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The overlooked occurrence of environmentally persistent free radicals in an area with low-rank coal burning, Xuanwei, China Peng Wang, Bo Pan, Hao Li, Yu Huang, Xudong Dong, Fang Ai, Lingyan Liu, Min Wu, and Baoshan Xing Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05453 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 10, 2018
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Environmental Science & Technology
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The overlooked occurrence of environmentally persistent free radicals in
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an area with low-rank coal burning, Xuanwei, China
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Peng Wang 1, Bo Pan *1, Hao Li 1, Yu Huang 1, Xudong Dong 2, Fang Ai 2, Lingyan Liu 2,
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Min Wu 1, Baoshan Xing 3
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Technology, Kunming 650500, P. R. China
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Faculty of Environmental Science & Engineering, Kunming University of Science &
The affiliated Hospital of Kunming University of Science and Technology, Kunming,
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650500, China
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United States
Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003,
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*Corresponding authors: Bo Pan, phone: 86-871-65102829, fax: 86-871-65170906, e-mail:
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[email protected] 16 17 18
ABSTRACT The mining and burning of low-rank coal in Xuanwei, China have attracted a great deal
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of research attention because of the generated polyaromatic hydrocarbons (PAHs) and the
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high incidence of lung cancer in this region. Given the abundant transition metals in the allitic
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soil, we hypothesized that environmentally persistent free radicals (EPFRs) are formed in this
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region and the potential risk had not been addressed. Strong electron paramagnetic resonance
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(EPR) signals of 3.20 × 1017 - 3.10 × 1019 spins/g were detected in environmental samples,
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including chimney soot, coal, soil and total suspended particles (TSP). These EPR signals did
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not significantly change after 18-months storage and had g-values in the range of
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2.0039-2.0046, suggesting typical organic free radicals. Similar strong EPR signals were
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observed in PAH (anthracene and pyrene as model compounds) degradation on simulated soil
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particles and lasted over one month even when the applied PAHs were 100% degraded. Based
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on g-value and bond width, we propose that EPR signals detected in TSP and soot originated
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from both coal combustion and PAH photodegradation. Further research is thus urgently
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required to investigate EPFR generation, exposure and risk in Xuanwei to better understand
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the cause of high lung cancer incidence.
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TOC Art
35 Environmental samples
Lab simulation
PAHs degradation
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EPR Characteristics
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EPFRs occurrence and risks in Xuanwei
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INTRODUCTION Quantitative identification of hazardous chemicals is the very first step for any risk
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assessment.1 The commonly applied method is to separate these hazardous chemicals from
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the environmental matrix through solvent extraction (such as for organic contaminants) or
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acid digestion (such as for heavy metals).2, 3 The obtained chemical concentrations are then
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used in the risk assessment modeling.
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Researchers often use this procedure to evaluate the risks of contaminants in a specific
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region. However, inconsistency between the measured contaminant concentrations and health
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risks is common. For example, in a joint analysis of 19 European cohorts, no statistically
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significant association was observed between long-term exposure to elemental constituents of
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environmental particles and total cardiovascular mortality.4 In a study focusing on the breast
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caner incidence in Hawaii over the past few decades, only 30% of the cancer risk could be
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explained by known factors.5 Franklin et al. also noted that acute cardiovascular events could
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not be fully explained by traditional risk factors when assessing the air pollution.6 These
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unexplained associations between the characteristics of conventional pollutants levels and
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major high health risks indicated that the risk assessment based on the pollutant concentration
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level is incomplete and even controversial.
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Previous investigators have noted that risk assessment is generally based on the toxicity
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database of individual contaminants. The toxicity of mixtures of various contaminants is not
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available yet, which may be the primary reason for the unexplained health risks as discussed
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earlier. In addition, some unknown toxic chemicals should be evaluated carefully. For
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example, environmentally persistent free radicals (EPFRs) have attracted some research
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attention because of their significant toxicity and risk7, but have not been monitored in
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studies involved with regional risk evaluations.
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In brief, organic free radicals, generated as the byproducts of organic chemical
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degradation, may interact with transition metals through electron transfer.8 These free radicals
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are stabilized and can last over 5 days, or even longer. Thus, they are called EPFRs, in
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contrast to common free radicals with short lifetimes, such as reactive oxygen species. EPFRs
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have been shown to induce strong oxidative stress in vitro, leading to cytotoxicity, pulmonary
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pathologies, and environmental asthma.9-13 Unfortunately, EPFRs have been detected in
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soil/sediment14, suspended particles15 and natural organic matter.16 How is EPFR formation
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dependent on soil composition? How is EPR signal strength related to environmental
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conditions and particle-pollutant association? These questions remain unanswered; thus,
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EPFRs have not been incorporated into regional risk assessment. Extended work is therefore
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urgently warranted to reveal EPFR generation in relation to geochemical environments.
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Xuanwei is a well-known area in Yunnan province, China with rich coal sources. Coal in
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this area belongs to lignite; thus, PAHs are generated widely during coal burning. It should be
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noted that the soils in this area belongs to a typical allitic soil, with abundant transition metals,
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such as Fe, Cu, and Zn. We thus hypothesized that PAH degradation byproducts may interact
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with these transition metals so that free radicals are stabilized. The formation of EPFRs may
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be ubiquitous in the environment, posing health risks to the residents. However, the
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occurrence and generation mechanism of EPFRs have never been studied in Xuanwei. We
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adopted both field investigation and lab simulation to study EPFR occurrence. This study
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may provide a new perspective for the future regional risk assessment in Xuanwei.
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EXPERIMENTAL SECTION
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Sample Collection. All the samples were collected from Laibin county (Latitude: 26°31',
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Longitude: 104°15') in Xuanwei, Yunnan, China between August 23-28, 2015. The soil
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samples were collected from sites far away from the residential area with limited agricultural
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activities. Five soil sub-samples (100 g each) were collected from the surface (
