contamination assessment of heavy metals in dust of selected roads in karaj, iran

air pollution is today a major problem for modern societies. it has long been recognized as a potentially lethal form of pollution. increasing pollution levels due to rapid urbanization and growth in emission related to vehicular transportation are now caused major concern. the mobilization of heavy metals into the biosphere by human activities has become an important process in the geochemical recycling of these metals. pollution of the natural environment by heavy metals is a worldwide problem because these metals are indestructible and most of them have toxic effects on living organisms. while some of these elements are essential for humans, at high levels they can also mean toxicological risks. according to numerous studies, the pollution sources of heavy metals in environment are mainly derived from anthropogenic sources. in urban soils and urban road dusts, the anthropogenic sources of heavy metals include traffic emission (vehicle exhaust particles, tire wear particles, weathered street surface particles, brake lining wear particles), industrial emission (power plants, coal combustion, metallurgical industry, auto repair shop, chemical plant, and etc.), domestic emission, weathering of building and pavement surface, atmospheric deposited and so on. in recent times, studies of air pollution especially in the urban environment have focused largely on road deposited dust. street dust, particles deposited on road, originates from the interaction of solid, liquid and gaseous materials produced from different sources. road dusts have been implicated to have the potential to carry a high loading of contaminant species such as heavy metals and organic pollutants. dust kicked up by vehicles traveling on roads may make up 33% of air pollution. street dust consists of deposition of vehicle exhausts and industrial exhausts, tire and brake wears, dust from paved roads or potholes, and dust from construction sites. street dust represents a significant source contributing to the generation and release of particulate matter into the atmosphere. control of street dust is a significant challenge in urban areas, and also in other spheres with high levels of vehicular traffic upon unsealed roads. it is easily re-suspended back into the atmosphere, where they contribute a significant amount of trace elements. dust borne heavy metals accumulate in topsoil due to atmospheric deposition by sedimentation, impaction and interception.   materials and methods karaj is as the capital city of alborz province in iran. at the 2011 census, population of the city was 1.9 million, making it the third-largest city in iran. it is situated 36 km west of tehran, at the foot of the alborz mountains. twenty sampling sites were selected for road dust sample collection. the samples were collected from karaj-qazvin highway (10 samples), mallard road (7 samples) and taleghani boulevard (3 samples). karaj-qazvin highway experiences intense traffic and mallard road is selected because a power plant is located in 7 km of the road. taleghani boulevard was selected for comparing, because it is a road with light traffic and away from any industrial activities. at each sampling point, approximately 50 g of road dust particles was collected from three points at the road sides with a brush and transferred into polyethylene tubes for transportation to the laboratory. all the samples were dried at 105 0c for 24 hr to drive out moisture. on cooling each sample was sieved through a sieve of 250 µm diameter. then, the sieved road dust samples were digested using the milestone ethos one microwave digestion system. finally, the total concentrations of cr, cu, ni, pb, zn, and mn were determined by aas. the assessment of soil or sediment enrichment can be carried out by many ways. in this work, the index of geoaccumulation (igeo), enrichment factor (ef), contamination factor (cf) and pollution load index (pli) have been applied to assess heavy metals (cr, cu, mn, ni, pb and zn) distribution and contamination in the road dust samples of the various categories of roads in karaj. the igeo is used to assess heavy metal contamination in urban soils by comparing current and pre-industrial concentrations. it is also employed in pollution assessment of heavy metals in urban road dust. geoaccumulation index is computed using the following equation:                                                        igeo = log2 ((cn) ×1/ 5bn enrichment factor (ef) has been employed for the assessment of contamination in various environmental media by several researchers. its version was adapted to assess the contamination of various environmental media as follows:                                                                   efx = [xs / es (ref)]/ [xc / ec (ref)] to assess the extent of contamination of heavy metals in road dust and also provide a measure of the degree of overall contamination along a particular road, contamination factor and pollution load index has been applied. enrichment factor (ef) of an element in the studied samples is based on the standardization of a measured element against a reference element. a reference element is often the one characterized by low occurrence variability. in this study, mn was selected for this purpose. the contamination factor (cf) and pollution load index parameters are expressed as: cf = c metal / c background                                           pli = n√ (cf1 × cf2 × cf3 × ........ cfn) the results of contamination assessment indices that mentioned above for three roads are showed in table 1.   table 1. enrichment factor, contamination factor and geo-accumulation index of heavy metals in urban road dusts in karaj cr zn cu ni pb mn index location 0.7 6.0 5.2 1.7 6.3 - ef malard road 0.8 6.7 5.7 1.8 7.0 1.1 cf -0.9 2.2 1.9 0.3 2.2 -0.4 igeo 2.76 pli 0.6 9.4 6.4 0.7 13.0 - ef karaj-qazvin freeway 0.7 10.6 7.1 0.7 14.5 1.1 cf -1.2 2.8 2.3 -1.0 3.3 -0.4 igeo 2.90 pli 0.4 3.6 3.2 0.4 2.9 - ef taleghani boulevard 0.5 4.6 4.0 0.5 3.6 1.3 cf -1.5 1.6 1.4 -1.7 1.3 -0.2 igeo 1.67 pli   discussion and conclusion   the range of heavy metals concentration in road dust samples is as follow: pb: 48-629 µg/g, ni: 19-224 µg/g, cu: 86-271 µg/g  , mn: 609-784  µg/g , zn: 282-1212 µg/g, cr:  40-117 µg/g. heavy metal contents and their possible sources in (the concentrations and sources of the heavy metals) road dust samples collected from the selected roads in karaj have been examined in this study. four contamination indices namely, enrichment factor (ef), geo-accumulation index (igeo), contamination factor (cf) and pollution load index (pli) were used in the assessment of level of metal contamination in the study area. the igeo, ef, and cf obtained for pb, zn, and cu in all the roads give an indication of anthropogenic origin. the ef calculated for the elements showed that cu, zn, and pb gave moderate to significant enrichment of the elements in road dust. the computed index of igeo gave values in the range of unpolluted (for ni, cr and mn) to extremely polluted (for zn, cu and pb) indicating the accumulation of zn, cu, and pb from anthropogenic sources. in some stations (m5, m6 and m7) at the mallard road the concentrations of cr and ni were found to be higher than the alert values. it is probably due to industrial activity of the power plant, because the electric utility industry is the number two emitter of nickel and chromium. however, the average concentration of these two elements in all the roads showed that they have low level of pollution. to compare whether the three roadways suffer contamination or not, the pollution load index (pli) described earlier was applied. the pli is aimed at providing a measure of the degree of the overall contamination at all the sampling sites and along the various roadways. the results of the pli indicated that k1 (kalak) and m6 (power plant) are the most contaminated site followed by k7(kalak) and k2 (fardis) in that order. the results of pli for all the roads show that the pollution load in karaj-qazvin freeway is the highest. ef, igeo, and cf values for pb in karaj-qazvin were higher than others. karaj-qazvin freeway is a road with high traffic capacity in vicinity of karaj. chen et al attributed this high pb contamination to the emission of pb from automobile exhaust and its deposition near highways and roads which has been reported worldwide. the major industrial source for zn is smelting. therefore, zinc used as a vulcanization agent in tires was the most likely source. the source of cu in street dust was indicated by research as being due to corrosion of metallic parts of cars derived from engine wear, thrust bearing, brushing and bearing metals.