Document Type : Original Article
1 university of hormozgan
2 university of malayer
3 university of Tehran
Introduction: Emissions of particulate matter from car exhaust, tire wear, engine lubricants and street fence wear contain heavy metals which is dangerous for urban ecosystem; However, most of the particles are deposited on the canopy and soil of plant which absorbed by the plant. Pollution-resistant species with high capacity to absorb suspended particles can be used to remove suspended particles from the urban environment. For this reason, in this study, concentrations of heavy metals in leaves and soil of tree species in Bandar Abbas were measured and evaluated. Also the species with the highest potential for adsorption of heavy metals were identified.
Materials and methods: In order to achieve the research approach, at first three tree species including: Azadirachta indica, Conocarpus and Prosopis juliflora were selected as the dominant plants in Bandar Abbas and in 30 points of the fields with 5 points of non-polluted environment outside the city (control), and sampled from the surface soil and leaves of these species, and after preparing and digesting the samples, the concentration of heavy metals (cadmium, manganese, zinc and lead) emitted from urban traffic, were measured using atomic absorption spectrometry in soil and tree leaf samples. Next, the difference in heavy metal concentrations between tree species was analyzed by ANOVA and between soil and leaves and between urban and control environments was analyzed by t-student test. Then, to determine the tree species with high accumulation of heavy metals, two indices of heavy metal accumulation (BCF) and bioaccumulation index (MAI) were used.
Results and discussion: The results showed that the pattern of heavy metals in soil and leaves of tree species was Mn> Zn> Pb> Cd. The maximum concentrations of heavy metals in the soil and leaves were Conocarpus, Azadirachta indica and Prosopis juliflora, respectively. Spatially, the highest concentration of heavy metals in the sampling points was related to Imam Hossein street, which has an east-west extension and passes through the north of Bandar Abbas and the north-south streets of the city are connected to it; However, the minimum concentration of heavy metals was observed in the Persian Gulf streets. The concentrations of Mn and Pb in the soil of plant species were different and their differences were significant at 95% level, but concentration of Cd and Zn wasn't different in the soil of tree species. in Pb, Mn and Zn concentrations weren't differences in the shoots of the studied tree species and their differences were not significant at 95% level; However, the concentration of Cd in the shoots of the species was different and the difference between the species was significant at 95% level. There was a significantly difference in the concentration of heavy metals between soil and leaves at 95 and 99%; Also, the concentration of heavy metals in soil and leaves between urban and control field was significantly different at 95%. The pattern of BCF index in all species was Zn> Pb> Mn> Cd and heavy metals had moderate accumulation for tree species. Bioaccumulation index (MAI) showed that Conocarpus has the highest adsorption potential of heavy metals and the lowest adsorption potential was observed in Prosopis juliflora. In the spatial distribution of this index, Imam Hossein street and the streets that originate from it, have the highest index of bioaccumulation of heavy metals.
Conclusion: Conocarpus, by absorbing pollutants from urban traffic, purifies the polluted air of Bandar Abbas; but Prosopis juliflora, which is one of the migratory trees in the city of Bandar Abbas, with strong and deep roots, dries up the surface and groundwater of the region. On the other hand, its potential to absorb urban air pollutants is very weak. Therefore, Conocarpus despite their resistance to drought and heat, have a high potential in purifying urban pollution, can replace Prosopis juliflora.