Heavy metal profile with health risk peculiarities in Enugu State and their long-term challenges in drinking water Open Access

The survival of all life forms depends on water and water absorption is an important means of nutritional nourishment for the human body (Peletz et al. 2018). The main objective of the campaign by the concerned health organizations is the use of safe and uncontaminated drinking water, which is considered a health priority (Cronk et al. 2015). Because the majority of illnesses in undeveloped countries are linked to the consumption of contaminated and dirty water (Peletz et al. 2018). As a consequence, water poisoning ultimately accounts for nearly a third of deaths in most poor countries (Dijkstra & de Roda Husman 2014; Kant & Graubard 2017). However, the contamination of drinking water sources by heavy metals (HMs) in the Enugu metropolis and the associated health risks (Nduka et al. 2023) are studied by the scarcity of data. Therefore, bottled water is a reasonable alternative when the quality of the drinking water or the water treatment is questionable (Cohen et al. 2022).

HMs become dangerous when their levels exceed the recommended safe limits (Agwu et al. 2023). Humans can be exposed to HM ions from both direct and indirect sources, including food, drinking water, exposure to industrial activities, and exhaust fumes (Pujari & Kapoor 2021). Several mineral elements, particularly metal ions, play a dual role in the functioning of the human body; although some are harmful in large doses, others are important in trace amounts (Ogamba et al. 2021). The entry and accumulation of HMs in the internal organs are the cause of numerous chronic diseases (Kapoor & Singh 2021). If the packing material leaks, the bottled water could potentially be contaminated. Previous studies have shown that toxic metals can get into plastic water bottles from polyethylene terephthalate (PET) (Ahimbisibwe et al. 2022). Also, the risk of water contamination from metals penetrating the bottle wall can increase due to prolonged storage of these bottled water at ambient temperature. This constraint applies to all the samples discussed in this study area (Molaee Aghaee et al. 2014). As a result, it is essential to continuously evaluate the quality of bottled water to preserve public health (Amarachi et al. 2023).

HM poisoning due to bottled drinking water contamination is a serious global problem. According to Ayodhya (2023), the most commonly analysed HM ions are zinc (Zn), copper (Cu), lead (Pb), tin (Sn), cadmium (Cd), chromium (Cr), iron (Fe), and mercury (Hg). Their toxicity comes from the bond formation of metals with the thiol group of proteins which alters the biochemical life cycle when they get into the cell. The current method of detecting HM ions by bottled water industries relies on shipping samples to a licensed laboratory for analysis. The method has complicated processing, expensive instruments, and time-consuming operations; but simple and cheap methods for identifying metal ions are highly desirable for detecting metal ions in bottled drinking water for consumers (Ayodhya 2022).

Additionally, studies on packaged water quality in Enugu, Nigeria, have not utilized the techniques applied in the current investigation to evaluate water quality and identify potential health risks associated with consumption. Moreover, despite these water firms' continual expansion, there has never been a comprehensive evaluation of the quality of the bottled water samples in the region. The Enugu region of Nigeria's coal mining industry has large amounts of mine tailings generated, which are dumped on the ground and in landfills, where it eventually enter surface runoff and groundwater (Obiadi et al. 2016). Another important factor is that, despite the findings of previous work (Osinowo 2016; Umoafia et al. 2023), water quality is still a concern within the community and the works suffered from the conventional approach, hence we propose to handle this difficulty through another approach. Therefore, this study aims to use flame atomic absorption spectrometry to measure the levels of toxic HM ions such as cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), nickel (Ni), and zinc (Zn) in commercial bottled water brands sold in the Enugu market and to compare the results with Nigerian and global drinking water standards.

The matrix used for the method blank was Li₂CO₃ (99%, Kiran Lighi, Laboratories, India). Standard stock solutions (1,000 ppm) of the HMs, Cd, Cr, Cu, Pb, Ni, and Zn, were adopted to generate calibration standards and spiking standards. Deionized water was used for the study's operation. Glass and plastic wares used in the analysis were washed with distilled water and then immersed overnight in a 10% (v/v) HNO₃ solution before being repeatedly rinsed with deionized water.

Fifty-one samples of 0.75 L bottled water from 17 different commercial domestic bottled water brands were randomly collected from shops and supermarkets in the Enugu metropolis, Nigeria in the first quarter of 2018. Three samples with different production dates for each brand were collected. The bottles were transferred to the Chemical Analysis Laboratory of the Projects Development Institute (PRODA), Enugu, where they were stored at 4°C until analysis (performed within a week). Among the popular brands of bottled water are Swan, Bigi, Ragolis, Lucozade Hydropure, Eva, Aquafina, Nestle, La Sien, Event, Cascade, Aquarapha, Ivy, Rovia, Trinity, Exalté, Ezbon, and Jasmine.

Five millilitre of concentrated nitric acid (HNO₃) and 5 mL of concentrated sulphuric acid (H₂SO₄) were added to each bottled water sample in the laboratory (Lu et al. 2022). The solution was then boiled until the volume was reduced to 15–20 mL, whereupon a clear solution resulted (Huang et al. 2020). After digestion, this sample was allowed to cool down to room temperature. Then, it was filtered with Whatman's 0.45 μm filter paper. The final volume was diluted to 100 mL with deionized water and then stored for analysis.

For the measurement of HMs (Cd, Cr, Cu, Pb, Ni, and Zn) a flame atomic absorption spectrophotometer Buck Scientific Model 210/211 VGP (East Newark, USA) with deuterium lamp background correction and hollow cathode lamps was used. A pH meter (Mettler Toledo Seven Compact pH/Ion metre and an In-lab Expert Pro-ISM pH electrode) was used to measure the pH of each sample of bottled water. The instrumental parameters were adjusted according to the manufacturer's recommendations. A calibration curve was constructed by plotting the analytical signal versus the HM concentration in a series of working standard solutions (Tibebe et al. 2022).

The operating conditions for a flame atomic absorption spectrophotometer are shown in Table 1.

The determination of pH of the samples of bottled water is presented in Table 3. pH does not directly harm human health unless mixed with other chemicals and when at high acidity or alkalinity levels. Consequently, the WHO guidelines stated that the recommended pH limit is 6.5–8.5 and same as NSDWQ but USEPA set pH limits of 6.5–9.0 (Hansen et al. 2018). Bottled water samples with pH levels below 4 taste sour and are acidic, and water samples above 8.5 have an unsightly alkaline taste. Dangerous trihalomethanes are formed at lower pH levels (Kumari & Gupta 2023). When the pH falls below 6.5, rust begins to form in pipes releasing toxic metals such as Cu, Pb, Cd, and Zn. The pH values of eight bottles of water, including those from Bigi (6.15), Aquafina (6.13), Nestle (6.43), Event (4.91), Cascade (6.45), Trinity (5.93), Exalté (5.90), and Ezbon (5.62), were outside the WHO recommended range of 6.5–8.5.

Contrarily, samples from Bigi, Aquafina, Nestle, Event, Cascade, Trinity, Exalté, and Ezbon table water samples were less alkaline than 6.5, with ‘Event’ samples' pH of 4.91 causing the most concern. According to a study by Wright (2015), using acidic water to cleanse and treat skin and hair can be beneficial as it has antibacterial properties. However, acidic water exposes the user to HMs and damages dental and bone health. Low pH levels may be due to chlorination and requires additional purification such as reverse osmosis and carbon distillation in the bottled water treatment.

Using Windows-based SPSS version 25.0 (IBM Corp., USA) software, descriptive statistics were used to analyse the data. To check if there were any apparent changes in the mean amounts of HMs in samples of bottled water, an analysis of variance (ANOVA), factor analysis, etc. were also carried out. In addition, post hoc tests were performed to determine where the differences within groups occurred once a statistically significant ANOVA result was obtained and a P-value ≤ 0.05 was considered significant according to the definition of statistical significance.

Screen plotting in SPSS at an eigenvalue of ≥1 was used to identify the number of PCs accounting for the selected HM concentrations in different water samples. The scree plot result is shown in Figure S1. Depicts the strong statistical significance of five PCs extracted at eigenvalue ≥1 which are used for the principal component analysis (PCA) of this study. The identified PCA commonalities represent the percentage of variances of the HMs extracted to account for the five PCAs adopted. The result from Table S1 showed 100% of variances of the HMs extracted to account for the 16 varying water sample companies. The total variance shown in Table S2 represents the extracted five major component factor solutions. The eigenvalue for the first PCA is 5.500 representing about 34.373% of the total variance while the second PCA has an eigenvalue of 4.075 making up for 25.471% of the total variance with a cumulative of 59.844%. Thus, this process continued till the fifth PCA with an eigenvalue of 1.011 representing 6.318% of total variance on a total cumulative variance of 100%.

The component matrix arising from the second PC (Table S3) showed that in the first PC, eight water sample companies (Swan, Eva, Aquafina, Lasien, Aquarafa, Ivy, Rovia, and Exalté) loaded strongly with a high correlation coefficient of 86.9. 88.2, 65.1, 50.4, 88.2, 88.2, 74.7, and 58.6%, respectively. While Swan, Eva, Aquarafa, and Ivy showed a strong negative correlation, Aquafina, Lasien, Rovia, and Exalté maintained a strong positive correlation. However, on the second PC, Nestle, Event, Cascade, Exbon, and Jasmine showed a strong positive correlation at 79.5, 87, 55.7, 60.2, and 74.8%, respectively, Locazade hydropure maintained a strong negative correlation at 79%. This relationship established above continued until the fifth PC.

Tables S4 and S5 show that the hierarchical cluster analysis was performed to observe how the sampled six HMs (Pd, Ni, Zn, etc.) correlated with the various water samples (Swam, Ivy, Eva, etc.). The result (dendrogram) showed five clusters, with Cu being more related to Zn than Pd, Ni, Cd, and Cr. Similarly, Pd is related to Zn and Ni more than it relates to Cu, Cd, and Cr as shown in Tables S4, S5, and Figure S2.

Pearson correlation matrix (Table S6) showed a strong positive correlation between Swam and Eva, Ragolis and Locozade, Cascade and Aquarafa, Lasien and Nestle, Event and Nestle, Lasien and Event, Swan and Aquarafa, Swan and Ivy, Locade and Rovia, Aquafina and Roviaragolis and Exalté, Nestle and Exbonevent and Exbon, Jasmire and Aquafina, Jasmire and Event, and Jasmire and Cascade at 99.9, 58.1, 99.9, 75.2, 77.1, 83.2, 75.2, 99.9, 55.4, 69.7, 93.9, 80.8, 96.8, 85.4, 58.5, 80.6, and 93.4%, respectively. However, a strong negative correlation was only encountered between Jasmire and Locade at 55.4%. Similarly, the pattern matrix which represents the loadings of variables in an oblique rotation was used. Each row of the pattern matrix is essentially a regression equation where the standardized observed variable is expressed as a regression coefficient. Thus, from the first PC (Table S7) Swan, Eva, Ivy, and Aquafina loaded negatively strongly with a regression coefficient of −100% each. In the second PC, Exbon, Lasien, Nestle, and Event bottled water samples showed strong positive regression coefficients at 98.8, 94.5, 91.3, and 90.4%, respectively. Considering the third PC, Trinity, Locade, and Rovia loaded strongly with a positive regression coefficient of 91.8, 82.9, and 66.7%, respectively, while Ragolis and Exalté loaded strongly positively at 97.2 and 92.3%, respectively, at the fourth PC. The fifth PC had the remaining bottled water variables loaded strongly with a positive correlation coefficient.

The contamination of bottled water samples by HMs in the Enugu metropolis and the associated health risks are serious health issue due to leakage, exposure, absorption, consumption, and lack of data. Using FAAS to analyse the samples of each of the three replicates by spiking and homogenizing selected samples, LOD, LOQ, IDL, and standard deviation were taken into account. From the results, it can be concluded that trace metals of concern occur in bottled water sold in the Enugu metropolis. Out of the 17 analysed samples, Cd ions were detected in 11 samples, Pb ions in 8 samples, Cu ions in 7 samples, and Ni ions in 6 samples. Cr was detected in all the samples except Aquafina and Exalté, while Zn was only detected in Aquafina and Trinity. All HM ions were detected in Aquafina except Cr and Cu ions; La Sien except Ni and Zn ions; Jasmine except Cu and Zn ions; and Cascade except Pb and Zn ions. In contrast, Rovia and Trinity showed a higher additional presence of metals because of only Zn and Ni, respectively. Hence, from the present findings, the least contaminated bottled water samples were Eva > Aquaphor ≤ Ivy ≤ Nestle, while Cd and Cr were the most frequently occurring HM ions and Zn was the least. Therefore, the study confirmed the presence of HM components in commercial bottled water samples, which can interact in more complicated ways at higher or lower pH values. The PCA showed strong statistical significance, while the hierarchical component analysis showed a strong correlation with the various samples. Hence, the use of activated carbon, ion exchangers, and nanofiltration is recommended to reduce all HM ions to the barest minimum. It is expected that these measures when implemented will help to protect the health of the population and significantly improve the quality of bottled water in Enugu. Further work can be carried out according to these recommendations.

The study, therefore, highly recommends the following for the effective removal of these toxic metals:

• The producer can apply selective separation of species at controlled pH levels to improve water quality.

• The human health effects of co-exposure to metals and other HMs (such as Pb and Cd) need to be better understood. Regulatory authorities may relate co-exposure situations and other aspects to the maximum metal levels in drinking water (e.g. treatability and toxicological implications).

• Significant amounts of HMs can escape from tanks and piping systems, resulting in significant leaks significantly. There is a good chance that several factors, including water pH, dwell time, temperature, pipe material, sanitizer type and dosage, and the age of the tanks and pipes, will affect the release of metals. The results of their interactions as well as the effects of these substances on HM leaching could be clarified by additional research.

• Technologies based on adsorption, coagulation, precipitation, and filtration are expected to generate significant amounts of HM-contaminated waste when applied in combination with one another. Saturated media and liquid wastes with HM contents should be disposed of properly. Several strategies could be employed to solve this issue. One such technique is to combine HM-contaminated materials with solid waste or engineered components, such as glass, brick, concrete, or cement blocks, and then carefully dispose of the resulting mixture.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.