Document Type : Research Paper
M.Sc., Soil Sciences, Isfahan University of Technology, Isfahan-Iran
Prof., Soil Sciences, Isfahan University of Technology, Isfahan-Iran
Dust is a signiﬁcant environmental medium that can provide information about the level, distribution and fate of contaminants present in the surface environment. Dust could be an indicator of pollutants such as heavy metal in the atmosphere. Furthermore, the elemental composition and concentrations in dust reflects the characteristics of short and long term activities in the area. Atmospheric dust is investigated as an important source of heavy metals in urban and industrial areas. Heavy metals can accumulate in dust and disperse in large scale. Thus, in these areas dust contains considerable amount of heavy metals. Both dust and heavy metals may cause human health issues and adverse environmental effects.
Various and conventional methods have been used to determine heavy metal contamination in soil, dust, water, and sediment. Almost all of these methods are based on chemical analysis and particularly acid digestion. In recent years, researchers have used magnetic susceptibility (MS) technique as an acceptable approach to identify pollution sources. Magnetic measurements are fast, cost-effective, non-destructive, sensitive and informative. Strong correlation between heavy metal concentration and magnetic susceptibility in dust can be used for fast assessment of heavy metals contamination and their spatial distribution. Studies have shown that magnetic susceptibility is a useful tool to identify industrial pollutants, traffic pollutants and other atmospheric pollutants.
Magnetic susceptibility is defined as the ratio of the total magnetization induced in a sample relative to the intensity of the magnetic field that produces the magnetization. Five basic types of magnetic behavior are commonly recognized. They include ferromagnetic materials (e.g., pure iron) exhibiting highly ordered magnetic moments aligned in the same direction, producing strongly positive MS; ferromagnetic materials (e.g., magnetite and maghemite) also having strongly aligned moments, but contains unequal opposing forces; anti ferromagnetic materials (e.g., goethite and hematite) having well-aligned but opposing moments, producing moderately positive MS; paramagnetic materials (e.g., biotite and pyrite) exhibiting weakly positive MS; and diamagnetic materials (e.g., quartz, calcite and gypsum) exhibiting weakly negative MS. Therefore, MS measurement can be used as a complementary tool for conventional geochemical approaches. The objective of this study was to evaluate this method in identifying heavy metal polluted dust in Isfahan metropolitan areas.
Materials and methods
Isfahan province is an important arid to semiarid area in Iran where natural and industrially derived dust deposition is an issue. In this study, dust samples were taken from 3 areas in Isfahan including an industrial area (Mobarakeh and Zarinshahr cities, n=7), an urban area with a heavy traffic (Isfahan city, n=20) and an urban but non-industrial area with a low traffic (Khomeynishahr and Falavarjan cities, n=9). Dust samples were taken from all the 36 locations for a period of 5 months and in 4 different periods including August, September, October, and November-Mid December 2010 by a dust sampler already tested in the area.
Magnetic parameters including magnetic susceptibility at low frequency (χlf), magnetic susceptibility at high frequency (χhf) and frequency-dependent susceptibility (χfd) were measured by a Bartington MS2 dual frequency sensor.
To determine heavy metal content, dust particles were first dried at 105°C.
Then, 0.5 g of each dust sample was accurately weighed and digested with 10 ml of HNO3, 60% solution and left overnight. The digested samples were heated for half an hour at 80°C. The final extracts were filtered into a 25 ml polyethylene volumetric flask through Whattman filters no. 42 and then diluted with 1% HNO3 solution. Heavy metals were determined using an atomic absorption spectrophotometer.
Magnetic minerals are mainly derived from industry, domestic and vehicle emissions or from abrasion of product such as asphalt and brake linings. Hence, to determine magnetic minerals extraction with acid ammonium oxalate was performed. Statistical analyses were performed using SPSS 17.0 and SAS 9.1 software.
Discussion of results and conclusions
The descriptive statistics of magnetic parameters and heavy metals concentration (Table 1) show that the highest concentration of heavy metals in dust follow the order of Fe > Mn > Zn > Pb > Ni > Cu > Co > Cr > Cd. Mean values of heavy metals in atmospheric dust were 70.9, 540, 82, 470.3, 223.5, 3.5, 26.5, 24.4 and 4525.9 mg/kg for Cu, Mn, Ni, Zn, Pb, Cd, Co, Cr and Fe, respectively. Distributions of almost all the heavy metals were non-normal likely indicating a great contribution of industrial and traffic sources of heavy metals to atmospheric deposition.
Table 1: Statistical description of magnetic parameters and heavy metals in dust samples.
Magnetic susceptibility at low frequency showed a significant linear relationship with %χfd, Mn and Fe contents (r = -0.78, 0.54 and 0.9, respectively). Previous studies have shown that anthropogenic magnetic minerals, mainly magnetite, are usually large magnetic grains in multi domain state. Significant correlations between χlf and χfd% implies that the dust is dominated by anthropogenic multi domain (MD) and stable single domain (SSD) grains. χlf variation among different regions was statistically significant (p<0.05).The highest χlf was found in dust from industrial area indicating that dust in industrial area contains a considerable quantity of magnetic particles originated from anthropogenic activities. Fe and Mn in dust is likely derived from industrial emission, particularly metallurgical industry. The lowest χfd% and the highest χlf were observed in Zarinshahr and Mobarake, where industrial activity is common. The mean value of χlf and χfd% was 603.6 × 10-8 m 3 kg -1 and 0.7, respectively.
In comparison with the data reported for Isfahan urban top soils (a mean value of 74.3 × 10-8 m 3kg -1 and 3.96 for χlf and χfd% , respectively), the dust particles are severely affected by anthropogenic activities.
Heavy metals concentration in dust is very high but the magnetic susceptibility is only well correlated with Fe and Mn concentrations (figure 1). A significant correlation between Fe and Mn concentrations in dust and magnetic parameters indicates that this method could be used to recognize dust polluted by these two heavy metals.
Studies have shown that Fe and Mn could be emitted from vehicles, steel industrial processes and from the Earth’s crust. In this study, a high level of Fe and Mn in deposited dust could be attributed to metallurgical industries.