Environ. Sci. Technol. 2003, 37, 3531-3536
Effect of Chelators and Reductants on the Mobilization of Metals from Ambient Particulate Matter HEE SANG SONG, WON GI BANG, AND NAMHYUN CHUNG* College of Life and Environmental Sciences, Korea University, Anam-dong 5-1, Sungbuk-ku, Seoul, 136-701, Korea YONG SUNG CHO AND YOON SHIN KIM Institute of Environmental and Industrial Medicine, Hanyang University, Hangdang 17, Sungdong-ku, Seoul, 133-791, Korea MYUNG HAING CHO College of Veterinary Medicine, Seoul National University, Suwon, 441-744, Korea
Ambient urban particulate matter (PM) contains various transition metals. When the PM is inhaled into the lung, not all but some part of metals from the particles might be mobilized to participate in a reaction that can damage various biomolecules, such as DNA and proteins. The dust particle size as well as organic acids may influence the metal mobilization. Thus, the mobilization of the metal from two standard reference materials (SRM; NIST, USA) and urban PM (PM2.5 and PM10) collected in the Seoul area was measured in the presence of artificial or biological chelator with or without reductant. The degree of the mobilization was higher with the artificial or biological chelator than the control with saline. In some cases, a reductant increased the mobilization as much as about 5 times the control without the reductant. Especially, the mobilization of Fe was greatly influenced by the presence of reductants. In general, the degree of the mobilization of the transition metal was higher with PM2.5 than with PM10. Therefore, it is expected that, considering the previously known toxicities of the transition metals, PM2.5 is more damaging to various biomolecules than PM10. The results also suggest that not the total amount but the mobilizable fraction of the metal in the ambient PM should be considered with regard to the toxicity of the urban particulate matter.
Introduction Numerous epidemiological studies have shown that elevated levels of ambient air pollution are associated with an increase of asthma (1), chronic obstructive pulmonary disease, emphysema, chronic bronchitis, and cardiovascular disease (2). Besides increasing these nonmalignant respiratory diseases, the ambient air pollution may also be responsible for the increasing rate of lung cancer (3). Ambient air pollution, comprised of particles derived from natural and anthropogenic origin, is a complex mixture of organic and inorganic compounds, including soluble or free transition * Corresponding author phone: +82-2-3290-3026; fax: +82-2923-8183; e-mail:
[email protected]. 10.1021/es025981p CCC: $25.00 Published on Web 07/22/2003
2003 American Chemical Society
metals (4). The toxic effects of the particulate matter are mainly attributed to small inhalable particles with an aerodynamic diameter of less than 10 µm (PM10) or to fine particles below 2.5 µm (PM2.5) (5). Because of their large specific surfaces, these particles can adsorb transition metals as well as various organic substances such as polycyclic aromatic hydrocarbons (PAHs). These compounds and transition metals might enhance the carcinogenic potency of the PM (6). The main focus of research on the toxic effects of PM has been on the organic constituents, including PAHs such as benzo[a]pyrene. However, in recent years, research on the toxic effect of the transition metals of particulate matter has increased (4, 7, 8). Biochemical mechanisms for particle-induced health effects are poorly understood; however, it has been hypothesized that such adverse health effects from particle exposure can result from metal-mediated generation of reactive oxygen species (ROS) (4, 7), which can cause severe oxidative stress within cells or tissue through the oxidation of nucleic acids, proteins, and lipids (9, 10). Ambient PM contains transition metals (such as Fe, Cu, Ni, V, Co, and Cr) in different amounts, forms, and oxidative states, of which Fe especially can be involved in the generation of highly reactive hydroxyl radical OH• in the presence of O2 (or H2O2) and/or a reductant such as ascorbate or cysteine (9). Human lung epithelial lining fluid is estimated to contain 160 µM ascorbic acid (11). Thus, the ascorbic acid in the lung epithelial lining fluid may be involved in the production of ROS from inhaled air particles. Not only iron but also other metals (Cu, Ni, Co, Cr, and V) have been known to become redox-active under certain conditions and to cause oxidative damage to biomolecules (8, 12). Donaldson et al. (13) have suggested that the adverse health effects of PM10 could be due to the Fe present in substantial quantities. Further epidemiological studies showed a stronger correlation between respiratory diseases and even smaller particles with an aerodynamic diameter of less than 2.5 µm (PM2.5) (14). Some authors have hypothesized that ultrafine particles, even if present in the atmosphere at very low mass concentrations, are involved in the observed pathologies because of their large number per unit volume and their extremely high surface/volume ratio (15). However, the mechanisms underlying these adverse effects are not well understood, and major questions concerning the specific size fraction, chemical composition, and causative mechanisms leading to the observed health effects remain to be answered (16). It is questionable whether the metal remains associated with the particle or must be mobilized in order to promote the oxidative damages. If the metal remains with the particle, the potentials for DNA damage and lipid peroxidation by metal-mediated oxidative stress should be limited to the immediate vicinity of the particle. However, if the metal is mobilized from the particle, the redox activity of the metal may be altered and the number of sites for the intracellular damage may become increased. At physiological pH, chelators (especially physiological low molecular weight chelators such as citrate, acetate, or phosphate compounds) were required for the mobilization of iron from asbestos in vitro (17). These suggested that iron mobilization in vivo was the result of chelation (18). Furrer and Stumm showed further detailed information about the role of surface complexation on reductive and nonreductive dissolution kinetics of minerals (19). However, while their results are very useful for the interpretation of metal mobilization from minerals, we have found that they have a limited usability in the VOL. 37, NO. 16, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Total Amount (µg/g) of Metals in SRM1648 and SRM1649aa metal Co Cr Cu Fe Mn Ni Pb V Zn b
SRM1648 18b
403 ( 12 609 ( 27 39 100 ( 1 000 860b 82 ( 3 6 550 ( 80 140 ( 3 4 760 ( 140
SRM1649a 16.4 ( 0.4 211 ( 5 223 ( 7 29 800 ( 700 237 ( 8 166 ( 7 12 400 ( 400 345 ( 13 1 680 ( 40
a Data Quoted in Certificate of Analysis published from NIST. Noncertified values of constituent elements.
interpretation of the metal mobilization from the ambient PM. This is because SRM and PM are highly heterogeneous physicochemical mixtures of organic and inorganic compounds, the composition of which varies within and between cities and with seasons and time of day (4, 20). This means that kinetics studies with a specific type of PM do not have much meaning and application. Although there is an increasing interest in the toxicity of the transition metals in ambient PM, most of the previous studies emphasized the total extractable amount of the metals from PM (20). However, we believed that only a fraction of metals are mobilized to be responsible for biological damages. That is, for toxicological risk assessment, it is logical to consider the mobilized thus bioavailable fraction of metals from PM. Thus, for the first time, we have decided to investigate the degree of the mobilization and the factors that affect the mobilization of various metals from different sizes of urban PM. We found that only a fraction of the metal was mobilized from urban PM and that the mobilization was influenced by both particle size and season.
Materials and Methods Particulate Matter. Standard reference material (SRM) 1648 and 1649a were purchased from the National Institute of Standard & Technology (Gaithersburg, MD). SRM1648 is an urban air PM sample, and SRM1649a is an urban air dust/ organics sample. Approximately 50% and 30% of the particulates in SRM 1648 and 1649a, respectively, are less than 10 µm in mean diameter. The total amount of each metal contained in SRM 1648 and 1649a is shown in Table 1. Ambient PM was collected at Seoul metropolitan area with a high-volume air sampler (Andersen Inc., USA) at a flow rate of 1.2 m3/min, using a quartz microfiber filter (20.3 × 25.4 cm) (Whatmann International Ltd., U.K.). The highvolume sampler is designed to collect particles into two fractions, PM10 (particles
