Introduction
Pollution, a pervasive global issue, has cast a looming shadow over environmental and public health concerns across the world. It represents the undesirable alteration or contamination of various environmental components, including soil, water, air, and natural ecosystems, caused by the introduction of harmful substances into these mediums. Pollution, stemming from a multitude of sources, has wide-ranging implications for ecosystems and human well-being. Pollution comprises the release of noxious substances into the environment, leading to environmental degradation, ecosystem disruption, and health hazards for populations worldwide [1,2]. While pollutants vary widely, they often manifest as industrial emissions, agricultural runoff, urban waste, and vehicular exhaust [3].
The challenge of pollution is further compounded by the intricate interplay of geographies and socioeconomic dynamics. Industrialization and urbanization are prominent drivers of pollution, as densely populated areas, industrial zones, and agricultural regions collectively contribute to emissions of pollutants into the environment [4,5]. In particular, the rapid expansion of industries has accelerated the discharge of toxic metals into local ecosystems, thereby escalating the risks associated with heavy metal pollution [6]. Each nation grapples with its unique blend of pollution sources, driven by factors such as industrial activities, transportation systems, and energy generation [7]. National policies and regulations also play a significant role in shaping the environmental landscape, with varying degrees of success in curbing pollution. A critical concern within the nation is the contamination of natural resources, particularly soil and water, by heavy metals originating from anthropogenic activities [8]. Pollution takes on distinct regional identities. Local pollution sources, including industrial facilities, agricultural practices, and urban areas, release pollutants that intimately affect nearby communities [9,10]. A pervasive issue within local contexts is the contamination of environmental matrices: soil, water, and air with heavy metals [11].
Heavy metals, a class of metallic elements with high atomic weights, have garnered significant attention in the context of pollution. These metals, which include lead (Pb), cadmium (Cd), mercury (Hg), and arsenic (As), among others, have distinctive properties that render them toxic, mutagenic and teratogenic effects even at very low concentrations [12]. Their persistence in the environment and propensity to accumulate in biota make them notorious pollutants of concern [2]. The origins of heavy metal contamination are often anthropogenic, stemming from industrial processes, mining activities, and agricultural practices [13]. The sources of heavy metal pollution are multifaceted and rooted in human activities. Industrial emissions release heavy metals into the atmosphere, where they can be dispersed over large areas before settling onto soil and water bodies [14]. Mining operations, a significant contributor, introduce heavy metals into soil and water systems [15]. Agricultural practices, particularly the use of metal-laden fertilizers and pesticides, elevate heavy metal levels in soil and crops [11]. In addition, inadequate waste disposal and improper management of hazardous materials result in the leaching of heavy metals into groundwater, posing a direct threat to human health [16].
Moreover, an ecological risk assessment revealed the presence of low concentrations of heavy metals, including lead, nickel, copper, zinc, chromium, cadmium, and iron in the soil of Lagos State University, Epe, Lagos State, Nigeria. [17]. Heavy metals may be present either as a deposit on the surface of vegetables or may be taken up by the crop roots and incorporated into the edible part of the plant tissues. Heavy metals deposited on the surface can often be eliminated simply by washing before consumption, whereas bioaccumulated metals are difficult to remove and are of major concern [18].
In addition, Nigeria is a major producer of green leafy vegetables. Vegetables grown along roadsides are exposed to heavy metals from vehicular emissions and other anthropogenic activities [19]; therefore, this elicits a response from researchers at Lagos State University and ensures proper investigation. This is very important because the Nigerian population relies heavily on vegetables in its diet and knows that different types of vegetables absorb and/or adsorb heavy metals differently. Monitoring the levels of heavy metals in vegetables is important for ensuring food safety and reducing the risk of health problems.
While classical analytical methods, including gravimetry, titration, and colorimetry, have historically been employed to quantify heavy metals in green leafy vegetables [20] these methods exhibit limitations. They lack the sensitivity to detect trace levels of heavy metals and may be prone to interference from other ions in the sample matrix [21]. Furthermore, they are time-consuming, require extensive sample preparation, and may not be suitable for multi-element analysis [22]. In contrast, modern spectroscopic techniques offer advantages such as high sensitivity, selectivity, accuracy, speed, and the ability to analyze a wide range of elements simultaneously [23]. This study aims to assess the levels of heavy metals, including lead (Pb), zinc (Zn), cadmium (Cd), and chromium (Cr), in vegetables cultivated in proximity to roadside farms and raise awareness about heavy metal contamination and its implications for food safety and public health in the environment. This will be achieved by quantifying the concentrations of these metals in various types of green leafy vegetables.
Materials and Methods
Study area and sampling
The study area encompassed specific locations along the Lagos–Badagry Expressway, including Abule Ado, Abule Oshun, and Oluti. These locations were selected based on their proximity to the roadside, which is associated with vehicular emissions and other anthropogenic activities and is characterized by significant agricultural activity, including the cultivation of vegetables, making them suitable for assessing heavy metal contamination in agricultural produce. To provide a clear understanding of the sampling sites, a study area map, illustrating the precise locations from which samples were collected, was developed.
Geographic coordinates of Abule Ado, Abule Oshun, and Oluti; Position, Latitude (width), and Longitude (length) Shown on the map, respectively,°6°27′09.2′N (6.4525600°), 3°14′42.2′E (3.2450500°); 6°26′36.3′N (6.4434300°), 3°13′36.7′E (3.2268600°), 6°27′13.2′N (6.4536800°), and 3°16′11.4′E (3.2698400°).
Sampling of leafy vegetables
Sampling of leafy vegetables was carried out following a standard random sampling procedure United Nations Environment Programme (UNEP) [24]. The choice of vegetables for this study focused on three commonly consumed types in the Nigerian diet: lettuce, spinach, and pumpkin. These vegetables are representative of leafy over-ground varieties commonly grown and consumed in the region.
In each of the three selected locations (Abule Ado, Abule Oshun, and Oluti), nine samples of each vegetable species were systematically collected and labeled with I1, I2, and I3 representing Abule Ado, L1, L2, and L3 representing Abule Oshun, and P1, P2, and P3 representing Oluti, amounting to a total of 36 samples. The rationale for collecting nine samples of each vegetable species lies in the need to obtain a statistically robust dataset that accurately reflects the heavy metal content across these locations. By collecting multiple samples, we aimed to capture potential variations and establish a comprehensive assessment of heavy metal levels in the selected vegetables.
Sample treatment
Collected samples were first washed as fresh vegetables using sterile distilled water. All washed samples were carefully air-dried, cut into small pieces, and weighed immediately (fresh weight, Wf) before drying in the oven at 120°C for 24 hours. The drying process employed was carefully controlled to minimize the loss of heavy metals, including Pb. Drying at this temperature is a standard procedure that facilitates the removal of water content from the samples while minimizing the volatilization of metals. The choice of this drying temperature and duration was based on established methods for sample preparation to ensure accurate analysis Tappi Test Method [25]. When samples became fully dry, they were weighed (dry weight Wd) so as to determine the original water content in each sample. This was followed by grinding and homogenization of the dried samples into fine powder using an electric grinder. The powdered samples were then stored in closed containers in the absence of humidity before analysis.
Determination of heavy metals in plant samples
The analysis of heavy metals in the vegetable samples was performed following a modified wet digestion method [25]. A one-gramme portion of each vegetable sample was ground into a homogenous mixture using a pestle and mortar. Subsequently, the ground samples underwent wet digestion with a solution composed of 15 ml of nitric acid (HNO3) and 5 ml of perchloric acid (HCLO4). This mixture was heated to a temperature of 100°C and maintained for a duration of 3–4 hours until complete evaporation and the attainment of a whitish dry mass.
Following cooling, the precipitate was subjected to extraction in an acid-water mixture in a 3:1 ratio. The resulting solution was then filtered through Whatman filter paper No. 42, and the final volume was adjusted to 50 ml. This filtered solution, referred to as the filtrate, was subsequently analyzed for its metal content. The analysis of heavy metals in the filtrate was carried out using a Flame Atomic Absorption Spectrophotometer (Buck Scientific 210 VGP model) at the Federal Institute of Industrial Research, Oshodi, Lagos. Before analyzing the collected samples, quality control measures were implemented. Blanks and standards were included in the analytical process to ensure the accuracy and reliability of the results, and the instrument was calibrated according to standard operating procedures and protocols. The specific heavy metals assessed included chromium (Cr), cadmium (Cd), lead (Pb), and zinc (Zn).
Statistical analysis
All data from the study were analysed statistically using Analysis of variance (ANOVA) and chi-square test for different parameters which include: the comparison of heavy metal (Pb, Zn, Cd, and Cr) concentrations in the vegetables among the three regions and regions which exceed the WHO standards.
Results
The levels of heavy metals varied among the different vegetable types and sampling locations. Table one shows the range of concentration of Pb, Zn, Cd, and Cr in the samples.
Table 1.
Range of concentration of Pb, Zn, Cd, and Cr in the samples in mg/kg.
| Samples | Pb | Zn | Cd | Cr |
|---|---|---|---|---|
| Lettuce | 14.05–49.10 | 12.40–40.00 | 0.40–8.00 | 5.75–13.30 |
| Spinach | 12.60–51.75 | 31.15–64.20 | 0.20–1.00 | 6.10–27.75 |
| Pumpkin | 1.05–42.90 | 29.55–51.80 | 0.20–1.40 | 6.60–43.90 |
| WHO standard (mg/kg) | 50.00 | 50–150 | 0.1–7 | 50.00 |
Figure 1.
Heavy metal concentration in lettuce samples from the three different locations. Key: I1, I2, and I3—Lettuce samples from Abule Ado; L1, L2, and L3—Abule Oshun; and P1, P2, and P3—Oluti.
Figure 2.
Heavy metal concentration in spinach samples from the three different locations. Key: I4, I5, and I6—Spinach samples from Abule Ado; L4, L5, and L6—Abule Oshun; and P4, P5, and P6—Oluti.
Figure 3.
Heavy metal concentration in pumpkin samples from the three different locations. Key: I7, I8, and I9—Pumpkin samples from Abule Ado; L7, L8, and L9—Abule Oshun; and P7, P8, and P9—Oluti.
From the three sampled locations, lettuce samples had the highest concentration of lead (Pb), 49.10 mg/kg, followed by zinc (Zn), 40.0 mg/kg, then chromium (Cr), 13.56 mg/kg and cadmium (Cd), 8.00 mg/kg the least (Fig. 1).
Figure 2 shows that spinach samples analysed had the highest concentration of zinc (Zn), 64.20 mg/kg followed by lead (Pb), 47.0 mg/kg, then chromium (Cr), 27.75 mg/kg and cadmium (Cd), 1.0 mg/kg the least.
Pumpkin samples studied had the highest concentrations of zinc (Zn), 51.80 mg/kg, followed by chromium (Cr), 43.90 mg/kg, then lead (Pb), 42.90 mg/kg and cadmium (Cd), with the least concentration of 1.40 mg/kg (Fig. 3).
The mean concentration of the heavy metals from the 36 samples analysed, lettuce had the highest concentration of lead (Pb), followed by spinach, while pumpkin had the least. Pumpkin had the highest concentration of Zn, followed by spinach, and lettuce the least. Lettuce had the highest concentration of cadmium (Cd), followed by pumpkin, while spinach had the least Cd concentration. Spinach (Cr) had the highest concentration of chromium, followed by lettuce and pumpkin had the lowest Cr concentration (Fig. 4). Nigerian regulatory organization: the National Agency for Drug Administration and Control adopts WHO standards.
Figure 4.
Summary of the mean concentration of Pb, Zn, Cd, and Cr in the vegetables.
Table 2.
ANOVA results table.
| Heavy metal | F-value | p-value | Result |
|---|---|---|---|
| Pb | 5.570 | 0.006 | Significant (p < 0.05) |
| Zn | 0.243 | 0.049 | Significant (p < 0.05) |
| Cd | 4.398 | 0.014 | Significant (p < 0.05) |
| Cr | 4.408 | 0.014 | Significant (p < 0.05) |
Table 3.
Chi-square test (Exceeding WHO Standards).
| Heavy metal | Chi-square value | p-value | Result |
|---|---|---|---|
| Pb | 3.067 | 0.217 | Not Significant (p > 0.05) |
| Zn | 3.200 | 0.202 | Not significant (p > 0.05) |
| Cd | 8.900 | 0.162 | Not significant (p > 0.05) |
| Cr | 7.800 | 0.051 | Not significant (p > 0.05) |
ANOVA results of the heavy metals; Pb, Zn, Cd, and Cr show that there is a significant difference (p < 0.05) and the mean concentrations of each metal in the vegetables vary significantly among the three regions (Table 2). Chi-square results show that none of the regions exceed the WHO standards (Table 3).
Discussion
These results show that there are significant differences in heavy metal concentrations in the vegetable samples among the regions for Pb, Zn, Cd, and Cr based on the ANOVA test (Table 2). However, none of the regions exceed the WHO standards for Pb, Zn, Cd, and Cr based on the chi-square test (Table 3). Abule Oshun consistently has the highest concentrations of Pb, Zn, Cd, and Cr across the different vegetables in the study area.
In the analysis of various vegetables, lead (Pb) levels displayed a wide range, from 1.05 to 51.75 mg/kg, with the highest concentration observed in lettuce (Table 1). These findings are consistent with a similar study that reported lead concentrations ranging from non-detectable levels from 2.695 to 2.805 mg/kg of dry vegetable samples [26]. The consumption of lead-contaminated vegetables can result in associated health risks when lead particles adhere to the plant surface, leading to indirect ingestion, or through direct uptake by the plant tissue. It is essential to note that lead exposure can lead to neurological impairments, including cognitive deficits, decreased intelligence quotient, and learning disabilities, especially in vulnerable children [27]. In adults, lead poisoning has been associated with neurobehavioral changes and an increased risk of neurodegenerative diseases [28]. Given these potential health risks, it is concerning that individuals in this study environment may unknowingly consume such vegetables without awareness of associated hazards.
Cadmium (Cd) concentrations in the studied samples ranged from 0.2 to 8.00 mg/kg (Table 1), with lettuce exhibiting the highest concentrations. These results align with the findings reported by Singh and Sharma [29]. The tolerable levels of cadmium, as reported by Singh et al. [30] are 0.01 mg/kg of fresh weight and 57–71 mg/kg/day in dry weight, respectively. Excessive cadmium intake has been associated with memory loss and an elevated risk of cardiovascular diseases, including hypertension and atherosclerosis, as indicated in research conducted by 31. Tellez-Plaza et al. [31] where 1,084 cardiovascular events, including 400 deaths, were recorded among participants in the Strong Heart Study.
The chromium (Cr) content in the studied samples ranged from 5.75 to 43.90 mg/kg of dry weight (Table 1). Although this range falls below the acceptable dietary intake levels for chromium in adults [32]. It is crucial to note that chromium deficiency can have health implications, including impaired glucose metabolism, cardiovascular issues, weight gain, mood swings, and mental disorders [8]. Excessive chromium intake, particularly in its hexavalent form (Cr(VI)), can be toxic and pose serious health risks. Hence, dietary chromium levels should remain within a healthy range. Children exposed to high levels of chromium may experience developmental delays, stunted growth, and adverse effects on cognitive function [33].
The zinc (Zn) concentrations in the samples ranged from 12.40 to 64.20 mg/kg, as presented in Table 1, and exceeded the WHO standards (50–150 mg/kg). Spinach and pumpkin leaves accumulated higher levels of zinc. While zinc is essential for the nutrition of humans, animals, and plants as a micronutrient, its presence above the required concentration could reduce the bioavailability of lead. Conversely, zinc deficiency can result in impaired growth and development, a weakened immune system, impaired wound healing, loss of appetite, hair loss, and cognitive impairment [34]. However, excessive zinc presence in food can also be harmful, potentially causing gastrointestinal distress, copper deficiency, and interference with mineral absorption, such as dietary iron, potentially leading to iron-deficiency anemia [35].
Although the concentrations of lead (Pb), zinc (Zn), cadmium (Cd), and chromium (Cr) in our study fell within WHO standards, the long-term effects of bioaccumulation and biomagnification could lead to severe health consequences for consumers over time. To avoid scenarios similar to the Minamata incident in Japan, where the effects of heavy metal deposition took years to manifest [36], continued monitoring and public awareness of the potential health hazards associated with heavy metal contamination in vegetables are essential.
Conclusion
The levels of heavy metals in green leafy vegetables grown along roadsides in Nigeria were below the WHO’s permissible limits. However, the consumption of contaminated vegetables could pose serious health risks to consumers, especially to vulnerable groups which may result in bioaccumulation and biomagnification leading to human disorders with children at higher risk than adults. The results highlight the need for regular monitoring of heavy metal levels in vegetables and the implementation of measures to reduce heavy metal contamination by relevant authorities. Future research in this area should consider including investigation of other heavy metals such as mercury, arsenic, and nickel, and implementation of effective agricultural and soil management practices that reduce heavy metal uptake by crops can also contribute to enhancing food safety and public health.
Acknowledgments
The authors express their appreciation to Lagos State University Ojo, Lagos, Nigeria, and especially, LASU-HUMBOLDT KOLLEG Conference, 2023.
Conflicts of interest
The authors declare no conflict of interest.
Funding
This research received no external funding, so was fully funded by the authors.
Authors’ contributions
DDM conceived the idea and was responsible for designing the research; OAO supervised the laboratory analysis; ZCA, HKS, and MOU collected the samples from the three locations and analysed the data. DDM wrote the manuscript and ZCA carried out the statistical analysis.
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