Concentration, Distribution and Comparison of Total and Bioavailable Heavy Metals in Top Soils of Bonab District in Zanjan Province

doi:10.4236/ojss.2012.22018

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Open Journal of Soil Science, 2012, 2, 123-132

http://dx.doi.org/10.4236/ojss.2012.22018 Published Online June 2012 (http://www.SciRP.org/journal/ojss)

123

Concentration, Distribution and Comparison of Total

and Bioavailable Heavy Metals in Top Soils of Bonab

District in Zanjan Province

Abdolhossein H. Parizanganeh1*, Vahid Bijnavand1, Abasali A. Zamani2, Ali Hajabolfath3

1Department of Environmental Sciences, Faculty of Science, Zanjan University, Zanjan, Iran; 2Department of Analytical Chemistry,

Faculty of Science, Zanjan University, Zanjan, Iran; 3Department of Geology, Faculty of Science, Zanjan University, Zanjan, Iran.

Email: *[email protected]

Received October 8th, 2011; revised November 10th, 2011; accepted December 28th, 2011

ABSTRACT

Heavy metal contamination of soils is a widespread problem in Zanjan province located in North West Iran due to

natural pedo geochemical background and anthropogenic sources. The province is highly polluted by some heavy met-

als due to the presence of mineral resources notably Lead and Zinc, their improper utilization, and also the development

of a number of related industries. Bonab district was selected for detail study and the objectives of the study were set to

evaluate the total as well as bioavailable fraction of heavy metals in surficial soils within the studied area. 72 soil sam-

ples were collected and analyzed for heavy metal contamination by Atomic Absorption Spectrometry (AAS). The ob-

tained results when compared with WHO and USEPA standards show a very high concentration of some toxic metals.

Soils were basically polluted by Pb, Zn, Cu, and Cd. Bioavailable fraction of studied metals were also measured with

formation of metal complex with Diethylene tri amine penta acetic acid (DTPA) agent. To find the share of the anthro-

pogenic sources in the contamination of soils, the Enrichment factor (EF) and Geological Accumulation Index were

measured and the distribution maps were drawn using Arc GIS (9.3). Highest concentrations of some toxic metals were

found as isolated patches around the Bonab Industrial Town as well as within and around the most populated areas of

the district notably Zanjan city indicating its anthropogenic origin.

Keywords: Soil Pollution; Pedogeochemical; Anthropogenic; Heavy Metals; Zanjan-Iran

1. Introduction

Heavy metal contamination of soils is widespread and

there is a risk of transfer of toxic and available metals to

human, animals, and agricultural crops [1]. Heavy metals

are natural constituents of the Earth crust. A number of

these elements are biologically essential at trace levels

and play an important role in human health [2]. The po-

tential risk for the environment and population due to soil

heavy metals arising from metallic mining has been well

described [3]. Heavy metals can induce toxicity in wild-

life if the soil level reaches critical values; also plant ac-

cumulation in above-ground tissues can result in an in-

crease of metal accumulation in top-soil, via leaf deposi-

tion, or can create an exposure pathway for metal intro-

duction into the food chain [4]. In contrast with soils in

agricultural areas, soils in urban environment, particu-

larly in parks and gardens, have a direct influence on

public health not related with production of food. This is

due to that they come easily in contact with humans and

are transferred to them, either as suspended dust or by

direct contact [5]. Two main sources of heavy metals in

soils can be considered: 1) the natural pedo-geochemical

background, which represents the heavy metal concen-

tration inherited from the parent rock [6] and 2) anthro-

pogenic contamination, which can be directed via wastes,

animal manure [7], compost [8], sewage sludge [9], or

diffuse via aerosol deposition [10]

Angoran area of Zanjan province located in North

West of Iran has a large metalliferous site and has been

considered as a traditional mining region since antiquity.

There are still large reserves of lead (Pb) and zinc (Zn) in

the area. Both mines and smelting units in the province

present a risk of contamination of soils, plants, surface

and ground water by dissemination of the particles car-

rying metals by wind action and/or by runoff from the

tailings. Heavy metal contamination in Zanjan province

has also been previously reported in the vicinity of Lead

and Zinc mining and smelting sites [11,12]. The total

heavy metal contents can indicate the extent of contami-

nation, but is not usually an accurate indication of the

*Corresponding author.

Concentration, Distribution and Comparison of Total and Bioavailable Heavy Metals in Top Soils of

Bonab District in Zanjan Province

124

phyto-toxicity; so many latest studies investigated the

heavy metal fractions of mine soils [13] and evaluated

the phyto-toxic risk for human receptors. However, the

determination of heavy metal fractions is a more com-

plex task than the determination of the total contents of

heavy metals [14]. The main purpose of this research is

the measurement of the quantity of heavy metals existing

in top soils of the study area, the determination of the

rate of the bioavailable elements in top soil is also one of

the pillars of this research, since this part aside from pol-

luted or unpolluted the soils, show the absorption ability

of the plants.

2. Materials and Methods

2.1. The Study Site

In this study the Bonab district located near the city of

Zanjan was selected for detailed study. Soil samples were

collected from the studied area during second week of

October 2010 (Figure 1).

2.2. Sample Collection

Seventy two soil samples were collected through system-

atic random sampling (top 0 - 30 cm soil layer). The

study area was divided into a 4*4 km grid and samples

were collected from every corner of the grid. Based on

the information gathered through field trips and taking

into account the type of landuse and population concen-

tration, the numbers of samples were increased by subdi-

viding the grid in selected locations (2.2 km grid).

Soil samples were collect from the surface of the soil

(0 - 30 cm deep) and preserved by using the methods of

soil analysis [15]. From each sampling points, four soil

samples were gathered and mixed properly to obtain a

composite sample mixture. The soil sampling spots were

cleared of debris before sampling. Each composite soil

samples were placed in cellophane bags labeled then

taken to the laboratory for pre-treatment and analysis.

The sampling tools, were washed with soap and rinsed

with distilled water after each sampling [16].

2.3. Soil Analysis

In the laboratory, bulk soil samples were spread on trays

and were air dried at ambient conditions for two weeks.

The samples were then grounded by mortar and pestle,

sieved through a 2 mm mesh, and oven-dried at 50˚C for

about 48 hours. The samples were then stored in poly-

ethylene bags and re-homogenized before being used.

The soil samples were digested using the 11466 ISO

standard methods (the aqua regia digestion method) [17].

3 g of soil was placed in a 100 ml round bottom flask

with 21 ml of concentrated HCl (35%) and 7 ml concen-

trated HNO3 (65%). The solution was kept at room tem-

perature overnight before a water condenser was attached

and the solution heated to boiling point for 2 hours.

Added 25 ml water to the condenser before filtration of

the mixture through using a Whatman (No. 42) filter. The

filtered residue was rinsed twice with 5 ml of water and

the solution was made up to 100 ml. The bioavailable

content of metals were determined using Lindsey method

(Lindsay, 1978), 10 g of soil was added in 20 mL mix-

ture of 0.005 mol·L−1 DTPA and 0.01 mol·L−1 CaCl2 and

0.01 mol·L−1 triethylamine (pH 7.3), shaken for 2 h and

then filtered through using a Whatman (No. 42) filter.

Figure 1. Location map of the studied area indicating sampling points.

Concentration, Distribution and Comparison of Total and Bioavailable Heavy Metals in Top Soils of

Bonab District in Zanjan Province

125

The metals (Zn, Pb, Cu, Fe, Ni, Co and Cd,) in the soil

extracts were analyzed by Atomic Absorption Spectro-

metry (AAS). Three analytical replicates were measured

for each sample. Data with replicates were presented as

mean-standard error and difference test was made using

SPSS 14 software. The pH and EC (solid: distilled water

= 1:5) of the soil samples were also measured by with pH

and EC meters (Metrohm, Germany). Finally, using the

geostatistical methods with Arc GIS (9.3) the heavy me-

tal distribution maps were drawn.

3. Results and Discussion

Total Heavy Metal in Soils

Comparing the results with standard range, shows that

the concentrations of heavy metals Pb, Zn, Cu, Cd in the

studied area are higher than standards. Summary statis-

tics for the analyzed elements in all the studied samples

are presented in Table 1. Though, Cadmium has the low-

est mean concentration (1.49 mg/kg), it is well above the

maximum permissible limits given by all existing stan-

dards (Table 2). While the highest contents are for Fe

(66933.7 mg/kg), Pb, Zn, and Cd are also much higher

than standards and do not relate to the pedo geochemical

sources. These points out that Zinc industry are most

possibly the major anthropogenic source for the enrich-

ment of these heavy metals in the top soil. The extremely

high metal levels in tailings made them a potentially

hazardous source of soil contamination. Soil pH aver-

aged 7.78 (from 7.16 to 8.08) indicating a neutral to al-

kaline nature. Electrical conductivity (EC) in the samples

is quite variable ranging from 248 to 1721 µs·cm–1.

The concentration of the studied metals are in the or-

der of Fe > Zn > Cu > Pb > Ni > Co > Cd.

The bioavailable heavy metals [19] content in soil

samples are less than total metal content. With increasing

total heavy metal content in soils, the bioavalaible con-

tent of metals are also increase and there are a significant

correlation between them, (Table 3).

Figure 2 shows the concentration and distribution of

each metal within the studied area. Zinc concentration

(Figure 2(a)) is very high in Bonab industrial town and

its damping site located in the center of the studied area,

indicating its anthropogenic source. Lead is also found in

some localized areas with extremely high concentrations

among them the Bonab industrial town and its damping

area and can be related to Zanjan Zinc and Lead smelting

plant. Lead is also found in high concentration in Zanjan

city area which is mainly due to traffic and use of leaded

gasoline and also may by air borne (Figure 2(b)).

Table1. The nature of the soil samples and total metal concentrations in the top soil (mg·kg–1).

pH EC (µs·cm–1) Pb Zn Cd Cu Co Ni Fe

Avg. 7.80 491.1 58.18 299.31 1.4 67.68 18.53 28.11 66933.7

Max. 8.08 1721.0 615.3 13461.67 5.6 3106.6 41.97 69.20 157916.7

Min. 7.16 248.0 14.33 25.10 0.3 0.83 8.53 1.07 30950.0

Median 7.80 425.0 38.16 93.96 1. 3 20.65 18.87 25.85 61583.3

STDEV 0.17 251.9 91.42 1574.91 0.7 346.51 4.92 14.35 23102.9

Table 2. Metal concentrations were compared with existing standard (mg·kg–1) [18].

Standard Pb Zn Cd Cu Co Ni Fe

USEPA1 10 NR - 30 8 40 NR

GLC2 20 NR - NR NR 20 NR

WHO 20 50 0.3 4 19 68 47200

Average in the study area 58.18 299.311.4 67.68 18.53 28.11 66933.70

1United State Environmental Protection Agency; 2Great London Council.

Table 3. Bioavailable heavy metal content in the top soil (mg·kg–1).

Pb Zn Cd Cu Co Ni Fe

Avg. 4.68 2.00 0.15 1.92 0.32 0.59 6.88

Max. 71.00 10.50 1.04 13.30 1.01 2.57 38.2

Min 0.06 0.07 0.05 0.14 0.09 0.08 1.68

Median 2.80 1.21 0.10 1.48 0.26 0.49 5.47

STDEV 8.77 2.23 0.16 1.48 0.20 0.42 5.39

Concentration, Distribution and Comparison of Total and Bioavailable Heavy Metals in Top Soils of

Bonab District in Zanjan Province

126

Figure 2. Concentration & distribution of (a) Zn; (b) Pb; (c) Co; (d) Cu; (e) Cd; (f) Fe; (g) Ni.

Concentration, Distribution and Comparison of Total and Bioavailable Heavy Metals in Top Soils of

Bonab District in Zanjan Province

127

Significant correlation between concentration of Cu,

Co, and Fe in the north east of the studied area (Figures

2(c), (d) and (f)) are mainly natural pedo-geochemical

background and maybe related to the Zaker (Fe) mining

area .

Nickel is also found in high concentration in south east

of the area its high concentration may be related to agri-

cultural fertilizer used in the cultivated lands in this part

of the studied area (Figure 2(g)).

4. Enrichment Factor (EF)

A common approach to estimate how much the sediment

is impacted (naturally and anthropogenically) with heavy

metal is to calculate the Enrichment Factor (EF) for me-

tal concentrations above un-contaminated background

levels [20]. Pollution will be measured as the amount or

ratio of the sample metal enrichment above the concen-

tration present in the reference station or material [21,22].

The EF method normalizes the measured heavy metal

content with respect to a samples reference such as Fe,

Al or Zn [21]. The EF of a heavy metal in sediment can

be calculated with the following formula [20]:

metal normalizer

metal controlnormalizercontrol

EF= CC

where Cmetal and Cnormalizer are the concentrations of heavy

metal and normalizer in sediment and in unpolluted soil.

Enrichment factor (EF) can be used to differentiate be-

tween the metals originating from anthropogenic activi-

ties and those from natural procedure, and to assess the

degree of anthropogenic influence. Five contamination

categories are recognized on the basis of the enrichment

factor as follows: [23].

EF < 2 is deficiency to minimal enrichment,

EF 2 - 5 is moderate enrichment,

EF 5 - 20 is significant enrichment,

EF 20 - 40 is very high enrichment,

EF > 40 is extremely high enrichment.

As the EF values increase, the contributions of the an-

thropogenic origins also show an increase [22]. Figure 3

shows the EF for heavy metals Cd (a), Pb (b), Cu (c), Co

(d), Zn (e), and Ni (f) in the study area.

5. Index of Geo-accumulation

Index of Geo-accumulation (Igeo) has been used widely

to evaluate the degree of metal contamination or pollu-

tion in terrestrial, aquatic and marine environment. The

Igeo of a metal in sediment can be calculated with the

fallowing formula [21]:



metalmetal Control

Igeo log2C1.5C

where Cmetal is the concentration of the heavy metal in the

enriched sample and Cmetal(control) is the concentration of

the metal in the unpolluted or control sample. The factor

1.5 is introduced to minimize the effect of the possible

variations in the background or control values which may

be attributed to lithogenic variations in the sediment [21].

The degree of metal pollution is assessed in terms of

seven contamination classes based on the increasing nu-

merical value of the index as follows: [20].

Igeo < 0 = means unpolluted

0 ≤ Igeo < 1 means unpolluted to moderately polluted,

1 ≤ Igeo < 2 means moderately polluted,

2 ≤ Igeo < 3 means moderately to strongly polluted,

3 ≤ Igeo < 4 means strongly polluted,

4 ≤ Igeo < 5 means strongly to very strongly polluted,

Igeo ≥ 5 means very strongly polluted.

Figure 4 shows the Index of Geo-accumulation for

heavy metals Pb (a), Co (b), Cu (c), Fe (d), Zn (e), Cd (f)

and Ni (g) in the studied area.

6. Conclusions

72 soil samples were analyzed to determine the concen-

tration and distribution of heavy metals (Cd, Pb, Ni, Cu,

Co, Fe and Zn) in top soils of Bonab district in Zanjan-

Iran. Determination of bioavailability of heavy metal in

soils with DTPA method has been shown that by increas-

ing total amount of heavy metals in soil, the bioavailable

content of heavy metal also will show an increased. The

dispersion maps provided for lead shows that the most

concentration of lead is near the industries specially near

Iran Lead and Zinc Factory and also observed in the

Zanjan City so it is possible to conclude that the concen-

tration of lead in the soils of study area is related to in-

dustries on the other hand the pollution is because of

human activities.

According to the dispersion maps provided for nickel

and landuse map of study area it was observed that the

concentration of nickel has a direct relationship with

landuse in study area as such as the most concentration

of nickel existed in the agricultural lands confirm of the

study area which it is due to the over use of chemical

fertilizer in this area.

For Cd and Zn the maximum concentration observed

near the Iran Lead and Zinc Factory and industrial areas

but for the Fe, Cu, and Co heavy metals obviously ob-

served that the concentration of this metals in top soils is

just related to the structural geology and the rock kinds

of the study area as it was observed that the average of

Fe, Pb, Zn, Cu, and Cd in sequence in top soils are

66933.70, 58.18, 299.31, 67.68, 1.49 mg · k g –1 and the

average of this metals according to the WHO standard in

sequence are 47,200, 20, 50, 45, 0.3 mg·kg–1. This means

that the study area is polluted by these heavy metals. For

Co and Ni, the average concentrations in sequence are

Concentration, Distribution and Comparison of Total and Bioavailable Heavy Metals in Top Soils of

Bonab District in Zanjan Province

128

Concentration, Distribution and Comparison of Total and Bioavailable Heavy Metals in Top Soils of

Bonab District in Zanjan Province

129

Figure 3. Enrichment Factor (EF) for heavy metals Cd (a), Pb (b), Cu (c), Co (d), Zn (e), and Ni (f) in the study area.

Concentration, Distribution and Comparison of Total and Bioavailable Heavy Metals in Top Soils of

Bonab District in Zanjan Province

130

Concentration, Distribution and Comparison of Total and Bioavailable Heavy Metals in Top Soils of

Bonab District in Zanjan Province

131

Figure 4. Index of Geo-accumulation for heavy metals Pb (a), Co (b), Cu(c), Fe (d), Zn (e), Cd (f) and Ni (g) in the study area.

28.11 and 18.53 mg·kg–1 and the WHO standards for

these metals are 68 and 19 mg·kg–1 respectively.

The percentage of bioavilable metals to the total con-

centration of them for each studied metal are Fe 0.01%,

Cd 10.06%, Co 1.72%, Ni 2.04%, Cu 2.83%, Zn, 0.66%

and Pb 8.04%.

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