene have a relative abundance of 51.90 %. The percentage abundance of the four rings PAHs, fluoranthene, Chrysene, benzo(a)anthracene, and pyrene was 34.80 %, and the three rings sized PAHs; acenaphthylene, fluorene, phenanthrene, anthracene, acenaphthylene have the relative abundance of 7.87 %. The six-ring size and the two-ring size recorded the least relative abundance of 4.13 % and 1.30 %, respectively.
3.3. Total Organic Carbon content
Figure 2 shows the distribution of total organic carbon (TOC) content in the samples. It is generally observed that the values of TOC decrease with increasing distance from the tobacco curing site. This implies that tobacco curing is the major contributor to the increasing organic matter level in the study area. The percentage of total organic carbon of the soil and crop samples shows a wide variation in their values. The percentage of total organic carbon in the soil and crop samples show a wide variation in their values that ranged from 8.3 wt. % to 13.6 wt. %. The maximum percentage TOC (13.6 wt. %) was recorded at the tobacco curing site, while the distant farmland FL3 recorded the lowest value of 8.3 wt. %. These results are in line with 1.0 wt. % detention limit of Soil Guidelines Values (SGVs) in CLEA Model published by DEFRA and the Environment Agency (E.A.) in March 2002 which sets a framework for the appropriate assessment of risks to human health from contaminated land, as required by Part II (A) of the Environmental Protection Act 1990. This is also justified with a 0.5 wt% (US EPA) detection limit. The soil from the tobacco curing site showed a relatively higher TOC percentage (13.6 wt. %) than the farmlands' soil samples. The farmland FL1 closest to the curing site recorded the second-highest value with (10.5 wt. %) while all the food crop samples (yam, cassava, maize) obtained from FL1 recorded the highest values with 10.4 wt. %, 10.1 wt. %, 8.9 wt. % respectively. The farmland FL3, the distant farmland from the curing site, recorded the smallest value with 8.3. wt. % while all the food crop samples (yam, cassava, maize) obtained from FL3 recorded the lowest values with 3.1 wt. %, 5.4 wt. % and 1.9 wt. %, respectively. This is because of the contribution of various proportions of combustible residues, non-aqueous phase liquids, and natural organic matter (NOM) that have a strong affinity for contaminants. As the contaminants are released in the soil matrix, they bind to the surface and become sequestrated into the soil matrix. The hydrophobicity of these compounds constitutes the main factor determining their persistence in the environment, and they tend to be strongly absorbed by soil particles with low bioavailability and possibly accumulate in the food chain. The scatter plot in Fig. 3 (b) was used to assess the relationship between the percentage total organic carbon and the total concentration of PAHs in the soil sample were shown by the linear regression curve in Fig. 3 (b). The R2 value was 0.937, showing that there is a positive correlation between the TOC and PAHs. This confirmed that the soil sample's organic carbon content increases the PAHs' adsorption at the study site.
3.4. The diagnostic ratio of PAHs
Different researchers have employed some PAHs isomers ratios to distinguish between petrogenic, biogenic, and pyrogenic sources of PAHs in the environment (Adedosu, Adeniyi, and Adedosu 2015; Liu et al. 2010; Zakaria et al. 2002). Petrogenic sources are characterized by the predominance of low molecular weight (LMW, 2-3 ring) PAHs over the high molecular weight (HMW, 4-6 ring) PAHs. The ratio of low molecular weight PAHS to high molecular weight PAHs greater than 1.0 (LMW/HMW ˃1.0) indicates a petrogenic source of PAHs, while pyrogenic sources are characterized with the predominance of high molecular weight PAHs over the low molecular weight PAHs and LMW/HMW < 1.0 (Adedosu et al. 2015). The diagnostic ratio of PAHs in the study area is presented in Table 4. The values of LMW/HMW PAHs for the TBS (0.46), FL1 (0.30), FL2 (0.12), and FL3 (0.19) were less than 1.0, suggesting a pyrolytic source of PAHs contamination. The Phe/Ant ratio allowed the separation of the pyrolytic (combustion origin) and petrogenic (unburned petroleum products) PAH sources. A Phe/Ant ratio lower than 15.0 is assumed to be of a pyrolytic origin from the combustion of plants, wood, grass, and others, whereas a value higher than 15.0 is assumed to be of combustion of petroleum hydrocarbons such as coal, crude oil, and others. The values of Phe/Ant at TBS, FL1, FL2, and FL3 were lower than 15.0 and confirmed that the PAHs were from the combustion of plants, wood, and leaves, possibly from the tobacco-curing activities in the area.
Table 4
Diagnostic Ratio of PAHs in the soil of the Tobacco-agro industrial area of Igboho
PAHs ratio
|
Diagnostic reference values
|
Sampling point
|
Petrogenic
|
Pyrogenic
|
TBS
|
FL1
|
FL2
|
FL3
|
Phe /Ant
|
> 15
|
< 15
|
2.04
|
1.04
|
1.30
|
0.01
|
Flu /Pyr
|
< 0.4
|
> 0.4
|
0.53
|
0.94
|
0.43
|
0.44
|
BaA/Chr
|
< 0.25
|
> 0.25
|
ND
|
0.93
|
ND
|
ND
|
BeP/BaP
|
< 1
|
> 2
|
7.56
|
3.60
|
5.10
|
2.98
|
BbF/BkF
|
> 1
|
< 1
|
0.27
|
0.11
|
ND
|
ND
|
Ant/ Ant + Phe
|
< 0.1
|
> 0.1
|
0.33
|
4.90
|
0.48
|
ND
|
LMW/HMW
|
> 1
|
< 1
|
0.46
|
0.30
|
0.12
|
0.29
|
Similarly, an Flt/Pyr ratio PAHs greater than 0.4 indicate the influence of pyrolytic PAHs, and the ratio of PAHs lower than 0.4 indicates petrogenic sources. The BaA/Chr ratio PAHs that more significant than 0.25 are assumed to be pyrolytic sources, and the BaA/Chr ratio PAHs lower than 0.25 are assumed to be petrogenic sources. The TBS, FL1, FL3 but FL2 was 0.93, which was greater than 0.25. The BeP/BaP ratio PAHs greater than 2.0 is assumed to be of pyrolytic origin. Similarly, other PAHs ratio in Table 4 confirmed that the source of PAHs contamination in the study area is from the combustion of biomass, woods, and other organic matter due to the tobacco curing process.
3.5. Carcinogenic potency and toxicity potential of PAHs in the Soil and food crop samples
The International Agency for Research on Cancer (IARC) and the United State Environment Programme, (USEPA) reported that chrysene, benzo(a)anthracene, dibenzo(a,h)anthracene, benzo(a)pyrene, benzo(b)fluoranthene, benzo(k)fluoranthene and indeno(1,2,3-cd)pyrene are potential human carcinogens. All the listed carcinogenic PAHs are detected in the soil samples of the study area. The total concentration of carcinogenic PAHs in the tobacco curing site soil (TBS) was 473.19 ng.g-1. This represented 53.2 % of the total PAHs concentrations detected in the tobacco curing site. The total concentrations of the carcinogenic PAHs at each of the sampling points were presented in Table 5. The highest concentration of carcinogenic PAHs (473.19 ng.g-1) was recorded in soil from the tobacco curing site (TBS). The concentration of carcinogenic PAHs decreased as the farmlands were farther away from the curing site with FL1 (388.72 ng.g-1), FL2 (109.41 ng.g-1), and FL3 (38.10 ng.g-1). The lowest concentrations of the carcinogenic PAHs were recorded at the distant farmland FL3. The total concentrations of the carcinogenic PAHs at each of the sampling points decreased from the curing site across the farmlands, which indicated that the tobacco curing site was the contaminant source within the study area's vicinity. Although benzo[a]pyrene (B(a)P) can have toxic effects, a major concern is the ability of the reactive metabolites, such as epoxides and dihydrodiols, of some PAHs to bind to cellular proteins and DNA. The level of B(a)P in the soil samples was 11.31 ng.g-1 for TBS, 12.63 ng.g-1 for FL1, 17.12 ng.g-1 for FL2, and 13.15 ng.g-1 for FL3 (Table 5). The B(a)P concentrations were lower than the European Union's maximum contaminant level (MCL) of 25 ng.g-1. The B(a)P equivalent (B(a)Peqv) was calculated using the toxicity equivalent (T.E.) for each PAH, as presented in Table 5. The calculated total B(a)Peqv at the TBS and surrounded farmlands FL1, FL2, and FL3 ranged from 13.84 ng.g-1 to 268.85 ng.g-1. The highest B(a)Peqv was found at the TBS with 268.85 ng.g-1, and the distant farmland FL3 recorded the least value of 13.84 ng.g-1. It is observed that the tobacco curing site TBS (268.85 ng.g-1) has been polluted while the farmland FL1 (112.21 ng.g-1), FL2 (92.29 ng.g-1), and FL3 (13.84 ng.g-1) were slightly polluted. Table 6 shows the total concentration of carcinogenic PAHs in the Manihot esculenta samples as the farmland closer to the tobacco curing site FL1C recorded the highest value of 193.79 ng.g-1. The concentration of carcinogenic PAHs adsorbed decreased as the farmlands were farther away from the curing site with FL1C (193.79 ng.g-1), FL2C (63.70 ng.g-1), and FL3C (12.03 ng.g-1), respectively. The lowest concentrations of the carcinogenic PAHs were recorded for the Manihot esculenta collected from the distant farmland FL3. The total concentrations of the carcinogenic PAHs at each sampling point decreased from the curing site across the farmlands indicated that the tobacco curing site was the source of contaminants within the study area's vicinity.
Table 5
Carcinogenic potency and toxicity potential of PAHs in the soil of the tobacco processing industry Igboho (ng.g− 1)
PAHs
|
TEF
|
Sampling sites
|
TBS (ng.g− 1)
|
FL1 (ng.g− 1)
|
FL2 (ng.g− 1)
|
FL3 (ng.g− 1)
|
ΣCPAHs
|
B(a)Pequ
|
ΣCPAHs
|
B(a)Pequ
|
ΣCPAHs
|
B(a)Pequ
|
ΣCPAHs
|
B(a)Pequ
|
B(a)p
|
0.1
|
ND
|
0.00
|
242.35
|
24.23
|
ND
|
0.00
|
ND
|
0.00
|
Chr
|
0.01
|
60.54
|
0.06
|
0.47
|
0.469
|
10.73
|
0.11
|
19.41
|
0.194
|
B(k)f
|
0.1
|
21.00
|
2.10
|
ND
|
0.00
|
ND
|
0.00
|
ND
|
0.00
|
B(b)f
|
0.1
|
5.74
|
0.57
|
4.77
|
0.477
|
7.22
|
0.72
|
5.54
|
0.554
|
B(a)f
|
1
|
11.31
|
11.31
|
12.63
|
1.63
|
17.12
|
17.12
|
13.15
|
13.15
|
Ind
|
0.1
|
133.10
|
13.31
|
38.00
|
3.80
|
ND
|
0.00
|
ND
|
0.00
|
D(ah)A
|
1
|
241.50
|
241.50
|
43.78
|
43.78
|
73.34
|
74.34
|
ND
|
0.00
|
Total
|
|
473.19
|
268.85
|
388.72
|
112.21
|
109.41
|
92.92
|
38.10
|
13.84
|
Table 6
Carcinogenic potency and toxicity potential of PAHs in Dioscorea alata from the farmlands (ng.g− 1)
PAHs
|
TEF
|
Sampling sites
|
FL1 (ng.g− 1)
|
FL2 (ng.g− 1)
|
FL3 (ng.g− 1)
|
ΣCPAHs
|
B(a)Pequ
|
ΣCPAHs
|
B(a)Pequ
|
ΣCPAHs
|
B(a)Pequ
|
Chry
|
0.01
|
52.48
|
0.542
|
33.80
|
0.338
|
1.99
|
0.02
|
B(k)f
|
0.10
|
10.45
|
1.045
|
ND
|
0.00
|
ND
|
0.00
|
B(b)f
|
0.10
|
7.24
|
0.724
|
5.83
|
0.583
|
1.89
|
0.19
|
B(a)p
|
1.00
|
51.01
|
51.01
|
12.86
|
12.86
|
10.14
|
10.14
|
D(a,h)A
|
1.00
|
20.94
|
20.94
|
ND
|
0.00
|
ND
|
0.00
|
Total
|
|
142.12
|
74.24
|
52.49
|
13.78
|
14.02
|
10.35
|
The calculated total B(a)Peqv obtained from Manihot esculenta samples at the farmlands was within the range 11.10–51.12 ng.g− 1. This range was above the legally permissible limit of 1.0 ng.g− 1 criteria of the European Union for processed cereal-based foods (Dennis et al.; 1984).
In Table 7, the concentration of carcinogenic compounds in the Dioscorea alata ranged from14.02-142.12 ng.g-1. As previously observed for other crops, the farmlands closer to the curing site FL1 recorded the highest concentration, and the distant farmland FL3Y recorded the lowest concentrations of PAHs in the Dioscorea alata. The concentrations of PAHs in the soils determined the amount adsorbed by plants and stored in the roots. This process is called phytoaccumulation. The calculated total B(a)Peqv obtained from samples at the sampling farmlands was within the range 10.14 – 51.01 ng.g-1. The European Union above the legally permissible limit (1.00 ng.g-1) processed this range for processed cereal-based foods (Dennis et al.; 1984). Table 8 shows that the concentration of carcinogenic PAHs in the Zea mays samples ranges from 0.00 ng.g-1 to 95.17 ng.g-1. The highest concentration was recorded at the farmland closer to the curing site, and the concentration at other farmlands was below the detection limit. It was observed that maize could undergo phytovolatilization by mineralizing carcinogenic PAHs into harmless products such as carbon (iv) oxide, methane, and water, making the maize from FL2 and FL3 free from contamination (Brady, 1990).
Table 7
Carcinogenic potency and toxicity potential of PAHs in Manihot esculenta (cassava) from the farmlands (ng.g− 1)
PAHs
|
TEF
|
Sampling sites
|
FL1C(ng.g− 1)
|
FL2C (ng.g− 1)
|
FL3C (ng.g− 1)
|
ΣCPAHs
|
B(a)Pequ
|
ΣCPAHs
|
B(a)Pequ
|
ΣCPAHs
|
B(a)Pequ
|
Chry
|
0.01
|
53.14
|
0.5314
|
33.26
|
0.3326
|
2.85
|
0.0285
|
B(k)f
|
0.10
|
10.02
|
1.002
|
ND
|
0.00
|
ND
|
0.00
|
B(b)f
|
0.10
|
7.27
|
0.272
|
6.02
|
0.602
|
1.92
|
0.192
|
B(a)p
|
1.00
|
51.12
|
51.12
|
12.36
|
12.36
|
11.10
|
11.10
|
D(a,h)A
|
1.00
|
21.12
|
21.12
|
ND
|
0.00
|
ND
|
0.00
|
Total
|
|
193.79
|
79.61
|
63.70
|
14.50
|
25.97
|
12.03
|
Table 8
Carcinogenic potency and toxicity potential of PAHs in Zea mays (maize) from the farmlands (ng.g− 1)
PAHs
|
TEF
|
Sampling sites
|
FL1M(ng.g− 1)
|
ΣCPAHs
|
B(a)Pequ
|
Chry
|
0.01
|
11.64
|
0.11
|
B(k)f
|
0.10
|
58.12
|
5.812
|
B(b)f
|
0.10
|
13.12
|
1.312
|
D(a,h)A
|
1.00
|
4.13
|
4.13
|
In(123)P
|
0.1
|
8.70
|
0.87
|
Total
|
|
95.17
|
12.24
|