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Assessment of metal immission in urban environments using elemental concentrations and Zn isotope signatures in leaves of Nerium oleander Ana Martín, Cristina Caldelas, Dominik Jakob Weiss, Iker Aranjuelo, and Enrique Navarro Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00617 • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018
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Environmental Science & Technology
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Assessment of metal immission in urban environments using elemental concentrations and zinc isotope signatures in leaves of Nerium oleander
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Martín, A.1, 2, Caldelas, C.3*, Weiss, D.4, Aranjuelo, I.5 and Navarro, E.1*
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* Corresponding authors:
[email protected];
[email protected] Pyrenean Institute of Ecology-CSIC. Avda. Montañana, 1005, 50.059 Zaragoza, Spain.
San Jorge University. Campus Universitario Villanueva de Gállego. Autovía A-23 Zaragoza-Huesca Km. 299, 50.830 Villanueva de Gállego (Zaragoza), Spain. Department of Evolutionary Biology, Ecology, and Environmental Sciences, University of Barcelona, Avda. Diagonal, 643, 08028 Barcelona, Spain. Department of Earth Science and Engineering, Imperial College of London, London SW7 2AZ, UK.
Agrobiotechnology Institute (IdAB)-CSIC-UPNA-GN, Avenida Pamplona 123, Mutilva Baja (Navarra), Spain.
ABSTRACT
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A thorough understanding of spatial and temporal emission and immission patterns of air
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pollutants in urban areas is challenged by the low number of air-quality monitoring stations
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available. Plants are promising low-cost biomonitoring tools. However, source identification of
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the trace metals incorporated in plant tissues (i.e. natural vs. anthropogenic) and the
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identification of the best plant to use remain fundamental challenges. To this end, Nerium
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oleander L. collected in the city of Zaragoza (NE Spain) has been investigated as a
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biomonitoring tool for assessing the spatial immission patterns of airborne metals (Pb, Cu, Cr,
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Ni, Ce, and Zn). N. oleander leaves were sampled at 118 locations across the city, including the
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city center, industrial hotspots, ring-roads, and outskirts. Metal concentrations were generally
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higher within a 4 km radius around the city center. Calculated enrichment factors relative to
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upper continental crust suggest an anthropogenic origin for Cr, Cu, Ni, Pb, and Zn. In addition,
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zinc isotopes showed significant variability that likely reflects different pollution sources. Plants
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closer to the industrial hotspots had heavier isotopic compositions (δ66ZnLyon up to +0.70‰),
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likely indicating significant contributions of fly ash particles, while those far away were
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isotopically light (up to -0.95‰), likely indicating significant contributions from exhaust
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emissions and flue gas. This information may be applied for improving the environmental and
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human risk assessment related to the exposure to air pollution in urban areas.
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Keywords: Zinc stable isotopes, air pollution, biomonitoring, trace element, galvanization,
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pollution tracing, aerosols, aluminum, copper, nickel, lead
1. INTRODUCTION 1 ACS Paragon Plus Environment
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More than 70% of the European population lives in urban areas, often exposed to elevated levels of air pollutants. These are emitted from a variety of sources, the most important of which are road traffic, domestic dwellings, industrial facilities, and power generation plants 1. Traditional air pollutants include sulphur dioxide, nitrogen oxides, carbon monoxide, lead (Pb), particulate matter (PM), ozone, and volatile organic compounds. Nowadays a new category of hazardous air pollutants (HAPs) is raising concern for its potential impacts on human health including metals, mineral fibers (i.e. asbestos), gases, polycyclic aromatic hydrocarbons, and dioxins. Industrial activity, traffic, urbanization, and agricultural activities have been identified as target sources of environmental pollution. Metals are not biodegradable and tend to accumulate in living organisms. Many metal ions are known to be either toxic or carcinogenic. While some metals like zinc, iron, and copper are essential for plants and animals, above a certain concentration threshold they can be very toxic to humans2. The respiratory and cardiovascular systems are strongly compromised by elevated levels of particulate matter, resulting in increased morbidity and mortality rates in urban areas. Monitoring efforts generally concentrate on the “traditional” pollutants such as (name them here), while HAPs are often only measured on a spatially and temporally restricted scale in monitoring programs 1. So, even if the European Environment Agency recommends: “to present and map air quality in Europe at relevant spatial and temporal scales” and “to provide quantitative relationships between air quality and the source emissions responsible”1, most EU cities have not yet implemented monitoring programs addressing these recommendations. One particular aspect of concern in urban air pollution is particulate matter (PM) 3. The long residence time of fine PM facilitates the dispersal over large areas away from their emission sources 4. The emission of Pb, Cu, Cd, Ni, and Zn has increased in urban environments due to rising industrial production (waste incineration, power plants, metal refining, galvanization, etc.), traffic (tires, exhaust particles, brake discs, and others), and run-off from road furniture (road signals, paints) 5. Immission patterns based on road traffic density assessments are often unusable because of the limited number of air-quality monitoring stations available in cities. However, a detailed understanding of immission patterns is a requisite for developing targeted policies to prevent human and environmental exposure to such pollutants. Knowledge on local and regional distribution of airborne particle-bound substances and their toxic, genotoxic, and ecotoxic potential is still very limited. For this reason, biomonitoring of ambient pollution using bioindicator plants has been suggested by European authorities (i.e. ‘‘Directive relating to arsenic, cadmium, mercury, nickel, and PAHs in ambient air’’, EU 2004). Bioindicator plants proportionate valuable information of the spatial and temporal distribution of air pollutants. Moreover, the use of plants is well established 6 and is generally less expensive than conventional methods of air sampling based on automated stations. That allows for more detailed spatial and temporal studies of pollutants immission 7 and for the identification of emission sources and the verification of the dispersion routes of pollutants 8, 9. Pollutant absorption by plants takes place through roots and leaf stomata 10 depending on the form of the pollutants in the environment. When pollutants are present in either soil or water, they are taken up together with other essential and non-essential elements in response to
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concentration gradients induced by the selective uptake of ions by roots, or by element diffusion in the soil. Plants have evolved specific mechanisms to absorb nutrients from soil, and transport them from roots to shoots by radial transportation and xylem loading. While pollutant absorption by the roots has been most frequently described in the literature, stomatal opening is also an important entrance of air pollutants within the leaves 11. Moreover, stomatal responsiveness to air pollutants is a complex process conditioned by pollutant concentration, environmental conditions, plant developmental stage, and species 12. In fact, stomatal density and opening increase at low pollutant concentrations 11; inversely both factors show a marked decrease in highly polluted environments, to reduce exposure to pollution 13. Nerium oleander L is a attractive plant to be used for biomonitoring for several reasons: (i) it is widely distributed in urban areas and a perennial; (ii) sampling, identification, and cultivation are easy and inexpensive, and (iii) it is resistant to pollution and droughts. This species has been used in atmospheric pollution studies in various European cities 7, 8, 14-19, showing a good capacity for bio-accumulating Pb, Cr, Cu, Li, Ni, and Zn 14. However source identification of the trace metals incorporated in plant tissues (i.e. natural vs. anthropogenic) remains a fundamental challenge. Stable isotope ratios can provide important insight with respect to the sources of pollutants. This has been demonstrated successfully for Zn, which has been used to identify anthropogenic sources in marine and terrestrial environments 20-22. Since geological and industrial processes modify the isotopic ratios23-26 these can be used for tracing the origin of the metals. Elevated Zn content combined with isotopic signatures (δ66Zn) reported in lichens and particulate matter collected from urban locations, have pointed to traffic as an important Zn source 27, 28. This isotope system has also been measured in lichens around a mining facility, which displayed Zn signatures similar to those of the ore and heavier than the natural dust originated from the local host rocks 29. In other study, Zn isotopic signature allowed for identifying the industrial sources of pollution (a mining and smelting site) affecting peat surface layers 30, or for identifying different urban sources 28. According to different air quality guidelines (US Environmental Protection Agency, World Health Organization, European Commission) Zaragoza is classed as a moderately polluted city 31 , and Zn the metal with highest concentrations, i.e. a mean value of 212 ng m-3 31. The air quality of Zaragoza is influenced by the main industrial valley located in the north-east, where various metal processing industries are located (metallurgical, alloy, and galvanizing industries). The last report published by the local government suggested that most of the Zn emitted to the atmosphere would come from these industrial activities, although road traffic was suggested as a potentially relevant contributor 32. Therefore, the working hypothesis of this study is that the differences in metal content and Zn isotopic signatures between samples and their spatial distribution will allow for a screening of the different pollution emission sources in the urban area. Accordingly leaves of N. oleander were sampled across the city and their Al, Cr, Cu, Ni, Pb, and Zn concentration and Zn isotopic signature were characterized. Special attention was paid to the traffic density and the location
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of metal-related industries as potential pollution sources particularly for Zn. Finally, detailed metal immission maps were created. To the best of our knowledge this is the first time that such maps are created based on a plant biomonitoring approach.
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Zaragoza (NE Spain, 700,000 inhabitants) is located in a river valley. It presents an annual average temperature of 15oC and a precipitation of 367 mm with a mean relative humidity of 67%. It is a very sunny city (2,824 h year-1), situated at 200 m above sea level and influenced by the Mediterranean Sea and the Atlantic Ocean. A wind called Cierzo is predominant and blows regularly from North-West to South-East direction. The data of the districts population density has been obtained from the web of the city (https://www.zaragoza.es/sedeelectronica/).
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N. oleander is widely distributed in Zaragoza. Plants were sampled during July 2011 at 118 locations including the city center, industrial areas, ring-roads, and outskirts (Fig. 1). All the 15 districts dividing the city were sampled; see district distribution in Fig. 1. In the city center, 6 plants were sampled at Casco (HIS), 13 at Centro (CEN), and 9 at Delicias (DEL). Adjacent to the city center, 3 plants were sampled at Oliver (OLI), and 11 at Universidad (UNI). In the periphery, 10 plants were sampled at Casablanca (CAS), 4 at Almozara (ALM), 5 at Fuentes (FUE), 3 at Miralbueno (MIR), 8 at San Jose (JOS), 6 at Torrero (TOR), and 2 in the rural outskirts (RUR). In the industrial area across the river, 18 plants were sampled at Actur (ACT), 8 at Rabal (RAB) and 12 at Santa Isabel (ISA).
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About 50 g of fresh, mature leaves were cut off from each plant from all sides of the canopy at 1.5 m above the ground level. This height was selected considering the maximum plant height (about 3 m) and the fact that they are regularly pruned to control height and diameter. This height is accessible and coincides with the maximum crown diameter of the plants. Moreover, similar studies found that road traffic derived metals as Pb and Fe accumulated preferentially at plant leaves located at 0.3 and 1.5-2 m; these last values are within the adult head height range 33. Leaf samples were split in two subsamples, one of which was carefully washed with 0.1 M EDTA to remove surface dust and metals. Both subsamples, washed and unwashed, were dried for 48 h at 70 °C to determine dry weight, ground, and stored until analysis.
2. MATERIALS AND METHODS 2.1. Study area and sampling description
2.2 Metal analysis For the concentration analysis, 100 mg of material (from unwashed and washed plants) and 5 ml of hydrochloric-nitric acid mixture (4:1) were digested in an open vessel microwave digestion system (UltraCLAVE Milestone microwave) for 20 minutes at 220°C 16. Metals were determined by simultaneous inductively coupled plasma (ICP-OES, Mod. ICAP 6500 DUO THERMO). Calibration curves were performed using standard stock solutions 1.0 mg l-1 diluted in 10% nitric acid. Recovery for metals ranged from 80% (Cr, Pb) to 92% (Zn). The detection limit for all metals was established at 0.5 mg kg-1 dry weight.
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To discriminate anthropogenic from natural sources in the leaves of N. oleander, enrichment factors (EF) were calculated: EF= [metal /Al] sample/ [metal /Al] upper continental crust Aluminum was used because of (a) its higher concentrations in plants compared to other lithogenic elements, (b) its low detection limits using plasma source mass spectrometry, and (c) it has no large contributions from anthropogenic sources, making it an ideal ‘lithogenic’ reference element. The metal/Al ratios for upper continental crust were taken from 34. If EF ≈ 1, natural mineral dust from upper continental crustal material is likely the predominant source; if EF > 10, there is a significant contribution from anthropogenic sources 35. While many metal/Al ratios are higher in soils than in upper continental crust, EF > 10 are robust evidences of the anthropogenic origin of the materials 36.
2.3 Determination of elemental concentration and isotope ratios of zinc Plants from nine sites belonging to three potentially different exposure scenarios were selected for further Zn isotope analysis (see Fig. S4, at Supporting Information -S.I.-): city center (mainly exposed to traffic), industrial areas (exposed to industrial emissions as combustion and metal related industries), and outskirts (cleanest sites exposed to transport from long-range aerosol sources). The samples were selected to represent a wide range of internal leaf Zn concentrations (9-76 mg kg-1). The material (350 mg of leaf dry weight) was digested at 90°C in HNO3:H2O2 (1:1 v/v) over night, then 1 ml of 40% HF was added and material was digested for 2 h at 90°C. Digests were evaporated to dryness on a hotplate at 120°C and the residues were dissolved in 3 ml of 7 M HCl (bi-distilled). This protocol has been tested in previous studies 37. HNO3 and HCl were sub-distilled in the laboratory and HF and H2O2 were purchased as Suprapur (Merck) quality. Each solution was split into three aliquots: 1 ml for Zn concentration measurements, 1 ml for Zn isotope analysis, and 1 ml for archive. The first aliquot was made up to 3.5 ml of 1 M HCl prior to concentration measurements on a Varian VISTA PRO inductively coupled plasma atomic emission spectrometer (ICP-AES), with typical precision
