当前世界环境

Introduction

Vegetables are vital sources of minerals, vitamins, dietary fibers, antioxidants, protein and carbohydrate.^{1, 2}Whereas, both essential and lethal elements (metals) are present in vegetables. Heavy metals in vegetables mainly occurred due to contaminated soil and irrigation.³Further, vegetables can easily absorb the metals accumulated on plant surfaces exposed to the tainted ambient.³the existence of heavy metals in fertilizers enhance the probability of metal pollution for vegetables.⁴Human activities like as farming, industrialization and urbanization amplify the level of heavy metal in the environment, as a result, elevate the metal concentration in the vegetables.⁵

Heavy metals from point sources, for instance metalliferous mining, smelting and industrial activities, on the other hand non-point source, for example fertilizers, pesticides, sewage sludge, organic manure and compost contribute to uplift the level of concentration in vegetables.⁶Emission from the industries and vehicles may be stored heavy metals on the vegetables surface during its production, transport and marketing. For instance, The level of heavy metals aggrandized into vegetables sold at Riyadh city in the Saudi Arabia due to atmospheric deposition.⁷Another reference, the amount of heavy metals rise up in the vegetables sold at open market in the Varanasi ,India because of same cases.^8,9

Plants grown in contaminated environment can aggregate heavy metals at elevated concentration causing serious risk to human health when consumed. however, heavy metals are toxic because they tend to bio accumulate in plants and animals, bioconcentrate in the food chain and attack specific organs in the body.^10、11Long time consumption of heavy metals through vegetables may causes disruption of numerous biochemical processes, Consequently occurred cardiovascular, nervous, kidney and bone diseases.^12,13Despite trace quantities of certain heavy metals such as Cr, Co, Cu, Mn and Zn etcetera are essential micronutrients for human and plants growth but their high concentration may cause health problem.^4,15Hence lot of works have been carried out to assess heavy metals bioaccumulation in different vegetables and plants all over the world.^16,17

The uptake of heavy metals in vegetables is trendy to be higher and accumulation of these toxic metals in human body generated warning in the present days. The daily vegetables consumption by an adult of Bangladesh is 130gm.¹⁸Varieties of vegetables are grown the whole year in tropical Bangladesh, but using little is known .about the heavy metal contents of vegetables.¹⁹Few articles^{18, 19}are available reporting the heavy metals concentration in different vegetables grown in various parts of our country. But such researches are not conducted in the metropolitan city of Chittagong, Bangladesh. The present study was undertaken with the aim to investigate the concentration of some specific toxic metals (Mn, Cr, Pb, Cd, Cu and Co) and essential nutrients (K, Ca, Mg, Fe and Zn) in selected vegetables available in this region.

Materials and Method

Study area and sampling locations

The present study was carried out in the Chittagong metropolitan area of Bangladesh. Samples of vegetables were randomly collected from central whole sale market of Chittagong metropolitan city named Reazuddin bazar (figure 1). All the vegetables were collected from three different locations (south, medium, and north) to provide replicate samples of each vegetable. All samples were stored in clean polyethylene bag according to their type and transported to the laboratory for further processing. The scientific, common and family name of each studied vegetables are listed (table 1).

Sample Pre-treatment and Digestion

The collected vegetables samples were washed with distilled water to remove dust particles. The samples were then cut into nearly uniform size using clean knife. This was done to facilitate drying of the pieces at the same rate. After air drying the samples were placed in clean acid washed porcelain crucibles according to the label and oven dried at 105⁰C for 24 hours until they were brittle and crispy. Now these dried samples of vegetables were grounded into fine particles using blender. The powdered samples were placed in labeled Petri dishes and dried to constant weight in desiccators until they acid digested. These samples were digested by using conc. H₂SO₄and 30% H₂O₂, in a three neck round bottom flask.

Figure 1: Map of the Chittagong metropolitan city Click here to View figure

Analytical technique

The samples were analyzed by an atomic absorption spectrophotometer (Thermo Scientific iCE 3300 AA Series Atomic Absorption Spectrophotometer), using an air acetylene flame. The analysis was carried out using hollow cathode lamps under standard instrument conditions.

Table 1: English name, Common name, scientific name and family name of different vegetables collected from Reazuddin bazar

Results and Discussion

All the twenty two vegetables sample contain more than 84% moisture except eddoe(68.35%), maximum amount of organic matter was found in eddoe (30.28%) due to presence of lots of carbohydrate whereas inorganic matter varies from 0.12%-3.94% (Table 2).

Table 2:Moisture content, organic and inorganic part of vegetables collected from Reazuddin bazar

Essential nutrients and Toxic metals (Table 3, Table 4) concentrations (mg/kg) of twenty two vegetables are depicted below. The potassium concentration in the vegetables samples varied from 572.64 mg/kg in green papaya to 4097.43 mg/kg in coriander leaf (Table 3).The concentration (mg/kg) of K is higher than the others essential nutrients because potassium is one of the main components of NPK fertilizer which is frequently used by farmer in cultivation field consequently vegetables under examination abundant with potassium since K can easily be absorbed by vegetables. Evidence Show that dietary potassium plays a role in decreasing blood pressure, controlling nerve and muscle function. Low potassium and high sodium level in food increase the probability of blood pressure.²¹

The concentration of calcium was found to be 4.99-242.56 mg/kg. All the vegetables except tomato contain reasonable amount calcium. Ca is essential for skeleton and central ion for vast range of biological process such as muscle contraction,glycolysis and gluconeogenesis ,ion transport ,cell division and growth.²²The Mg concentration (mg/kg) in most green vegetables is more than 150 mg/kg since Mg is the central element of chlorophyll structure. So that in leafy vegetables coriander leaf contain highest 4797.63 mg/kg of Mg. Lack of Magnesium can affect most of vital organ of human body.²³High content of iron were observed in cabbage (14.67mg/kg) since it is fully leafy vegetable .Iron is an inevitable element since it is the central ion for hemoglobin which is involved in oxygen and carbon dioxide transportation.²⁴A deficiency of iron influence the formation of hemoglobin consequently limits the oxygen delivery to cells, resulting in fatigue, poor working efficiency and immunity. On the reverse side, excess iron can result in toxicity and even death.^{25, 26}

Table 3: Essential nutrients concentration (mg/kg) in different vegetables collected from Reazuddin bazar

*All the values of concentration are expressed in Average±SD

Cu, Mn, Cr, Pb, Cd and Co concentration (Table 4) of twenty two vegetables collected from Reazuddin bazar, central vegetables market, Chittagong metropolitan city, Bangladesh. The

average concentration of toxic metals estimated in twenty two vegetables ranged from 0.18-2.49 mg/kg for Cu, 0.04-3.92 mg/kg for Mn, 0.05-1.59 mg/kg for Cr, 0.04-2.97 mg/kg for Pb, 0.01-0.07 mg/kg for Cd, 0.02-0.35 mg/kg for Co. Heavy metals content in the examined vegetables found in the decreasing order as Mn>Cu>Pb>Cr>Co>Cd(Table 4).Though trace amount of Mn, Cu and Cr are accounted as important electrolyte for several biological processes in human body, but higher levels of these metals can affect the major organs on consumer health.¹³Manganese plays major role in oxygen evolution catalyzed by the proteins of the photosynthetic reaction center and pyruvate carboxylase in human being and also involved in bone concentration, protein, lipid and metabolism.²²The Adequate Intake (AI) of Mn for adult men and women is 2.3mg/day and 1.8 mg/day respectively.²⁶Copper plays an important role as biocatalyst in the body [13]. Generally soil contain inadequate amount of copper so that copper is provided to plants through artificial or organic fertilizers.²⁷Further, the copper toxicity can induce lipid peroxidation, iron deficiency and destruction of cell wall [28]. The highest value of copper is found 2.49 mg/kg in string and lowest value is found 0.11mg/kg in tomato (Table 4). Chromium can cause ulceration, liver and kidney damage.^{27, 28}In addition, Chromium in trace amount controls the glucose level in blood.²⁸Chromium concentration is highest in string bean (1.59 mg/kg) and lowest in radish (0.05 mg/kg).

When Pb is up taken by crops from soil, it’s mainly stay at root area as it cannot effectively transport the endodermis of roots whereas leaves can absorb vast quantities of Pb from the atmosphere.³⁰Pb is toxic metals which have carcinogenic effects and teratogenic abnormalities in human, even at very low concentration.³¹Pb has been reported as a severe aggregative body toxin which enters the body through food, air and water, cannot be eliminated through washing vegetables.^{32, 33}In our tested vegetables all the vegetables contain huge (more than 0.50 mg/kg) amount of Pb which is really a great concern for consumer health issue. The high concentration of Pb in all the vegetables might be due to contaminants in irrigation water, soil or industrial and vehicular emission as lead occurs in the fuel as anti knocking agents.³³Artificial fertilizers are one of the major source of soil contamination by trace metals especially Cd, as it is naturally found as an impurity in phosphate rocks.^{34, 35}It was found that FAO/WHO for

Table 4: Toxic metals concentration in different vegetables collected from Reazuddin bazar

*All the values of concentration are expressed in Average±SD

The Cd concentration(Table 4) of all the vegetables were within the safe limit recommended by human consumption.³⁶The Co found in most of the vegetables were within the limit recommended by WHO/FAO [36]. Among all the tested vegetables Co was found highest in Brinjal (0.35 mg/kg) and lowest in potato (0.001 mg/kg).

Conclusion

尽管所有的蔬菜都存在的巨大源泉ntial nutrients such as K,Mg,Ca,Fe and Zn but the average concentration of heavy metals especially Mn, Cr and Pb exceeding the recommended limits whereas Cu, Cd and Co level maintained within the safe limits in all the vegetables. Consuming heavy metals contaminated vegetables for prolonged time might create different types of chronic diseases. Therefore regular monitoring of heavy metals in vegetables is crucial to avoid excessive uptake of these metals in the food chain and concern authority should take necessary steps to avoid heavy metals contamination in vegetables.

Acknowledgement

We are grateful to Bangladesh Council of Scientific and Industrial Research (BCSIR) and The Director of BCSIR Laboratories, Chittagong.

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