Bioaccumulation of Heavy Metals in Soil and Vegetables
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The present study was carried out to assess heavy metals Cadmium (Cd), Lead (Pb), copper (Cu), zinc (Zn), Chromium (Cr) and Nickel (Ni) levels in vegetables like Cauliflower (Brassica oleracea var. botrytis), Cabbage (Brassica oleracea), Carrot (Daucus carota), Brinjal (Solanum melongena), Spinach (Spinacia oleracea) and Radish (Raphanus sativus) irrigated with domestic wastewater. The vegetable samples were randomly collected from the farmlands irrigated with domestic wastewater around the Hisar district. Spinach, cabbage, carrot, brinjal and carrot accumulated higher Cd (1.30Â±0.31), Pb (4.23Â±0.32 mg kg-1), Cu (1.42Â±0.25 mg kg-1), Zn (3.4Â±0.28 mg kg-1), Cr (1.16Â±0.11 mg kg-1) and Ni (2.45Â±0.86 mg kg-1) respectively. Transfer Factor (TF) of Cd, Pb, Cu, Zn, Cr and Ni are more in spinach (0.0306), cabbage (0.4448), spinach (0.2642), cauliflower (0.2494), carrot (0.0764) and spinach (0.7469) respectively. The health risk assessment has been calculated followed by Estimated Daily Intake Metal (EDIM) and Estimated Health Risk Index (EHRI). The present study highlights that both adults and children consuming vegetables grown in wastewater irrigated soils accumulate significant amount of these metals. However, the values of these metals were lesser than recommended maximum tolerable levels proposed by the FAO/WHO (1999).
Keywords: Daily intake, Heavy metals, Plant uptake, Risk Assessment, Reference dose, EDIM, EHRI
Indian economy is based on agriculture and having second largest population in the world. Most of its states are depends on the monsoon. There are two main sources for irrigation. First one is canal and second is ground water but the quality of ground water is so poor for the long sustainability of agriculture system. This water is not adequate to fulfill the crop water requirement and needs additional extra water for agricultural purposes. To cater the need of the present demand for irrigation, use of municipal domestic wastewater, is becoming a common practice in urban areas of Haryana, India. This causes serious problems of salinity and ultimately reduction in agriculture production (Marshall et al., 2007, Singh et al., 2010). The large amounts of untreated industrial and domestic wastewater are used for year round irrigation of vegetables. Such waste water usually contains heavy metals that accumulate in the soil. The use of such untreated wastewater has been reported to cause contaminations of the food chain (Wang et al., 2004; Mapanda et al., 2005).
Some trace amount of heavy metals such as Zn and Cu are essential for the growth of organisms while others such as Cd and Pb are toxic (McBride, 1994, Kabata-Pendias and Mukherjee, 2007). Dietary uptake pathway could be through crops irrigated with contaminated wastewater and have been reported to contain large amount of toxic heavy metals which may lead to health disorders in humans depending on the uptake of these metals into plant and consumed by animals or humans (Bosso and Enzweiler, 2008; Fu et al., 2008; Lim et al., 2008; Agbenin et al., 2009). Consumption of metal contaminated vegetables may lead to a weakened immune system, intra-uterine growth retardation, impaired psycho-social behavior, high prevalence of upper gastrointestinal cancer and other disorders typically associated with malnutrition (Arora et al., 2008).
Potential health risks to humans from consumption of vegetables can be due to heavy metal uptake from contaminated soils via plant roots as well as direct deposition of contaminants from the atmosphere onto plant surfaces (McBride, 2003). A number of previous studies from developing countries have reported heavy metal contamination in wastewater and wastewater irrigated soil (Cao and Hu, 2000; Mapanda et al., 2005; Nyamangara and Mzezewa, 1999; Singh et al., 2004; Nan et al., 2002). Dietary intake is the main route of exposure of heavy metals for most people (Tripathi et al., 1997). The information about heavy metal concentrations in different type of vegetables and their dietary intake is very important for assessing their risk to human health. Heavy metals in the nutrient cycle have seriously threatened health and environmental integrity, therefore, problem of heavy metal contamination in vegetables should be studied in details to develop central strategies. The objective of present study was bioaccumulation of heavy metals in vegetables irrigated by domestic wastewater and assessment of health rick due to these heavy metals.
Materials and methods
Study area and sampling
All samples were randomly collected from the farmlands irrigated with domestic wastewater around the three different locations such as Rishi Nager (L1), New Police Line (L2) and Ludass village (L3) of Hisar district of Haryana, India. Soil samples were collected at the surface depth of 10cm using stainless steel spade sampling tools and plastic buckets to avoid any contamination of samples with traces of elements from the tools. At each sampling site, scrape away surface debris and remove a core sample to the appropriate depth. Soil samples were air dried, ground, passed through a 2mm sieve and stored in plastic bags for further analysis. Five ground Water and seven wastewater samples were also collected from the each location. All samples were collected and stored and kept at 4oC for further analysis in polythene bags/canes according to their type and brought to the laboratory for metal quantification.
All the collected Vegetables were washed with double distilled water to remove airborne particles. The edible parts of the samples were weighed and soils samples were air-dried at room temperature, to reduce water content. All the samples were then oven-dried in a hot air oven at 70Â±5 oC for 24 h. Dried samples were powdered using a pestle and mortar and sieved through muslin cloth.
Digestion of the vegetable and soils samples
For each vegetable, three powdered samples from each source of domestic wastewater irrigation (1.0 g each) were accurately weighed and placed in crucibles, three replicates for each sample. The soil and vegetable ash samples were digested with perchloric acid and nitric acid (1:4) solution. The samples were left to cool and contents were filtered through Whitman filter paper No. 40. Each sample solution was made up to a final volume of 50 ml with distilled water and concentration of heavy metals were analyzed by atomic absorption spectrophotometer (ASS: model AA6300, Shimadzu).
Transfer Factor (TF)
The transfer factor (TF) of Cadmium (Cd), Lead (Pb), copper (Cu), zinc (Zn), Chromium (Cr) and Nickel (Ni) from the soil to vegetables were calculated using below given equation 1(Cui et al., 2004; Gupta, et al., 2010):
Estimated Daily Intake of Metal (EDIM)
The Estimated daily oral intake of metals from soil through vegetables in mg was calculated by equation 2:
Where; Cm is heavy metals conc. in vegetable plants (mg/kg), CF is conversion factor, DI is daily intake of vegetables (kg/ person/day) and BAw is Average body weight (kg). The conversion factor used to convert fresh green vegetable weight to dry weight was 0.085, as described by Rattan et al., 2005; Khan et al., 2008, Arora et al., 2008). The average daily vegetable intakes for adults and children were considered to be 0.250 and 0.150 kg/ person/day, respectively, while average body weights were taken as 55 and 25 kg of the age of 35 and 16 years respectively for adult and child.
Estimated Health Risk Index (EHRI)
Estimated health risk index (EHRI) is the ratio of estimated daily intake of metal (EDIM) to the reference dose (RD) is defined as the maximum tolerable daily intake of a specific metal that does not result in any harmful health effects. If the value of EHRI less than one than the exposed population is said to be safe and if greater than one indicating that there is a potential risk associated with that metal (IRIS, 2003) was calculated by below given equation:
Result and discussion
Metal concentration level in water and soil
The quality of domestic wastewater and ground water (tube well) was assessed for irrigation with respect to their pH, EC, OC and some of the heavy metals. The pH of the sewage water in the range of 6.8-7.3 (7.03Â±0.07) was lower than the ground water as collected from the nearby area in the ranged of 7.2- 7.9 (7.6Â±0.12) while its salt content (EC) was in the ranged of 175.8-195.3 mS/m considerably higher than those of ground waters (148.9-158.6 mS/m). The concentration for heavy metal contents in domestic wastewater and in ground water samples shows that Cd, Pb, Cu, Zn, Cr and Ni (figure 1) are well within the limits permissible limit set by FAO (1985) and PFA (2000). In general, concentrations of heavy metals were higher side in domestic wastewater than in the ground waters (Tube Well) which could be toxic to some crops and human health. In the studied area, the concentration of all the heavy metals in water and domestic wastewater were found to be higher except Zn from the permissible limits of Indian standards (PFA, 2000) and (WHO/FAO, 2007).
The total concentrations of heavy metals (Cd, Pb, Cu, Zn, Cr and Ni) in soils sampled at the three different sites are presented in figure 2. The average pH of the location-I (7.04Â±011), location-II (7.02Â±0.19) and location-III (7.0Â±0.03) soil are almost nearly neutral. The electrical conductivity (EC) was 190.2-273.6 mS/m for location-I, 202.8-247.2 mS/m for location -II and 170-271.3 mS/m for location-III. The percent organic carbon contents in soil were higher due to constant domestic wastewater irrigation and ranged from 3.48-5.2% for location-I, 3.85-5.54% for location-II and 3-69-5.6% for location-III. As the sub soils were clayey the organic carbon was found to be in high percent. Soil organic carbon is the most important indicator of soil quality and in addition to acting as a store-house of the plant nutrients plays a major role in nutrient cycling (Rattan et al., 2005).
Across the study area, wide ranges of soil heavy metal concentration were observed and are shown in figure 3. The observed concentration of heavy metal in ranged between 37.76-73.5 mg kg-1 for Cd, 2.74-22.44 mg kg-1 for Pb, 0.54-24.16 mg kg-1 for Cu, 0.96-12.44 mg kg-1 for Zn, 3.21-72.85 mg kg-1 for Cr, 1.37-6.47 mg kg-1 for Ni, and 249.3-773.4 mg kg-1 for Fe for location-I. For location-II, heavy metals ranged between 23.7-103.7 mg kg-1 for Cd, 5.67-30.99 mg kg-1 for Pb, 3.04-16.47 mg kg-1 for Cu, 3.16-9.16 mg kg-1 for Zn, 3.19-41.35 mg kg-1 for Cr and 4.68-13.77 mg kg-1 for Ni; 6.31-66.61 mg kg-1 for Cd, 3.20-19.60 mg kg-1 for Pb, 1.35-11.81 mg kg-1 for Cu, 4.47-15.55 mg kg-1 for Zn, 1.38-3.54-46.64 mg kg-1 for Cr and 1.38-9.99 mg kg-1 for Ni for location-III. The domestic wastewater irrigated soil in all locations, the heavy metal (Cd, Pb, Cu and Cr) concentrations was not significantly and for Zn and Cr is significant at (P<0.05) among the locations.
The results have clearly indicated higher concentration of metals in soils which is contributed prolonged irrigation by sewage wastewater. If the same trend continues the concentrations of metals will accumulate in the soil. Relatively higher amounts of these heavy metal which attributes to the reduction in soil pH to moderately acidic conditions as well increase in organic carbon due to the continuous use of sewage effluents. This may be a cause of prime concern in near future. Long-term application of domestic wastewater will result in increase in organic carbon and decrease in pH.
Heavy metal accumulation in vegetables
The concentrations of heavy metals in edible part of vegetables (Table 3 and figure 2) were investigated in vegetables which is commonly grown. The bioaccumulations of heavy metals concentration in all the vegetables are different, so no similar trend have been observed for heavy metal concentration. The heavy metals bioaccumulation in cauliflower was Zn > Pb > Cu > Ni > Cd > Cr, for cabbage was Pb > Zn > Cd > Cr > Cu > Ni, for carrot was Zn > Pb > Ni > Cu > Cr > Cd, for brinjal Zn > Pb > Cd > Ni > Cr > Cu, for spinach was Zn > Ni > Pb > Cd > Cu > Cr , a similar trend have also been observed by Pandey and Pandey (2009) and for radish Zn > Ni > Pb > Cr > Cu > Cd.
Cauliflower accumulated in the range of 0.15-0.27 mg kg-1 Cd, 0.14-0.34 mg kg-1 Pb, 0.7-1.75 mg kg-1 Cu, 2.9-5.9 mg kg-1 Zn, 0.10-0.35 mg kg-1 Cr and 0.01-0.60 mg kg-1 Ni, cabbage accumulated 0.30-1.20 mg kg-1 Cd, 3.37-5.37 mg kg-1 Pb, 0.06-1.10 mg kg-1 Cu, 0.74-4.90 mg kg-1 Zn, 0.06-1.23 mg kg-1 Cr and 0.01-0.34 mg kg-1 Ni, carrot accumulated 0.20-0.70 mg kg-1 Cd, 0.98-3.25 mg kg-1 Pb, 0.55-1.95 mg kg-1 Cu, 2.65-4.02 mg kg-1 Zn, 0.96-1.50 mg kg-1 Cr and 0.85-1.36 mg kg-1 Ni, brinjal accumulated 0.50-1.10 mg kg-1 Cd, 0.80-2.64 mg kg-1 Pb, 0.30-0.46 mg kg-1 Cu, 3.26-3.90 mg kg-1 Zn, 0.30-1.02 mg kg-1 Cr and 0.64-0.86 mg kg-1 Ni, spinach accumulated 0.7-1.88 mg kg-1 Cd, 1.26-3.8 mg kg-1 Pb, 0.90-1.45 mg kg-1Cu, 1.6-6.75 mg kg-1 Zn, 0.65-1.05 mg kg-1 Cr and 0.01-3.80 mg kg-1 Ni, and radish accumulated 0.22-0.38 mg kg-1 Cd, 0.64-1.24 mg kg-1 Pb, 0.19-1.30 mg kg-1 Cu, 0.56-2.70 mg kg-1 Zn, 0.76-1.25 mg kg-1 Cr and 0.21-2.40 mg kg-1 Ni.
The range of Cd concentration in brinjal (0.50-1.10 mg kg-1) recorded in this study was lower than the range (1.10 - 9.20 Î¼g g-1) reported by Sharma et al. (2006). Vegetables (spinach and cabbage), range of Ni concentration was highest in spinach (0.01-3.80 mg kg-1). These values were lower than the range (5.55 - 15.00 Î¼g g-1) reported by Sharma et al., 2006 and Singh et al., 2010 in spinach from Dinapur area as well as the range (0.2 - 3.0 mg kg-1) in spinach from waste water irrigated areas of Hyderabad reported by Sridhara Chary et al. (2008). In cabbage concentrations of Pb, Cu, Zn, Cr, Ni and were lower during the present study as compared to those obtained by Sridhara Chary et al. (2008). The present concentration (mg kg-1) of 0.30Â±0.03 for Cd, 0.94Â±0.10 for Pb, 0.83Â±0.18 for Cu and 0.94Â±0.09 for Cr in radish were lower than the values obtained for radish collected from a suburban area of Zhengzhou city, Henan Province, China (Liu et al. 2006). Khan et al. (2008) have reported higher concentrations of Cd, Pb, Cu, Cr and Ni and in radish plants grown at waste water irrigated areas of Beijing than the clean water irrigated ones. Among all the heavy metals, Zn concentration (4.26Â±0.06) was higher and Cr (0.09Â±0.07) was lower in cauliflower. Radwan & Salama (2006) have also found highest concentration of Zn in vegetables collected from Egyptian markets.
The bioaccumulation of Cd in spinach is higher than all the tested vegetable samples and similar result have also observed by Zhuang et al., 2009 in chaina. The bioaccumulation of Pb, Cu, Zn, Cr and Ni were found higher in cabbage, carrot, cauliflower, carrot and radish respectively. The results indicate that the concentrations of heavy metals in all the vegetables below the Indian standard (PFA, 2000) and (WHO/FAO, 2007).
Heavy metal transfer factor
Due to industrialization and urbanization, the heavy metal concentration of soil has increased worldwide. Soil-to- plant transfer of heavy metal is major pathway of human exposure to soil contamination (Chi et al, 2004). The high transfer factor from soil to plants indicates a strong accumulation of the particular metals by vegetable crops (Khan et al., 2008). The results indicated that TF values were lower for Cd, Pb, Cu and Cr, and higher TF for Zn and Ni from those. The transfer factor values in soil to plant of studied metals such as Cd, Pb, Cu, Zn, Cr and Ni for various vegetables varied between vegetable plants and locations are shown in table 4. The results revealed that there is large variation in transfer factor of Cd, than other metals in all three locations. The spinach (0.0306) had very high transfer factor for Cd and followed by brinjal (0.0207), cabbage (0.0165), carrot (0.0108), radish (0.0071) and cauliflower (0.0049). Similarly, The cabbage (0.4448), had very high transfer factor for Pb and followed by carrot (0.2376), spinach (0.2187), brinjal (0.0207), cauliflower (0.1672) and radish (0.0988); The carrot (0.2977) had very high transfer factor for Cu and followed by, spinach (0.2642), cauliflower (0.2537), radish (0.1740), cabbage (0.0881) and brinjal (0.0818); The carrot (0.0764) had very high transfer factor for Cr and followed by radish (0.0619), spinach (0.0527), cabbage (0.0454), brinjal (0.0408) and cauliflower (0.0054) for location-III; The cauliflower (0.2494) had very high transfer factor for Zn and followed by carrot (0.1991), brinjal (0.1909), cabbage (0.1862), spinach (0.1739) and radish (0.0984) for location-II; The spinach (0.7469) had very high transfer factor for Ni and followed by carrot (0.4451), radish (0.4542), brinjal (0.2348), cauliflower (0.0732) and cabbage (0.0335) for location-I. The transfer factor of Cd, Pb, Cu and Cr is more in Location III, similarly, Zn in location II and Ni in location-I. Therefore, vegetable crops growing on polluted site can accumulate high concentrations of trace elements to cause serious health risk to consumers.
In order to check the health risk of any toxicity from wastewater, it is necessary to estimate the level of exposure of the heavy metals through food chain. In this studied six vegetables were selected and calculate the health risk assessment in terms of estimated daily intake of metal (EDIM) and estimated health risk index (EHRI) by considering the intake of metal through vegetables by the human being.
Estimated Daily Intake of Metal (EDIM)
The daily intake of heavy metals was estimated on the basis of the average consumption and concentration of metal in particular vegetable spices. The average concentration of Cd, Pb, Cu, Zn, Cr, Ni and Fe are shown in table 3. Based on the above concentrations, the estimated daily intake of metal (EDIM) for adults and children through food chain were calculated is shown in Table 5. The highest daily intake of metal such as Pb, Zn, Ni, Cu, Cd and Cr were from spinach, cabbage, cauliflower, spinach, carrot, spinach and Carrot respectively grown in domestic wastewater for both adults and children. The results of studied revealed that EDIM suggest that the consumption of vegetables grown in domestic wastewater polluted location is high but is free from any risk as the dietary intake of Cd, Pb, Cu, Zn, Cr and Ni in adults is below than the permissible limits (WHO, 1996).
Estimated Health Risk Index (EHRI)
In order to investigate the estimated health risk index (EHRI) associated with domestic wastewater irrigated soil, it is essential to estimate the level of exposure by quantifying the route of exposure of a heavy metal to the target person. The results indicate that the EHRI values were less than one for Cd, Pb, Cu, Zn, Cr and Ni in all tested vegetables consumption. Therefore, the health risks of heavy metals such Cd, Pb, Cu, Zn, Cr and Ni exposure through vegetables was no consequences and generally assumed to be safe. All the estimated dietary intake of Cd, Pb, Cu, Zn, Cr and Ni were far below the tolerable limits. The oral reference dose (RD) for Cd, Pb, Cu, Zn, Cr and Ni are 1.0E-03, 3.5E-03, 4.0E-02, 3.0E-01, 1.5E-00 and 2.0E-02 mg kg -1 day -1, respectively given by US-EPA (2002), IRIS (2003). The results of the study showed that EDIM and EHRI suggest that consumption of vegetable grown in polluted soil irrigated with domestic wastewater is nearly free of risk. But there are also other sources of metal exposure such as dust inhalation, vehicular exhaust which were not include in this study.
The prolonged domestic wastewater irrigation increases heavy metal accumulation in the soil has lead to contamination of food crops in the study area. This study shows that edible vegetables shows significant bioaccumulation of heavy metals in vegetables grown in sewage wastewater irrigation. These vegetables are supplied to local markets and there is possibility of health hazard associated with consumption of these contaminated vegetables over a long period of time. The concentration of all heavy metals in domestic wastewater was found to be higher except Zn from the permissible limits of Indian standards (PFA, 2000) and (WHO/FAO, 2007). The results indicated that higher concentration of metals in soils which is contributed prolonged irrigation by sewage wastewater. If it is continues, the concentrations of metals will accumulate in the soil. Relatively higher amounts of these heavy metal which attributes to the reduction in soil pH to moderately acidic conditions as well increase in organic carbon due to the continuous use of sewage effluents. The concentration of heavy metals in all the vegetables is lesser than Indian standard (PFA, 2000) and (WHO/FAO, 2007). The transfer factor of Cd, Pb, Cu and Cr is more in Location III, similarly, Zn in location II and Ni in location-I. Therefore, vegetable crops growing on polluted site can bio-accumulate high concentrations of trace elements to cause serious health risk to consumers. The highest daily intake of metal such as Pb, Zn, Ni, Cu, Cd and Cr were from consumption of the spinach, cabbage, cauliflower, spinach, carrot, spinach and Carrot respectively grown in domestic wastewater for both adults and children. The results of studied revealed that EDIM and EHRI suggest that the consumption of vegetables grown in domestic wastewater polluted location is high but is free from any risk. Therefore, the health risks from the heavy metals such Cd, Pb, Cu, Zn, Cr and Ni exposure through vegetables was no consequences and generally assumed to be safe.
Therefore, in order to reduce risks, plants with lower accumulative nature should be grown. In this scenario the present study significantly indicating the need for proper treatment and disposal of domestic wastewater with low cost and worldwide acceptable technology.
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