https://doi.org/10.3390/toxins13120870
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2.1. Quantification of Heavy Metals
Across the studied samples, a total of eight heavy metals were detected, which include cadmium (Cd), lead (Pb), cobalt (Co), manganese (Mn), mercury (Hg), Arsenic (As), chromium (Cr), and Nickel (Ni). The maximum permissible limits established by reputable international regulators, specific to the detected heavy metals in this current work are summarized in Table 1 [30,31,32,33,34,35]. Clearly, all the maximum permissible limits of these heavy metals differ from each other, with the exception of Ni and Cr.
Table 1. The maximum permissible limits established by reputable international regulators, specific to the detected heavy metals in this current work.
| Heavy Metal Types | Maximum Permissible Limits | Country/Regulator | References |
|---|---|---|---|
| Cadmium (Cd) | 0.3 mg/kg | Food and Agriculture Organization/World Health Organization, Joint FAO/WHO Expert Committee on Food Additives | [31,33] |
| Lead (Pb) | 1.0 mg/kg | Food and Agriculture Organization/World Health Organization, Joint FAO/WHO Expert Committee on Food Additives | [30,31,33] |
| Cobalt (Co) | 0.043 mg/kg | Food and Nutrition Board | [34] |
| Manganese (Mn) | 2.0 mg/kg | -Food and Agriculture Organization/World Health Organization, Joint FAO/WHO Expert Committee on Food Additives -Codex Alimentarius Commission Food Additives and Contaminants |
[30,32] |
| Mercury (Hg) | 0.5 mg/kg | Food and Agriculture Organization/World Health Organization, Joint FAO/WHO Expert Committee on Food Additives | [32,33] |
| Arsenic (As) | 1.4 mg/kg | Food and Agriculture Organization/World Health Organization, Joint FAO/WHO Expert Committee on Food Additives | [31,33] |
| Nickel (Ni) | 0.10 mg/kg | -Food and Agriculture Organization/World Health Organization, Joint FAO/WHO Expert Committee on Food Additives -U.S. Environmental Protection Agency |
[32,35] |
| Chromium (Cr) | 0.10 mg/kg | Food and Agriculture Organization/World Health Organization, Joint FAO/WHO Expert Committee on Food Additives | [31,33] |
2.1.1. Concentration of Heavy Metals Detected in the Studied Cereals (Maize, Rice, and Millet), Tubers (Cassava and Yam), and Legumes (Soybean)
The concentrations of different heavy metals (Cd, Pb, Co, Mn, Hg, As, and Ni) detected in cereals (maize, rice, and millet), tubers (cassava and yam), and legumes (soybean) are given in Table 2. When comparing the values of detected heavy metals across the studied cereals, tubers, and legume samples, some statistical differences (p < 0.05) were found with varying ranges of Cd from 0.000 to 0.015 mg/kg, Pb from 0.000 to 0.059 mg/kg, Co from 0.000 to 0.018 mg/kg, Mn from 0.004 to 0.146 mg/kg, Hg from 0.000 to 0.079 mg/kg, As from 0.000 to 0.028 mg/kg, as well as Ni from 0.003 to 0.023 mg/kg. Additionally, all the different heavy metals (Mn, Hg, Cd, Pb, As, and Ni) concentrations detected in the studied cereals (maize, rice, and millet) fell below the established permissible limits set by the international authorities [30,31,32,33,34,35]. Clearly, some heavy metals were not detected in some of the studied samples, which include: (a)The Pb was not detected in rice, yam; (b) The Co was not detected in maize, rice, and millet; (c)The Hg was not detected in yam, cassava, and soybean; and (d) The As was not detected in maize, yam, and soybean. Despite not being detected in rice and yam, the presence of Pb found in cassava > millet > maize> soybean should still be of consumer concern. The same can be said of Hg, which despite not being detected in yam, cassava, and soybean, its presence found in maize > millet > rice should equally be of consumer concern. The presence of Pb, for instance, could be due to it being released into the environment through lead-based gasoline combustion/painting as well as lead-containing pipes [36]. Notably also, the Hg in organic form is chemically toxic, and can bioaccumulate/biomagnify within the food chain [17].
Table 2. Concentrations of different heavy metals (Cd, Pb, Co, Mn, Hg, As, and Ni) detected in cereals (maize, rice, and millet), tubers (cassava and yam), and legumes (soybean).
| Detected Heavy Metals | Maize | Rice | Millet | Yam | Cassava | Soybean | Permissible Limits with Reference Sources |
|---|---|---|---|---|---|---|---|
| Cd (mg/kg) | 0.006 Ab ± 0.00 | 0.011 Bb ± 0.00 | 0.008 Ab ± 0.00 | 0.000 | 0.006 Ab ± 0.00 | 0.015 Cb ± 0.00 | 0.3 a [31,33] |
| Pb (mg/kg) | 0.046 Ab ± 0.00 | 0.000 | 0.053 Bb ± 0.00 | 0.000 | 0.059 BCb ± 0.00 | 0.041 Ab ± 0.00 | 1.0 a [31,33] |
| Co (mg/kg) | 0.000 | 0.000 | 0.000 | 0.003 Ab ± 0.00 | 0.005 Ab ±0.00 | 0.018 Bb ± 0.00 | 0.043 a [34] |
| Mn (mg/kg) | 0.009 Ab ±0.00 | 0.004 Ab ± 0.00 | 0.146 Bb ± 0.00 | 0.066 Cb ± 0.00 | 0.072 Db ± 0.00 | 0.077 Db ± 0.00 | 2.0 a [30,32] |
| Hg (mg/kg) | 0.079 Ab ± 0.00 | 0.068 Ab ± 0.00 | 0.069 Ab ± 0.00 | 0.000 | 0.000 | 0.000 | 0.5 a [32,33] |
| As (mg/kg) | 0.000 | 0.028 Ab ± 0.00 | 0.027 Ab ± 0.00 | 0.000 | 0.017 Bb ± 0.00 | 0.000 | 1.4 a [31,33] |
| Ni (mg/kg) | 0.009 Ab ± 0.00 | 0.003 Ab ± 0.00 | 0.014 Bb ± 0.00 | 0.009 Ab ± 0.00 | 0.023 Cb ± 0.00 | 0.021 Cb ± 0.00 | 0.10 a [32,35] |
Key: Each value from triplicate determinations, presented in mean ± standard error (SE); values with different superscripts (lowercase [a,b]) are statistically significant (p < 0.05) compared to the maximum permissible limits; values with different superscripts (uppercase [A–D]) are statistically significant (p < 0.05) across samples (of same heavy metals).
Notably, as the As concentrations in rice (~0.028 mg/kg) and millet (~0.027 mg/kg) resembled (p > 0.05), both values were significantly above (p < 0.05) those of cassava (~0.017 mg/kg), despite being absent in maize, which was contrary to a previous report [37]. Further, the Ni concentrations in cereals herein appear much lower than the data range reported by Onianwa et al. [38] (1.00–1.89 mg/kg). It is worthy to reiterate here that Ni toxicity largely manifests as skin allergies and lung cancer [39]. Table 1 also shows that the Cd concentrations, detected in maize (~0.006 mg/kg), rice (~0.011 mg/kg), and millet (~0.008 mg/kg) were much lower than the cereal range values reported by Onianwa et al. [38] (0.06–0.28 mg/kg). It is worthy to also reiterate that high Cd levels threaten human health and could damage the kidneys and liver despite its slow excretion [40]. Table 1 also shows that the Mn concentrations detected in maize (~0.009 mg/kg) and rice (~0.004 mg/kg) were much lower than the cereal range values reported by Akinyele and Shokunbi [41] (0.06–0.28 mg/kg), with the exception of the millet sample (~0.146 mg/kg) (Table 2).
Moreover, whereas previous researchers like Onianwa et al. [38] reported Ni concentration ranges in tubers (0.93–1.79 mg/kg) and legumes (3.47–7.00 mg/kg), and Akinyele and Shokunbi [41] detected Mn in yams (~4.42 mg/kg) and soybeans (~23.42 mg/kg), Orisakwe et al. [42] reported Pb (~0.33 mg/kg), Cd (~0.10 mg/kg), and Ni (~0.30 mg/kg) in cassava, as well as Pb (~0.46 mg/kg), Cd (~0.24 mg/kg), and Ni (~1.72 mg/kg) in soybeans. Importantly, all these above-mentioned reported values were far above those obtained in this current work. Despite being a trace element that serves as an enzyme cofactor, it appears the WHO limit for Mn is not yet established. Notably, when in excess of (>5 mg/dm3), Mn could cause neurological disorders [43]. Moreover, cassava contains Fe, an important element responsible for a variety of biological processes, for example, oxygen transport and electron transport. When Fe is excessively consumed, however, it can lead to tissue damage [44]. The prolonged consumption of foods contaminated with high levels of Pb can be deleterious to human health and would affect the cardiovascular, nervous, skeletal, muscular, and immune systems [45].
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