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American Journal of Applied Sciences
Year: 2009  |  Volume: 6  |  Issue: 3  |  Page No.: 534 - 542

Heavy Metals and Anion Levels in Some Samples of Vegetable Grown Within the Vicinity of Challawa Industrial Area, Kano State, Nigeria

J.C. Akan, F.I. Abdulrahman, V.O. Ogugbuaja and J.T. Ayodele    

Abstract: Problem section: Effluents from the surrounding industries such as tannery and textile are used by vegetable farmers for the irrigation of their crops. These effluents may contain some toxic metals which bioaccumulate along the food chain. Moreover the uptake of such toxic metals by crops is governed by their availability and concentration in the soil. Therefore such crops may accumulate heavy metals in excessive amount in their various parts. This may ultimately, adversely affect humans and other species that depend on such crops for food, hence the need to evaluate the pollutant levels in vegetables samples in these areas. Approach: Six vegetable samples of various organs were freshly harvested from ten farms within the vicinity of Challawa industrial areas. The concentration of heavy metals which include, Cu, Zn, Co, Mn, Mg, Fe, Cr, Cd As, Ni and Pb, were determined using Perkin-Elmer analyst 300 Atomic Absorption Spectroscopy (AAS). Levels of some anions (nitrate, nitrite, sulphate and phosphate were determined using Hach direct reading 2000 Spectrophotometer. Results: The results obtained from this analysis revealed that Cr and Fe shows the highest concentrations, while As shows the lowest levels in the whole vegetable organs studied. The concentrations of the anions ranged between 40.00-1300.00 µg g-1 nitrite, 65-1500 µg g-1 nitrate, 122.00-765.00µg g-1 and 12.00-60.00 µg g-1. The leaves contained much higher concentrations of heavy metals and anions than roots and stems. Conclusions: The concentrations of the above parameters were higher than the FAO, WHO/EU and FAO/WHO allowed limit. The high values might be attributed to the used of untreated effluents from textile and tanneries industries by farmers for the irrigation of these vegetables. Thus, the high values of these trace metals and anions in the vegetable samples could put the consumers of these vegetables at health risk. Further works should be carried out in the soil samples were the vegetables are grown.

Fig. 1a and b. The concentrations of heavy metals in different parts of carrot ranged from 3.66-9.23 µg g-1 Cr, 0.54-2.33 µg g-1 Mn, .00-4.22 µg g-1 Mg, 4.00-14.00 µg g-1 Fe, 0.32-2.00 µg g-1 Cu, 0.23-01.25 µg g-1 Co, 0.11-0.34 µg g-1 As, 2.00-7.44 µg g-1 Ni, 0.32-2.00 µg g-1 Pb, 2.00-4.00 µg g-1 Zn, 0.11-1.66 µg g-1 Cd for spinach. 1.00-5.00 µg g-1 Cr, 0.61-3.00 µg g-1 Mn, 0.22-3.12 µg g-1 Mg, 2.00-5.00 µg g-1 Fe, 0.02-2.00 µg g-1 Cu, 0.11-0.43 µg g-1 Co, 0.33-4.00 µg g-1 As, 1.00-4.06 µg g-1 Ni, 0.03-1.00 µg g-1 Pb, 2.00-o 4.00 µg g-1 Zn, 0.11-2.00 µg g-1 Cd for lettuce, 0.21-3.22 µg g-1 Cr, 1.54-2.43 µg g-1 Mn, 0.87-3.21 µg g-1 Mg, 1.22-4.33 µg g-1 Fe, 0.12-0.66 µg g-1 Cu, 0.05-0.54 µg g-1 Co, 0.05-0.11 µg g-1 As, 2.00-5.33 µg g-1 Ni, 0.54-2.11 µg g-1 Pb, 0.33-3.11 µg g-1 Zn, 0.01-0.44 µg g-1 Cd for cabbage Fig. 1a 2.44-0 7.22 µg g-1 Cr, 0.22-1.22 µg g-1 Mn, 0.12-2.44 µg g-1 Mg, 2.33-8.23 µg g-1 Fe, 0.22-1.42 µg g-1 Cu, 0.11-0.63 µg g-1 Co, 0.02-0.11 µg g-1 As, 1.02-6.33 µg g-1 Ni, 0.22-1.23 µg g-1 Pb, 1.07-3.22 µg g-1 Zn, 0.11-1.34 µg g-1 Cd carrot.3.00-12.00 µg g-1 Cr, 1.00-6.00 µg g-1 Mn, 0.22-4.00 µg g-1 Mg, 2.00-10.00 µg g-1 Fe, 0.34-2.23 µg g-1 Cu, 0.03-1.33 µg g-1 Co, 0.12-1.87 µg g-1 As, 1.55-5.67 µg g-1 Ni, 2.00-6.66 µg g-1 Pb, 2.00-6.00 µg g-1 Zn, 0.22-1.00 µg g-1 Cd tomato. 3.00-7.00 µg g-1 Cr, 1.00-4.00 µg g-1 Mn, 0.22-3.00 µg g-1 Mg, 2.00-7.00 µg g-1 Fe, 0.34-1.00 µg g-1 Cu, 0.03-0.22 µg g-1 Co, 0.12-1.00 µg g-1 As, 1.55-4.33 µg g-1 Ni, 2.00-5.33 µg g-1 Pb, 2.00-4.55 µg g-1 Zn, 0.22-0.45 µg g-1 Cd onion Fig. 1b.

The mean metals concentrations in different parts of vegetable samples during the rainy season are as presented in Fig. 2a and b. The concentrations of heavy metals in the vegetable samples range between 0.21-3.54 µg g-1 Cr, 0.20-1.34 µg g-1 Mn, 0.13-2.00 µg g-1 Mg, 0.56-6.45 µg g-1 Fe, 0.02-1.33 µg g-1 Cu, 0.20-0.57 µg g-1 Co, 0.01-2.00 µg g-1 As, 0.50-3.22 µg g-1 Ni, 0.01-1.01 µg g-1 Pb, 0.33-2.00 µg g-1 Zn, 0.01-1.00 µg g-1 Cd.

From Fig. 1, 2a and b, the maximum concentration of chromium was in spinach (1.00-9.23 µg g-1) and the minimum in lettuce (0.21-3.33 µg g-1). Mn was found in tomato (1.00-6.00 µg g-1) while cabbage had the least concentrations (0.22-1.22 µg g-1). The highest levels of magnesium were in spinach (1.00-4.22 µg g-1) and the least were observed in cabbage (1.00-2.44 µg g-1). Iron had the maximum concentration in spinach (4.00-14.00 µg g-1) and minimum in cabbage (1.22-4.33 µg g-1). Copper content was relatively higher in tomato (0.34-2.23 µg g-1) while carrot had the least (0.10-1.42 µg g-1). Cobalt level was highest in spinach (0.23-1.23 µg g-1) and least in onion (0.03-0.11 µg g-1). Arsenic showed maximum concentration in lettuce (0.33-4.00 µg g-1) and the minimum in cabbage (0.02-0.11 µg g-1). Nickel concentration was highest in spinach (2.00-7.44 µg g-1) and lettuce shows the least (1.00-4.06 µg g-1). Lead showed the maximum concentrations in tomato (2.00-6.66 µg g-1) and the minimum concentrations in lettuce (0.03-1.00 µg g-1). The maximum concentrations of zinc were found in tomato (2.00-6.00 µg g-1) and the minimum in cabbage (0.33-3.11 µg g-1). Cadmium showed maximum concentration in lettuce (0.11-2.00 µg g-1) and the minimum in cabbage (0.01-0.44 µg g-1).

The mean concentrations of nitrate for all the organs of different vegetable samples in the dry and rainy seasons are presented in Fig. 2. The concentrations of nitrate ranged from 244.00-432.00 µg g-1 carrot, 500.00-1300.00 µg g-1 spinach, 444.00-765.00 µg g-1 lettuce, 232.00-377.00 µg g-1 cabbage, 40.00-133.00 µg g-1 tomato and 477.00-633.00 µg g-1 onion (dry season), while rainy season ranged from 174.00-237.00 µg g-1 carrot, 310.00-543.00 µg g-1 spinach, 273.00-436.00 µg g- lettuce, 123.00-231.00 µg g-1 cabbage, 38.00-121.00 µg g-1 tomato and 237.00-394.00 µg g-1 onion. Figure 4 show the concentrations of nitrite in vegetable samples in the dry and rainy seasons, which ranged from 365.00-533.00 µg g-1 carrot, 600.00-1500.00 µg g-1 spinach, 522.00-867 µg g-1 lettuce, 256.00-433.00 µg g-1 cabbage, 65.00-211.00 µg g-1 tomato and 544.00-700.00 µg g-1 onion (dry season) and rainy season ranged from 122.00-212.00 µg g-1 carrot, 253.00-474.00 µg g-1 spinach, 222.00-352 µg g-1 lettuce, 121.00-187.00 µg g-1 cabbage, 33.00-63.00 µg g-1 tomato and 232.00-376.00 µg g-1 onion. For sulphate concentration (Fig. 5), carrot ranged from 412.00 to476.00 µg g-1, 743.00-1765.00 µg g-1 spinach, 588.00-900.00 µg g-1 lettuce, 311.00-511.00 µg g-1 cabbage, 122.00-344.00 µg g-1 tomato and 765.00-899 00 µg g-1 onion (dry season), while rainy season ranged from 206.00-293.00 µg g-1 carrot, 372.00-812.00 µg g-1 spinach, 253.00-456 µg g-1 lettuce, 156.00-253.00 µg g-1 cabbage, 112.00-156.00 µg g-1 tomato and 385.00-453.00 µg g-1 onion.

Fig. 1a: Trace element concentrations (μg g-1) in different parts of vegetable samples (Spinach, lettuce and cabbage) in dry season from kano industrial area

Fig. 1b: Trace element concentrations (μg g-1) in different parts of vegetable samples (Carrot, tomato and onion) in dry season from kano industrial area

Fig. 2a: Mean metals concentrations (μg g-1) in different parts of vegetable samples (Spinach, lettuce and cabbage) in rainy season from kano industrial area

Fig. 2b: Mean metals concentrations (μg g-1) in different parts of vegetable samples (Carrot, tomato and onion) in rainy season from kano industrial area

Fig. 3: Seasonal variation of Nitrate concentrations (μg g-1) in different parts of vegetable samples from kano industrial area

Fig. 4: Seasonal variation of Nitrate concentrations (μg g-1) in different parts of vegetable samples from kano industrial area

Fig. 5: Seasonal variation of Sulphate concentrations (μg g-1) in different parts of vegetable samples from kano industrial area

Phosphate levels ranged from 42.00-80.00 µg g-1 carrot, 30.00-60.00 µg g-1 spinach, 22.00-80.00 µg g-1 lettuce, 30.00-55.00 µg g-1 cabbage, 12.00-45.00 µg g-1 tomato, 21.00-35.00 µg g-1 onion (dry season) and rainy season ranged from 42.00-80.00 µg g-1 carrot, 30.00-60.00 µg g-1 spinach, 22.00-80.00 µg g-1 lettuce, 30.00-55.00 µg g-1 cabbage, 23.00-45.00 µg g-1 tomato and 21.00-44.00 µg g-1 onion Fig. 6.

Fig. 6: Seasonal variation of Phosphate concentrations (μg g-1) in different parts of vegetable samples from kano industrial area

From Fig. 3, the maximum concentration of nitrate was found in spinach (543.00-1500.00 µg g-1) and the minimum in tomato (38.00-211.00 µg g-1). Nitrite had the maximum concentration in spinach (478.00-1300 µg g-1) and minimum in tomato (33.00-344.00 µg g-1) Fig. 4. Sulphate content was relatively higher in spinach (812.00-1765.00 µg g-1) while tomato shows the least values (112.00-344.00 µg g-1) Fig. 5. Phosphate showed the maximum concentrations in carrot (80.00-80.00 µg g-1) and the minimum concentrations in tomato (12.00-45.00 µg g-1) Fig. 6.

DISCUSSION

In the whole plants studied during the dry and rainy season, leaves contained higher concentrations of heavy metals than roots and stems. Similar study carried out by[9] shows that the heavy metal content of various parts of plant differs. They reported that in vegetables organs the concentrations of heavy metals are in the order of leaf> stem> root> tuber> bulb> fruit> seed. [10]reported high level of heavy metals (Pb, Fe, Cu and Zn) in their study of the long term effects of municipal waste disposal on soil properties and productivity of sites used for urban agriculture in Abakaliki, South eastern part of Nigeria. [11] studied plant uptake of heavy metals on a similar site at University of Ife dump site and reported that Pb uptake by water leaf (Talinum triangulare), okra (Albennucus esculentus) increased in leaves and roots of water leaf and in the fruit of okra relative-those grown in the non-dump sites. Similar work by[12] reported that vegetables accumulate considerable amount of heavy metals especially Pb, Cr, Cu, Zn in roots and leaves.

The concentrations of heavy metals in all the vegetable organs in the dry season were higher than the rainy season. The high concentration of heavy metals in the dry season when compared with the rainy season might be due-the use of untreated effluents from textile and tanneries by farmers for the irrigation of these vegetables during the dry season[6,13]. The concentrations of heavy metals in all the vegetable samples in the dry and rainy season were higher than the FAO/WHO guideline values of 0.1-0.2 µg g-1 Cr, 0.3 µg g-1 Fe, 0.1 µg g-1 Pb, 0.1 µg g-1 Cu, 0.1 µg g-1 Zn, 0.1 µg g-1 Ni, 0.02 µg g-1 Cd and 0.3 µg g-1 Mn, with exception of Co in the rainy season. The presence of these metals in the vegetable samples during the rainy season might be due-some metal residues left behind in the dry season farming. The result of this study agreed with the data reported by[14]. Results from present and earlier reports[15] demonstrated that plants grown on wastewater-irrigated soils are contaminated with heavy metals and pose health concern. Absorption and accumulation of heavy metals in plant tissues depend upon many factors. These include temperature, moisture, organic matter, pH and nutrient availability. The wastewater from tanneries and textile industries used for irrigation showed higher organic matter. The sites irrigated with textile and tanneries effluents also showed similar contamination levels for heavy metals, hence the high content of organic matter. Organic complexing molecules of Low Molecular Weight (LMW) serve as carriers of micronutrients[15]. Low Molecular Weight (LMW) has been shown-increase heavy metals uptake[16], while the presence of organic matter has been reported-increase the uptake of zinc, chromium, lead, iron and copper in the wheat plant[16].

The alkalinity range of soil is known-restrict the mobilization of heavy metals, thus reducing their uptake. However, the high nutrient input of irrigated water from these sites could contribute-the high uptake of heavy metals. The field data support this argument in that a higher uptake was recorded at the time when these vegetables were irrigated with textile and tanneries effluent as compared-the time when these effluents were not used. Long-term application of waste effluent is associated with increase in plant uptake and bioavailability of heavy metals[15]. In the present study many soil factors such as pH, organic matter, nitrogen bioavailability, soil moisture and temperature have interacted-impact on uptake.

The values of nitrate, nitrite, sulphate and phosphate in the vegetable samples show that the leaves are rich in this anion content than other organs studied. Similar study was carried out by [9] stated that nitrate and nitrite content of various parts of a plant differs in the order of leaf> stem> root> tuber> bulb> fruit> seed. [17]reported that vegetables that are consumed with their roots, stems and leaves have a high nitrate and nitrite accumulation, whereas melons and those vegetables with only fruits as consumable parts have a low nitrate accumulation This observation was also noted by[9] where leaf and stem accumulate the most nitrate, sulphate and nitrite followed by stem and roots. [18]in their study, Nitrate and nitrite accumulation in vegetables, noted that leafy vegetables such as spinachand lettuce contains nitrate, sulphate and nitrite at significant levels. They noted that plants that develop fruits or storage organs, such as potato and tomato, usually have low nitrite and nitrate concentrations. Nitrite content in vegetables is usually very low compared-nitrate[19]. Result from this study agreed with the above observations. The concentrations of these anions were higher in the dry season when compared-rainy season. The high levels of these anions in the dry season is attributed-the used of untreated effluents from textile and tanneries by farmers for the irrigation of these vegetables, while the low concentrations of these anions during the rainy season are due-the fact that this effluent are not used for the irrigation of these vegetables. The concentrations of these anions in all the vegetable organs study during the dry season were higher than the limits set by WHO. Result of analysis of variance (ANOVA) showed that variation between vegetables and organs were statistically significant (p<0.05).

CONCLUSION

The results indicate that all the vegetable samples analyzed in this study had high levels of heavy metals and anions. Heavy metal levels were higher than those recommended by Food and Agricultural Organization (FAO) and the WHO/EU joint limits. The high levels of these heavy metals and anions place the consumers of these and other vegetable crops grown within vicinity of Challawa industrial area at health risk with time unless an urgent step is taken by relevant agencies-address this issue.

ACKNOWLEDGEMENT

I would like-acknowledge Mr. Fine Akawo for his laboratory assistance.

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