Heavy metal concentration in root vegetables

Concentrations of 8 different heavy metals (Cr, Cd, Pb, Ni, Cu, Zn, Fe, and Mn) in 4 frequently consumed root vegetable species are determined (Table 2). The root vegetables studied for heavy metal are beet, radish, carrot, and turnip. All heavy metal concentrations are expressed on a dry weight basis. Also, the effects of early (1st phase), mid-time (2nd and 3rd phases), and late (4th phase) harvesting on the metal concentrations are evaluated.

Table 2 Concentration of heavy metals in root vegetables

In the case of chromium (Cr), the highest concentration (49.60 mg/kg dw) was recorded in turnip that was harvested in the late-season (4th phase) among all four root vegetables. On the other hand, the lowest chromium concentration (0.50 mg/kg dw) was recorded in the carrot that was harvested in the mid-time season (2nd phase). The concentration of Cr was also found to be higher in turnip collected in the 1st and 4th phases. The overall concentration of Cr was found to be in turnip > carrot > radish > beet order. The very high Cr value (49.60 mg/kg dw) made us concerned, and we did the analysis several times and found a similar value. This high value can be ascribed to the high use of preservatives at the end of the season. One study from Bangladesh also observed lower concentrations of heavy metals in carrots collected from 30 agro-ecological zones of Bangladesh (Shaheen et al. 2016). Among all four types of root vegetables, the only turnip exceeded the maximum permissible limit (MPL) over the season.

Average cadmium (Cd) concentration exceeded the MPL in all root vegetables except turnip. Our data also shows that beet, radish, and carrot harvested during the mid-time season (3rd phase) had higher heavy metal content than all other harvesting periods. On the contrary, late-harvested root vegetables resulted in having the lowest heavy metal concentrations. While the highest Cd concentrations were recorded in radish and carrot (0.50 mg/kg dw), turnip had the lowest concentration (0 mg/kg dw) overall. Some recent studies from Bangladesh also reported the Cd concentrations found to be 0.001–1.60 mg/kg in 12 vegetable species (Islam et al. 2015), 0.006–0.3 mg/kg in 16 vegetable species (Rahman et al. 2013), and 0.60 mg/kg in 5 vegetable species (Ahmad and Goni 2010).

Lead (Pb) contamination was relatively lower than the other heavy metals for the root vegetables. The lead was detected only in cases of carrot (3.0 mg/kg dw) and turnip (1.0 mg/kg dw) harvested during the 2nd phase and exceeded the MPL as well. Otherwise, there was no lead contamination. Pb concentration in five commonly grown vegetables (eggplant, chili, ladies’ finger, tomato, and green cabbage) was reported by Ahmad and Goni (2010) to be 3.9 mg/kg (Ahmad and Goni 2010).

Nickel (Ni) contamination in the root vegetables was also within the maximum permissible limit except for radish (3.30 mg/kg dw) harvested in the early season and turnip (4.50 mg/kg dw) harvested in the mid-time (3rd phase) season. Surprisingly, all the root vegetables collected in 2nd phase did not contain any of those heavy metals. The highest Ni concentration was recorded in turnip (4.50 mg/kg dw) harvested in the mid-time (3rd phase) season. Recent literature reported Ni concentrations in vegetables from different regions of Bangladesh as 0.02–12 mg/kg in 12 vegetable species (Islam et al. 2015), 0.3–4.7 mg/kg in 16 vegetable species (Rahman et al. 2013), and 3.0 mg/kg in 5 vegetable species (Ahmad and Goni 2010).

Among all four root vegetables, the beet only exceeded the MPL of copper (Cu) set by the FAO/WHO. While the highest Cu concentration was recorded in beet (19.30 mg/kg dw) during the 3rd phase, radish had the lowest concentration (2.0 mg/kg dw) during the 2nd phase. A study from Bangladesh reported the Cu concentration in common vegetables (tomato, green chili, brinjal, bean, potato, onion, and carrot) in the range of 2.254–9.718 mg/kg (Shaheen et al. 2016).

The concentration of zinc (Zn) was also measured in all four species of root vegetables. During the early season harvesting, all except carrot exceeded the MPL. Maximum Zn concentration was found in radish (68.03 mg/kg dw) during the late harvesting season. The lowest concentration was found in carrots (7.28 mg/kg dw) during the 3rd phase of harvesting. The average Zn concentration only in the beet exceeded the MPL throughout the season. A previously mentioned study also found the lowest concentration of Zn in carrot 0.074–4.75 mg/kg (Shaheen et al. 2016). In another study, the median concentration of Zn in common vegetables (16 vegetable species) in Bangladesh was reported to be 50 mg/kg (Rahman et al. 2013).

Iron (Fe) concentration was significantly higher in all root vegetables concerning other heavy metals that are analyzed in this study. But under no circumstances the concentration of Fe exceeded the maximum permissible limit. The highest concentration of Fe was found in late-harvested beet (446.4 mg/kg dw), while the lowest was found in radish (37.4 mg/kg dw) harvested during the mid-time season (2nd phase). The overall concentration of Fe was found to be in beet > radish > carrot > turnip order. One study from Tanzania has analyzed four different vegetables (potato leaves, African spinach, ladies’ finger, and brinjal) and observed a mean Fe concentration of 48.40–136.40 mg/kg (Kacholi and Sahu 2018).

Like Fe, manganese (Mn) also did not exceed the MPL in any of the root vegetables and was well below the MPL. Maximum Mn concentration was found in radish (64.7 mg/kg dw) during the early harvesting season. The overall concentration of Mn was found to be in radish > beet > turnip > carrot order. In a study, the median concentration of Mn in common vegetables in Bangladesh was reported to be 65 mg/kg (Rahman et al. 2013).

Heavy metal concentration in leafy vegetables

Five different leafy vegetables (mustard, cabbage, spinach, coriander, and mint) were studied for 8 different heavy metals (Cr, Cd, Pb, Ni, Cu, Zn, Fe, and Mn). Mean concentrations of the heavy metals in five studied leafy vegetables revealed significant variation (p < 0.05) for Cd, Ni, Zn, and Fe but revealed no variation for Cr, Pb, Cu, and Mn (Table 3). Table 3 summarizes the metal concentrations found in all leafy vegetable samples throughout the season.

Table 3 Concentration of heavy metals in leafy vegetables

Maximum Cr concentration was found in mint (16.7 mg/kg dw) harvested in the 3rd phase, while the lowest concentration was recorded in cabbage (0 mg/kg dw) harvested in the 2nd phase. The average concentration of Cr exceeded the MPL in mustard (4.70 mg/kg dw), coriander (3.88 mg/kg dw), and mint (6.18 mg/kg dw). Also, all the leafy vegetables harvested in the late season exceeded the MPL of Cr concentration. Besides, mustard has exceeded the MPL in all harvesting seasons.

Higher Cd contamination was observed during the first two phases of harvesting compared to the latter two. All the leafy vegetables harvested during the 2nd phase exceeded the MPL. The highest concentration of Cd was found in coriander (1.80 mg/kg dw) harvested in the 2nd phase. Also, the concentration of Cd in coriander exceeded the MPL in all harvesting seasons.

Though there was no Pb contamination found in early harvested leafy vegetables, the average concentrations of Pb exceeded the MPL in all leafy vegetables by the end of the season. Maximum Pb concentration was found in mint (6.0 mg/kg dw) harvested in the 3rd phase. Overall concentrations of Pb in the leafy vegetables are found to be in mint > coriander > mustard > cabbage > spinach order.

Contamination by Ni was relatively lower than the contamination by chromium, cadmium, and lead. During the first two phases, the Ni concentrations were below the MPL set by FAO/WHO. Considering all four phases, the average Ni concentration only in mustard (3.23 mg/kg dw) and coriander (3.05 mg/kg dw) exceeded the MPL. The highest concentration of Ni was recorded in coriander (4.50 mg/kg dw) harvested in the late season. Research from Sri Lanka also found that Ni, Cd, Cr, and Pb levels exceeded the MPL set by FAO/WHO (2001) in five different green leafy vegetables (Kananke et al. 2014).

Maximum Cu concentration was found in mustard (44.30 mg/kg dw) harvested in the mid-time season (3rd phase), while the lowest concentration was recorded in cabbage (1.50 mg/kg dw) harvested in the early season. Also, Cu contaminations were much higher in the second half of the harvesting season than in the first half. Considering all four phases, the average Cu concentration in mustard (18.3 mg/kg dw), coriander (14.5 mg/kg dw), and mint (12.7 mg/kg dw) exceeded the MPL. Besides, mint has exceeded the MPL in all harvesting seasons.

Zn has also demonstrated higher concentrations in the second half of the harvesting season than in the first half. The average Zn concentrations in all leafy vegetables except cabbage have exceeded the MPL. The highest concentration of Zn was found in spinach (233.82 mg/kg dw) harvested in the early season, while the lowest concentration was found in cabbage (12.25 mg/kg dw) harvested in the late season. The average concentration of Zn decreased in the order of spinach > coriander > mustard > mint > cabbage.

Fe concentration was significantly higher in all leafy vegetables concerning other heavy metals analyzed in this study. The highest concentration of Fe was found in late-harvested coriander (2365.7 mg/kg dw), while the lowest was found in cabbage (63.7 mg/kg dw) harvested during the mid-time season (3rd phase). Also, coriander and mint have exceeded the MPL in all harvesting seasons. One study reported that the average concentration of Fe, Pb, and Cd in the leafy vegetables collected from the Satkhira district of Bangladesh exceeded the permissible limit (Uddin and Dhar 2019). Overall concentrations of Fe have decreased in the order of coriander > mustard > mint > spinach > cabbage order.

Under no phases and conditions, Mn has exceeded the MPL in any leafy vegetables. Maximum Mn concentration was found in mustard (389.30 mg/kg dw) harvested in the mid-time season (3rd phase). The lowest concentration was found in cabbage (13.7 mg/kg dw) harvested in the early season.

The major sources of heavy metals in the studied root and leafy vegetables might be due to soil contamination. Besides, solid waste disposal, sludge applications, vehicular exhaust, and agrochemicals are the most prominent reasons behind soil contamination. Therefore, contamination of agricultural soils through these anthropogenic activities leads to excessive heavy metal uptake by vegetables, which in turn affects food quality and safety (Muchuweti et al. 2006). And the difference in heavy metals among different phases of sampling can be attributed to the land in which the particular vegetable had grown, probably having a different level of contamination, or at the end of the season when there is scarcity for a particular vegetable, the tendency to use preservative increases.

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