Comparison of Three Tree Leaves as Biomonitors of Heavy Metals Contamination in Dust, A Case Study of Isfahan

Author Name(s): Alireza Aghaei, Hosein Khademi, Saeid Eslamian
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Abstract

The major challenges facing atmospheric pollution assessment are the difficulties and the high costs associated with direct sampling. Instead of dust samples, tree leaves have been used as biomonitors to overcome these difficulties. In this study, leaves were collected from the three tree species of Platanus orientalis L, Ulmus boissieri, and Morus alba at 22 sites to monitor pollution in the city of Isfahan. Sampling was carried out on a monthly basis over a period of 7 months (May to November2014). The difference between concentrations of such heavy metals as Zn, Cu, Ni, Mn, Fe, Co, and Pb on washed and unwashed leaf samples was determined to assess the capability of the leaves in monitoring atmospheric dust. The difference between the dust accumulated each month and that of the previous month was found to increase. From a certain point in time onwards, this increasing trend was observed to shift to a decreasing one because of a heavy rainfall event. Maximum differential concentration of heavy metals between washed and unwashed leaves in seven consecutive months was observed in the leaves of Platanus orientalis L. On other hand, a significant correlation was found between the concentrations of heavy metals collected in dust traps and those in the dust accumulated on the leaves of Platanus orientalis L. This indicates that Platanus orientalis L leaves offer a better potential for tracing atmospheric heavy metals and can, thus, be used as a cheap and available tool for monitoring atmospheric heavy metals.

Keywords

Heavy metals, Atmospheric dust, Tree leaves, Biomonitors

Introduction

The increasingly intensified droughts in recent years due to the climate change and desertification in the Middle East have led to increasing dust pollution in the region. This is complicated by the increasing desertificationthat has dried more soil and released tiny particles, known as dust (Sing and Sing, 2010), to form aerosols joining the airflow by wind currents or other physical processes. The main concern about air pollution in cosmopolitan areas is huge dust emission, or aerosols, produced in a short time (Wang et al., 2016). Depending on the ecological conditions, aerosols containing heavy metals can remain in suspension in the air for only a limited time before they are quickly deposited on soil, water, or other surfaces and accumulate in the form of dry or wet deposits on the surfaces of moss, lichen, or tree trunks (Fujiwara et al., 2011).

One of the most recent and reliable methods used for assessing the amounts and types of environmental pollutants, especially air pollutants, is the use of natural biomonitors. Many plants are able to absorb and store most pollutants through their aerial tissues while they also absorb those in the soil and water. This makes them an especially useful proxy for instruments to estimate and monitor atmospheric pollutants (Celik et al., 2005; Madejon et al., 2004). Since trees are fixed and do not move, they serve as the best means of measuring dust pollution in the air (Ram et al., 2012). It has also been observed that tree branches and leaves are very efficient in trapping atmospheric particles, especially in urban areas (Nowak et al., 2006; Qiu et al., 2009). Over the last few decades, tree leaves have been increasingly used as biomonitors of heavy metals in the environment (Al-Khashman., 2004). Heavy metal concentrations, particularly in urban trees, depend on the uptake of their roots or the amount of dry and wet deposits on their aerial tissues (Lehndorff and Schwark. 2010). Several studies have been conducted on the deposition of urban atmospheric heavy metals on tree leaves. In all of these studies, trees have been introduced as cheap and accessible means for assessing atmospheric heavy metals.

Korori et al (2010) reported that the increased number of motor vehiclesas a result of the growing population in Tehranhas led to elevated heavy metal concentrations in the herbal fibers of plants growing in the city. In another study of atmospheric heavy metals, Al-Khashman et al. (2011) used Phoenix dactylfiera leaves as the best and cheapest biomonitor for assessing air-borne heavy metals in arid and semi-arid areas, particularly in crowded industrial cities in arid areas having high loads of dust. The authors found that increased heavy metal concentrations in leaf samples had a positive and significant correlation with increasing industrial activities in the area.

In assessing the temporal trend and aggregation of some heavy metals using Quercus robur and Tilia cordata in Belgrade, Aniĉić et al. (2011) found that atmospheric subsidence had a stronger correlation with heavy metal concentrations in Quercus robur leaves than with those in Tilia cordata leaves. The authors also observed that the temporal trend of Zn, Cu, Ni, Mn, Fe, Co, and Pb accumulation could be better traced in Quercus robur leaves than in those of Tilia cordata. Deljanin et al. (2014) studied the deposition of Pb isotopes in the four tree species of Tilia cordata, Aesculushippocastanum, Acer platanoides, and Betula pendula in Belgrade, Serbia, to find Betula pendula leaves as the best biomonitor for identifying the source of Pb. The study also revealed that Betula pendula leaves were capable of storing the highest amount of Pb in the cortical layer. Madejón et al. (2006) studied absorption of heavy metals in the leaves and fruits of Oleaeuropaea and Quercus robur in Spain. They found that Quercus robur leaves were imbued with heavy metals from atmospheric subsidence and that Quercus robur served as a better bio-aggregator than Olea europaea. Studying urban heavy metals in the leaves of tree species in Istanbul, Turkey, Baycu et al. (2006) reported the two species of Fraxinusexcelsior and Ailanthus glandulosa played important roles in trapping such heavy metals as Ni and Pb so that Fraxinusexcelsior leaves absorbed a large amount of Ni from the atmosphere while Ailanthus glandulosa absorbed the greatest amount of atmospheric Pb on its leaf surfaces. The present study focuses on tracing the sources of heavy metals in atmospheric dust using the three tree species commonly grown in Isfahan. In order to discriminate between the heavy metals absorbed from the soil and those accumulated on tree leaves from the atmosphere, the tree leaf samples were divided into washed and unwashed.

Conclusions

The study revealed that the leaves of Platanus orientalis L leaf are better bio-indicators of heavy metals in the urban atmosphere than are those of Morus alba and Ulmus boissieri. More particularly, it was shown that Zn, Fe, Mn, Ni, and Cu could be better traced by the leaves of this species than the two metals of Co and Pb. In order to understand whether heavy metals existing on plant leaves are the result of root uptake or atmospheric dust accumulated on tree leaves, the difference between metal concentrations in washed and unwashed leaves was determined for each sampling site and each tree species. The evidence obtained showed that the closer we drew to the end of the sampling period when leaf growth reached its culmination, the greater would become the difference between heavy metal concentrations in washed and unwashed leaves. This was ascribed to the temporal trend in aggregation of dust deposition on tree leaves and the increase in tree leaf surface area. During the early months of the sampling period, the difference between the heavy metal concentrations in washed and unwashed leaves was found to be insignificant for some metals; however, the difference became significant toward the later months of the sampling period. Contrary to our expectation, the difference between washed and unwashed leaves disappeared in the last month of the sampling period (T7), which was attributed to the rainfall event which washed the dust off the leaves. Another finding of the study was the higher heavy metal concentration in the dust on the leaves of Platanus orientalis L than on those of the other two species. Moreover, the correlation found between the heavy metal concentrations in the dust collected on traps and that in the dust accumulated on the leaves of Platanus orientalis L ‒ particularly during the final months of the sampling period ‒ was greater than that established between the concentrations of the dust traps and those on Morus alba and Ulmus boissieri leaves. It was, hence, concluded that Platanus orientalis L was a better replacement for the costly measuring instruments.

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