Introduction

Spices are mainly considered dried seeds, fruit, root, bark, or vegetable substances and are primarily used as ingredients for flavouring, colouring, or preserving food [1,2,3,4,5]. Various indigenous and exotic spices are cultivated widely by smallholder farmers and grow naturally in wilds in Ethiopia due to suitable and diverse agro-climate conditions [6, 7]. The country mainly produces and exports ginger, black pepper, fenugreek, coriander, black cumin, Korarima, turmeric, red pepper, and so forth [7]. Although they are used mainly as flavouring and colouring agents, they are also used in food preservation and extending shelf life, as well as in the treatment of diseases such as cancer, metabolic, neurological, cardiovascular, and inflammatory diseases, and aging [3, 8]. However, these spices can also contain some harmful components, such as mycotoxins, pesticides, and heavy metals [9]. These spices are easily contaminated with these components during production, storage, transportation, preparation, and handling, which has raised public and scientific concern due to their serious health effects on humans [1, 10, 11].

Widespread contamination with heavy metals has occurred in the last decade due to the use of fertilizers, pesticides, and contaminated water for irrigation as well as contaminated soils [12,13,14]. Contamination of spices with heavy metals and the addition of contaminated spices to foods can increase the accumulation of these metals in human organs [11]. Heavy metals, such as zinc (Zn), iron (Fe), copper (Cu), chromium (Cr), and cobalt (Co), are essential for humans and play an important role in biological systems[15]. These elements can become toxic when their intake exceeds certain permissible levels. However, lead (Pb), arsenic (As), and cadmium (Cd) are nonessential metals that cause serious health concerns since they are non-biodegradable, have long biological half-lives, accumulate in the body, and cause adverse effects [16,17,18,19]. Exposure to these heavy metal ions above the permissible levels can cause acute and chronic toxicity and damage to blood components, the lungs, kidneys, and liver, the central nervous system, and the immune system. Therefore, it is necessary to analyse these spices sold in commercial markets to ensure that the levels of these contaminants meet agreed international requirements to protect human health.

Recently, several studies have been conducted to determine heavy metals in several spices cultivated in Ethiopia. These studies include levels of selected metals in the sweet basil (Ocimum basilicum L.), black cumin (Nigella sativa), and tenadam (Ruta chalepensis) species [20], levels of essential and nonessential metals in ginger (Zingiber officinale) [21,22,23,24], in the korarima seeds [3], in the bulb and leaves of garlic (Allium sativum L.) [14], health risk assessment of heavy metals in spice samples (ajwain, black cumin, coriander, and fenugreek) [17], and levels of selected metals in the leaves of Ruta chalepensis L. (Rue) [25]. These studies clearly show the importance of strict and periodic monitoring of heavy metals in spices sold in commercial markets to protect human health and the food industry. However, there were no published reports that revealed the concentration of heavy metals in spices available in the local market in Fiche town in the North Shewa zone, Oromia regional state, Ethiopia. This study therefore seeks to bridge that gap by providing information on the levels of heavy metals in these most consumed spices.

Hence, the objectives of this study were to determine the levels of Fe, Zn, Cu, Cr, Mn, Ni, Cd, and Pb in ginger, red pepper, korarima, and turmeric spices sold in the local market of Fiche town using inductively coupled plasma‒optical emission spectrometry (ICP‒OES). Additionally, the levels of the investigated metals were compared with levels recommended by international organizations.

Materials and Method

Description of Sampling Site

The study was conducted in Fiche town of North Shewa Zone, Oromia regional state, Ethiopia. The edible parts of ginger, turmeric, korarima, and red pepper used in this study were collected from the available local market of Fiche town. The distance of Fiche town from Addis Ababa, the capital city of Ethiopia, is 114 km with a latitude of 9° 47′ 59.99″N and a longitude of 38° 43′ 59.99″ E. It has an elevation between 2738 and 2782 m above sea level (Fig. 1).

Fig. 1
figure 1

North Shewa Zone Map showing the study sites [50]

Full size image

Sample Collection and Preparation

Four different species samples, namely, ginger, korarima, red pepper, and turmeric, were collected from the local market, recognized, and classified according to their English name, scientific name, and the part of the plant used in Table 1.

Table 1 The botanical description of the spices used in this study
Full size table

For the collection of samples, four sites were randomly selected in the Fiche open market where the spices are sold. Then, 250 g of red peppers was collected from each location and mixed well to have a 1-kg sample and carefully packed in polyethylene plastic bags, labelled, and transported to the laboratory for processing and preservation at Salale University. The sample collection used for red pepper samples was also used to collect 1 kg of ginger, 1 kg of korarima, and 1 kg of turmeric samples (separately). For analytical purposes, only the edible parts of each plant were used. The samples were washed several times with tap water to remove all contaminants, rinsed with deionized water, dried in air, and in an oven to obtain a constant mass of the samples. Finally, the dried spices were ground, sieved (0.425 mm), and homogenized using a mortar and pestle. The dried, sieved, and homogenized samples were kept in clean polyethylene bags and stored in desiccators until digestion. Botanical descriptions of the four spices used in this study are shown in Table 1.

Apparatus and Instruments

A digital analytical balance (Mettler Toledo, Model AG 204, Switzerland) with a precision of ± 0.0001 g was used to weigh red pepper, korarima, ginger, and turmeric samples. A drying oven (Digitheat, J. P. Selecta, and Spain) was used to dry the spice samples (red pepper, korarima, ginger, and turmeric samples). A refrigerator (Hitachi, Tokyo, Japan) was used to store the digested sample until analysis. A 250-mL round bottom flask fitted with a reflux condenser was used in the Kjeldahl apparatus (Gallenkamp, England). Micropipettes (1–10 µL and 100–1000 µL) were used to measure the volume of reagents and standards. All measurements of the metals (Fe, Cu, Zn, Cr, Mn, Ni, Cd, and Pb) were performed with a SPECTRO ARCOS optical emission spectrometer (SPECTRO Analytical Instruments, Kleve, Germany) with radial plasma observation.

Reagents and Chemicals

Analytical grade (analytical reagent) chemicals and reagents were used throughout the study. Nitric acid (HNO3, 69–72%, Sigma Aldrich, Steinin, Germany) and perchloric acid (HClO4, 70%, Research Laboratory Fine Chemical Industry, Mumbai, India) were used for the digestion of spices in different matrices. Stock standard solutions (Buck Scientific Puro-Graphic, USA) with concentrations of 1000 mg L−1 in 2% HNO3 of the metals Mn, Cu, Fe, Zn, Cr, Ni, Cd, and Pb were used for the preparation of calibration standards in the analysis of the samples in nonspiking and spiking experiments. Deionized water (chemical purity below 1.5 μScm−1) was used for sample preparation, dilution, and rinsing of the apparatus before analysis.

Optimization of the Digestion Procedure of Spice

The digestion processes using HNO3 and HClO4 acid mixtures were optimized by varying the volume of the acid mixture, digestion time, and temperature for environmentally friendly reactions. The optimized method was chosen based on obtaining a clear solution of the resulting spice digest using a small amount of reagent, a shorter digestion time, and a moderately mild temperature, as shown in Table 2.

Table 2 Optimization of digestion procedure for 0.5 g of korarima, ginger, turmeric, and red pepper samples
Full size table

Digestion of Spice Samples

Korarima seed samples were digested according to a previously reported protocol with slight modifications [3]. Briefly, 0.5 g of the powdered and homogenized korarima seed samples was placed in a 100-mL round-bottomed flask, and 3 mL concentrated HNO3 (69–72%) and 1 mL of HClO4 (70%) were added. The mixtures were digested using a Kjeldahl digestion apparatus by setting the temperature to dial 1 (120 °C) for 30 min, followed by dial 6 (210 °C) for 120 min until a clear solution was obtained, following the optimized digestion procedure (Table 2). After a total of 150 min, the digested solutions were allowed to cool for 10 min without dismantling the condenser and then for an additional 10 min after removing the condenser from the flask. The 10-mL portions of distilled-deionized water were added to the cooled solution to dissolve the precipitate formed on cooling. The cooled digested samples were filtered into a 50-mL standard flask through Whatman filter paper (110 mm) to remove any suspended or turbid matter. The filtrate was subsequently rinsed with 5 mL of distilled deionized water until the volume reached approximately 45 mL. The volume was then brought to the mark (50 mL) with distilled deionized water. For each korarima seed sample, triplicate digestions were carried out, and blank solutions were also digested in triplicate. The digested sample was kept in the refrigerator until the levels of all the metals in the sample solutions were determined by inductively coupled plasma‒optical emission spectrometry. The optimized digestion procedure used for the korarima seed samples was applied to the red pepper, ginger, and turmeric samples as well. The inductively coupled plasma‒optical emission spectrometry (ICP‒OES) operating conditions for determinations of heavy metals are given in Table 3, and the instrumental detection limit (mg L−1) for investigated metals in spice samples is given in Table 4.

Table 3 Instrumental operating conditions for the determination of heavy metals using ICP‒OES
Full size table
Table 4 Instrumental detection limit (mg L−1) for the investigated metals in spice samples
Full size table

Method Validation

Method validation is the process of proving that an analytical method is acceptable for its intended purpose [26]. The criteria used for evaluating analytical methods are called figures of merit. These figures of merit are linearity, accuracy, precision, detection limits, and limit of quantitation, which were used for validating the analytical procedure[27].

The limit of detection (LOD) is defined as the concentration that gives an instrument signal (y) significantly different from the ‘blank’ or ‘background’ signal [26]. In the present study, the LOD for the analysis of heavy metals using ICP‒OES in ginger, korarima, red pepper, and turmeric samples are supposed to be determined using a reagent blank, i.e. 3.00 mL of 69–72% HNO3 and 1.00 mL of 70% HClO4. However, all blank samples determined gave signal intensities below the instrumental detection limit.

Therefore, an approach based on the standard deviation of the signal (random errors in the y-direction, sy/x) and the slope of the calibration was used to calculate the LOD (\(\frac{3\mathrm{Sy}/\mathrm{x}}{\mathrm{m}}\)) [28]. On the other hand, the limit of quantitation (LOQ) is the lowest concentration of the analyte that can be measured in the sample matrix at an acceptable level of precision and accuracy and was calculated using \(\frac{10\mathrm{Sy}/\mathrm{x}}{\mathrm{m}}\)[28]

The accuracy of the method of the analytical procedures helps to ensure that loss or contamination occurred during the sample preparation steps and matrix interferences and was established by spiking experiments. Due to the absence of standardized certified reference materials, the accuracy of the method was validated by spiking experiments (recovery test). A recovery test was performed by spiking the spice samples with standard solutions of heavy metals. The spiked samples were prepared by adding a small known quantity of metal standard solutions to 0.500 g of ginger, red pepper, korarima, and turmeric samples. The spiked spice samples were digested separately in triplicate following optimized procedures used for the digestion of unspiked spice samples. The resulting digest of the spiked samples was then analysed for their respective metal contents using ICP-OES. From the recovered quantity of the analyte, the percent recovery was calculated using the following equation:

$$\mathrm{\% Recovery }=\frac{\mathrm{Cspiked sample}-\mathrm{C unspiked sample }}{\mathrm{C added}}\times 100$$

The relative standard deviation for replicate analyses of the same sample was obtained by dividing the standard deviation by the mean value of the analytical data according to the following equation:

$$\text{R}\text{S}\text{D}=\frac{{\varvec{S}}}{\overline{x} }\times 100$$

where,

S = standard deviation and x̄ the mean of the replicate analysis

Statistical Analysis

Data were analysed using the statistical software package SPSS version 20.0, and the value is presented as the mean ± standard deviation (SD). One-way ANOVA was used to determine the significance of variation between samples analysed for heavy metal contents.

Results and Discussion

Method Validation

Calibration Curves, Limit of Detection, and Limit of Quantitation

Table 5 shows the working standard concentrations, regression equation, and correlation coefficient for the determination of heavy metals using ICP‒OES in ginger, turmeric, korarima, and red pepper samples. For all analytes, the analytical curves showed correlation coefficient (R) values ranging from 0.989 to 0.999, indicating a good linear correlation between the analytical signal and the analyte concentration (Fig S1–8). From Table 5, the limit of detection (LOD) values for all the metals analysed ranged from 0.0250 to 0.176 mg kg−1, and the limit of quantitation (LOQ) values for all the metals analysed ranged from 0.0770 to 0.587 mg kg−1. The LOD and LOQ methods obtained were low enough to detect the presence of metals of interest at trace levels in all samples. The limit of detection (LOD) and limit of quantitation (LOQ) value by the ICP‒OES method of the studied element are given in Table 5.

Table 5 Working standard concentrations, regression equations, correlation coefficients, limits of detection (LODs), and limits of quantitation (LOQs) were obtained for each element using ICP‒OES
Full size table

Accuracy and Precision

The accuracy of the optimized procedure was evaluated by spiking the spice samples with a known amount of standard solution. The percent recovery for the spice samples was found to be in the range of 80.5–121% (as shown in Table 6) for red pepper, 80.6–113% (as shown in Table 7) for ginger, 86.1–124% for korarima (as shown in Table 8), and 93.1–118% (as shown in Table 9) for turmeric spice, which is within the acceptable range for all metals [29], showing that the method is accurate and applicable for metal determination in spice samples, so the optimized procedure has good accuracy. The precision of the heavy metal analysis was evaluated by the relative standard deviation (RSD) for three replicate measurements and was found to be less than 20% for most heavy metals, which indicates that the proposed method was precise.

Table 6 Recovery results (% mean ± SD, mgkg−1, n = 3) were obtained for validation of the optimized procedure for the red pepper sample
Full size table
Table 7 Recovery results (% mean ± SD, mgkg−1, n = 3) were obtained for validation of the optimized procedure for the ginger sample
Full size table
Table 8 Recovery results (% mean ± SD, mgkg−1, n = 3) were obtained for validation of the optimized procedure for the korarima sample
Full size table
Table 9 Recovery results (% mean ± SD, mgkg−1, n = 3) were obtained for validation of the optimized procedure for the turmeric sample
Full size table

Heavy Metal Concentrations in Spice Samples

The concentrations (mgkg−1) of heavy metals (Fe, Mn, Zn, Cu, Cr, Cd, Pb, and Ni) in red pepper, ginger, turmeric, and korarima samples were determined using ICP-OES. The final concentration of elements in the samples was calculated as:

$${\mathrm{Concentration }(\mathrm{mg kg}}^{-1})=\frac{\mathrm{Concentration}\left(\mathrm{mg}/\mathrm{L}\right)\times \mathrm{V}}{\mathrm{W}}$$

where,

V is the final volume of the digested solution (50 mL) and W is the weight of the sample (0.5 g).

The following concentration ranges of Fe (38.7–98.9), Mn (10.5–257), Zn (7.30–29.2), Cu (1.70–6.50), Cr (5.40–9.70), Cd (1.50–2.90), Pb (28.4–14.5), and Ni (6.70–3.90) were obtained. The maximum permissible limits (MPLs) of Pb, Cd, Ni, Cr, Cu, Fe, Mn, and Zn are 2.50–10.0, 0.200, 50.0, 30.0, 20.0, 300, 100, and 50.0 mg kg−1, respectively [30,31,32]. Among the analysed heavy metals, chromium, nickel, iron, zinc, and copper were lower than the permissible value reported by WHO. However, Pb and Cd in all spices and Mn (only in ginger) were above the permissible value reported by WHO [30,31,32]. The concentration values of all the heavy metals analysed and their maximum permissible values are given in Table 10.

Table 10 Heavy metal concentration (mean ± SD, mg kg−1, n = 3), percent relative standard deviations (% RSD) in different spice samples, and their maximum permissible limits (MPL)
Full size table

As shown in Fig. 2, the trend of metal content in red pepper samples was in descending order of Fe > Zn > Pb > Mn > Ni > Cu > Cr > Cd, and the metal concentration level in Korarima samples was in descending order of Mn > Fe > Pb > Zn > Cr > Cu > Ni > Cd. However, the trend of metal content in the ginger samples was in descending order of Mn > Fe > Zn > Pb > Cr > Ni > Cu > Cd, while the descending order of turmeric samples was Fe > Mn > Pb > Zn > Cr > Ni > Cd > Cu.

Fig. 2
figure 2

Concentrations of heavy metals in red pepper, ginger, korarima, and turmeric samples (mg kg.−1 dry weight)

Full size image

Comparison of the Metal Levels Found in the Present Study with Those Found in the Literature

In the present study, the levels of heavy metals found in red pepper, ginger, turmeric, and korarima samples were compared with the values found in the literature for similar spices (as shown in Table 11). The data presented in Table 11 indicated that some of the values reported in the literature were higher or lower than those found in the present study.

Table 11 Comparison of the concentration of heavy metals in the spice sample (mg kg−1) in this study and the reported values
Full size table

Copper is an essential element for the functioning of many major enzyme systems [33]. However, a high level of copper affects respiration and the metabolism of carbohydrates, lipids, and proteins and affects mainly the blood and kidneys [17]. Copper was found in all spices analysed, and the copper concentration for the spices analysed is shown in Table 10. The level of Cu in the spice samples was found to be in the range of 1.70–6.50 mgkg−1. Concentrations of copper between samples were compared using one-way ANOVA, which indicated that there were significant differences in the levels of copper between the four samples. The maximum Cu concentrations were found in the korarima samples (6.50 ± 0.300), and the minimum Cu concentrations were found in the turmeric samples (1.70 ± 0.200 mg kg–1). The results obtained for all spices are within the acceptable copper limits set by the WHO (20.0 mg kg −1) for edible plants [32]. Compared with reports (as shown in Table 11), the concentrations of Cu in red pepper were lower than those reported from Saudi Arabia [16], Ghana [10, 34], Iran [35], and Nigeria [36], which are not in agreement with our findings. While the concentrations of Cu in red pepper were higher than those reported from Ethiopia [13] and Turkey [37], indicating that none of the results of our study was consistent with these findings. On the other hand, the concentrations of Cu in ginger were higher than those reported from Ethiopia [38], Iraq [39], and Poland [34]. However, the concentration of Cu in ginger is lower than that reported in Serbia [4], Ethiopia [21], and Nigeria [36]. In turmeric samples, the concentrations of Cu were lower than those reported in most studies from Iran [35], Eritrea [31], Saudi Arabia [16], Bangladesh [1], Ghana [10], and Iraq [39]. However, the copper concentration reported in this study was higher than that in Poland [34] and Greece [18]. In general, all spices studied have a low copper content and are not dangerous for human consumption.

Zinc (Zn) is one of the crucial micronutrients for growth and the immune system. It is a cofactor of more than 300 enzymes and plays a prominent role in cellular growth and tissue differentiation [40]. However, its high levels in the human body can be toxic due to its interference with copper metabolism [34, 41]. In this study, the concentration of Zn was found in the range of 7.3–29.2 mg kg−1 in the spice samples analysed. The ANOVA analysis indicated the existence of significant differences (p < 0.05) between means of Zn in red pepper, ginger, turmeric, and korarima samples. Among the investigated spices, rhizomes of ginger accumulated higher zinc concentrations (29.2 ± 1.70 mg kg−1). The minimum concentration of zinc was obtained in turmeric (7.30 ± 0.200 mg kg−1). The zinc contents of the studied spices were compared with the limit proposed by FAO/WHO in edible plants (50 mg kg−1) [31]. It was found that all the spice samples accumulated zinc below this limit. The concentration of zinc in red pepper was higher than that in Turkey (7.84 ± 0.600), Saudi Arabia [16], Nigeria [42, 43], and Iran [35]. However, higher concentrations of Zn in red pepper were reported from Ethiopia [13] and Ghana [10]. On the other hand, the concentration of Zn in ginger in this study was found to be higher than reported values from Poland [34], Nigeria [10], and Ethiopia [13]. However, higher concentrations of Zn in ginger were reported from Ethiopia [24, 38] and Ghana [10]. However, a comparable concentration was reported from Iraq [39]. In the turmeric sample, a higher concentration of Zn was reported from Eritrea [31], Iraq [39], and Poland [34], but a lower concentration than the current study was reported from Iran [35].

Lead is considered a non-essential heavy metal for any organism and can reach the human system through air, water, and food [17, 44]. It has no beneficial role in human metabolism and produces progressive toxicity even at lower levels, and there is no known safe exposure level. It accumulates in humans and animals mainly in the liver and kidneys. It reacts or complexes with many biomolecules and adversely affects the reproductive, nervous, gastrointestinal, immune, renal, cardiovascular, skeletal, and muscular systems as well as developmental processes [39]. In this study, the concentrations of lead in red pepper, ginger, korarima, and turmeric were found to be 14.50 ± 0.400, 24.8 ± 0.0800, 28.4 ± 4.800, and 16.1 ± 0.0500 mg kg−1, respectively. Statistically, there were significant differences (p < 0.05) in the means of Pb in red pepper, korarima, ginger, and turmeric samples. The lead content in red pepper, korarima, ginger, and turmeric was above the permissible limit compared to the value reported by the WHO (2.50–10 mg kg−1) [30, 31]. In general, all spices studied have a high lead content and are dangerous. The content of lead investigated in red pepper was higher than that reported in the literature from Nigeria [36, 42], Iran [35], Turkey [37], and Ethiopia [13]. The levels of Pb obtained in this study are lower than those recorded in Ghana [10]. From the data given in Table 11, the levels of Pb in ginger samples from Iraq [39], Poland [34], Nigeria [42], Ethiopia [13, 23, 38], and Serbia [4] are much lower than those of ginger samples in the present study. In the present study, the levels of Pb are much lower than those reported in Ghana [10]. The amount of lead found in the turmeric sample was greater than what was reported in the literature from Eritrea [31], Bangladesh [45], Poland [34], Iraq [39], and Iran [35].

The concentrations of Cr in red pepper, rhizomes of ginger, seeds of korarima, and rhizomes of turmeric samples were found in the range of 5.40 ± 0.00700, 8.60 ± 0.00400, 9.700 ± 0.0130, and 5.40 ± 0.00200 in mg kg−1, respectively. The highest value (9.70 ± 0.0130) was detected in korarima seeds, while the lowest value (5.40 ± 0.00200) was observed in turmeric samples. However, in all analysed spice samples, the chromium contents were below the WHO permissible limit (30 mg kg−1). Statistically, there were significant differences (p < 0.05) in the means of Cr in four samples. The concentration of Cr in red pepper was higher than that in previous studies from Saudi Arabia [16] and Nigeria [36, 42]. However, it was lower than that reported in Ethiopia [13] and Turkey [37]. In ginger samples, the obtained Cr concentrations were found to be relatively higher than those reported from Nigeria [42] and Ethiopia [13, 38]. The levels of Cr obtained in this study are lower than those recorded in Iraq [39]. However, the Cr contents in ginger are comparable with those reported from Ethiopia [21]. On the other hand, the Cr concentration in the turmeric sample was found to be higher than those recorded from Eritrea [31] and Bangladesh [46]. However, the Cr contents in turmeric are lower than those reported in Iraq [39] and Sri Lanka [32]. Therefore, it is concluded that the chromium contents in all the spice samples collected from the Fiche open market are within the normal limit. These are safe for human consumption.

Nickel is considered a health hazard, causing damage to kidney tissues, liver disease, and damage to lung and blood circulation [47]. The results showed that the nickel concentration range in the studied spices was between 3.90 and 6.70 mg kg−1. The highest values were determined in red pepper samples, while the lowest concentration value was obtained in turmeric samples. Statistically, there were significant differences (p < 0.05) in the means of Ni in red pepper, korarima, ginger, and turmeric samples. The nickel levels determined in this study (Ethiopia) were below the internationally permissible value of 50.0 mgkg−1. Therefore, these spices are not considered to pose a risk to human health. Nickel concentrations in red pepper were higher than those reported in Ethiopia [13], Turkey [37], Saudi Arabia [16], Nigeria [42], and Iran [35]. The current study is still lower than the nickel content (36 mg kg−1) obtained in Ghana [32]. Nickel concentrations in ginger reported from Ethiopia [38] are comparable to those in this study. On the other hand, nickel concentrations in turmeric samples were higher than those reported in Eritrea [31] and Iran [35].

Iron is an important nutrient for the body, including oxygen transport, deoxyribonucleic acid (DNA) synthesis, and electron transport [48]. However, iron deficiency anaemia is one of the major health problems that affect cognitive performance, physical capacity, immune status, and reproductive performance. On the other hand, excess iron can lead to iron poisoning and early symptoms of abdominal pain, nausea, and vomiting, in addition to cirrhosis, heart failure, and diabetes [47]. Among the heavy metals, the iron content of all spices was higher than that of the other heavy metals except Mn. The iron concentrations in red pepper, ginger, korarima, and turmeric were 71.2 ± 0.00300, 47.7 ± 0.0110, 38.7 ± 0.0270, and 98.9 ± 0.0230 mg kg−1, respectively. The result indicated that significant differences were obtained (p < 0.05) at the 95% confidence level for Fe in red pepper, ginger, korarima, and turmeric. Among the spices studied, turmeric had the highest iron content, while korarima had the lowest iron content. The iron content obtained in this study was lower than the WHO permissible limit for iron of 300 mg kg−1 for edible plants [31]. Iron concentrations in red pepper samples were lower than those reported in Ethiopia [13], Ghana [10], Turkey [33], and Saudi Arabia [16] but still higher than those reported in Nigeria [10, 42] and Iran [35]. The iron concentrations in ginger samples were lower than those reported in Ethiopia [13], Ghana [10], and Iraq [39]. However, it was found to be higher than the value recorded from Nigeria. The concentration of Fe in ginger reported [38] in Ethiopia is comparable to that in the present study. In turmeric samples, the concentrations of Fe were higher when compared to literature reports from Iran [35]. However, the iron content in turmeric was lower than that in the samples reported in Eritrea [31], Sri Lanka [32], and Iraq [39].

Manganese acts as an activator of enzymes, respiration, and nitrogen metabolism. It also plays a role in chlorophyll synthesis and plant enzyme systems. Redox reactions in the photosynthetic electron transport system also involve manganese species. In this study, the manganese concentrations in red pepper, ginger, korarima, and turmeric were found to be 10.5 ± 0.00100, 257 ± 0.105, 69.6 ± 0.0410, and 43.6 ± 0.00300 in mg kg−1, respectively. The result indicated that significant differences were obtained (p < 0.05) at the 95% confidence level for Mn in red pepper, ginger, korarima, and turmeric. Except for ginger, the results are below the WHO permissible limit for manganese (100 mg kg−1) [31]. In this study, ginger had the highest manganese content, 257 ± 0.105 mgkg−1, which was within the range of Mn (184–401 mg kg−1) reported in Ethiopia [21]. The highest concentration can be attributed to the presence of heavy metals in the relatively acidic soils of the farmland. The manganese content of red pepper obtained in this study is higher than that reported in Saudi Arabia [16], Ethiopia [13], and Nigeria [10, 42] and lower than that obtained from Turkey [37] and Iran [35]. Manganese concentrations in red pepper reported in Nigeria [42] are comparable to those in this study. Compared with literature values, the Mn content in ginger obtained in this study is higher than the results reported in Iraq [39] and Nigeria [10]. Eritrea [31], Sir Lanka [32], and Iraq [39] reported lower Mn content in turmeric samples compared to those values obtained in this study. However, the Mn content reported by Iran [35] is higher than the result of this study.

Cadmium is a highly toxic metal that accumulates in the kidneys and liver and can cause a variety of serious diseases even at low concentrations [1]. The cadmium concentrations in red pepper, ginger, korarima, and turmeric were 1.50 ± 0.00200, 2.30 ± 0.00500, 2.90 ± 0.00300, and 2.10 ± 0.00600, respectively. The statistical analysis of levels of Cd showed a significant difference (p < 0.05) between the means of Cd in red pepper, ginger, korarima, and turmeric samples. The levels of the toxic metal cadmium found in this study were lower than those of any heavy metal examined in all spice samples. The WHO permissible limit for cadmium is 0.200 mg kg−1 for spices [31]. Compared with the results reported in the literature, the cadmium level in red pepper was higher than that reported in Turkey [37], Saudi Arabia [16], Iraq [39], and Ethiopia [13]. Meanwhile, Gaya and Ikechukwu reported higher levels of Cd in Nigeria [36] than the results obtained in this study (Table 10). The cadmium levels found in ginger samples were slightly higher than those reported in Poland [39] and Iraq [39]. However, the results are lower than those in Ethiopia [38] and Nigeria [36]. The cadmium concentrations in turmeric are higher than those reported in Iran [35], Eritrea [31], Bangladesh [45], Poland [34], and Iraq [39]. The results obtained from the present study show that all the spice samples contain cadmium above permissible limits and are not safe for human consumption.

In summary, the differences between the heavy metal contents of each of the various spices studied and those reported in the literature are attributed to the difference in environmental conditions, soil, and geographical locations of the source of planting, growing plants, and analytical methods used for analysis. There is a general scarcity of literature that addresses these types of internal compositions of spices; hence, comparisons between similar spices are difficult [49].

Conclusions

In this work, the ICP-OES method was used to determine the presence of essential (Fe, Zn, Cu, Cr, Mn, and Ni) and non-essential (Cd and Pb) metals in some spices collected from a local market. In all spices, lead and cadmium were found in the range of 14.5–28.4 mg kg–1 and 1.50–2.90 mg kg–1 respectively, while manganese (only in ginger) was found to be 257 mg kg–1, all of which are above the permissible levels. Therefore, further research on spice samples based on their cultivation is recommended to determine which hotspots are responsible for lead, cadmium, and manganese contamination. However, the concentrations of essential metals are within the permissible limits allowed by the WHO, so there is no risk of using the studied spices in food. Therefore, special attention should be given to the continuous monitoring of heavy metals, particularly non-essential metals, since they accumulate over time and pose a threat to consumers.