Heavy Metals Contents and Their Mobility in Various Land Uses
for Vegetables Cultivating in Bali, Indonesia
I Made Adnyana
1*and I Made Siaka
21. Agricultural Soil Department of Agricultural Faculty, Udayana University, Jl. P.B. Sudirman Denpasar-Bali, Indonesia
2. Chemistry Department of Maths and Sciences Faculty, Udayana University, Bukit Jimbaran Campus -Bali, Indonesia.
Abstract:A study on the mobility of heavy metals in various types of land uses for vegetables cultivating at Candi Kuning, Bedugul, Bali has been carried out. Heavy metals contaminating agricultural soils could contaminate the vegetables growing on the lands. The movements of heavy metals to the vegetables depend on their mobility and the land characteristics. This study was aimed to determine the mobility of heavy metals being available in the lands uses by investigating the most bioavailable fractions of the metals under study. Digestion and sequential extraction methods were applied to establish the mobility of heavy metals. All metals measurements were carried out with the use of AAS. The results showed that the concentrations of total Pb, Cu, Cd, Cr, and Zn were in the range of 27.952-102.248, 112.759-179.664, 4.593-16.201, 5.512-36.473, and 110.289-238.498 mg/kg, respectively. The mobility of each metal in the soils was varied. The mobility of Pb (the highest) was found in the soil for cultivating potatoes (37.13%), Cu and Zn in soil for lettuce (25.68 and 36.65%), Cd and Cr in soil for tomatoes (31.61 and 42.38%). Statistically, the mobility of the metals was significantly different (p<0.05) among the metals themselves and the land uses.
Keywords:heavy metals, mobility, land use, vegetable soils.
1
Introduction
Candi Kuning is one village located in the region of Bedugul, Bali - Indonesia which is well known as the center for vegetables and fruits production. This area produces vegetables intensively in order to meet the increase needs of the market. This causes the farmers stepping up the production of the vegetables in such a way so they could get more products with better performances. For such purposes, there are several ways they usually do such as applying agrochemicals excessively (inorganic fertilizers and synthetic pesticides) in addition to using organic fertilizers. Unfortunately the farmers are not aware to the destructive impacts of agrochemicals to both vegetables and the soil itself.
Some researchers suggested that the increased use of inorganic fertilizers, organic fertilizers, and synthetic pesticides can cause soil and plants contaminated by chemical substances, including heavy metals such as As, Pb, Cd, Cu, Co, Cr, Mo, Sr, Ti, V, Mn, Fe, Ni, Hg, Ba, Sc, and Zn while some elements such as Cu, Zn, Mn and Fe in low concentrations are essential micro-nutrients[1-7]. Heavy metals in agricultural soils could be potentially absorbed by the plants in the soil and accumulated in plant parts such as in fruits, leaves, stems, and roots. The concentration, types, and mobility of the metals in the soil are factors that control the accumulation of heavy metals in plants[8]and this has been proved by Hindersah, et al. that there was a positive correlation between Pb and Cd accumulation in tomato fruits grown in soil that was applied with doses of domestic dried sludge[9].
mineral phases[15-17]. With the exception of the stable mineral phases, such as silicates, all solid phases associated with heavy metals are bioavailable and potentially bioavailale, so this will be closely related to the mobility of these metals .
Mobility is the capacity of an element to move in the liquid after the dissolution. There are several factors that can control the mobility including pH, reaction solubility, absorption reactions, and redox state. Oxidation state of an element greatly affects mobility and interaction of the element[18]. Mobility of metals in the soil can be determined by running metal speciation/fractionation as proposed by Smith, inwhich understanding the metal speciation is the key for understanding the mobility of the metals as well as understanding the bioavailability and toxicity of the metals themselves[18]. The information about the total heavy metals contained in soils does not provide adequate information for both bioavailability and mobility of these metals. Therefore, the determination of species (speciation) to the presence of these metals in the soils is required prior to understanding their interactions with the biotic components in the environment. For this purpose, the knowledge of partitioning of these metals will be very useful to see the species and thebiological availability[19].
Some researchers have suggested a sequential chemical extraction technique for performing speciation of metals associated in various chemical phases in the soils[16,20-26]. By applying this technique, various metal fractions in the soils such as ionic and carbonate fractions (EFLE: Easily, freely, leacable, and exchangeable), Fe/Mn oxide (reducible), organic and sulfide (oxidizable), and stable minerals (inert) fractions could be determined. These informations could be interpreted for establishing the bioavailability and mobility of the metals. From this, the amount of heavy metals absorbed by the vegetables growing on the lands in the up-take process could be predicted.
Based on the above informations, it is necessary to analyze the total contents of Pb, Cd, Cu, Cr, and Zn in various types of lands for cultivating some vegetables such as tomatoes, lettuce, spring onion, potatoes, and carrots in the region of Candi Kuning, Bedugul. In addition, the heavy metals speciation were also required for determining the chemical phases of the metals and therefore, the mobility of those heavy metals in such lands could be determined as well.
2
Materials and Methods
2.1 Chemicals and Apparatus
All chemicals used for experiment were of analytical grades. The stock solutions of 1,000 mg/L Pb, Cu, Cd, Cr, and Zn standards were prepared from Pb(CH3COO)2, CuSO4.5H2O, Cd(NO3)2, K2Cr2O7, ZnSO4.7H2O respectively. Digested
solution of reverse aqua regia was made by mixing HNO3and HCl (3:1). Sequential extraction was achieved with the
use of NH2OH.HCl, H2O2, and CH3COOH. Dilute solutions of the metals were prepared with distilled water, and all
filtrations were carried out with Whatman No. 42 filter paper.
Ultrasonic bath (Elma S 450H, Elmasonic) and a hotplate were used for sample digestions, as well as, shaker was used for sequential extraction. All flame-AAS measurements were performed using a Shimadzu Model AA-7000 operated in a double beam mode, and the background correction was accomplished with a deuterium lamp.
2.2 Study Site and Sampling
vegetable land. Each sample was taken from the surface soil (0 – 20 cm) at specific typical vegetable land and is defined as a composite one from eight sample points of each part. All soil samples were air dried at ambient temperature and portions of them were ground into powder. A 63 µm nylon mesh screen was used for sieving the powdered samples to separate the coarse and fine fractions. The homogeneity fine fractions were stored in plastic bags at room temperature prior to chemical analysis.
2.3 Sample Digestion
The fine powdered samples (1 g each) were accurately weighed and dissolved in 10 mL of reverse aqua regia (HNO3-HCl, 3:1). The samples were digested by using an ultrasonic bath at 60oC for 45 minutes followed by a hotplate
treatment at 140oC for 45 minutes[28]. The digestion of samples was performed in triplicate. Each sample solution was filtered through Whatman filter paper No. 42 and made up to a final volume of 50 mL with distilled water. The determination of Pb, Cu, Cd, Cr, and Zn was then carried out using flame-AAS (Shimadzu AA-7000). Quantification of each metal was calculated involving calibration curve method.
2.4 Sequential Extraction
Sequential extraction comprises four steps extraction procedures and the three-step of the sequential extraction procedure developed by Davidsonet al. and Thomaset al.[29-31]was followed to fractionate the soil phase-bound trace metals into three fractions (F1, F2, and F3), while the residuals were treated with the acid digestion method described in section 2.3 for quantification of F4. Each fraction shows the bound of the metals to the soil phase, including F1 of carbonates, exchangeable, water and acid soluble metals, F2 of Fe/Mn oxides and reducible metals, F3 of metals bound to organic matter, oxidisable, and metal sulfides, and F4 of residual metals such as silicates. The result of these fractions was used to establish the bioavailability and predict the mobility of heavy metals in soils. The extraction steps/fractions, reagent used in each step, and soil phases were summarized in Table 1. Each extraction step was performed in triplicate. All the extracts were centrifuged for 10 minutes at 3000 rpm, and the supernatants filtered through Whatman filter paper No. 42. Metals present in extracts were determined by flame-AAS (Shimadzu AA-7000).
Table 1 Reagents used at each step in the sequential extraction procedure and the soil phases extracted
Extraction step *Digestion of the residual material was followed digestion method described in section 2.3[28].
3
Results and Discussion
3.1 Total heavy metals in various types of vegetable lands
in the normal value for rural areas except that the land for cultivating lettuce has a slightly higher Pb. However, this content could potentially contaminate the vegetables produced in this area with lead, as reported by Siakaet al. that is, all the vegetables that were investigated from the Bedugul area containing Pb exceeds the maximum allowed by FAO/WHO[32].
The average of total Cu found in five types of soils under study ranged from 112.759 to 179.664 mg/kg. According to Aloway, this metal concentration was in the concentration range for agricultural land (2-250 mg/kg), however Cu in all five lands were found to be exceeded the maximum value of common value for farming in the rural area (20-30 mg/kg)[1]. The source of copper in the agricultural land may be derived from agricultural activities themselves, as a result of intensive use of agrochemicals. The highest Cu content was found in the soil of potatoes, followed by soils of spring onion, lettuce, carrots, and tomatoes (as seen in Table 2). The presence of Cu in soils is actually very useful because Cu is an essential metal. However, if the amount of this metal is high, it could be a threat for health as this metal can be absorbed by the vegetables grown in the soil containing a large amount of the metal or the metal in the soil may exceed the maximum allowed limit. In fact, Siakaet al. reported that Cu contained in ten types of vegetables investigated were within the allowed value, except for Cu in cabbage and lettuce which were about 2-6 times the maximum value allowed by FAO/WHO[32]. This is highly dependent on the nature of the metal itself and its availability in the soils.
In contrast to Pb and Cu, the highest Cd content, which was 16.201 mg/kg (presented in Table 2), was found in the soil for tomato. The concentrations order of cadmium in the five vegetable soils is as follows: Cd concentration in tomato soil > lettuce soil > potato soil > carrots soil > spring onion soil. In general, the average concentration of Cd in soils investigated was above the range for both common value (0.2 1 mg/kg) and range for agricultural soils (0.01 -2.4 mg/kg )[1]. This revealed that Cd could be a threat to health, if it in high availability therefore, it could contaminate the vegetables grown in the soils. In addition, Cd can also compete with Zn or in the absence of Zn, Cd substitutes this metal resulting in malfunctions in metabolic processes. Cd is located in the same group with Zn in the periodic table of elements, so that the metals have similar properties[33]. Absorption of Cd by plants including vegetables is highly dependent on the bioavailability of the metal. The introduction of Cd into agricultural lands could be derived from the use of synthetic pesticides, inorganic fertilizers and manure[1-7]intensively in the area.
Chromium found in the vegetable lands in Bedugul area ranged from 5.512 to 36.473 mg/kg. These concentrations were below the lowest common value Cr range in agricultural soils,i.e. 70-100 mg/kg[1]. The highest Cr
concentration was found in the soil for tomato while the lowest concentration was in lettuce soil. Although Cr content was low enough, but a preliminary research found that the content of Cr in 8 out of 10 types of vegetables exceeded the maximum limit allowed by FAO/WHO[32]. This indicated that the chromium in the soil has a high bioavailability so that at high concentration this metal could be absorbed by the vegetables.
regulation[32].
Table 2 The average of total heavy metals containing in various vegetable soils
Soils for
Tomato 27.952 0.088 112.759 0.223 16.201 0.224 36.473 0.143 212.545 0.620
Lettuce 102.248 0.014 156.126 0.077 9.462 0.165 5.512 0.008 238.498 0.831
Spring onion 34.928 0.024 174.871 0.005 4.593 0.005 20.612 0.057 183.808 0.367
Potatoes 29.596 0.338 179.664 0.681 8.387 0.681 16.587 0.091 129.487 1.103
Carrot 31.198 0.059 119.421 0.018 4.916 0.018 18.046 0.022 110.289 0.019
3.2 Fractionation of heavy metals in different types of lands for vegetable cultivation
Fractionation results of Pb, Cu, Cd, Cr, and Zn in soil cultivated with tomato, lettuce, spring onion, potatoes, and carrots are presented in Table 3. Fractionation of the metals is the result of a sequential extraction of four steps in accordance with the strength of the metal bonding to the soil particles. From the table, it can be seen that the tomatoes soil had the highest Zn and Cr extracted in F1, while the other metals, Pb, Cu, and Zn in F1 were found to be the highest in lettuce soil. F1 is the fraction that has the weakest bond so that the metals in this fraction are easily separated from the bonding in soil particles. Heavy metals in this fraction are classified as dissolved metals or ionic forms, metals bound to carbonate, and exchangeable metals[29-31]. Therefore, these metals are very unstable so they can be readily available for the plants growing on the soil. It was evident that the soils showing the highest total metal contents they also showed the highest bioavailable fractions, except that Cu showed the highest total metal content in the soil for potatoes.
Fraction 2 and 3 are the fractions of the heavy metals that are more tightly bound to the soil particles than that of in fraction 1 and they bound to Fe/Mn oxides (F2) and organic and sulfide (F3). The metals in these fractions/bonding are more stable than those in F1, this causes the metals are less bioavailable but they can potentially change becoming bioavailable as the conditions of the soils changes, such as pH decreases (for F2) and increased oxidation of soil (for F3). The soil for tomatoes was dominated by Cu, Cd, Zn, and Cr those were potentially bioavailable, while lettuce soil was dominated by Pb and Zn. Spring onion and potato soils were dominated by Cr and Cu that were potentially bioavailable. Evidently, the soil for carrot did not show a metal that dominates each other. Several factors that can affect the entry of heavy metals into plants including soil pH, cation exchange capacity (CEC), organic matter content, soil texture, and interaction between elements of the target (soil pH, cation exchange capcity, organic matter content, soil texture, and interaction among the targets elements)[35].
Tabel 3 Fracionation of heavy metal Pb, Cu, Cd, Cr, dan Zn on vegetables soil
Soils for Cultivating
Fraction 1 (mg/kg) Fraction 2 (mg/kg)
Pb Cu Cd Cr Zn Pb Cu Cd Cr Zn
Tomato 6.321 19.651 5.121 15.457 63.716 4.477 31.325 2.311 3.722 47.449
Lettuce 35.058 40.087 1.308 2.047 87.416 13.288 11.694 1.447 0.665 57.458
Spring onion 8.260 33.141 1.369 8.646 36.695 11.858 15.013 0.423 4.265 44.238
Potatoes 10.989 18.811 0.887 1.641 14.163 3.034 5.603 3.160 1.393 25.767
Carrot 4.916 30.042 1.518 2.732 38.326 7.601 12.869 1.457 3.331 16.088
Fraction 3 (mg/kg) Residual Fraction*(mg/kg)
Tomato 8.978 36.551 4.247 13.500 42.405 8.175 25.175 4.375 3.581 57.609
Lettuce 39.711 53.268 2.171 1.352 77.691 14.118 51.010 4.396 1.447 15.060
Spring onion 10.497 78.174 1.846 6.336 65.086 4.306 48.543 0.955 1.362 37.788
Potatoes 6.192 83.960 2.272 7.766 19.588 9.339 71.279 2.065 5.771 21.761
Carrot 8.437 50.145 1.557 6.138 25.953 10.236 26.355 0.384 5.846 29.923
*Residual fraction: non-extractable
3.3 Mobility of heavy metals in different types of lands for vegetable cultivation
The mobility of Pb, Cu, Cd, Cr, and Zn in the soils for cultivating of vegetables investigated is the ratio of the concentration of heavy metals extracted at the first fraction to the total concentration of the metals. The mobility of an element is hard to predict quantitatively in the environment surface, so it should be considered as a relative matter by comparing the behavior of the elements under environmental conditions changes empirically[18]. The mobility of each metal in each area including the land for cultivating tomatoes, lettuce, spring onion, potatoes, and carrots showed different levels as illustrated in Figure 1. The average mobilities of Pb, Cu, Cd, Cr, and Zn in all types of the vegetable soils were 15.76 to 37.13, 10.47 to 25.68, 10.57 to 31.61, 9.90 to 42.38, and 10.94 to 36.65 %. Statistically, the mobility of each metal in each field was significantly different at p < 0.05 or at confidence limit of 95% (as shown in Table 4).
The most mobile Pb was found in the soil for cultivating potatoes and the lowest was in the land for carrots. Lead mobility in the soils decreased as follows: potato soil > lettuce soil > spring onion soil > tomato soil > carrots soil. The high mobility of Pb in the potatoes soil may be due to lack of Fe/Mn oxides (% F2) and organic matter or sulfides (% F3) that can bind these metals so that they tend to stay in the form of ions or easily dissolved in the soil solution. Likewise, the presence of Cr is high enough on the soil (as shown in Table 2) allowing Pb to precipitate chromium in the form of Cr2O72-and CrO42-as PbCrO4which is in the soil solution ( pH < 7 ) likely to dissolve as Pb2+(pH soil =
6.20, recorded).
Table 4 Analysis statistic for heavy metals in different vegetable soils
Duncanabc Spring onion 23.6700 18.9533 29.8067 41.9467 19.9633
Potatoes 37.0633* 10.4700 10.5733 9.8967 16.7500
Carrot 15.7467 4.1167 30.8833 15.1400 34.7500 Mean Square 0.026 0.005 0.022 0.181 0.060
Sig. 1.000 1.000 1.000 1.000 1.000
*
The highest percentage heavy metal Significant for Cr in tomato and spring onion soils was 0.244
Ӯ = the average mobility of metal
Figure 1 Mobility of Pb, Cu, Cd, Cr, and Zn in various cultivating lands
In contrast to Pb, Cu, and Cd, Chromium has a fairly large mobility (37.13 to 42.38%) in three types of soils that were tomato, lettuce and spring onion soils. The mobility of Cr in tomatoes soil was the highest, followed by spring onion soil, lettuce soil and decreased sharply in carrots soil (15.14%), while the lowest was found in potatoes soil. The level of Cr mobility in these cases can not be explained by the existence of the second and third factions, because it can not explain consistently as the discussions of Pb, Cu, and Cd. This is probably because of Cr has very different properties with the other four metals. For example, Cr can form anion compounds with other elements such as CrO4
2-and Cr2O72-. Likewise, Cr has two oxidation numbers which are +3 and +6 with its ionic radius of the smallest (75.5
and 58 pm) among other metals. The ionic radii of Pb+2, Cu+1, Cu+2, Cd+2, and Zn+2are 133, 91, 87, 109, and 88 pm respectively. Theoritically, the smaller the atomic radius, the more mobile the metal is[18]. Therefore, it is reasonable that chromium was the most mobile metals among the five metals under study. It was supported by the average of the mobility percentage of Cr on all soils was the highest i.e. 29.30 %. The average of the mobility percentages of Pb, Zn, Cd, and Cu were 26.69, 26.46, 23.24, and 19.54 % respectively.
Zinc was the metal that showed the third highest mobility after Cr and Pb, as presented in Figure 1. The mobility of Zn was quite high in the three soil types including: lettuce, carrots, and tomatoes soils. Mobility of Zn was found to be the highest in the lettuce soil, while the lowest was in the potatoes soil. The mobilities of Zn in vegetable soils were found in the range of 10.94 to 36.65 % with the average of 26.46 %. This metal mobility in vegetable soils can be arranged as following the order of lettuce soil > carrots soil > tomato soil > spring onion soil > potatoes soil.
In general, the results of this study indicated that Cr metal mobility was the highest, followed by Pb, Zn, and Cd, and the lowest was Cu. The soils for tomatoes and lettuce had four metals with relatively high mobilities, namely Pb, Cd, Cr, and Zn in tomatoes soil and Pb, Cu, Cr, and Zn in lettuce soil. The soil for carrot had 3 metals with high enough mobility, i.e. Cu, Cd and Zn, while spring onion soil had two metals with relatively high mobilities. Potatoes soil was the only had one metal with high mobility,i.e. Pb and the other metals were relatively low (< 11 %). This suggested that the soil for potatoes was the most secure land for cultivating all types of vegetables.
4
Conclusion
Based on the above discussions, it can be concluded that the range of total heavy metals including Pb, Cu, Cd, Cr, and Zn in various soil types for cultivating vegetables were: 27.952 - 102.248, 112.759 - 179.664, 4.593 - 16.201, 5.512 - 36.473, and 110.289 - 238.498 mg/kg, respectively. The highest Pb and Zn contained in the soil for cultivating lettuce, while the highest concentrations of Cd and Cr were found in soils for tomato. The highest Cu content was found in the soil for cultivating potatoes. The mobilities of all heavy metals were significantly different between metals and the soils. The average mobilities of Pb, Cu, Cd, Cr, and Zn in all types of the vegetable lands were 15.76 - 37.13; 10.47 - 25.68; 10.57 - 31.61; 9.90 - 42.38; and 10.94 -36.65%, respectively. Statistically, the mobility of each metal in each field was significantly different at 95% confidence limit (p<0.05). The highest Pb mobility was found in the land of potatoes (37.13%), Cu and Zn in lettuce soil (25.68 and 36.65%), and Cd and Cr in tomato soil (31.61 and 42.38%). The heavy metal mobilities decreased in the order of Cr > Pb > Zn > Cd > Cu.
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