Concentration of Heavy Metals in Soil and Staple Crops and the Associated Health Risk

The non-biodegradable and persistent nature of heavy metals results in their accumulation in human body and destruction of vital organs such as kidney, bones and liver (1). Accumulation of heavy metals in agricultural soils may not only result in soil contamination but also the subsequent transfer of these elements to food crops (2). One of the main food items in many countries around the world are cereals such as wheat, rice, barley, rye, corn etc. Among them, wheat and rice accounts for 4/5 parts of the total food consumption of the world's population (3). Total consumption of wheat in Iran has risen from 15.8 MT in 2010 to 17.5MT in 2015 (4). Iran is the ninth largest consumer of wheat in the world and wheat bread is a staple in the Iranian diet. Due to the high tolerance of wheat and barley against elevated levels of heavy metals in the soil, they have been proposed for the phytoremediation of heavy metal contaminated soils in earlier researches (5); however, this accumulation can pose a high risk to consumers as well. Since, wheat and barley are considered as staple products so; the health risk associated with their consumption has a great importance in view of A-R-T-I-C-L-EI-N-F-O A-B-S-T-R-A-C-T Article Notes: Received: Nov 11, 2016 Received in revised form: Apr 18, 2017 Accepted: Jun 29, 2017 Available Online: Jul 21, 2017 Background & Aims of the Study: The main objectives of the current research were (1) to study the extent of soil pollution by heavy metals (Ag, Cd, Cr, Cu, Mn, Ni, Pb, Zn, Ba and Li) in sampling locations (2) to investigate the extent of heavy metal pollution and soil to plant transfer of these elements in wheat and barley (3) to study the health risk of heavy metals via consumption of wheat grains for adults and children. Materials &Methods: The levels of heavy metals in 29 sampling locations in soil and different parts of wheat and grains of barley were quantified in Shahrood and Damghan of Iran by inductively coupled plasma (ICP) optical emission spectroscopy (ICP-OES). Different statistical methods including ANOVA were used for the comparison of heavy metals in different soil groups and plant's tissues. Results: All of the bio concentration factors (BFs) were lower than one indicating the low level of accumulation in wheat and barley. The BFs were higher in aerial parts than that of grains showing the restricted translocation of these elements by wheat and barley. The highest accumulation was obtained for Zn, Ag and Cu. Conclusions: Considering Hazard Quotients (HQs), there was not any detrimental effect due to the consumption of wheat grains in the study area. The HQs of all heavy metals for adults were higher than that of children. The highest and lowest HQs values were for Mn and Cr in both age groups, respectively. The Hazard Index (HI) associated with wheat grains for children and adults were 1.36 and 2.06, respectively which is indicated the adverse health effects due to the consumption of this staple crop.

The non-biodegradable and persistent nature of heavy metals results in their accumulation in human body and destruction of vital organs such as kidney, bones and liver (1). Accumulation of heavy metals in agricultural soils may not only result in soil contamination but also the subsequent transfer of these elements to food crops (2). One of the main food items in many countries around the world are cereals such as wheat, rice, barley, rye, corn etc. Among them, wheat and rice accounts for 4/5 parts of the total food consumption of the world's population (3). Total consumption of wheat in Iran has risen from 15.8 MT in 2010 to 17.5MT in 2015 (4). Iran is the ninth largest consumer of wheat in the world and wheat bread is a staple in the Iranian diet. Due to the high tolerance of wheat and barley against elevated levels of heavy metals in the soil, they have been proposed for the phytoremediation of heavy metal contaminated soils in earlier researches (5); however, this accumulation can pose a high risk to consumers as well. Since, wheat and barley are considered as staple products so; the health risk associated with their consumption has a great importance in view of the health of natives. There have been multiple studies on the levels of heavy metals in wheat and barley all of the world. In a study by Moradi et al. (6), the average concentrations of Cu, Mn and Zn of wheat grain in three different cities of Isfahan province (Zarinshahr, Mobarakeh, Natanz), Iran, were 4.8,34.0 and 38.3 mgkg -1 DW, respectively. Singh et al. (7) considered the heavy metal concentrations, associated health risk of consumption of cereals (wheat and rice) and some other vegetable crops in India. Salehipour et al. (8) conducted a study to assess the risks of human health from exposure to arsenic, lead, nickel, zinc and copper through consumption of wheat, rice and some vegetables in Isfahan province, Iran, using the total non-carcinogenic hazard quotient (THQ) and cancer risk assessment estimates.

Aims of the study:
In the present study, the concentrations of heavy metals in locally produced wheat and barley plants in addition to soil samples in 29 locations of Shahrood and Damghan, Semnan province of Iran were quantified. The main objectives were (1) to study the extent of soil pollution by heavy metals (Ag, Cd, Cr, Cu, Mn, Ni, Pb, Zn, Ba and Li) in sampling locations in the study area (2) to investigate the extent of heavy metal pollution and soil to plant transfer of these elements in wheat and barley (3) to study the health risk of heavy metals via consumption of wheat grains for adults and children. To the best of our knowledge, it is one of the most comprehensive published papers on the health risk of wheat grains (as a staple crop) in Iran.
27 sampling locations were selected in arable fields of Shahrood and Damghan where wheat and barley as staple crops are cultivated. A view of the study area has been illustrated in Figure1.Top-soil samples (0-25) were collected from the same sampling locations to take into account the contribution of soil to the pollution of plants and calculate the associated bio concentration factor as well. In a laboratory, the collected soil samples were air-dried and sieved through a 2-mm stainless steel mesh to remove stones and plant roots. Following digestion of soil samples with nitric acid (HNO 3 ) and hydrochloric acid (HCl) in a ratio of 3:1 (HNO 3 :HCl), the total heavy metal concentrations of Ag, Cd, Cr, Cu, Mn, Ni, Pb, Zn, Ba and Li were analyzed by inductively coupled plasma (ICP) optical emission spectroscopy(ICP-OES). Dried samples were grounded, using a stainless steel grinder (<0.25 mm) and the total content of the abovementioned heavy metals were detected by ICP-OES. In order to compare the heavy metal concentrations in the considered parts of wheat (grain, leave and stalk, aerial part) at the studied sites, a one-way analysis of variance (ANOVA) was performed. For this purpose, the normality requirement of data was assessed by Shapiro-Wilk test of normality before the ANOVA test and the transformation of data was implemented to fulfill the normality requirements for those data which were not normal according to this test. In addition, for each data set, the homogeneity of variance was tested by Levene statistic. The significant level was 5 percent for each test. Furthermore, to consider the significant difference between heavy metal levels in wheat and barley grains, a one-sample t-test was carried out. The significant difference among soil groups related to each plant part was considered, using ANOVA test. All of the statistical tests in this study were performed by SPSS. Health risk associated with wheat grain consumption Risk to human health due to the consumption of wheat grains is characterized, using a Hazard Quotient (HQ) (9) which is calculated by the following equations:

Materials & Methods
in which the Chronic Daily Intake (CDI) is the exposure expressed as the mass of a substance per unit body weight per unit time, averaged over a long period of time (a lifetime) whereas RfD o is the oral reference dose (mg kg −1 day −1 ). CF is the median concentration of a heavy metal in wheat grain (mg.kg −1 ); IR is the ingestion rate of wheat grain (kg.person −1 .day −1 ) in which the per capita wheat consumption is about 170 kg per year (10) for adults and for children (0-6 old) it was assumed to be one third of that of adults (11); EF is the exposure frequency (365 days year −1 ); ED is the exposure duration (70 years for adults and 6 years for children); BW is the average body weight (70 kg for adults and 35 kg for children), and AT is the average exposure time for non-carcinogenic effects (ED × 365 days year −1 ). If the CDI exceeds the threshold (i.e., if HQ exceeds unity), potential non-cancer effects may be a concern. The RfD o values were 0.005,0.001,0.04,0.14,0.02,0.3,0.2,0.02 mg.kg -1 day -1 for Ag, Cd, Cu, Mn, Ni, Zn, Ba and Li (12), 1.5 mg.kg -1 .day -1 for Cr (13) and 0.004 mg.kg -1 .day -1 for Pb (11), respectively.
The concentrations of heavy metals in plants (wheat and barley) and soil samples have been given in Table1 and Table 2, respectively. Among the studied heavy metals in different parts of wheat samples, there was a significant different at 5 percent in the levels of Cu and Ba. A method for expressing the accumulation of metals from soil to above-ground tissues of plants is through bio concentration factor (BF) which is generally obtained by dividing the concentration of the element in plant tissues (grains, shoots, straw, etc) to the total concentration of the same element in the rooted soil (14). The bio concentration factors of wheat and barley have been shown in Table 3. On the contrary, the results of hazard quotient (HQ) for individual heavy metals related to adults and children have been illustrated in Figure 2.  The distribution pattern of copper in different soil horizons is mainly influenced by its accumulation in the top horizons (15) resulting in its subsequent uptake by plants. As a whole, the behavior, phytobio availability and toxicity of Cu are influenced by its species and are not the function of its total concentration (16). The rate of Cu uptake by plants completely depends on the plant species. For example, Chlopecka (17) concluded that anthropogenic sources of copper are more available for barley uptake than geogenic sources. Since, there was no significant difference between the copper levels in the four studied soil groups (  (28). This is in consistent with the results of this study as well.

Figure1) A view of the study area in which the regions with intensive agricultural activity has been highlighted
The tracking of Zn movement in plants by radionuclide showed that Zn has the highest mobility as compared with As, Cd and Cu (29). In summary, as an essential element, like that of Cu (which is also an essential element) none of the analyzed samples have exceeded the permissible limits recommended in this regard. Cadmium is a metal with unknown essential function in higher plants but it is easily absorbed by plant roots and transferred to the above-ground parts (30). When the growing takes place on the same type of soil, the cadmium accumulation in different species decreases as follows: Grains<Root<Vegetables<Leaf vegetables (31). In a similar research, Eriksson et al. (32) showed that Cd distribution in cereal grains is in the following order: wheat>oat>barley. The results of the current study indicated elevated levels of Cd in leave and stalk as compared to that of grain in wheat. Hornburg, Bümmer (33) and Adams et al. (34) showed that there is a linear relationship between soil and plant levels of cadmium; however, in this study there was a weak correlation (r=0.068) between the concentrations of cadmium in plant crops and its soil levels. Jafarnejadi et al. In mining areas, Pb may be dispersed due to the erosion and chemical weathering of tailings. The severity of these processes depends on chemical characteristics, and the minerals present in the tailings. Like that of silver, the prevalent mining activities in the area are one of the main sources of lead next to motor vehicles in the soils of the region. These metals are continuously dispersed downstream and downslope from the tailings by movement through wind (40). Referring to plant samples, the Pb concentrations in all of the wheat samples were higher than the permissible value of 0.3 mg.kg -1 set by WHO/FAO ( Table 1). The mean concentration of lead in wheat grain in 16 different studies conducted in various countries was mostly high ranging from 0.015 to 22.6 mg.kg -1 (41). The mean value of this element in wheat grain samples from Pakistan were at the same range as that of this study with the mean value of 0.35 mg.kg -1 (41). In the study of Bermudez et al. (14) conducted in Argentina, the range of Pb in wheat samples fluctuated between 0.022 and 0.269 mg.kg -1 with mean value of 0.088 mg.kg -1 which was lower than the reported value in this study. In addition to soil, airborne Pb is a major source of Pb in plants through uptake by foliage. In this respect, Dalenberg and Van Driel (42) suggested that 73-95% of the concentration of Pb in wheat grain was a result of atmospheric deposition. By contrast, only 21% of the concentration of Cd measured in wheat grain was from atmospheric sources. As reported by Kabata-Pendias and Pendias (20), the mean concentration of lithium in soil samples is about 32 mg.kg -1 which is within the range of the findings of this study. In the arid climatic zones like that of the study area, Li follows the upward movement of the soil solution and may precipitate at top horizons along with easily soluble salts of chlorites, sulfates, and borates. Since the lithium in soil solution is easily available for plant absorption so, the plant content of this element is believed to be a good guide to the Li status of the soil (43). The findings of the present study confirm this as the levels of lithium in leaves and stalk are far higher than that of wheat grains (Table  1).
In the absence of anthropogenic sources, the elevated levels of chromium next to nickel have been proved to originate from the ultramafic rocks and the developed soils over them (44). The presence of these formations in the study area may be one of the reasons for this higher than normal average levels of this element (e.g. 92.550) ,though less than the standard values, in the region. a study on temperate trees confirmed that Cr was poorly taken up into the aerial tissues but was held in the root. Despite these conclusions, the results of the current study showed higher than standard values of Cr in all of the studied organs as compared with the permissible value of 0.02 mg.kg -1 DW suggested by WHO/FAO (Table 1). Due to the carcinogenic properties of this element, it looms large for the consumers in the study area. In this field, López-Luna et al. (48) found that roots of wheat, oat and sorghum accumulated more Cr than shoots; however, in spite of that, wheat, oat and sorghum showed Cr translocation from roots to shoots.  Ag>Zn>Cu>Cd>Mn>Li>Ni=Pb>Ba>Cr and Zn>Ag>Cu>Cd>Mn>Li>Ni>Pb>Ba>Cr, respectively. All of the calculated bio concentration factors were lower than one indicating the low amount of accumulation of heavy metals in wheat and barley.As a whole, the BFs were higher in aerial parts than that of grains showing the restricted translocation of these elements by the wheat and barley in the study area.The highest accumulation were obtained for Zn, Ag, Cu and the following order belonged to Cd.The results of this study agreed with that of Boussen et al. (57), in which Zn was the most abundant element in wheat grains and also higher BFs were found in aerial compared with that of grains. The bio concentration capabilities of silver have been earlier discussed by Ratte (58).The least accumulated elements in this study were Cr next to Ba in wheat and barley grains.This finding completely confirms the results of Huang et al. (11) and Gigliotti et al. (59)who found that Cr is the least mobile element in wheat grains and corn plants,respectively.The results of this study were also in consistent with that of Brunetti et al. (60) who found that Cr next to Ni are the least mobile elements in wheat and barley samples in Italy. As can be seen in Figure2, the HQs of all heavy metals for adultsare higher than that of children. The HQs decreased in the following order for adults: Mn(0.78)>Pb(0.49)>Cu(0.30)>Zn(0.21)>Ni(0.0 8)>Li(0.07)>Cd(0.05)>Ba(0.04)>Ag(0.03)>Cr( 0.001) and the order for children they were as follows:Mn(0.50)>Pb(0.33)>Cu(0.20)>Zn(0.14 )>Ni(0.05)>Li(0.04)>Cd(0.03)>Ba(0.02)>Ag(0 .01)>Cr(0.001). If the risk exceeds unity there will be a concern for potential health hazard for the consumers (11). The HI associated with wheat grains consumption for children and adults were 1.36 and 2.06, respectively indicating adverse health effects due to the consumption of wheat grains with respects to the levels of all considered heavy metals in both age groups.The values obtained for HI in this study were higher than those reported by Huang et al. (11) but less than the HI values in Zheng et al. (61) and Bermudez et al. (14).
The results of this study indicated that zinc next to silver have the highest bio concentration capabilities to transfer from soil to wheat and barley.The hazard quotients of heavy metals in wheat grain decreased in the following order: Mn>Pb>Cu>Zn>Ni>Li>Cd>Ba>Ag>Cr. Considering the hazard quotient of individual heavy metals considered in this study, there is no health threat due to the consumption of wheat grains for adults and children whereas, hazard index for combined heavy metals was higher than one indicating that children and adults will probably experiencing health risks due to the dietary intake of heavy metals.