این مطالعه با هدف منشاءیابی و تعیین میزان آلودگی گرد و غبار خیابانی شهر اهواز به فلزات سرب، روی و مس انجام شد. بدین منظور از گرد و غبار معابر و خیابانهای 69 نقطه و خاک سطحی (5-0 سانتیمتری) 23 نقطهی شهر اهواز در بهمنماه 1393 نمونهبرداری و الگوی توزیع اندازه ذرات آن-ها به روش پراش پرتو لیزر، مقادیر آنیونها و کاتیونهای محلول به روش کروماتوگرافی یونی و غلظت فلزات سنگین در نمونههای جمع-آوری شده با استفاده از دستگاه ICP تعیین گردید. نتایج نشان داد که توزیع اندازه ذرات گرد و غبار در نمونههای مورد بررسی دارای الگوی توزیع دوقلهای با فراوانی بیشتر ذرات در اندازه سیلت و سپس شن ریز است. بهگونهای که 57 تا 89 درصد ذرات گرد و غبار جمعآوری شده را سیلت و 5 تا 16 درصد ذرات را شن ریز تشکیل میدهد. بررسی ترکیب شیمیایی ذرات گرد و غبار از لحاظ میزان آنیونها و کاتیونهای محلول نیز حاکی از فراوانی یونهای سدیم، کلسیم، کلراید و سولفات در نمونههای جمعآوری شده است که در مقایسه با خاکهای منطقه مقادیر بسیار بیشتری را نشان میدهد. نتایج نشان داد که میانگین غلظت تمامی فلزات مورد بررسی از متوسط خاکهای منطقه و پوستهی فوقانی زمین بیشتر بوده و بهصورت روی ˂ مس ˂ سرب میباشد. همچنین نتایج نشان داد که گرد و غبار خیابانی شهر اهواز در رابطه با عناصر مطالعه شده در وضعیت آلودگی زیاد قرار دارد. متوسط شاخص آلودگی یکپارچهی نمرو 26/7 بهدست آمد که آلودگی زیاد گرد و غبار خیابانی را در شهر اهواز نشان میدهد.
عنوان مقاله [English]
Assessment of Street Dust Pollution Status to Some Heavy Metals and their Origins in Ahvaz City
Khuzestan province in southwestern Iran is one of the most critical areas affected by dust storms due to the arid climate and the abundance of desert areas in its western and southern parts. Dust storms in these areas are among the most critical environmental issues. Air pollution, the development or increase of respiratory diseases, reduced soil fertility, damage to crops, and reduced solar radiation are among the most critical consequences of dust storms. Dust particles can absorb significant amounts of heavy metals, which facilitate their transport on a large scale due to their fine particle size distribution. Street dust is considered the major source of pollutants from a wide range of traffic, industrial emissions, pesticides, and mining activities. Although many studies have been conducted to identify the origin and pollution status of dust particles in the country, the assessment of pollution and source of street dust particles during dust storms, especially in Ahvaz city, has received less attention. .Thus, this study was conducted to: (1) identify the source of street dust in Ahvaz city, and (2): determine the level of contamination to Pb, Zn, and Cu.
Materials and Methods
Dust and soil samples were collected respectively at 69 and 23 points from streets and the surface soil (0-5 cm) in Ahvaz city in February 2015. To determine the particle size distribution pattern in the dust samples, they were first dispersed in 1 M sodium hydroxide and 10% sodium hexaphosphate solutions for 2 hours. Then, they were analyzed using A Malvern Hydro 2000g laser diffraction device. The ionic compositions of the dust and soil samples were also determined after extraction from 1 (dust/soil): 5 (water) suspension with an advanced Meterohm 861 model ion chromatography apparatus. The heavy metal contents of soil and dust particles were determined using inductively coupled plasma (ICP) spectroscopy. To determine the Pb, Fe, Cu, and Zn contents, 0.5 g of the dust or soil samples were digested with 60% nitric acid, and after 24 hours, the samples were heated for 0.5 hours at a temperature of 80 ° C. Then, they were filtered with Whatman 42 paper and finally were examined using an Agilent 7000 inductively coupled plasma (ICP) spectrometer. To assess the degree of street dust pollution in Ahvaz city, various indicators, including the single element pollution index and Nemerow integrated pollution index, were calculated. A pollution index is expressed as the ratio of the concentration of an element in soil or dust samples to the same component's baseline value in soil or dust sample. If this index is greater than 1, it indicates different levels of pollution.
Results and Discussion
The particle size distribution in the studied samples showed a bimodal pattern with more abundance of particles in the size of silt and fine sand. Accordingly, 57 to 89% of the particles were in the silt size, and 5 to 16% were in the size of fine sand. The results also indicated that the abundance of sodium, calcium, chloride, and sulfate ions was comparably higher than the local soils. Similarly, the average concentration of each heavy metal was higher than those of the local soils and the upper earth crust, which followed the order Zn> Cu> Pb. Accordingly, The average Pb, Cu, and Zn concentrations were 5.23, 6.37, and 6.89 times more than their corresponding values in the earth's upper crust. Accordingly, and based on the values obtained from the pollution index (PI), all the studied elements in the street dust of Ahvaz city could be categorized as highly polluted. The average of Nemrow integrated pollution index was found 7.26, which shows a high pollution level for street dust in Ahvaz city
It seems that dust particles collected from streets and sidewalks of the Ahvaz city are mainly originated from regional focal points in eastern and southeastern parts of the city. When Pb, Cu, and Zn concentrations in the street dust of Ahvaz city and those reported from different cities in Iran and other countries are compared, it is concluded that dust particles deposited over the streets and sidewalks in Ahvaz county have a higher degree of pollution. Therefore, Prompt actions are needed to lower the risk of these elements for the environment.