Stud. Geophys. Geod., 56 (2012), 879−887, DOI: 10.1007/s11200-011-0265-1 879 © 2012 Inst. Geophys. AS CR, Prague

Ficus benjamina leaves as indicator of atmospheric pollution: a reconaissance study

BERTHA AGUILAR REYES1,3, RUBÉN CEJUDO RUIZ3, JUAN MARTÍNEZ-CRUZ2, FRANCISCO BAUTISTA3, AVTO GOGUITCHAICHVILI1,3, CLAIRE CARVALLO4 AND JUAN MORALES1,3

1 Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Geofísica – Sede

Michoacán, Universidad Nacional Autónoma de México, Campus Morelia, 58089 Morelia, México (baguilar@geofisica.unam.mx)

2 Centro de Investigaciones en Ecosistemas, Universidad Nacional Autónoma de México, Campus Morelia, 58089 Morelia, México

3 Laboratorio Universitario de Geofísica Ambiental, Centro de Investigaciones en Geografía Ambiental, Universidad Nacional Autónoma de México, Campus Morelia, 58089 Morelia, México

4 Institut de Minéralogie et de Physique des Milieux Condensés, Université Pierre et Marie Curie, Paris, France

Received: October 20, 2011; Revised: January 12, 2012; Accepted: January 19, 2012

ABSTRACT

We present a diagnostic study to evaluate the suitability of Ficus benjamina tree leaves as a captor of heavy metal particles from atmospheric dusts in urban areas. Leaf samples were taken at 16 localities within three areas in Morelia, state of Michoacán (Mexico’s medium size city, 830000 inhabitants). Measurements of magnetic susceptibility were conducted to determine the magnetic enhancement using samples from green, relatively unpolluted area, as a reference. The samples collected at areas with heavy traffic (main avenues) yielded values almost ten times higher than the values obtained for the unpolluted reference. Isothermal Remanent Magnetization curves are proportional to the degree of pollution. Associations of almost pure magnetite with heavy metals were revealed by scanning electron microscope.

Ke y wo r d s : urban tree, air pollution, magnetic enhancement, heavy metals

1. INTRODUCTION

The atmospheric particulate pollutants (particulate matter, PM10) are principally related to several sources like industry, construction material and the traffic/vehicle emission. These particules, which are smaller than 10 µm, can be inhaled; they contain some hazardous pollutants associated with magnetic PM which can be dangerous for the human health. In the absence of heavy industry, the main source of magnetic particles are the vehicle emissions (Muxworthy et al., 2001; Moreno et al., 2003; Gautam et al., 2005).

To determine the spatial and temporal distribution of atmospheric dust containing magnetic particulate matter, the magnetic properties of various tree leaves have been

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recently studied; Platanus sp. and Quercus ilex (Moreno et al., 2003); plane maple (Acer platanoides) and sycamore maple (Acer pseudoplatanus) (Hanesch et al., 2003); pine (Lehndorff et al., 2006); lime (Tilia platyphyllos) (Mitchell and Maher, 2009). It has been demonstrated that magnetic measurements can be used as a direct indicator of pollution level in atmospheric dusts, because magnetic particules can be associated with metallic pollutants like Cu, Cr, Zn and Pb (Hunt et al., 1984; Gautam et al., 2005; Lu et al., 2007).

This relationship could be due to the fact that heavy metals are incorporated into the lattice structure of the ferrimagnetic minerals which are produced during combustion processes, or because they are adsorbed onto the surface of ferrimagnetic minerals already present in the environments (Petrovský and Elwood, 1999; Kukier et al., 2003; Lu et al., 2008).

Most of the magnetic studies of tree leaves have used the susceptibility as the principal parameter for rock-magnetic analysis. However, the Isothermal Remanent Magnetization (IRM) seems to be a better indicator of pollution level since diamagnetic minerals do not have a significant contribution in this parameter, as is the case of the measurement of magnetic susceptibility. Recently, both magnetic susceptibility and IRM have been used as an optimum proxy of air pollution (Moreno et al., 2003; Hanesch et al, 2003; Maher et al., 2008; Mitchell and Maher, 2009).

All previous magnetic studies of tree leaves have been carried out with tree species which are almost inexistent in Mexico. Therefore, we present a detailed magnetic survey on Ficus benjamina leaves - an abundant species in urban landscapes in Mexico. The secretion of latex and morphologic characteristics of Ficus benjamina leaves, makes this species an excellent adsorbent of pollutant particles (by fixing of atmospheric dust), whose magnetic signal is detectable using common magnetic equipments.

Morelia is a medium city where industry is not an important activity, but where the traffic is very heavy in some areas. The interest of this study is to show how vehicle emissions are absorbed by tree leaves and discuss the potential use of Ficus benjamina to monitor atmospheric pollution using magnetic parameters. A subsequent objective would be to extend the use of the described specie in two of the most important cities in the country: Guadalajara (approx. 5 million inhabitants) and the Mexico City (approx. 22 million inhabitants), considered as critical zones due to pollution.

2. METHODS

2 . 1 . T r e e C h a r a c t e r i s t i c s

Ficus benjamina, native to Asia (Riffle, 1998), belongs to Moraceae family. This is a popular tree worldwide cultivated for reforestation and ornamental purposes (Starr et al., 2003), probably because its drought tolerance (Riffle, 1998). Description: trees evergreen; roots adventitious; bark gray, smooth; branchlets brown; leaf blade oblong, elliptic, lanceolate, or ovate, 4−6(−11) × 1.5−6 cm (Wunderlin, 1997).

2 . 2 . S a m p l i n g

The sampling sites were chosen considering the traffic density, according to the number of roads. We sampled five sites with heavy traffic (1, 2, 3, 5 and 8), located in avenues with 5−6 roads; five other sites with medium traffic (10, 11, 14, 15 and 17),

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located in avenues with 3−4 roads; five sites (4, 9, 12, 13, 16) with low traffic, in avenues with 1−2 roads. Two more sites (6 and 7) were sampled at areas without traffic which we consider as a baseline in present study (Table 1, Fig. 1). Sampling was made in winter (January 2009), after three months without rain. The tree leaf samples were taken at the same height (1.5−2 m) and they were chosen according to their maturity by considering their color and their size. About five mature leaves were collected per site. If we consider that this species (evergreen) keeps its leaves up to two years, samples have remained in the tree at least one rainy season before dry period. Leaves were first weighted, then dried in an oven at 50°C during 24 hours (Szönyi et al., 2008), weighted again, and then crushed in a mortar. They were placed into standard plastic cubes (11 cm3) in order to proceed with magnetic measurements.

2 . 3 . M a g n e t i c , M i c r o s c o p i c a n d C h e m i c a l A n a l y s e s

The susceptibility and IRM measurements were carried out at the Laboratorio Interinstitucional de Magnetismo Natural (LIMNA) and Laboratorio Universitario de Geofísica Ambiental (LUGA), National University of Mexico, Campus Morelia. We used a Bartington MS2B susceptibility meter to measure the susceptibility at low frequency (κLF at 470 Hz). We made a second measurement by using an AGICO MFK1 Kappabridge. Values obtained from both equipments are similar. From these κLF values, mass-specific susceptibility χLF was calculated. IRM was induced in the samples by placing them under the influence of increasing magnetic fields at room temperature, using a pulse magnetizer ASC IM-10. The IRM acquired at 700 mT is referred to as the

Table 1. Description of sampling sites and the density of traffic according to the number of nearby roads.

Site Location Description Number of Nearby Roads

1 Avenue Camelinas Heavy traffic

6 2 6 3 6 4 Park in Avenue Camelinas Slightly exposed to traffic 1 5 Avenue Camelinas Heavy traffic 6 6 Orchidarium park Non traffic 0 7 Planetarium park 0 8 Avenue Ventura Puente Heavy traffic 6 9 Cathedral garden, in the middle Slightly exposed to traffic 1

10 Cathedral garden N Medium traffic 4 11 Cathedral garden S 4 12 Villalongin garden W Gentle traffic 2 13 Villalongin garden N 2 14 Villalongin garden E Medium traffic 4 15 San Diego road , extremity E Medium traffic 3 16 San Diego road, middle Slightly exposed to traffic 1 17 San Diego road, extremity W Medium traffic 3

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saturation isothermal remanent magnetization (SIRM). Following the acquisition of SIRM, the S−200 was measured by applying an opposing DC field of 200 mT (IRM−200). We chose this value for opposing field for to have the possibility to detect small differences in coercivity because the S−300 is too close to 1.0 for all samples. The normalized results are expressed as 200 200S IRM SIRM− −= . All the remanent magnetizations were measured using an AGICO JR6 spinner magnetometer.

The observations under the scanning electron microscope (SEM) were performed on selected samples using a SEM/FRG ZEISS Ultra 55 scanning electron microscope, and the concentration of heavy metals was determined with the help of Noran System 7 EDS detector at SEM laboratory in the Institut de Minéralogie et de Physique des Milieux Condensés, Université Pierre et Marie Curie, Paris, France.

3. MAIN RESULTS AND DISCUSSION

The IRM curves show a low to medium coercivity behavior for all samples, because saturation is achieved before 300 mT. The IRM curves were averaged through interpolation method (OriginPro software). They are shown in Fig. 2 for each group of samples.The “slightly exposed to traffic” and “gentle traffic” samples (Table 1) were grouped in “low-traffic” category. Thus we have 5 samples in each group exposed to traffic, and 2 samples in the group of samples non affected by traffic. The SIRM gave almost seven times higher values for heavy traffic areas comparing to zones without traffic.

Fig. 1. Location of studied sites in Morelia: (1−5) at the periphery, (6−7) in a green area near to a main avenue (8), and (9−17) two zones in downtown.

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The S−200 values range between 0.7 and 1.0 for the majority of samples, confirming the presence of a soft magnetic (ferrimagnetic) phase (Fig. 3).

It has been reported that remanence parameters combined with magnetic susceptibility measurements indicate with more accuracy the presence and the relative amounts of

Fig. 2. Averaged IRM acquisition curves for each group of samples according to density of traffic (see Table 1).

Fig. 3. Plot of mass-specific susceptibility χLF vs. S−200. Dashed line indicates the boundary between minerals with low and high S−200 ratio. A predominance of low coercivity minerals as the magnetic carriers (high S−200 ratio) is observed.

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magnetic particles derived from anthropogenic sources in place of only magnetic susceptibility (Chaparro et al., 2006; Yang et al., 2007). The SIRM against χLF is plotted in Fig. 4, a reasonably good correlation is observed, which indicates very similar magnetic carriers (Thompson and Oldfield, 1986).

We assume that values of magnetic parameters are mainly due to anthropogenic ferrimagnetic minerals, coming mainly from vehicle fuel combustion. The contribution of magnetic particles coming from ground is negligible. The highest values correspond to Samples 1, 3, 5 and 8, which were taken along main avenues (see Table 1). The SIRM values are 3 to 6 times higher than SIRM values from parks, and their corresponding χLF values are up to 12 times higher than χLF values from parks. The correlation observed shows different levels of magnetic concentration.

The association between magnetite and heavy metals was observed by scanning electron microscopy. We analyzed 2 samples from each group. Fig. 5 shows the magnetite particles as irregular shaped-aggregates (<1 μm). The chemical spot analysis indicates the presence of magnetite associated with S and heavy metals as Cr, Ni, Cu, and Pb. This association is produced by fuel-combustion (Abdul-Razzaq and Gautam, 2001) as well as the abrasion or corrosion of the vehicle (Kim et al., 2007; Maher et al., 2008). At low magnifications, these particles are observed covering most of the surface of the leaves. The stomata, saturated with particles of similar nature, are visible with difficulty. In contrast, samples not exposed are almost clean, and its stomata are visible.

The good correlation between χLF and SIRM shows that magnetic methods applied on Ficus tree leaves can be used as a proxy to determine different degrees of pollution by heavy metals not only in the studied area, but in any city where this species is used for urban landscapes.

Fig. 4. SIRM versus χLF, a good correlation is observed. The highest values correspond to heavy traffic.

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The magnetic survey of Ficus benjamina opens new alternatives to atmospheric pollution monitoring in Mexican cities. Ficus benjamina is an ornamental plant that also functions as an environmental barrier to protect households against urban dust pollution.

4. CONCLUSIONS

The evaluation of magnetic susceptibility and saturation isothermal remanent magnetization in Ficus benjamina leaves from Morelia shows that these parameters reflect the relative levels of pollution: samples exposed to heavy traffic are up to 6−12 times higher, respectively, than the reference values (non exposed samples). The magnetic signal is almost exclusively due to magnetite produced by vehicular combustion, which was confirmed by SEM observations. The good correlation between magnetic parameters and traffic density suggests that this species can be used as a monitoring material, not only because of its capability of catching atmospherically deposited particles, but also due to its abundance in urban zones within entire Mexico and other countries with similar climate.

Aknowledgments:This work was supported by the Universidad Nacional Autónoma de México

trough the Internal Project UM02, and the Consejo Nacional de Ciencia y Tecnología (CONACYT) through the projects CONSOL-SNI 118971 and Ciencia Básica-169915. The authors would like to thank Thomas Ihl for the map design.

Fig. 5. Morphological characteristics of magnetic particles (< 2 μm) and their association with heavy metals. Bright particles labeled with Mgt are identified as magnetite.

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