Determination of Lead and Cadmium Content of Dill (Anethum graveolens) and Onion (Allium Cepa L.) Cultivated in Khozestan/Iran

1. Introduction

       Lead and cadmium are among the toxic heavy metals that are stable in the enviroment, and their biodegradation does not easily occur [1]. One of the major sources of exposure of humans to heavy metals, such as Cd and Pb is their daily diet [2-4]. People extensively have vegetables in their daily food basket. Many studies have shown that vegetables are contaminated by Cd and Pb [5, 6]. Medicinal herbs and accordingly their preparations are also polluted by these heavy metals [7, 9].

       Cadmium and lead enter agricultural products through contaminated air, soil, and water [10-12], so the level of heavy metals in vegetables and medicinal plants can be a good indication of the extent of pollution in the area. Besides, the health hazards resulting from consumption of these agricultural products can be predicted, and reasonable diet can be suggested to prevent food poisoning.

Table 1. Lead and cadmium content of dill and onion of Shooshtar and Aghili/ Khozestan province obtained by FAAS. Results are the mean of four measurements.

ND: Not Detected

      In folk medicine, onion has also been used for whooping cough, chest pain (angina), gallbladder complaints, dehydration, menstrual problems, parasitic infections, and diabetes. It is applied externally for insect bites, wounds, mild burns, warts, boils, and bruises. Its sulfur compound thins the blood and helps prevent dangerous clots while lowering blood pressure and possibly reducing cholesterol levels. Onions also possess the ability to kill a wide variety of germs. And, for asthma victims, the onion extract may even relieve allergy-induced bronchial constriction. Dill is a familiar kitchen spice, best known as a flavoring for pickled cucumbers. They act medicinally by relieving spasms and blocking the growth of bacteria. Both dill seed and dill leaf have been used medicinally, but only dill seed has been proven effective. Dill leaf has been used for stomach and intestinal problems, kidney and urinary tract conditions, spasms,  and  sleep  disorders,  but  its effectiveness for these conditions remains unproven.

       Dill and onion in addition to their medical application are extensively used in Iranian diets both in cooked and uncooked forms. Khozestan has a vast cultivation area for these vegetables. Since many environmental sources of pollution are known in Khozestan province, analysis of these agricultural products for the level of pollution seems to be necessary. So in these study vegetables from two cultivated areas, namely Shushtar and Aghili, are analyzed for their Pb and Cd contents.

Table 2. Mean values of Pb and Cd of five randomly selected dill samples obtained by both calibration and standard addition.

       Different analytical methods are used for the determination of heavy metals in foodstuffs, namely flame and furnace analytical atomic spectrometry [13, 14], stripping voltammetry [15], inductively coupled plasma - time of flight mass spectrometry [16]. Among these, flame atomic absorption spectrometry (FAAS) is simple, and economic but the main limitation on its application is the high detection limits compared to other techniques such as graphite furnace atomic absorption spectrometry and stripping voltammetry.

       In this study, reliability of FAAS for determination of lead and cadmium in dill and onion is evaluated. Finally the mean values of these metals in the area are determined.

2. Experimental

2.1. Reagents and materials

       All chemicals used in this study were purchased from Merck, Germany. Stock solution of 1000 μg.ml^-1 of lead and cadmium were purchased from Chem Lab N.V. (Belgium). Double distilled water was used throughout this study.

2.2. Sample collection

       Dill (Anethum graveolens L.) and onion (Allium Cepa L.) samples were collected from Shooshtar and Aghili located in Khozestan province. Figure 1 shows sampling positions on the map. For this purpose, 30 farms (15 dill and 15 onion farms) were randomly selected from different locations in agricultural district of Shooshtar and Aghili. Twelve samples were collected from each farm, total of 180 samples. Samples from each farm were divided into two sets of six. Each set were mixed to make composite sample. However, it was two composite samples for each farm and overall of 30 composite samples for dill and the same number for onion. The samples were washed thoroughly with distilled water, cut into pieces, and dried in shadow on a clean place and then grinded. Powders were kept in clean plastic vessels till analysis.

2.3. Sample preparation

       The burning procedure was as reported in AOAC [17]. To briefly describe, 15 g powder of each sample was placed in a 50 ml crucible. It was preliminary burned using conventional flame and then transferred to furnace. Furnace was programmed to reach 450 °C, stay at this temperature till a white residue was observed. The residue was dissolved in minimum amounts of HNO₃, then filtered and made up the solution to the mark in 50 ml volumetric flask. The same procedures were applied to empty crucibles to prepare the blank. Pb and Cd contents of five randomly selected samples were determined by standard addition method and compared with external calibration results. Two samples were spiked with different concentration of standard of these elements. Two samples were simultaneously (both Pb and Cd added) and separately (two paired samples, one spiked with Pb and one with Cd) spiked with standards and the results were recorded. Precision of the determination were investigated by analyzing randomly selected samples four times in a day and for three consecutive days.

 Table 3. Mean values of Cadmium of five Onion samples calculated by both calibration and standard addition curves.

Table 4. Recovery of FAAS for the determination of Pb and Cd in dill and onion.

a added concentration of Pb was 2 µg.ml ^-1; b added concentration of Cd was 1 µg.ml ^-1; c whenPb and Cd standard were separately added to the solutions.; d when both Pb and Cd standards were spiked in a single sample.

2.4. Apparatus and heavy metal measurement

       Flame atomic absorption spectrometer, CTA-3000, Anal. Tech (UK), and Naber Therm electro-thermal furnace (Germany) were used. All glass wares used in this experiment were soaked in 10% nitric acid for 24 h, then rinsed completely with double distilled water, and dried in an oven before use. Concentrations of Pb and Cd in the 60 samples were determined by FAAS at wavelength of 217 and 283.8 nm for Cd and Pb, respectively.

Figure 1. Sampling area of the onion and dill (Iran, Khuzestan province).

3. Results and discussions

       As determined by pilot studies, the levels of Pb and Cd in dill cultivated in Khozestan province were relatively high and FAAS was suitable for the determination. Only the Pb contents of onions were not in the calibration range of FAAS and cannot be determined by this method without preconcentration. In order to show the reliability of data, accuracy and precision of FAAS method for the determination of Pb and Cd in these vegetables were also evaluated.

       Concentrations of Pb and Cd in the 60 samples were determined by FAAS at wavelength of 217 and 283.8 nm for Cd and Pb, respectively. The results are shown in Table 1. No lead was detected by FAAS in onion samples. In order to check the limit of Pb in onion samples some randomly selected samples were analyzed by graphite furnace but not quantitative amount were detected. It means that the Pb content of onion was even lower than the range of determination of graphite furnace.

       In order to evaluate the reliability of the results obtained by the calibration curves, Pb and Cd contents of five randomly selected samples were determined by standard addition method and compared with external calibration results. The results are summarized in Tables 2 and 3. As the results indicate, external calibration can be used as a determination method and no serious matrix effect was seen. To further evaluate the reliability of the results and check the extent of matrix effect, two samples were spiked with different concentrations of standard of these elements. Two samples were simultaneously (both Pb and Cd added) and separately (two paired samples, one spiked with Pb and one with Cd) spiked with standards and the results were recorded. Calibration curve was used to calculate the concentration of unknown and standards. Table 4 shows the results for these kinds of measurements. The recoveries for standards were in the range of 90% to 110%.

      Precision of the determination were investigated by analyzing randomly selected samples four times in a day and for three consecutive days. The results have shown good relative standard deviation ranging from 2.5% to 10% for onion and dill (Table 5).

       According to results shown in Table 1 and Figure 2, mean values of Pb contents in dill cultivated in Shooshtar were higher than permitted standard limit mentioned in Codex for vegetables (0.3 mg/kg) but Cd level is a bit higher than reported limit (0.2 mg/kg for cadmium). Student’s t-test also emphasized that there was significant differences between Pb contents of dill and the codex limit. These contamination levels can be hazardous for consumers and must be controlled carefully. Mean value of Cd in onion is relatively low and far from standard limit mentioned in the Codex as also confirmed by statistical tests.

 Table 5. Reproducibility for determination of lead and cadmium by FAAS.

Figure 2. Graphical representation of Pb and Cd content of Khozestan Dill and Onion compared to the limit reported in codex.

4. Conclusion

        As the results demonstrate, dill samples cultivated in Shooshtar/Khozestan is contaminated by both Cd and Pb. That is mainly due to polluted soil, atmosphere, and irrigation water resulting from so many industrial and urban activities in the province. The main source of onion contamination is contaminated soil and irrigation water. Cadmium contents of soil are relatively high because of uncontrolled use of phosphate, nitrogen and potash fertilizers. So, it is recommended to analyze atmosphere, soil and irrigation water to find a relation between their heavy metals contents and those of agricultural products. Precise diet is also required.

        This study also demonstrates the reliability of FAAS method for the determination of lead and cadmium in above mentioned vegetables.

Acknowledgments

        The financial supports of Jundishapur University of Medical Sciences and research center of Azad University, Ahvaz branch is gratefully acknowledged.