Spatial Distribution and Contamination of Heavy Metals in Street Dust from Hamedan, Iran of Heavy Metals in Street Dust from Hamedan,

In the Anthropocene, researchers are to pay special attention to heavy metal pollution associated with urban dust particles, amid overwhelming concerns that heavy metals might exert considerable impacts on ecosystem and human health. In this research, 60 street dust samples were taken from five different urban locations namely high, and low traffic streets, parks, residential, and industrial areas of Hamedan, Iran, as well as background city soils. The samples were analyzed for Zn, Cu, Pb, Mn, Cr and Co, using the atomic absorption spectrophotometric technique and ICP MS. Data showed that mean values for Zn, Cu, Pb, Mn, Cr, and Co in the dust samples were 189.9, 63.8, 63.0, 378.5, 33.0, and 19.8 mg Kg -1 , respectively. The street dust samples were found to contain significant levels of Cu, compared to the background; however, their geo-accumulation indices (I geo ) suggested them to be uncontaminated with Zn, Pb, Mn, Cr and Co, and moderately contaminated with Cu. The Igeo values were in the following order: Cu >Pb, Zn>Mn>Cr>-Co. The enrichment factor (EF) was estimated for all studied heavy metals by using Mn as the reference element. Except from Cu, the mean values of EF were less


Introduction
In recent decades, characterized as the Anthropocene epoch, much attention has been paid to heavy metal contamination with dust particles in many parts of the world [1,2]. Dust chemicals vary from elemental wastes to organic and inorganic compounds. Natural and anthropogenic heavy metals comprise the most important part of the dusts' inorganic components [3,4]. Disproportionate accumulation of heavy metals is a serious threat to living organisms.
They pollute the natural environment and enact toxic impacts on living humans. Some heavy metals are essential for life; however, it must be taken into consideration that at higher levels, they can also impose some toxicological risk [4,6]. When studying urban atmospheric pollution, identification of source and location of the dust stands out as a high priority [7]. Hence, such studies should determine origin, distribution, environmental damage and health effects of the concerned heavy metals [8]. Dust storms with their important impacts on human health have turned into a frequent phenomenon of Iran's weather system. Their adverse effects on humans causing respiratory and cardiovascular diseases, and infections, in one hand, and on the environment, reducing visibility, creating agricultural losses, affecting the industry, and making satellite imagery complications are well-known [9]. On global scale, most of the dust particles come from arid and semiarid areas [10].
Recently, the amount of dust particles coming to Iran from Arabian countries has been dramatically increasing. They have viciously affected western and even central parts of Iran [11].
They are an important way of exposing people to heavy metals.
In recent years, the large amounts of atmospheric dust particles have been depositing in many cities, crossing the western borders of Iran. The most dominant winds of the country blow from west to east. Therefore, as sources of Iran's dust storms; mainly western adjacent countries must be blamed for [12]. Saeedi et al. reported that although there is no confirmed origin, it is suspected that most of the dust particles originate from the dry wetlands of south eastern Iraq and the desserts of Iran's western neighbors [13]. The growth of population, industrial activities, and vehicles in large cities are the other major causes of pollution in urban environments. Road dust particle play as the leading path in terms of subjecting people to the toxic elements [14]. Cities have become source points of toxic chemicals from the unrestrained use of fossil fuels. Urban people are the most affected and the traffic policemen are the worst sufferers, because they are particularly close to the fumes of automobile exhaust [15]. The complexity of dust particles makes their characterization and source identification difficult [16]. Dust particles could be introduced as an indicator of heavy metal contamination from atmospheric deposition [17][18][19][20].
The close association of heavy metals with dust particles may be enhanced in the presence of anthropogenic sources of heavy metals.
Depending on the population and economical activities, the level of contamination in a city varies from place to place. The presence of heavy metals in high concentrations in the environment lead to health hazards such as its adverse effects on the nervous, blood forming, renal, and reproductive systems. The toxicity of elements such as Cu, Cd, and Zn, have been identified as being able to alter the purpose of the human central nervous and respiratory system, while also disturbing the endocrine [21].
Others include reduced intelligence, attention deficit, and behavioral abnormality, as well as increased cardiovascular diseases in adults [22]. Dust particles may also cause other kind of problems such as reduced soil fertility, damage to crops and reduced solar radiation [23]. While there are many published studies on the concentration of heavy metals in street dust of major cities in the developed countries; few research projects have been conducted addressing the issue in smaller cities of developing countries. Therefore, contamination and spatial distribution of heavy metals associated with street dust particles has become a major environmental issue in many western cities of Iran including Hamedan. The main objective of this study; therefore, was to determine the concentration of heavy metals (Zn, Cu, Pb, Mn, Cr and Co) in street dust samples collected from different parts of Hamedan city; and then examine their spatial distribution.

Sample Collection
The street dust samples were collected and placed into polyethylene bags using a clean brush and dustpan. The sampling was conducted in May (prior to rainy season) to avoid heavy metals being washed out by rain. Each street dust sample was 300-700 g in mass and collected from a 1 m2 area which measured by a ruler. Then, the samples were transferred to soil science laboratory of Bu-Ali Sina university, Hamedan, Iran; and were dried at room temperature for 3 days. Later, they were sieved through a 1 mm stainless steel sieve.

Dust Analysis
At first, 0.6 grams from every sample was digested [24] with a mixture of perchloric, nitric, and sulfuric acid in proportion of 1:5:1 and heated at 215°C until white fumes given off and a creamy color appeared. Then, 10ml deionized water was added into each sample and heated at 100°C for 1 hour. The solutions were allowed to cool, then filtered and made up to 100 ml. Finally, the concentrations of Zn, Cu, Pb, Mn, Cr, and Co were determined using the atomic absorption spectrophotometric technique.

Contamination Assessment Methods
A number of calculation methods have been put forward for quantifying the degree of metal enrichment or pollution in dust particles [25]. In this study, geo-accumulation index (I geo ), enrichment factor (EF), and pollution index (PI) were calculated to assess the heavy metal contamination level in the road dust particles. However, the geo-accumulation index was originally used with bottom sediments [25]. It is also widely used to determine the pollution degree of heavy metals in dust particles and soil [26]. I geo is computed by the following equation (eq. 1): Where Cn represents measured concentration of the element n in sampled street dust and Bn is geochemical background value of the element n in background sample. The geo-accumulation index is classified as shown in Table 1. The enrichment factor (EF) is used in studied samples to determine degree of metal contamination [27]. It was based on standardization of a determined element against a reference element. A reference element is often the one characterized by low occurrence variability such as Fe, Al, Ti, Mn, Sc, etc. [28][29][30]. The EF calculation is expressed below as eq.2:  Table 2). The Pollution index (PI) was used to assess pollution degree and environment quality in this study.
The PI is defined as eq.3: PI= Cn/ Cref eq.3 Where Cn and Cref stand for the concentration of examined element and the reference value, respectively (Table 3).

Methods of Mapping Heavy Metal Concentrations
The spatial distribution maps of heavy metal concentrations were generated through the Inverse Distance Weighted (IDW) interpolating data from 60 street dust samples using Arc map software.

Results and Discussion
Heavy Metal Concentration  Shi and Wang reported that in their studied area, mean content of Cu particles was found to be almost 6 times higher than the background. Lu found the maximums Cu in the samples from heavy of Pb in Hamedan city. It is believed that Pb is responsible for many negative effects on human bodies such as damaging kidneys, the nervous, and reproductive systems. It is also the element of most concern in environmental heavy metal pollution studies [32].

Spatial Distribution of Heavy Metals
The spatial distribution of metal concentrations is a useful method to assess the possible sources of enrichment, and to identify hot-spot areas with highest metal concentration. According  (Table 6). The order of EF values was as the following: Cu >Zn, Pb> Cr, Co, similar to the I geo order, which could also be interpreted as decreasing order of their overall contamination of street dusts from Hamedan city. The anthropogenic pollution is clearly identified when the maximum EF of each heavy metal is larger than 3. The mean EF of Cu was higher than 2, while the mean EF of Zn, Pb, Cr, and Co were less than 2. These findings showed that Cu in street dusts of Hamedan city was the main pollutant, and largely originated from anthropogenic sources.    The high values of I geo , EF and PI for Cu in the sampled street dusts indicated that there was a considerable Cu pollution, which mainly originated from traffic and industry activities. The I geo , EF and PI of other studied heavy metals were low and revealed low levels of these heavy metals' pollution in street dusts from studied area. These findings stressed on the fact that more attention should be paid on heavy metal contamination in the street dusts of the city, especially on the Cu. Some protective measures such as encouraging use of public transport; replacing liquid fossil fuels with gaseous fuel, and planting more green areas are suggested to combat the problem.