Heavy metal bioavailability
The results, as shown in Fig. 2, represent the levels of the heavy metals studied in the various fractions (\({\text{F}}_{1}- {\text{F}}_{5}\)) at the sampling sites. In Enugu, the percent bioavailability ranged from 6.65% (Cr) to 78.47% (Zn). Zn was mostly partitioned in the \({\text{F}}_{1}\), while Cr was mostly partitioned in the \({\text{F}}_{5}\). Similar results were presented by Tytler (2019), who indicated that Zn was concentrated in the mobile fraction. Fe showed the highest concentration of the total fractions (168.47 mg kg− 1) but it was mostly partitioned in the \({\text{F}}_{2}\)and least in the \({\text{F}}_{1}\). This was supported by research conducted earlier (Ajala and Onwukeme 2012; Onwukeme et al. 2013). At Aba, the percent bioavailability of the HMs was found to be in the following order: Ni > Cd > Pb > Cu > Zn > Mn > Fe > Cr. Most of the Ni content was bioavailable, while a substantial amount of Cd and Cr were not immediately available to biota. This observation was also confirmed by other researchers (Tytler 2019; Osakwe, 2011).
Onitsha had a higher fraction of Fe occluded in the α and β forms, which are not readily available. This signifies that Fe had medium mobility. However, over time, the decomposition and/or oxidation of organic matter may make it available (Osakwe 2011). Conversely, Mn, Zn, and Cr showed lower percent bioavailability while the toxic HMs (Pb and Cd) had higher percent bioavailability because a larger portion of their contents were present in \({\text{F}}_{1}\) and \({\text{F}}_{2}\). In Afikpo, the residual form of Fe (653 mg kg− 1) had the highest concentration of HMs, while Pb was mostly found in the oxidizable form, followed by the exchangeable form, and least in the residual form. In Owerri, the residual form of Cr (11.85 mg kg− 1) accounted for the highest fraction out of the total (16.73 mg kg− 1), resulting in a very low percent bioavailability (9.15%). Cu and Cd were heavily partitioned in the \({\text{F}}_{2}\), and \({\text{F}}_{4}\), respectively. This finding is consistent with the results reported by Tytler (2019).
Pollution and ecological risk assessments
When considering the chemical forms of the HMs examined and taking into account their immediate pollution effects (Table 2), it was observed that in some cases, they could pose a hazard to the environment, while in other cases, they did not. This could be observed in Fe with high bioavailability for all sampled sites, whereas Cr was the least bioavailable, except in Enugu. The high bioavailability of Fe could be attributed to the percolation of waste materials, such as municipal waste and medical or industrial waste, which have the potential to release Fe into the environment (He et al. 2021).
The entire sites analyzed were practically uncontaminated, except for Cd, as can be seen in their respective Igeo (Table 2). Cd contaminated Aba and Onitsha moderately but showed moderate to heavy contamination in Afikpo and Owerri, and on the other hand, uncontaminated Enugu. The significant contamination observed in Aba and Onitsha could be attributed to the extensive industrial activities occurring in those areas (Guo et al. 2012). On the other hand, the high levels of Cd contamination found in Afikpo and Owerri may be linked to agricultural practices involving the use of fertilizers and pesticides (Wang et al. 2011).
Table 2
Immediate pollution levels of HMs in the FSS of industrialized areas of South-East, Nigeria
Site | Parameter | Fe | Mn | Cu | Zn | Pb | Ni | Cd | Cr | mPI | mCd | ERI |
Enugu | \({(\text{F}}_{1}+ {\text{F}}_{2})\) | 869.47 | 8.89 | 8.59 | 60.20 | 37.86 | 6.83 | 0.75 | 1.44 | 0.11 | 0.28 | 33.09 |
CF | 0.02 | 0.01 | 0.24 | 0.43 | 0.45 | 0.20 | 0.94 | 0.01 |
Igeo | –6.06 | –7.16 | –2.65 | –1.80 | –1.75 | –2.94 | –0.68 | –6.70 |
ERF | NC | 0.01 | 1.2 | 0.43 | 2.23 | 1.00 | 28.20 | 0.02 |
Aba | \({(\text{F}}_{1}+ {\text{F}}_{2})\) | 864.97 | 19.71 | 7.04 | 5.69 | 107.02 | 3.92 | 3.87 | 0.10 | 0.09 | 0.80 | 153.03 |
CF | 0.02 | 0.02 | 0.20 | 0.04 | 1.26 | 0.11 | 4.84 | 1.0E− 3 |
Igeo | –6.06 | –6.06 | –2.94 | –5.21 | –0.25 | –3.73 | 1.70 | –10.55 |
ERF | NC | 0.02 | 0.98 | 0.04 | 6.30 | 0.56 | 145.13 | 2.0E− 3 |
Onitsha | \({(\text{F}}_{1}+ {\text{F}}_{2})\) | 215.00 | 1.42 | 3.55 | 3.01 | 25.13 | 5.22 | 4.73 | 0.19 | 0.04 | 0.81 | 180.11 |
CF | 5.66E− 3 | 1.67E− 3 | 0.10 | 0.02 | 0.30 | 0.15 | 5.91 | 1.9E− 3 |
Igeo | –8.05 | –9.82 | –3.92 | –6.06 | –2.34 | –3.33 | 1.98 | –9.62 |
ERF | NC | 1.67E− 3 | 0.49 | 0.02 | 1.47 | 0.75 | 177.38 | 3.8E− 3 |
Afikpo | \({(\text{F}}_{1}+ {\text{F}}_{2})\) | 802 | 6.23 | 7.80 | 15.13 | 35.99 | 4.06 | 5.50 | 2.20 | 0.11 | 0.98 | 210.19 |
CF | 0.02 | 7.33E− 3 | 0.22 | 0.11 | 0.42 | 0.12 | 6.88 | 0.02 |
Igeo | –6.16 | –7.68 | –2.79 | –3.84 | –1.82 | –3.69 | 2.20 | –6.09 |
ERF | NC | 7.33E− 3 | 1.08 | 0.11 | 2.12 | 0.58 | 206.25 | 0.04 |
Owerri | \({(\text{F}}_{1}+ {\text{F}}_{2})\) | 328.09 | 23.00 | 75.15 | 14.85 | 47.79 | 1.85 | 4.85 | 1.53 | 0.15 | 1.12 | 195.56 |
CF | 8.63E− 3 | 0.03 | 2.09 | 0.11 | 0.56 | 0.05 | 6.06 | 0.02 |
Igeo | –7.45 | –5.80 | 0.48 | –3.84 | –1.42 | –4.83 | 2.01 | –6.62 |
ERF | NC | 0.03 | 10.44 | 0.11 | 2.81 | 0.26 | 181.88 | 0.03 |
NC: Not calculated since there was no data for toxic response factor (Tr). \({(\text{F}}_{1}+ {\text{F}}_{2})\) represents bioavailable fractions. \({\text{F}}_{1}\) = metal content bound to exchangeable fraction, \({\text{F}}_{2}\) = metal content bound to carbonate fraction. CF, Igeo, ERF, mPI, MCd and ERI are contamination factor, geo-accumulation index, ecological risk factor, multiple factor pollution index and risk index, respectively.
Consequently, the sampled sites in Enugu had a low risk of Cd contamination on the scale of ERF, sites in Aba were at a considerable risk, while Onitsha, Owerri, and Afikpo were at a higher risk of Cd contamination. Thus, Aba, Onitsha, Owerri, and Afikpo were at a moderate risk of immediate HM contamination, while sites in Enugu were at a low risk of immediate HM contamination.
In terms of long-term pollution indices of HMs analyzed in the FSS (Table 3), the results showed that Onitsha and Owerri had a low risk of Pb contamination, while Enugu, Aba, and Afikpo exhibited moderate contamination risk. It was also observed that all the major cities sampled in this study were at a very high risk of Cd contamination due to projected heavy contamination, especially in Enugu, Aba, Owerri, and Afikpo.
Table 3
Long-term pollution levels of HMs in the FSS of industrialized areas of South-East, Nigeria
Site | Parameter | Fe | Mn | Cu | Zn | Pb | Ni | Cd | Cr | Mpi | mCd | ERI |
Enugu | *Total Fraction | 1685.47 | 40.75 | 19.23 | 76.72 | 108.50 | 12.59 | 19.46 | 6.62 | 0.40 | 3.37 | 731.79 |
CF | 0.05 | 0.05 | 0.54 | 0.54 | 1.29 | 0.38 | 24.00 | 0.07 |
Igeo | –5.01 | –4.97 | –1.49 | –1.45 | –0.23 | –2.06 | 4.02 | –4.50 |
ERF | NC | 0.05 | 2.68 | 0.54 | 6.47 | 1.91 | 720.00 | 0.14 |
Aba | *Total Fraction | 1986.30 | 46.58 | 11.39 | 14.39 | 160.32 | 5.46 | 12.23 | 3.81 | 0.26 | 2.24 | 470.38 |
CF | 0.05 | 0.06 | 0.32 | 0.11 | 1.89 | 0.17 | 15.28 | 0.04 |
Igeo | –4.84 | –4.81 | –2.25 | –3.87 | 0.33 | –3.27 | 3.35 | –5.30 |
ERF | NC | 0.06 | 1.59 | 0.11 | 9.44 | 0.85 | 458.25 | 0.08 |
Onitsha | *Total Fraction | 2453.71 | 25.15 | 7.28 | 23.44 | 31.74 | 7.98 | 7.68 | 5.88 | 0.19 | 1.32 | 291.93 |
CF | 0.07 | 0.03 | 0.20 | 0.18 | 0.14 | 0.23 | 9.63 | 0.06 |
Igeo | –4.54 | –5.67 | –2.89 | –3.16 | –2.01 | –2.72 | –2.68 | –4.67 |
ERF | NC | 0.03 | 1.02 | 0.18 | 0.68 | 1.15 | 288.75 | 0.12 |
| *Total Fraction | 3562.81 | 32.96 | 15.80 | 31.13 | 104.75 | 7.38 | 20.01 | 12.40 | 0.37 | 3.51 | 779.81 |
CF | 0.09 | 0.04 | 0.44 | 0.23 | 1.29 | 0.23 | 25.65 | 0.12 |
Igeo | –4.01 | –5.27 | –1.77 | –2.76 | –0.28 | –2.83 | 4.06 | –3.60 |
ERF | NC | 0.04 | 2.21 | 0.23 | 6.44 | 1.16 | 769.50 | 0.23 |
Owerri | *Total Fraction | 3075.61 | 42.36 | 105.87 | 53.37 | 77.83 | 6.55 | 18.08 | 16.73 | 0.48 | 3.41 | 678.76 |
CF | 0.08 | 0.05 | 2.94 | 0.38 | 0.92 | 0.19 | 22.53 | 0.17 |
Igeo | –4.21 | –4.92 | –0.97 | –1.98 | –0.71 | –3.00 | 3.91 | –3.16 |
ERF | NC | 0.05 | 0.82 | 0.38 | 0.47 | 0.95 | 675.75 | 0.34 |
NC: Not calculated since there was no data for iron toxic response factor (Tr). CF, Igeo, ERF, mPI, MCd, and ERI are contamination factor, geo-accumulation index, ecological risk factor, multiple factor pollution index, and risk index, respectively. *Total fraction represents the total metal contents from chemical fractionation steps: exchangeable, carbonate, reducible, oxidizable, and residual. Additional information can be found in the ESM Table S1–S5.
Generally, in the long-term pollution assessment, Onitsha would be at a moderate risk of HM pollution, Aba would be at a considerable risk, while Enugu, Afikpo, and Owerri would be at a high risk of HM pollution due to the bioaccumulation of these HMs in the FSS. This poses a grave danger to human health and the ecosystem if proactive measures are not taken to prevent this negative trend (Khalid et al., 2017; Wang et al., 2016).
Table 4
Comparison between the immediate and long-term pollution levels in relation to hazard classification
Site | Index | Immediate | Long-term | Additional comment |
Enugu | CF | Low contamination (≤ 1) with respect to the entire HMs | Moderately contaminated with Pb, very highly contaminated with Cd ( > > 6) | Only 34.89 and 39.83% Pb and Cd, respectively are immediately available to biota |
Igeo | Practically uncontaminated (≤ 0) with respect to all the HMs | Heavily contaminated with Cd |
ERF | Low-risk with respect to all the HMs (≤ 40) | Very high-risk with Cd (> 320) |
mPI | Low contaminated class | Low contamination class |
mCd | Low degree of contamination | Low degree of contamination |
ERI | Low-risk class (≤ 150) | High-risk class (> 600) |
Aba | CF | Moderately contaminated with Pb, considerably contaminated Cd | Moderately contaminated with Pb, very highly contaminated with Cd | 66.75% Pb and 70.33% Cd were bioavailable. Cr had the least bioavailability of 2.62% |
Igeo | Moderately contaminated Cd | Uncontaminated to moderately contaminated with Pb |
ERF | Considerable risk with respect to Cd | Very high-risk with respect to Cd |
mPI | Low contaminated site | Low contaminated site |
mCd | Low degree of contamination | Low degree of contamination |
ERI | Moderate risk class | Considerable risk class |
Onitsha | CF | Cd was in considerable contamination | Cd was grouped under very high contamination class | 61.59% of Cd at this site was available. Mn, Zn, and Cr had very % bioavailability of 9.37, 9.00, and 3.23%, respectively |
Igeo | Cd was moderately contaminated the site | Practically uncontaminated |
ERF | Cd was in high-risk class | Cd was in the high-risk class |
mPI | Low contamination of the site | Low contamination of the site |
mCd | Low degree of contamination of the site | Low degree of contamination of the site |
ERI | Moderate risk | Moderate risk |
Afikpo | CF | Cd had a very high contamination | Moderately to heavily contaminated with Pb, very high with Cd | 34.36 and 50.22% of Pb and Cd partitioned, respectively in \({\text{F}}_{1} \text{a}\text{n}\text{d} {\text{F}}_{2}.\)Cr had low bioavailability of 11.69%. % bioavailability at this site ranged from 11.69 (Cr) to 50.22% (Cd) |
Igeo | Moderately to heavily contaminated with Cd | Heavily contaminated with Cd |
ERF | Cd was in a high-risk class | Cd was in a high-risk class |
mPI | Low contamination of the site | Low contamination of the site |
mCd | Low degree of contamination | Low degree of contamination |
ERI | Moderate risk | High risk |
Owerri | CF | Moderate and very high contamination with Cu and Cd, respectively | Moderate and very high contamination with Cu and Cd, respectively | The available Cu was partitioned in the exchangeable and carbonate fractions making about 70.98%. Only about 26.83% of Cd was bioavailable. Cr had the least % bioavailability (9.15%) |
Igeo | Uncontaminated to moderately contaminated with Cu and moderately to heavily contaminated with Cd | Heavily contaminated with Cd |
ERF | Cd was in high-risk class | Cd was in high-risk class |
mPI | Low contamination of the site | Low contamination of the site |
mCd | Low degree of site contamination | Low degree of site contamination |
ERI | Moderate risk | High risk |
The risk analysis conducted using immediate and long-term pollution assessments showed that Cd posed the highest ecological risk among the analyzed HMs in both pollution assessment schemes and across the studied sites. When considering the two approaches, Cd can be hazardous to the environment in certain cases, while in others, it may not be. This can be observed when comparing the pollution levels of Cd with respect to CF, ERI, and ERF in both approaches, as opposed to its pollution levels in relation to Igeo, mCd, and mPI. For more details, Table 4 provides summaries of the pollution levels of the selected HMs at specific sites in relation to the risk assessment schemes used in this study.
Agglomerative hierarchical clustering
The AHC dendrograms for the immediate and long-term pollution assessments of HMs in the FSS of Southeast Nigeria are presented in Fig. 3. The bioavailable fractions (Fig. 3a) indicated that the pollution in the Owerri axis was noticeably distinct from the other industrialized areas being studied. Meanwhile, at the dissimilarity index of 0.05, the Aba, Onitsha, Enugu, and Afikpo axes had similar pollution experiences for the immediate HM pollution. Concisely, the dendrogram indicated that Onisha and Aba had similar HM pollution scenario. On the other hand, Enugu and Afikpo were also similar but reflected lower HM pollution. The above trend was expected because Afikpo and Enugu have fewer industrial activities compared to Onitsha and Aba. Although Owerri has fewer industrial activities than Onitsha and Aba, the higher index recorded for Owerri could be due to the significant amount of domestic activities in the studied area. Most effluents from industries are usually treated to reduce environmental toxicity, whereas those from domestic areas are typically discharged into the environment without treatment. This could have increased the pollution rate in Owerri. The long-term HM pollution (Fig. 3b) indicated that Aba and Enugu were generally similar, while Onitsha and Afikpo showed similarities as well. However, Owerri stood out as being different from the other cities. However, all the five selected industrialized areas had similar long-term pollution experiences, with a dissimilarity index of 0.05, which contrasts with the immediate pollution level. The reason could be that some heavy metals HMs) may be more bioavailable over time in Owerri compared to other investigated sites. This assertion corroborated our earlier observation.
Principal component and correlation analyses
The PCA was used to illustrate the overview and deviations among the industrialized areas in Southeast Nigeria. Figure 4(a–d) shows the core plots and biplots for the CF of immediate and future HM pollution in the FSS of the study areas. The distribution of HM responsible for immediate pollution (Fig. 4a and b) indicated that Cu, Mn, Cr, and Pb were more prevalent in Owerri; Zn and Fe in Enugu; Cd in Afikpo, while Ni was more prevalent in Onitsha and Enugu. In the future, there may be an accumulation of HMs in Onitsha. Generally, the accumulation of Fe, Cu, and Cr was highest in Afikpo and Owerri. Meanwhile, Zn, Cd, Mn, and Ni were the most HMs found in Enugu, while Pb was the most prevalent heavy metal in Aba.
The results in Figs. 5(a–d) indicate that various biochemical reactions occur at industrial areas, leading to changes in CF for both immediate and future pollution levels. This may have led to the generation of new bio-contaminants, resulting in a shift in the concentration of HM pollution among the sites. Again, the ERF results indicated that Owerri had the highest concentration of Cr for both immediate and future pollution levels, while Enugu had the highest concentration of Zn and Ni for the same reason.
In Fig. 6(a–d), the mPI, mCd, and ERI are presented. Owerri had the highest mPI for both immediate and future pollution. In general, for mCd and ERI, Cr levels were higher in Owerri for both immediate and future pollution effects, whereas Enugu recorded the highest levels of Zn for the same reason given above. The results of this study could serve as a guide for determining the appropriate measures needed to control HM pollution in industrialized areas of Southeast Nigeria.