Heavy Metal Content and Health Risk Assessment of Some Selected Medicinal Plants from Obuasi, a Mining Town in Ghana

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Introduction
Medicinal crops can be defned as plant species that possess therapeutic properties or exert benefcial pharmacological efect on the human or animal body. Herbal medications or concoctions are medicinal plant-derived substances that exist naturally and are used to cure ailments with minimal or no industrial processing [1]. Various parts of medicinal plants including the leaves, roots, barks, fruits, and seeds are found in herbal remedies.
It has been estimated by the World Health Organization (WHO) that 80% of patients in Africa use traditional medicine for their healthcare needs [2]. Te Ghana Health Service has increasingly assimilated the use of herbal medicines into mainstream health delivery to acknowledge the essential role herbal medicines play in Ghana's healthcare system [3].
In Ghana, some common medicinal plants are Moringa oleifera Lam (drumstick tree) for treating jaundice, asthma, cancer, constipation, diabetes, etc. Piper guineense (West African black pepper) is known to treat fevers, malaria, and infammation. Lemon is used to treat scurvy, a condition caused by lack of vitamin C. Dalbergia saxatilis (locally known in Ghana as ahoma kyem) is used as a decoction in traditional medicine for treating ailments such as cough, smallpox, skin lesions, bronchial ailments, and toothache. Tetrapleura tetraptera (Aidan fruit, locally known as prekese) is cooked in soup and fed to mothers to prevent postpartum contraction. It is also used to treat typhoid and asthma [4,5].
Mining activities are well known for their deleterious efects on the environment. Most tailings from gold mines are known to contain high levels of heavy metals like arsenic, mercury, cadmium, nickel, lead, copper, and cobalt. Te environmental impact of toxic metal pollution and the accompanying health efects remain great areas of concern [6,7]. Heavy metals may pose serious carcinogenic and noncarcinogenic health risks when consumed above their acceptable threshold limits.
Tese metals may be transported through soils to reach groundwater or may be taken up by plants, including crops [8]. Tese contaminants can accumulate during the cultivation, storage, and processing of herbs and may have adverse efects on consumer health [9]. Tis could result in heavy metal exposure, especially for residents in a mining town like Obuasi and its surrounding villages [10], where both legal and illegal mining are actively done. Heavy metals have been found in soils and groundwater wells [11], organs of sheep and goats (Akoto et al., 2014), and food [13] in Obuasi and other parts of Ghana [7].
Medicinal plants from Obuasi are also likely to be contaminated with these heavy metals. Several works have been done worldwide to ascertain the heavy metal content in some medicinal plants [14][15][16]. Te medicinal plants used in this study were selected based on their popularity amongst herbal medicine practitioners and the masses that frequently patronize them. Most of the plants utilized in Ghana are also frequently found as diverse excipients in local medicines. Tis work was therefore initiated to determine the levels of selected heavy metals in medicinal plants from Obuasi, a mining town in Ghana.

Study Area.
Obuasi was chosen particularly due to its long-term exposure to extensive mining activities. Artisanal mining has been documented in the area as early as 1471 [17]. Te chief industrial activity in the area is gold mining, and most of the inhabitants of Obuasi engage in illegal smallscale gold mining commonly known as "galamsey." AngloGold Ashanti, one of the wealthiest gold mining companies in Africa, operates an underground mining operation to a depth of 1,500 m in Obuasi [13].

Sample Collection.
Fresh commonly used medicinal crop samples comprised of leaves, stems, bark, and roots from the forest and nearby farms in Obuasi (Figure 1), Ghana, were selected. A total number of 20 diferent raw medicinal plant species were used for this study. Te medicinal plant samples were selected based on the information acquired from local traditional herbal practitioners about their usage in disease, diagnosis, and treatment and also based on their popularity amongst the masses that frequently patronize them. Te bulk of the samples was packaged in polythene bags and transferred to the laboratory. Plant identifcation was carried out at Dr. Hilla Limann Technical University's Department of Pharmaceutical Sciences. Table 1 shows a summary of the medicinal plants, codes, local names, their therapeutic indication, and parts that were used in this study. Samples were washed with double distilled water and dried in an oven at 30°C for a week before pulverization into fne particles using a porcelain mortar and pestle. Samples were then sieved in a 2 mm mesh sieve before the analysis to remove extraneous matter.

Reagents.
Analytical-grade reagents were used throughout the experiment. At a purity greater than 99%, As, Cd, Cr, Hg, Pb, Ni, and Mn were acquired from Sigma-Aldrich (Steinheim, Germany). Te metal concentration used to prepare the mixed standard solution was 100 mg/L. Te stock solution was diluted with double-deionized water to form the standard working solutions for the calibration curves. Te samples were digested with concentrated HNO 3 and HCl (Sigma-Aldrich, Germany) 2.1.3. Digestion of Samples. One (1.0) gram of the preweighed sample was placed in a 100 mL borosilicate beaker. A 10 mL mixture of concentrated nitric acid (HNO 3 ) and hydrochloric acid (HCl) in a ratio of 1 :1 was added. Te mixture was heated up to a constant temperature of 250°C until a clear solution was obtained. Te digest was then transferred into a 50 mL volumetric fask. Double distilled water was added to the mark, and the solution was fltered using a membrane flter (0.45 pm) or Whatman no. 42 flter paper. Te clear solution was transferred into previously washed polyethylene bottles for ICP-MS (Perkin Elmer, 2000 series). Te run conditions of the ICP-MS were as follows: plasma fow (15 L/min), auxiliary fow (1.2 L/ min), nebulizer fow (0.88 L/min), sampling depth (0.5 mm), and pump rate (9 rpm).
Blank and standard analysis was performed by analyzing a standard solution and a blank solution for each of the elements at intervals. To verify the quality of the results, standard solutions were analyzed with each batch of about 10 samples for each element.

Quality Assurance and Quality Control.
To ensure the precision, reliability, and accuracy of the results, various quality assurance and control measures were employed in  Journal of Chemistry this study. Te ICP-MS was calibrated based on a linear fourpoint calibration curve for each element. Te standard calibration curves (r 2 = 0.998 − 0.997) were run during measurement. Calibration verifcation was carried out by analyzing a blank and standard solution for each element at intervals. Te quality of the results was determined by analyzing matrix duplicate/matrix spike for each element. Matrix spike recovery was carried out by adding an increasing amount of standard solution to the sample matrix before digestion. Te results for the recovery of the metals analyzed in this study are presented in Table 2. Medicinal plant samples were handled with care by using nitrile gloves in the laboratory to avoid contamination. Polyethene bags containing the samples were carefully sealed and coded. All equipment and glassware used for sampling, milling, and drying such as blender, crucibles, and spatula were cleaned with soap water and double distilled water to reduce the possibility of cross-contamination. A blank solution was prepared by the same procedure as used for the digestion of plant samples. To assess the reproducibility of the obtained results, triplicate analysis was conducted for every sample. Results in the text are shown in tabulated form as mean ± SD (standard deviation).

Statistical Analysis.
Results obtained were subjected to analysis by SPSS software version 20, and data were reported as standard deviation, average, and 95% confdence interval. A sample t-test was performed at a signifcant level of 0.05 and then compared with standard values.

Health Risk Assessment.
Te risk associated with the consumption of medicinal plants contaminated with heavy metal was investigated based on the estimated daily intake (EDI), hazard quotient (HQ), and hazard index (HI). Te estimated dietary intake (EDI) in mg·kg −1 day was calculated using the following equation [7,18]: where C is the concentration of the toxic metal present (mg/ kg) in the herbal plant. IR is the ingestion rate of herbal plants per day (g/day), and the maximum dosage of 30 g specifed in the West African Herbal Pharmacopia [19] was applied. BW, the average body weight (kg), was considered to be 70 kg [20]. Te HQ was used to estimate the non-carcinogenic risk of a metal. Oral reference doses (Rfd) in mg/kg/day are as follows: As (0.003), Cd (0.001), Cr (1.5), Hg (0.0001), Mn (0.14), Ni (0.02), and Pb (0.004) [21,22]. Te HQ is expressed as An HQ value less than one is considered safe, and a value higher or equal to one is unsafe and poses a likely adverse health risk to a population.
Te hazard index (HI) is the sum of the individual hazard quotients of the metals (HQ of As, Cd, Cr, Hg, Mn, Ni, and Pb) and is expressed as where HQ 1 is the hazard quotient for the frst toxicant, HQ 2 hazard quotient for the second toxicant, and HQ n is the hazard quotient for the n th toxicant.

Estimated Carcinogenic Risk (ECR).
Te cancer risk, ECR, was estimated using the following equation: ECR represents the estimated lifetime of an individual to acquire cancer from exposure to potentially carcinogenic contaminated medicinal herbs. CSF (Pb (0.0085) and As (1.5), mg/kg/day) is the slope factor, which shows the likelihood of the consumer developing cancer when exposed orally to a cancer-causing substance over the average lifespan of 70 years for Ghanaians [23]. Carcinogenic risks within 10 −4 to 10 −6 are acceptable [24].  Table 3. Table 3, the mean arsenic concentration recorded was 0.503 ± 0.445 mg/kg with a range of 0.206 to 1.092 mg/kg. All the samples used for this study recorded As levels below the World Health Organization maximum permissible limit (MPL) of 3.0 mg/kg [25]. Moringa oleifera yielded the highest As level of 2.36 mg/kg which was about 78.6% below the MPL. Tawiah [26] did not detect any As in medicinal crops sampled within the Accra Metropolis, Ghana. Te variations in As fgures can be attributed to the mining activities that take place in Obuasi.

Arsenic (As). From
Begaa and Messaoudi [15] also recorded an As range of 0.18 to 5.44 mg/kg in selected medicinal crops from the Djelfa Region of Algeria. Vaculík et al. [27] reported an extremely high arsenic concentration range of 518.6 to 919.9 mg/kg for Fragaria vesca plant species on abandoned mining sites in Slovakia. In young adults, high exposure to As, however, causes cruel damage such as cancer of the lungs, liver dysfunction, multi-organ function, and adult respiratory syndromes [28]. Table 3 was 0.620 ± 0.187 mg/kg with a range of 0.253 to 1.341 mg/kg. Paullinia pinnata recorded the highest Cd concentration which is about 447% above the WHO MPL while Ocimum viride (OV) is about 84% below the WHO MPL of 0.3 mg/kg. Cadmium toxicity afects multiple organs in the human body, but it primarily accumulates in the kidneys, where it causes severe damage such as lung hyperinfation, renal tubular destruction, vascular immune system disruption, and kidney stones [29]. Tis study recorded higher levels of Cd compared to that of Nkansah et al. [30] for medicinal herbs from Kumasi, Ghana. Ogbonna et al. [16] recorded a mean Cd concentration of 2.00 mg/kg for medicinal crops from an industrial area in Enyimba City, Nigeria. Kulhari et al. [31] also had Cd values lower than the WHO permissible limit for frequently utilized medicinal plants from Northwestern India. Dinu et al. [32] recorded Cd content higher than the WHO and European Commission's permissible recommended limits for Ocimum basilicum L. in a mining contaminated soil.

Chromium (Cr).
Te mean chromium (Cr) concentration for the study was 3.150 ± 1.078 mg/kg with a range of 2.005 to 6.603 mg/kg. Newbouldia laevis (SM) yielded the highest Cr concentration which was about 507% above the World Health Organization maximum permissible limit (MPL), and Khaya senegalensis (KS2) had the least mean concentration which is about 30.04% below the World Health Organization (WHO) maximum permissible limit (MPL). Lartey et al. [33] reported a mean Cr concentration of 6.75 mg/kg for Moringa oleifera Lam. from selected areas in Accra, Ghana. Abosede et al. [14] recorded a mean Cr concentration lower than the WHO permissible limit for Piper guineense leaves collected from three markets in Lagos, Nigeria. Chromium, though an essential element in human metabolism, is also very toxic by inhalation and dermal route, causing lung cancer, nasal irritation, nasal ulcer, and hypersensitivity reactions like contact dermatitis and asthma [34]. Table 3, all the samples analyzed recorded mercury concentrations lower than the WHO permissible limit of 0.50 mg/kg. Te mean Hg concentration was 0.020 ± 0015 with a range of 0.001 to 0.045 mg/kg. Entandrophragma angolense (EA) recorded the least mercury (Hg) concentration which was 0.2% of the WHO permissible limit. Annan et al. [35] did not detect any mercury in medicinal crops sampled from diferent geographical locations in Ghana. Mercury (Hg), especially methylmercury, is known to cause damage to the kidneys and nerves, and it can rupture the placental barrier, causing harm to the fetus [36].

Lead (Pb).
Te mean concentration of Pb recorded in this study was 6.140 ± 5.933 mg/kg with a range of 0.629 to 26.410 mg/kg. Alchornea cordifolia (AC) recorded the highest mean concentration which was 264.1% greater than the WHO maximum permissible limit (MPL) while Ocimum viride (OV) recorded the least concentration. Results from this study were higher than those of Nkansah et al. [30] who reported lead concentrations in a range of 0.44 to 0.89 mg/kg for medicinal plants sampled from Kumasi, Ghana. Lead is one of the most toxic heavy metals and has no nutritive value [37,38]. Progressive exposure to lead results in a decrease in the performance of the nervous system and afects renal clearance [39]. Inorganic lead is also a carcinogen and may cause miscarriage in pregnant women.

Nickel (Ni).
Te mean nickel (Ni) concentration was determined to be 3.31 ± 63.648 mg/kg with a range of 1.679 to 4.666 mg/kg. All the samples used in this study recorded concentrations lower than the WHO maximum permissible limit (MPL) of 10.0 mg/kg [25]. Cola nitida recorded the highest concentration which was 46.7% less than the WHO maximum permissible limit (MPL). Piper guineense (PG) had the least nickel (Ni) concentration 1.679 ± 0.002 mg/kg which was 16.8% less than the WHO MPL of 10.0 mg/kg. Baba     Journal of Chemistry

Human Health Risk Assessment of Heavy Metals.
Te values for the estimated daily intake for all the heavy metals are presented in Table 4. From the EDI values recorded, the intake of these medicinal plants is therefore not likely to pose any signifcant health risk to the population. Te hazard quotient (HQ) or non-carcinogenic health risk which measures the risk associated with long-term exposure to a particular metal was calculated from the EDI and oral reference dose as shown in Table 5. HQ values >1 are considered to pose a health risk to Tis implies that the exposed population is not likely to experience adverse health efects if they continue to consume these plants.  Generally, cancer risk (CR) values lower than 10− 6 are considered negligible, those above 10 −4 are deemed unacceptable, and those between 10 −4 -10 −6 are considered an acceptable range [43]. From Table 6, CR values are all acceptable, meaning that they are not likely to cause cancer if consumers should ingest them for a prolonged period (see Table 7).

Conclusion
Te levels of heavy metals in twenty medicinal crops were determined. Heavy metals were detected in all the medicinal crop samples selected for this study. Only one medicinal crop sample (OV) recorded cadmium (Cd) level lower than the MPL of 0.3 mg/kg. Chromium (Cr) levels exceeded the WHO maximum permissible limit (MPL) of 1.3 mg/kg in all the samples. However, mercury (Hg), nickel (Ni), and arsenic (As) levels in all the samples were below the WHO permissible limits of 0.5, 10.0, and 3.0 mg/kg, respectively. Forty and ffteen percent of the samples recorded levels higher than the WHO permissible limit for Mn and Pb, respectively. EDI values were below their recommended tolerable intake values for all the samples. HQ values were also lower than 1. Carcinogenic risk values were in the range deemed acceptable (10 −4 ) for human consumption, indicating no potential longterm adverse health risk to consumers.

Data Availability
Te data used to support the fndings of this study are included within the article.

Conflicts of Interest
Te authors declare that they have no conficts of interest.

Authors' Contributions
Flora Amarh was responsible for conceptualization and methodology. Eric Selorm Agorku was responsible for original draft preparation. Ray Voegboelo was responsible for supervision. Gerheart Ashong was responsible for  Te degree of correlation and relationship between heavy metals was determined using Pearson's correlation in Microsoft Excel 2019. In Table 7, As shows a positive relationship with Cr while Mn shows a positive correlation with Cd. Hg showed negative correlations with Ni and Pb. Weak correlations of As were observed between Cd, Mn, and Ni. Te strongly positive relationship between the metals in Table 7 shows strong signifcant frequent interactions, toxicity profle, and a common source of pollution whereby weak correlations exhibit no strong signifcant association between the metals' sources of pollution. In addition, the negative correlations represent a non-signifcant relationship between the metals. software. Napoleon Mensah was responsible for validation. Enoch Nortey was responsible for data curation.