Formation and Evolution of Solvent-Extracted and Non-Extractable

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Characterization of Natural and Affected Environments

Formation and Evolution of Solvent-Extracted and NonExtractable Environmentally Persistent Free Radicals in Fly Ash of Municipal Solid Waste Incinerators Song Zhao, Pin Gao, Duo Miao, Lan Wu, Yajie Qian, Shanping Chen, Virender K. Sharma, and Hanzhong Jia Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b03453 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 12, 2019

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Environmental Science & Technology

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Formation and Evolution of Solvent-Extracted and Non-Extractable

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Environmentally Persistent Free Radicals in Fly Ash of

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Municipal Solid Waste Incinerators

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Song Zhaoa #, Pin Gaob #, Duo Miaoa, Lan Wua, Yajie Qianb, Shanping Chend, Virender K. Sharmac*, and Hanzhong Jiaa*

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

a College

of Resources and Environment, Northwest A & F University, Yangling 712100, China, Email: [email protected] b College of Environmental Science and Engineering, Donghua University, Shanghai 201620, China c Program for the Environment and Sustainability, Department of Occupational and Environmental Health, School of Public Health, Texas A&M University, College Station, Texas 77843, USA. Email: [email protected] d Shanghai Environmental Sanitation Engineering Design Institute Co. Ltd, Shanghai 200232, China

23 24 25 26 27 28 29 30 31 32

# S.

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*To whom correspondence should be addressed.

Zhao and P. Gao contributed equally.

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ACS Paragon Plus Environment


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ABSTRACT

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Environmentally persistent free radicals (EPFRs) are emerging contaminants

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occurring in combustion-borne particulates and atmospheric particulate matter, but

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information on their formation and behavior on fly ash from municipal solid waste

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(MSW) incinerators is scarce. Here, we have found that MSW-associated fly ash

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samples contain EPFRs concentration of 3-10 × 1015 spins g-1, line width (ΔHp-p) of ~

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8.6 G, and g-factor of 2.0032 - 2.0038. These EPFRs are proposed to be mixtures of

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carbon-centered and oxygen-centered free radicals. Fractionation of the fly ash-

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associated EPFRs into solvent-extracted and non-extractable radicals suggests that the

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solvent-extracted part accounts for ~ 45-73% of the total amount of EPFRs. Spin

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densities of solvent-extracted EPFRs correlate positively with the concentrations of Fe,

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Cu, Mn, Ti, and Zn, whereas similar correlations are comparatively insignificant for

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non-extractable EPFRs. Under natural conditions, these two types of EPFRs exhibit

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different stabilization that solvent-extracted EPFRs are relatively unstable, whereas the

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non-extractable fraction possesses long life span. Significant correlations between

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concentrations of solvent-extracted EPFRs and generation of hydroxyl and superoxide

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radicals are found. Overall, our results suggest that the fractionated solvent-extracted

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and non-extractable EPFRs may experience different formation and stabilization

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processes, and health effects.

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TOC Art

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INTRODUCTION

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An increase in world population with improving living standards has increased the

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amount of municipal solid waste (MSW). Incineration process is an effective

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technology to treat MSW.1 In China, 191.4 million ton MSW was delivered, of which

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32.3% subjected to incineration.2 There is a great concern in China and other parts of

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the world for the use of incineration technology due to the emissions of contaminants

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from MSW.3, 4 Fly ash of MSW containing toxic metals (e.g., Cr, Cu, and Ni) and

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persistent organic pollutants [e.g., polycyclic aromatic hydrocarbons (PAHs),

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polychlorinated dibenzo-p-dioxins, dibenzofurans, and biphenyls] has a significant

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contribution to atmospheric particulate matter (PM) surrounding the municipal waste

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incineration plants.5-7 An exposure of fly ash to humans may lead to respiratory and

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cardiovascular diseases.8-10 Numerous investigations have been performed on analyzing

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contaminants in fly ash, but information on knowledge of an emerging contaminant,

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environmentally persistent free radicals (EPFRs), is largely missing in literature.

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EPFRs have been widely detected at elevated concentrations in cigarette tar,

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atmospheric PM, contaminated soils, as well as combustion byproducts originated from

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diesel, raw coal, and biomass.11-17 In the past decade, many investigations have been

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carried out under laboratory conditions to understand origin, behavior, environmental

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significance, and health relevance of EPFRs.18 Thermally-driven transformation of

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organic contaminants on single transition metal-doped particles has been conducted to

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mimic the generation of combustion-related EPFRs.19-21 The results showed that metal

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oxides not only acted as a catalyst, but also stabilized the generated free radicals.21, 22 4


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Interestingly, different metal oxides with various redox potentials or oxidizing strengths

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might exhibit different abilities to produce and to stabilize EPFRs.19, 23, 24 Most of these

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studies were limited to model environmental samples, however, examining the

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properties of free organic radicals on real combustion byproducts, particularly the

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evolution and stabilization of fly ash-associated EPFRs under natural conditions, are

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mainly unknown.

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The potential impacts of EPFRs may not be learned from their total amounts in solid

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samples. Previous studies have shown that particulate-associated EPFRs can be

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fractionated into several components via mineral composition separating, particle-size

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grading, and solvent-extraction processes.7, 24, 25 Recently, solvent extraction has been

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applied to fractionate and subsequently characterize EPFRs onto PM.24, 26 For example,

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the major proportion of free organic radicals in atmospheric particulates is considered

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to be originated from solvent-resistant organic carbon.26 The study emphasized that the

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solvent-resistant fraction may have more importance than metallic oxide-formed or

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solvent-extractable EPFRs due to its high proportion of EPFRs.26 Different types of free

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organic radicals obtained from fractionation may exhibit distinct natural and

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environmental impacts. However, there is no study in literature to distinguish EPFRs in

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fly ash-associated samples. Therefore, more investigations are needed to understand the

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formation mechanisms, evolution behaviors, and environmental persistence of different

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types of EPFRs. Moreover, potential generation of reactive oxygen species (ROS) of

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fly ash may trigger the oxidative stress of cells, which has an adverse effect on human

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being and environment.27,

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However, there is no investigation on the possible 5



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generation of ROS from EPFRs of fly ash samples. Previous work reported determining

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persistent free radicals on carbonaceous materials and inorganic minerals.29-31 In

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addition, atmospheric PM has been found to induce a large amount of ROS, which are

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probably resulted from the redox cycling of EPFRs and/or reactions among the particle

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components.18, 32, 33

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In the present study, we collected eleven fly ash samples from multiple MSW

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incinerators in China to learn formation and evolution of EPFRs. Electron paramagnetic

108

resonance (EPR) spectrometer equipped with light accessories was utilized in-situ to

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observe the evolution of EPFRs, while the spin-trapping technique was applied to detect

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the generated ROS on fly ash samples. The objectives of this study are: (i) to gain

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insights into the spin density (S) and other properties of EPFRs in fly ash samples by

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distinguishing them as “solvent-extracted” and “non-extractable” EPFRs, (ii) to

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comprehend the formation of EPFRs by correlating S with the mineral compositions of

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fly ash, and (iii) to investigate the potential formation and decay of radical intermediates

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by tracing the evolution of EPFRs and ROS under atmospherically relevant conditions.

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EXPERIMENTAL SECTION

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Chemicals and sampling sites. Details of the chemicals used in this study are given

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in Text S1 (Supplementary Information). MSW-fly ash samples were collected from

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the fabric filter in eleven different incineration plants in China during a period from

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May to August, 2018 (see Figure S1). In these plants, grate furnace technology is

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applied to incinerate MSW. The combustion temperature is 850-1000 oC and the 6


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residence time in secondary combustion chamber is higher than 2 s. These samples were

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labeled as Sample A, B, C, D, E, F, G, H, I, J, and K, respectively. The obtained samples

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were immediately sealed in aluminum foil bags and transferred to laboratory in three

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days. During this period, some active species/radicals might decay, but our research

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focused on the long-lived free organic radicals (i.e., EPFRs), which have been well

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known to have persistent life time. In China, MSW are generally composed of kitchen

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and other organic wastes (45-55%), scrap paper and cardboard (10-20%), plastic (5-

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20%), scrap glass (1-8%), metal (1-4%), wood (2-4%), textiles, rubber, and leather (~

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3%).34, 35 The particle size of fly ash samples was analyzed by a laser particle size

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analyzer (Malvern Instruments Ltd., ZEN3600, UK). As shown in Figure S2, the mean

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particle size ranged from 1.94 µm (Sample J) to 4.30 µm (Sample E), which is

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comparable to the atmospheric PM.25

135 136

Composition analysis of fly ash samples. The collected fly ash samples were firstly

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air-dried for overnight before subjecting to inorganic and organic analyses. Element

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compositions of H, C, and N in fly ash samples were measured by an elemental analyzer

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(Vario MICRO, Heraeus, Germany), and the results are given in Table S1. The

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elemental analyses of Si, K, Ca, Al, and other trace metals were carried out using X-

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ray fluorescence (XRF) spectrometry (Thermo-ARL SMS-2500, Billerica, USA) at an

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accelerating current of 100 mA and a voltage of 60 kV, and the results are summarized

143

in Table S2 and Table S3.

144 7



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Identification of organic compounds in fly ash samples. Organic compounds

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adsorbed onto the collected samples were first Soxhlet-extracted with a mixture of

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methanol and dichloromethane (1:1 of v/v) for 24 h before gas chromatography-mass

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spectrometry (GC-MS) analysis. The extracts were concentrated to 1-2 mL using a

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rotary evaporator and diluted into hexane. The concentrated extracts were purified by a

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silica gel column and a basic-alumina column (Merck Silica gel 60). The column

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chromatography was eluted with mixture solvents of dichloromethane and hexane at a

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volume ratio of 20:3, and the elution flow rate was ~ 12 mL min-1. The GC-MS

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instrument was an Agilent 7890A-5975C gas chromatograph (Palo Alto, USA) coupled

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with a mass spectrometer operated on a full scan mode (30-500 amu).36 The detected

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compounds in fly ash samples are listed in Table S4. Polycyclic aromatic compounds

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(PACs) were the main identified organic compounds, based on the peak heights. Other

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compounds include alkylated benzene, alkylated PAHs, ketonized/hydroxylated PAHs

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and their derivatives. Total organic carbon (TOC) was measured by Elementar vario

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TOC cube (Hesse-Darmstadt, Germany), and the obtained results are given in Table

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S5.

161 162

EPR measurements. Samples were pretreated prior to analysis by the EPR

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technique. Solvent-extractable and extraction residue EPFR species were subjected to

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EPR measurements and differentiated with different extraction procedures as described

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in Figure S3. The corresponding method is summarized in Figure S4. Briefly, each

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sample of 0.3 g was first extracted by methanol (2 mL).24, 26, 37 The remaining portion 8


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of methanol-extracted samples was further treated by a mixture of acetone and

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dichloromethane (1:1 of v/v) (2.0 mL) to acquire PAHs-based EPFRs on fly ash.16 Both

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solvent-extracting samples were placed on an ultrasonic bath for 10 min and

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subsequently centrifuged for 5 min at 8000 rpm. The previous study conducted in our

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laboratory confirmed that the ultrasound extraction method had no significant impact

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on production and persistence of EPFRs.38 The supernatants were filtered through 0.22

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μm membrane syringe filters and mixed together (2 mL methanol + 2 mL acetone-

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dichloromethane), as solvent-extractable EPFRs. The extractable procedures in our

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study have high extraction efficiency for the adsorbed species. The extracted radical

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species are therefore named as the solvent-extracted EPFRs samples. The remaining

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solid residues after extraction by acetone-dichloromethane mixture are referred as non-

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extractable EPFRs samples. The EPR analysis of the original fly ash samples without

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any solvent extraction was carried out to gain information on the total spins, which

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include both solvent-extracted and non-extractable EPFRs. Before EPR measurements,

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the original fly ash samples and the non-extractable samples were dried by an LGJ-10G

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freeze dryer (Sihuan, Beijing). For EPR analysis, 0.15 g of solid fly ash was placed into

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an EPR tube. All EPR measurements were carried out at room temperature (25 ± 2 oC)

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by a Bruker EMX-micro spectrometer (Karlsruhe, Germany) with the method reported

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previously.16 Solid samples were analyzed with microwave power of 2.02 mW, X-band

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microwave frequency of 9.8 GHz, and modulation amplitude of 4.00 G. The instrument

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parameters were set as follows: modulation amplitude, 4.0 G; receiver gain, 3.54×104;

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sweep width, 200 G; sweep time 167.7 s; time constant, 41.0 ms; and at center field, 9



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3500 G. The g-factor and spin density were determined based on comparison to

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standard radical, 2,2-diphenyl-1-picrylhydrazyl (DPPH).25, 39, 40 The spectra of original

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fly ash samples and solvent-extracted EPFRs are presented in Figure S5 and Figure S6,

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respectively.

193 194

In situ light-irradiation experiments. To investigate the EPFRs evolution under

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light irradiation, 0.15 g of the original samples and the above collected non-extractable

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samples were placed into an EPR tube and fastened into the resonator. A xenon lamp

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with an intensity of 0.25 W cm-2 (λ = 420 nm), equipped with a filter of > 380 nm, was

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used as the light source and placed in front of resonator at a distance of 500 mm. The

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EPR signals were collected every three minutes after the light was turned on.

200 201

Stabilization of EPFRs under simulated conditions. The persistence and evolution

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of EPFRs associated with fly ash samples were investigated by aging the collected

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samples in the dark at a relative humidity of ~ 60%. S and g-factor were obtained from

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the EPR measurements of solid samples as a function of time.

205 206

ROS measurements. To quantify the concentration of ROS on MSW-fly ash, spin-

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trap reagent such as DMPO in aqueous solvent was applied to capture the potentially

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produced ROS and subsequently detected by EPR. Specifically, 0.15 g of the solid

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samples was individually mixed with 0.45 mL of freshly prepared DMPO (150 mM) in

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dimethyl sulfoxide (DMSO) or water, in order to trap superoxide radicals and hydroxyl 10


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radicals, respectively.41 The suspension was shaken on a vortex mixer (MX-S, DLAB,

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China) for 30 seconds in touch mode and then filtered using a 0.22 µm membrane

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syringe filter. After suspension, 20 μL of the extract was immediately (~ 60 seconds)

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transferred to an EPR capillary tube and sealed at one end by vacuum grease. The

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capillary was put into an EPR tube and fastened in the EPR resonator. The obtained

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EPR signal was fitted by Spin-Fit module of Xenon program in Bruker EMX-micro

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spectrometer. The exact spin densities were determined by comparison with the DPPH

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standard.42 Details of EPR parameters are described above.

219 220

RESULTS AND DISCUSSION.

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The solvent-extracted and non-extractable samples were subjected to detailed EPR

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analysis to learn the stability and evolution properties of EPFRs and their subsequent

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role in generating ROS.

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EPFRs on fly ash samples. The EPR spectra reveal the presence of single, broad,

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and symmetric signals with g-factor of ~ 2.0035 and line width distance (ΔHp-p) ranging

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from 8 to 10 G. Such spectra are characteristics of EPFRs, which are comparable to the

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formation of EPFRs on the simulated combustion-generated particles at reaction

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temperatures of 150–400 oC.21 The S of EPFRs range from 3.7 × 1015 to 10.3 × 1015

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spins g−1 for various fly ash samples (Table S6), which are lower than the reported S in

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the range of 1016–1018 spins g-1 on other substances such as ambient PM, soot particles,

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and diesel exhaust matters.19, 43 Similar single, symmetric, and paramagnetic signals of

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the solvent-extracted EPFRs were observed with S ranging from 2.8 × 1015 to 4.7 × 1015 11



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spins g-1, which accounted for about 45.6-73.7% of the total amount of EPFRs on fly

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ash samples (Table S6). The solvent-extracted EPFRs could be formed via one-electron

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oxidation during the interaction between metal oxide surfaces and organic contaminants,

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such as chlorinated and hydroxylated benzene molecules, substituted naphthalene, and

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PAHs.13, 21, 38 The solvent-extracted EPFRs possess g-factor ranging from 2.0035 to

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2.0045, which are higher than that of the original fly ash samples (see Table S6). The

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results indicate that the extractable EPFRs are likely adsorbed EPFRs, which mainly

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consist of oxygen-centered radicals and/or carbon-centered radicals with a nearby

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heteroatom such as oxygen.22

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The values of S of non-extractable EPFRs range from 0.9 × 1015 to 6.2 × 1015 spins

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g−1 (Table S6), manifesting the possible formation of non-extractable EPFRs may be

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structurally bound on fly ash during MSW incineration. Such persistent free radicals

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were also produced and stabilized during the combustion of organic compounds.44 The

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g-factor of non-extractable EPFRs is ~ 2.0029, suggesting that the unpaired electrons

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are located on carbon atoms of the structural moieties, which are confined in the

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structure of solid matrix.45 Sample E has the highest amount of non-extractable free

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radicals of 6.1 × 1015 spins g-1, which is approximately seven times higher than that of

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sample I. In addition, the total S value (or sum) of solvent-extracted and non-extractable

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EPFRs is quite close to the amount of EPFR of original MSW-fly ash samples (Table

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S6), indicating that the extraction process applied in our study was able to account all

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of the free organic radicals in fly ash samples.

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The different amounts of EPFRs on MSW-fly ash samples may be due to their 12


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differences in mineral compositions and pyrogenic carbonaceous materials, as well as

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combustion conditions. In the incineration process, thermal cleavage of covalent bonds

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initiates a series of radical reactions including generation, propagation, coupling, and

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condensation of radical fragments. It seems that these free organic radicals are confined

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in the structures of carbonaceous materials, making them persist for a long time under

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natural ambient conditions. In addition, incomplete combustion of organic matter may

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also contribute to these stable EPFRs.26 The non-extractable EPFRs have been found

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on graphene oxide, biochar, hydrochar, soots, black carbon, and fly ash generated

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during the thermal treatment process.26, 45, 46

264 265

Correlations between EPFRs and compositions of fly ash. The relationship of the

266

amount of EPFRs with carbon fraction on fly ash is sought. The difference in TOC of

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fly ash samples before and after solvent-extraction treatment can be considered as the

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extracted organic compounds, which are sources of solvent-extracted EPFRs. The

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major carbon components of non-extractable residues, recognized as carriers of non-

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extractable EPFRs, which include elemental carbon and residue organic carbon. The

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identified extractable compounds in fly ash samples include alkylated benzene, PAHs,

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alkylated/ketonized/hydroxylated PAHs, phenol and its derivatives. Among these

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detected organic compounds, the chlorine- and hydroxyl-substituted aromatic

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compounds have been demonstrated to form EPFRs.19,

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showed that PAHs (e.g., anthracene, pyrene, and benzo[a]pyrene) could yield EPFRs

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in the presence of metal ions under natural conditions.38 Additionally, the substituted 13


21

Our previous study also


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PAHs (alkylated, ketonized, and hydroxylated PAHs) may also act as precursors for the

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formation of EPFRs.23 In contrast, other observed compounds, such as alkylated

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benzene and naphthol in the present work, may have a minor contribution to the

280

formation of EPFRs. There is no attempt was made to quantitatively analyze the

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individual organic compounds due to complexity involved in the analytical procedures.

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The relationship between EPFRs and TOC of the fly ash samples was analyzed.

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Significantly, S of the solvent-extracted and non-extractable EPFRs samples correlates

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linearly with their respective TOC (Figure S7). This further demonstrates that the

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detected EPFRs are mainly arisen from organic components of fly ash samples.

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In addition to organic carbons, the inorganic components such as metal ion/oxides

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also can govern the formation of solvent-extracted and non-extractable free organic

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radicals on fly ash.26, 47 Therefore, correlation analyses were performed between the

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normalized spin densities (SN) (i.e., SN = S/[TOC]) and metal contents. Figure 1 shows

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the correlations of SN of EPFRs with metals and the correlation parameters of r2 and p.

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The SN values of solvent-extracted EPFRs significantly correlate with the

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concentrations of Fe and Cu (r2 = 0.812 and 0.667, respectively, p = 0.001), indicating

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that Fe and Cu may potentially contribute to the formation of EPFRs.48 In contrast, the

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correlations are not strong for Zn, Mn, and Ti (r2 = 0.586 - 0.643, p < 0.1) (Figure 1).

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These results support previous findings from laboratory investigations that the formed

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free organic radicals strongly bounded to FexOy, TiO2, and ZnO surfaces, and thereby

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are stabilized.21, 39, 49 It is noted that Mn may also play an important role in EPFRs

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formation during the combustion process. Cu (r2 = 0.667, p = 0.01) exhibits a 14


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comparable correlation although its content is insignificant compared with Fe. This may

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be due to the fact that such produced EPFRs are more easily formed and stabilized on

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solid surfaces associated with Cu compared with Fe species.16, 50, 51 Overall, it appears

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that the transition metal ions/oxides in fly ash involved in different processes such as

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interaction with the organics, single electron transfer, and further partial oxidation may

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induce the formation of solvent-extracted EPFRs.

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The potential role of metals in the formation of non-extractable EPFRs was also

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investigated by correlation analysis of SN of non-extractable EPFRs with the

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concentration of metals (Figure 1). Insignificant correlations were found between SN

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and contents of individual metals (r2 < 0.40), indicating that the generation and

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stabilization of the EPFRs, structurally bounded to fly ash, is independent of the

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inorganic minerals.

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The above results suggest that the solvent-extracted and non-extractable EPFRs

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experience totally different formation processes and mechanisms. Solvent-extracted

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EPFRs may be formed by the interaction or chemisorption of aromatic compounds on

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metal-doped surfaces under post-combustion condition.13 In comparison, the formation

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of non-extractable EPFRs may correlate with thermal cleavage of covalent bonds

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during the combustion of organic matters, which merely depends on the metal contents

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of fly ash.

318 319

Evolution of EPFRs under ambient conditions. The evolution and behavior of

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particle-associated EPFRs are highly sensitive to the environmental factors, such as 15



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dark/light conditions.52 In this study, we monitored the dynamic variation of S under

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the environmentally relevant conditions (RH = ~ 60% in the dark), and the results are

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shown in Table S7. As indicated, the S of EPFRs slowly decreases with aging process

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for all fly ash samples. After a period of 45 days, the S decreases by 25-50% of their

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initial concentrations (Table S7). As the aging process is continued for another one

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month, the S remains constant and finally accounts for nearly 70-80% of the total

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amount of solvent-extracted EPFRs. The g-factor slightly decreases from 2.0032-

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2.0038 to 2.0029-2.0032. The observed decay of fly ash-associated EPFRs is probably

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due to the transformation of free organic radicals to final products on the fly ash. As

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reported previously, the reactions between PAH-type free radicals and O2/H2O could

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induce EPFRs consumption for the formation of final oxygenic products.22, 50

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For the non-extractable EPFRs samples, the values of S of EPR signals maintain for

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an extended period of time (Table S8). The relatively high stability of non-extractable

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EPFRs indicates that the structural EPFRs associated with fly ash rarely react with other

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compounds or small molecules such as H2O or O2. The detected free radicals may be

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formed heterogeneously in particle structure and are confined in encapsulation and

337

protected by hydrophobic microenvironment in the matrix.53 Therefore, the small

338

molecules cannot easily access with the structural free radicals, exhibiting higher

339

stability than solvent-extracted free radicals, which is consistent with our previous

340

studies.53, 54

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The effect of light irradiation on the formation of EPFRs was investigated by in-situ

342

irradiation of fly ash samples. As shown in Figure 2(A)-2(C), the concentrations of 16


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EPFRs on the representative samples show an initial rapid increase to the maximum in

344

~ 100 min and keep constant for up to ~ 250 min. The variation extent of fly ash Sample

345

E is significantly greater than other samples during the ~250 min of evolution process.

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The S of the representative samples is followed by a fast decay with the elapsed reaction

347

time (Figure S8). Among them, the decreasing rates of EPFRs for Sample E (~ 69.2%)

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and Sample I (~ 30.5%) are much faster than Sample H (~ 2%). An increase in the

349

formation of free organic radicals can be attributed to the photo-enhanced electron-

350

transfer between organic contaminants and metal oxides. The light irradiation of

351

atmospheric PM was reported to cause an increase of more than one fold in the

352

concentration of free organic radicals.55 The newly formed EPFRs were considered as

353

“secondary EPFRs”, which exhibited different characteristics depending on the

354

precursors.56 Organic compounds, especially PAHs, are susceptible to photolysis and

355

chemical oxidation in atmospheric conditions.57-59 In our study, the concentrations of

356

PAHs decrease with increasing in irradiation time (Figures S9 and S10). The oxidation

357

products are primarily identified as oxygenated PAHs, such as semiquinone, quinines,

358

and diol compounds.50, 60, 61 The quinone-like structures may act as electron shuttles and

359

the semiquinone radicals are readily produced as intermediates. Thus, the

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transformation from PAHs to oxygenated aromatics is probably accompanied by the

361

formation of oxygenic EPFRs on particle surfaces, possessing a g-factor > 2.0040.22

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With the elapsed irradiation time, the gradual decrease in EPFRs concentration is

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mainly due to the photo-degradation of these organic intermediates on particle surfaces.

364

This is supported by a decrease in the content of extracted PAH compounds with 17



365

increasing reaction time (Figures S9 and S10).

366

Likewise, the non-extractable EPFRs samples are also subjected to light irradiation.

367

In contrast to the solvent-extracted EPFRs samples, the concentration of EPFRs yields

368

a higher maximum at a relatively short-time period, followed by a slow decay compared

369

with their initial growth (Figure 2(D)-2(F)). The dominant mechanism governing the

370

variation of free radicals probably is the photo-induced charge transfer on particle

371

surfaces. Carbonaceous particle-bound EPFRs are related to their surface functional

372

groups, such as aromatic ketones, phenoxyl, quinone, and semiquinone.46 The photo-

373

irradiation likely enhances the charge-transfer within functional groups, promoting the

374

formation of free radicals on particle surfaces. However, the structural relaxation into

375

the radical ion state is not pronounced generally55. The photo-induced free radicals are

376

relatively unstable and ready to convert back to the original state under natural

377

conditions.62 After a relatively short period, therefore, a relative constant regime is

378

observed (Figure 2(D)-2(F)).

379 380

Detection of ROS. The adverse health effects of EPFRs arise not only from their

381

molecular precursors and subsequent products, but also because of their potential to

382

form ROS.30 The EPR technique coupled with spin-trapping agent of DMPO was

383

applied to determine the generation of ROS in an aqueous suspension of MSW-fly ash

384

samples. Figure 3 displays the variable EPR spectral features with composition of

385

several overlapped lines, indicating that various types and abundances of ROS are

386

produced on different samples. The observed EPR spectra are de-convoluted and fitted 18


Page 18 of 37

Page 19 of 37


387

by DMPO-OH (hyperfine coupling constants of aN =14.3 G, aβH = 12.7 G, aγH = 0.61

388

G), DMPO-1O2 (aN =14.4 G, aH = 13.5 G), DMPO-OOH (aN = 14.3 G, aH = 8.1 G),

389

DMPO-R (aN = 15.2 G, aH = 21.6 G), and DMPO-OR (aN = 14.5 G, aH = 16.6 G). As

390

shown in Figure S11, the observed ROS is mainly composed of hydroxyl, singlet

391

oxygen, carbon-centered, and oxygen-centered free radicals.

392

The EPR peaks related to DMPO-OOH are undetectable in the aqueous media of

393

DMPO. This may be due to the low efficacy for trapping superoxide radicals in aqueous

394

media and the relatively short lifetime of DMPO-OOH.30,

395

solvent of DMSO was used as a media for spin trapping of superoxide. 30 As shown in

396

Figure 3, a four-line signal is detected in the suspension of fly ash samples. The

397

observed EPR spectra are typical for DMPO-OOH spin adduct (Figure S11),

398

confirming the involvement of superoxide radicals on fly ash samples. To further

399

investigate the potential generation of ROS, tert-butanol, NaN3 and p-benzoquinone

400

were applied as quenching agents for ●OH, 1O2, and O2●−, respectively. The obtained

401

results confirmed the generation of these ROS onto fly ash particles.

63

Therefore, an aprotic

402

Concentration of ROS is quantified with assumption that ROS reacted

403

stoichiometrically with DMPO to form DMPO-ROS spin adducts. As shown in Figure

404

4 and Table S9 and S10, total concentrations of the detected ROS range from 19.34 ×

405

1015 to 38.35 × 1015 spins g-1. The relative contribution of individual ROS to the total

406

amount of ROS varies with samples. Samples A and D contain high levels of superoxide

407

radicals with 22.89% and 22.90%. Hydroxyl radicals are detected as the main species

408

of ROS for Samples D (34.42%) and H (30.89%). The concentration of oxygen19



409

centered radicals is lower than other radicals in all samples. Levels of singlet oxygen

410

vary in different samples. Among these detected ROS, superoxide radicals have the

411

highest concentration (29.48-57.06%), which is in agreement with the results reported

412

in studies of atmospheric PM.15

413

The formation of ROS correlates with the particle components such as EPFRs,

414

organic quinones, peroxides, and metal species. As reported previously, EPFRs on

415

particle surfaces may activate molecular oxygen to generate ROS even under dark

416

condition.64 Electron transfer from particle-bound free radicals to oxygen molecules

417

can induce the formation of O2-●, HO2●, and H2O2. These radicals can be further

418

decomposed into ●OH by dismutation and Fenton-like reactions.65 The reaction

419

between O2-● and ●OH may produce 1O2.66, 67 Hydroxyl radical can also be generated

420

by the decomposition of hydro-peroxides/peroxides, simultaneously yielding oxygen-

421

centered radicals.68-70 The presence of Fe(II) or quinines may enhance the

422

decomposition of ROOH to yield RO● radicals.69, 71 ROS production also correlates

423

with some metal ions and quinone compounds on particulate surfaces. It was reported

424

that semiquinones could react with O2 to form 1O2 under light irradiation.57 The 1O2 can

425

also be produced by interaction between O2 and the reduced transition metals such as

426

Fe(II) and Cu(I).27 The role of transition metals is crucial to enhance radical formation

427

via Fenton-like reactions and by participating in the redox-cycling of quinines.41, 71, 72

428

Overall, the generation of various ROS in our study is likely due to the presence of free

429

organic radicals, organic peroxides/hydroperoxides, and organic quinones as well as

430

transition metals. 20


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431

As discussed above, the formation of ●OH might correlate with the redox cycling

432

process associated with EPFRs. The respective contributions of solvent-extracted

433

EPFRs and non-extractable EPFRs on the formation of ROS were investigated. As

434

shown in Table S9, the removal of solvent-extracted EPFRs reduces the total amount

435

of ROS by 43-78% for all fly ash samples. In particular, the relative amount of ●OH

436

induced by non-extractable EPFRs samples decreases to ~ 20% of that in the original

437

samples. More than 50% of O2-● disappeared after extraction. Due to the metal content

438

merely changed before and after extraction, the difference between DMPO-OOH or

439

DMPO-OH adducts formed in the original samples and the non-extractable solid (i.e.,

440

non-extractable EPFRs samples) is most likely due to the organic solvent-extractable

441

compounds or species such as solvent-extracted EPFRs or organic quinones. The

442

extracted organic compounds include PAHs, alkylated/ketonized/hydroxylated PAHs,

443

and substituted benzenes (Table S4). To demonstrate that free organic radicals rather

444

than phenoxyl or quinine-type compounds may produce ROS, the detected compounds

445

including phenol, 1-naphthol, anthraquinone and B[a]P-diones were used as model

446

compounds to investigate the potential formation of ROS. EPR measurements showed

447

that DMPO-ROS signals for these species were undetectable. This set of experiments

448

also ruled out the possibility that the extractable quinine-type compounds induce the

449

formation of ROS. Therefore, the detected ROS was mainly attributed to the free

450

organic radicals associated with the extracted compounds like PAH-type EPFRs,

451

semiquinone-type EPFRs or phenoxyl radical species. Other possibilities that may also

452

produce ROS, such as ●OH and RO● radicals, include the decomposition of hydro21



453

peroxides/peroxides.

454

To further investigate the role of EPFRs on ROS production, the dependence of

455

DMPO adducts on solvent-extracted EPFRs is depicted in Figures 5A and 5B. The

456

signal intensity of DMPO-OH adducts (or DMPO-OOH) exhibits a linear increase with

457

the concentration of EPFRs adsorbed on fly ash. This observation supports the

458

hypothesis that solvent-extracted EPFRs play a vital role in the generation of O2●- and

459

●OH

460

correlations are found between the concentrations of EPFRs and other ROS such as

461

carbon-centered or oxygen-centered radicals. The variation of these ROS may be due

462

to the removal of relevant organic compounds such as organic hydro-

463

peroxides/peroxides from particle surface during the extraction treatment. In addition,

464

non-extractable EPFRs are also recognized to be responsible for the ROS generation.73

465

In this study, however, insignificant correlations are observed between the

466

concentration of non-extractable EPFRs and O2●- and ●OH radicals induced by the

467

solvent-treated samples (Figures 5C and 5D). Similar phenomenon was observed

468

previously.74 The generation of ROS induced by EPFRs binding on carbonaceous

469

materials was negligible (limited), of which other active species such as metal species,

470

organic peroxides, organic quinones may play a dominate role in the formation of

471

ROS.74

radicals, and the formation of ●OH correlates with O2●-.30 However, no such

472 473

Environmental significance. Atmospheric PM and combustion-associated fly ash

474

are considered as vital carriers of atmospheric pollutants such as metals and organics, 22


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Page 23 of 37


475

including EPFRs. The EPFR properties and mean particle sizes of fly ash samples in

476

our study are comparable to the hazardous PM like PM2.5 or PM10. Previous work

477

reported that fly ash samples had a significant contribution to atmospheric pollutants

478

surrounding municipal waste incineration plants.75-77 Therefore, EPFRs may also be an

479

emerging pollutant associated with atmospheric PM in the same area. More importantly,

480

our study characterized the types and behaviors of persistent free radicals on MSW-fly

481

ash samples collected from various incinerators. The presence of solvent-extracted and

482

non-extractable EPFRs exhibits distinct formation mechanisms, evolution behaviors,

483

and persistence under natural conditions. These two types of EPFRs have different

484

abilities to generate ROS, especially ●OH. The EPFR-containing fly ash may potentially

485

interact with lung antioxidants upon inhalation and respiratory deposition.78, 79 Excess

486

concentrations of ROS may cause oxidative stress to lung cells and tissues, resulting in

487

chronic respiratory and cardiopulmonary dysfunction.80-83 Thus, EPFRs in fly ash can

488

exert adverse effects on human health and ecological safety.84, 85 Overall, our study

489

provides valuable insights into the underlying formation mechanisms and reaction

490

behaviors of EPFRs in fly ash produced from MSW incineration, emphasizing the role

491

of extractable EPFRs rather than the total amount of EPFRs in samples to assess

492

environmental pollution. Furthermore, this approach will aid to better evaluate the risks

493

associated with combustion residues.

494 495

ASSOCIATED CONTENT

496

Supporting Information 23



497

The supporting information is available free of charge on the ACS Publications website.

498

Chemicals and materials, location of fly ash samples, EPR spectra of fly ash samples,

499

concentrations of EPFRs versus organic carbon, GC-MS analyses of fly ash samples,

500

EPRs spectra of radical species, and variation of EPFRs under dark condition.

501

ACKNOWLEDGEMENTS

502

Financial support by the National Natural Science Foundation of China (Grants No.

503

41571446 & 41877126), and the “One Hundred Talents” program of Shaanxi Province

504

(SXBR9171) are gratefully acknowledged. We thank anonymous reviewers for their

505

comments, which improved the paper greatly.

24


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REFERENCES

507

1. Wang, X.; Guo, M.; Koppelaar, R.; van Dam, K. H.; Triantafyllidis, C. P.; Shah, N., A nexus approach for sustainable urban energy-water-waste systems planning and operation. Environ. Sci. Technol. 2018, 52, (5), 3257-3266.

508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537

2. Wang, G.; Deng, J.; Ma, Z.; Hao, J.; Jiang, J., Characteristics of filterable and condensable particulate matter emitted from two waste incineration power plants in China. Sci. Total Environ. 2018, 639, 695-704. 3. Edo, M.; Ortuno, N.; Persson, P. E.; Conesa, J. A.; Jansson, S., Emissions of toxic pollutants from co-combustion of demolition and construction wood and household waste fuel blends. Chemosphere 2018, 203, 506-513. 4. Silva, R. V.; de Brito, J.; Lynn, C. J.; Dhir, R. K., Environmental impacts of the use of bottom ashes from municipal solid waste incineration: A review. Resour. Conserv. Recy. 2019, 140, 23-35. 5. Fujimori, T.; Tanino, Y.; Takaoka, M., Role of zinc in MSW fly ash during formation of chlorinated aromatics. Environ. Sci. Technol. 2011, 45, (18), 7678-7684. 6. Shen, C. F.; Tang, X. J.; Yao, J.; Shi, D. Z.; Fang, J.; Khan, M. I.; Cheema, S. A.; Chen, Y. X., Levels and patterns of polycyclic aromatic hydrocarbons and polychlorinated biphenyls in municipal waste incinerator bottom ash in Zhejiang province, China. J. Hazard. Mater. 2010, 179, (1-3), 197-202. 7. Feld-Cook, E. E.; Bovenkamp-Langlois, L.; Lomnicki, S. M., Effect of particulate matter mineral composition on environmentally persistent free radical (EPFR) formation. Environ. Sci. Technol. 2017, 51, (18), 10396-10402. 8. Magnani, N. D.; Marchini, T.; Garces, M.; Mebert, A.; Caceres, L.; Diaz, L.; Desimone, M.; Evelson, P. A., Role of transition metals present in air particulate matter on lung oxygen metabolism. Inter. J. Bio. Cell Bio.2016, 81, (Pt B), 419-426. 9. Cormier, S. A.; Lomnicki, S.; Backes, W.; Dellinger, B., Origin and health impacts of emissions of toxic by-products and fine particles from combustion and thermal treatment of hazardous wastes and materials. Environ. Health Persp. 2006, 114, (6), 810-817. 10. Li, J.; Zhang, Y.; Sun, T.; Hao, H.; Wu, H.; Wang, L.; Chen, Y.; Xing, L.; Niu, Z., The health risk levels of different age groups of residents living in the vicinity of municipal solid waste incinerator posed by PCDD/Fs in atmosphere and soil. Sci Total Environ. 2018, 631-632, 81-91.

539

11. Lomnicki, S.; Truong, H.; Vejerano, E.; Dellinger, B., Copper oxide-based model of persistent free radical formation on combustion-derived particulate matter. Environ. Sci. Technol. 2008, 42, (13), 4982-4988.

540

12.

538

Gehling, W.; Dellinger, B., Environmentally Persistent Free Radicals and their lifetimes in 25



541

PM2.5. Environ. Sci. Technol. 2013, 47, (15), 8172-8178.

542

13. Dellinger, B.; Lomnicki, S.; Khachatryan, L.; Maskos, Z.; Hall, R. W.; Adounkpe, J.; McFerrin, C.; Truong, H., Formation and stabilization of persistent free radicals. Inter. Sym. Combu. 2007, 31, (1), 521-528.

543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572

14. Borrowman, C. K.; Zhou, S.; Burrow, T. E.; Abbatt, J. P., Formation of environmentally persistent free radicals from the heterogeneous reaction of ozone and polycyclic aromatic compounds. Phys. Chem. Chem. Phys. 2016, 18, (1), 205-212. 15. Arangio, A. M.; Tong, H. J.; Socorro, J.; Poschl, U.; Shiraiwa, M., Quantification of environmentally persistent free radicals and reactive oxygen species in atmospheric aerosol particles. Atmos. Chem. Phys. 2016, 16, (20), 13105-13119. 16. Jia, H. Z.; Zhao, S.; Nulaji, G.; Tao, K. L.; Wang, F.; Sharma, V. K.; Wang, C. Y., Environmentally persistent free radicals in soils of past coking sites: Distribution and stabilization. Environ. Sci. Technol. 2017, 51, (11), 6000-6008. 17. Zhu, K.; Jia, H.; Zhao, S.; Xia, T.; Guo, X.; Wang, T.; Zhu, L., Formation of environmentally persistent free radicals on microplastics under light irradiation. Environ. Sci. Technol. 2019. DOI: 10.1021/acs.est.9b01474 18. Vejerano, E. P.; Rao, G. Y.; Khachatryan, L.; Cormier, S. A.; Lomnicki, S., Environmentally persistent free radicals: Insights on a new class of Pollutants. Environ. Sci. Technol. 2018, 52, (5), 2468-2481. 19. Vejerano, E.; Lomnicki, S. M.; Dellinger, B., Formation and stabilization of combustiongenerated, environmentally persistent free radicals on Ni(II)O supported on a silica surface. Environ. Sci. Technol. 2012, 46, (17), 9406-9411. 20. Assaf, N. W.; Altarawneh, M.; Oluwoye, I.; Radny, M.; Lomnicki, S. M.; Dlugogorski, B. Z., Formation of environmentally persistent free radicals on α-Al2O3. Environ. Sci. Technol. 2016, 50, (20), 11094-11102. 21. Vejerano, E.; Lomnicki, S.; Dellinger, B., Formation and stabilization of combustiongenerated environmentally persistent free radicals on an Fe(III)2O3/silica surface. Environ. Sci. Technol. 2011, 45, (2), 589-594. 22. Jia, H. Z.; Nulaji, G.; Gao, H. W.; Wang, F.; Zhu, Y. Q.; Wang, C. Y., Formation and stabilization of Environmentally Persistent Free Radicals induced by the interaction of anthracene with Fe(III)-modified clays. Environ. Sci. Technol. 2016, 50, (12), 6310-6319.

574

23. Yang, L. L.; Liu, G. R.; Zheng, M. H.; Jin, R.; Zhao, Y. Y.; Wu, X. L.; Xu, Y., Pivotal roles of metal oxides in the formation of Environmentally Persistent Free Radicals. Environ. Sci. Technol. 2017, 51, (21), 12329-12336.

575

24.

573

Truong, H.; Lomnicki, S.; Dellinger, B., Potential for misidentification of environmentally 26


Page 26 of 37

Page 27 of 37

576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610


persistent free radicals as molecular pollutants in particulate matter. Environ. Sci. Technol. 2010, 44, (6), 1933-1939. 25. Yang, L.; Liu, G.; Zheng, M.; Jin, R.; Zhu, Q.; Zhao, Y.; Wu, X.; Xu, Y., Highly elevated levels and particle-size distributions of environmentally persistent free radicals in haze-associated atmosphere. Environ. Sci. Technol. 2017, 51, (14), 7936-7944. 26. Chen, Q.; Sun, H.; Wang, M.; Mu, Z.; Wang, Y.; Li, Y.; Wang, Y.; Zhang, L.; Zhang, Z., Dominant fraction of EPFRs from nonsolvent-extractable organic matter in fine particulates over Xi'an, China. Environ. Sci. Technol. 2018, 52, (17), 9646-9655. 27. Gehling, W.; Khachatryan, L.; Dellinger, B., Hydroxyl radical generation from environmentally persistent free radicals (EPFRs) in PM2.5. Environ. Sci. Technol. 2014, 48, (8), 4266-4272. 28. Ayres, J. G.; Borm, P.; Cassee, F. R.; Castranova, V.; Donaldson, K.; Ghio, A.; Harrison, R. M.; Hider, R.; Kelly, F.; Kooter, I. M.; Marano, F.; Maynard, R. L.; Mudway, I.; Nel, A.; Sioutas, C.; Smith, S.; Baeza-Squiban, A.; Cho, A.; Duggan, S.; Froines, J., Evaluating the toxicity of airborne particulate matter and nanoparticles by measuring oxidative stress potential--a workshop report and consensus statement. Inhal. Toxicol. 2008, 20, (1), 75-99. 29. Yang, J.; Pan, B.; Li, H.; Liao, S.; Zhang, D.; Wu, M.; Xing, B., Degradation of pnitrophenol on biochars: Role of persistent free radicals. Environ. Sci. Technol. 2016, 50, (2), 694700. 30. Khachatryan, L.; Vejerano, E.; Lomnicki, S.; Dellinger, B., Environmentally persistent free radicals (EPFRs). 1. Generation of reactive oxygen species in aqueous solutions. Environ. Sci. Technol. 2011, 45, (19), 8559-8566. 31. Fang, G.; Zhu, C.; Dionysiou, D. D.; Gao, J.; Zhou, D., Mechanism of hydroxyl radical generation from biochar suspensions: Implications to diethyl phthalate degradation. Bioresource Technol. 2015, 176, 210-217. 32. Hung, H. F.; Wang, C. S., Experimental determination of reactive oxygen species in Taipei aerosols. J. Aerosol. Sci. 2001, 32, (10), 1201-1211. 33. Venkatachari, P.; Hopke, P. K.; Grover, B. D.; Eatough, D. J., Measurement of particlebound reactive oxygen species in Rubidoux aerosols. J. Atmos. Chem. 2005, 50, (1), 49-58. 34. Liikanen, M.; Havukainen, J.; Hupponen, M.; Horttanainen, M., Influence of different factors in the life cycle assessment of mixed municipal solid waste management systems - A comparison of case studies in Finland and China. J. Clean. Prod. 2017, 154, 389-400. 35. Zhao, Y.; Wang, H. T.; Lu, W. J.; Damgaard, A.; Christensen, T. H., Life-cycle assessment of the municipal solid waste management system in Hangzhou, China (EASEWASTE). Waste Manage. Res. 2009, 27, (4), 399-406. 27



611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642

36. Ahn, S.; Werner, D.; Luthy, R. G., Physicochemical characterization of coke-plant soil for the assessment of polycyclic aromatic hydrocarbon availability and the feasibility of phytoremediation. Environ. Toxicol. Chem. 2005, 24, (9), 2185-2195. 37. Hills, J. W.; Hill, H. H.; Hansen, D. R.; Metcalf, S. G., Carbon-dioxide supercitical-fluid extraction of incinerator fly-ash with a reactive solvent modifier J. Chromatogr. A. 1994, 679, (2), 319-328. 38. Jia, H. Z.; Zhao, S.; Shi, Y. F.; Zhu, L. Y.; Wang, C. Y.; Sharma, V. K., Transformation of Polycyclic Aromatic Hydrocarbons and formation of environmentally persistent free radicals on modified montmorillonite: The role of surface metal ions and polycyclic aromatic hydrocarbon molecular properties. Environ. Sci. Technol. 2018, 52, (10), 5725-5733. 39. Kiruri, L. W.; Khachatryan, L.; Dellinger, B.; Lomnicki, S., Effect of copper oxide concentration on the formation and persistency of environmentally persistent free radicals (EPFRs) in particulates. Environ. Sci. Technol. 2014, 48, (4), 2212-2217. 40. Nwosu, U. G.; Cook, R. L., 13C Nuclear magnetic resonance and electron paramagnetic spectroscopic comparison of hydrophobic acid, transphilic acid, and reverse osmosis May 2012 isolates of organic matter from the suwannee river. Environ. Engin. Sci. 2015, 32, (1), 14-22. 41. Khachatryan, L.; Dellinger, B., Environmentally persistent free radicals (EPFRs)-2. Are free hydroxyl radicals generated in aqueous solutions? Environ. Sci. Technol. 2011, 45, (21), 9232-9. 42. Krzystek. J., S. A., Pardi. L., Brunel. L. C., DPPH as a standard for high-field EPR. J. Mag. Res. 1997, 125, 207-211. 43. Lim, J.; Yu, L. E.; Kostetski, Y. Y.; Lim, C.; Ryu, J.; Kim, J., Effects of driving conditions on diesel exhaust particulates. J. Air. Waste. Manag. Assoc. 2008, 58, (8), 1077-1085. 44. Pryor, W. A.; Arbour, N. C.; Upham, B.; Church, D. F., The inhibitory effect of extracts of cigarette tar on electron-transport of mitochondria and submitochondrial particles. Free Radical Bio. Med. 1992, 12, (5), 365-372. 45. Yang, J.; Pignatello, J. J.; Pan, B.; Xing, B. S., Degradation of p-Nitrophenol by lignin and cellulose chars: H2O2-mediated reaction and direct reaction with the char. Environ. Sci. Technol. 2017, 51, (16), 8972-8980. 46. Fang, G. D.; Liu, C.; Gao, J.; Dionysiou, D. D.; Zhou, D. M., Manipulation of Persistent Free Radicals in biochar to activate persulfate for contaminant degradation. Environ. Sci. Technol. 2015, 49, (9), 5645-5653.

644

47. Wang, P.; Pan, B.; Li, H.; Huang, Y.; Dong, X.; Ai, F.; Liu, L.; Wu, M.; Xing, B., The overlooked occurrence of Environmentally Persistent Free Radicals in an area with low-rank coal burning, Xuanwei, China. Environ. Sci. Technol. 2018, 52, (3), 1054-1061.

645

48.

643

Allouis, C.; Beretta, F.; D'Alessio, A., Structure of inorganic and carbonaceous particles 28


Page 28 of 37

Page 29 of 37


646

emitted from heavy oil combustion. Chemosphere 2003, 51, (10), 1091-1096.

647

49. Patterson, M. C.; Keilbart, N. D.; Kiruri, L. W.; Thibodeaux, C. A.; Lomnicki, S.; Kurtz, R. L.; Poliakoff, E. D.; Dellinger, B.; Sprunger, P. T., EPFR formation from phenol adsorption on Al2O3 and TiO2: EPR and EELS studies. Chem. Phys. 2013, 422, 277-282.

648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677

50. Zhao, S.; Jia, H.; Nulaji, G.; Gao, H.; Wang, F.; Wang, C., Photolysis of polycyclic aromatic hydrocarbons (PAHs) on Fe(3+)-montmorillonite surface under visible light: Degradation kinetics, mechanism, and toxicity assessments. Chemosphere 2017, 184, 1346-1354. 51. Khachatryan, L.; Lomnicki, S.; Dellinger, B., An expanded reaction kinetic model of the CuO surface-mediated formation of PCDD/F from pyrolysis of 2-chlorophenol. Chemosphere 2007, 68, (9), 1741-50. 52. Dellinger, B.; Khachatryan, L.; Masko, S.; Lomnicki, S., Free radicals in tobacco smoke. Mini-Rev. Org. Chem. 2011, 8, (4), 427-433. 53. He, W. J.; Liu, Z. Y.; Liu, Q. Y.; Ci, D. H.; Lievens, C.; Guo, X. F., Behaviors of radical fragments in tar generated from pyrolysis of 4 coals. Fuel 2014, 134, 375-380. 54. Shi, L.; Liu, Q. Y.; Guo, X. J.; Wu, W. Z.; Liu, Z. Y., Pyrolysis behavior and bonding information of coal - A TGA study. Fuel. Process. Technol. 2013, 108, 125-132. 55. Paul, A.; Stosser, R.; Zehl, A.; Zwirnmann, E.; Vogt, R. D.; Steinberg, C. E. W., Nature and abundance of organic radicals in natural organic matter: Effect of pH and irradiation. Environ. Sci. Technol. 2006, 40, (19), 5897-5903. 56. Maskos, Z.; Dellinger, B., Formation of the secondary radicals from the aging of tobacco smoke. Energ. Fuel. 2008, 22, (1), 382-388. 57. Mmereki, B. T.; Donaldson, D. J., Direct observation of the kinetics of an atmospherically important reaction at the air-aqueous interface. J. Phys. Chem. A. 2003, 107, (50), 11038-11042. 58. Chu, S. N.; Sands, S.; Tomasik, M. R.; Lee, P. S.; McNeill, V. F., Ozone oxidation of surface-adsorbed polycyclic aromatic hydrocarbons: role of PAH-surface interaction. J. Am. Chem. Soc. 2010, 132, (45), 15968-75. 59. Kwamena, N. O.; Staikova, M. G.; Donaldson, D. J.; George, I. J.; Abbatt, J. P., Role of the aerosol substrate in the heterogeneous ozonation reactions of surface-bound PAHs. J. Phys. Chem. A. 2007, 111, (43), 11050-8. 60. Yao, J. J.; Huang, Z. H.; Masten, S. J., The ozonation of benz[a]anthracene: Pathway and product identification. Water Res. 1998, 32, (11), 3235-3244.

678

61. Zhang, Y.; Shu, J.; Yang, B., Homogeneous and heterogeneous reactions of anthracene with selected atmospheric oxidants. J. Environ. Sci. 2013, 25, (9), 1817-23.

679

62.

Shiraiwa, M.; Selzle, K.; Poschl, U., Hazardous components and health effects of 29



680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711

atmospheric aerosol particles: reactive oxygen species, soot, polycyclic aromatic compounds and allergenic proteins. Free Radical Res. 2012, 46, (8), 927-939. 63. Zhao, H. T.; Joseph, J.; Zhang, H.; Karoui, H.; Kalyanaraman, B., Synthesis and biochemical applications of a solid cyclic nitrone spin trap: A relatively superior trap for detecting superoxide anions and glutathiyl radicals. Free Radical Bio. Med. 2001, 31, (5), 599-606. 64. Chen, N.; Huang, Y. H.; Hou, X. J.; Ai, Z. H.; Zhang, L. Z., Photochemistry of hydrochar: Reactive oxygen species generation and sulfadimidine degradation. Environ. Sci. Technol. 2017, 51, (19), 11278-11287. 65. Khachatryan, L.; McFerrin, C. A.; Hall, R. W.; Dellinger, B., Environmentally persistent free radicals (EPFRs). 3. Free versus bound hydroxyl radicals in EPFR aqueous solutions. Environ. Sci. Technol. 2014, 48, (16), 9220-9226. 66. Li, Y.; Li, L.; Chen, Z. X.; Zhang, J.; Gong, L.; Wang, Y. X.; Zhao, H. Q.; Mu, Y., Carbonate-activated hydrogen peroxide oxidation process for azo dye decolorization: Process, kinetics, and mechanisms. Chemosphere 2018, 192, 372-378. 67. Wang, T. C; Cao, Y.; Qu, G.; Sun, Q.; Xia, T.; Guo, X.; Jia, H.; Zhu, L., Novel Cu(II)EDTA decomplexation by discharge plasma oxidation and coupled Cu removal by alkaline precipitation: underneath mechanisms. Environ. Sci. Technol. 2018, 52, (14), 7884-7891. 68. Sanchez-Cruz, P.; Santos, A.; Diaz, S.; Alegria, A. E., Metal-independent reduction of hydrogen peroxide by semiquinones. Chem. Res. Toxicol. 2014, 27, (8), 1380-1386. 69. Zhu, B. Z., Zhao, H. T., Kalyanaraman, B., Liu, J., Shan, G. Q., Du, Y. G., Frei, B., Metalindependent decomposition of organic hydroperoxides and formation of alkoxyl radicals by halogenated quinones. PNAS 2007, 104, 3698-3702. 70. Huang, C. H.; Ren, F. R.; Shan, G. Q.; Qin, H.; Mao, L.; Zhu, B. Z., Molecular mechanism of metal-independent decomposition of organic hydroperoxides by halogenated quinoid carcinogens and the potential biological implications. Chem. Res. Toxicol. 2015, 28, (5), 831-837. 71. Tong, H. J.; Arangio, A. M.; Lakey, P. S. J.; Berkemeier, T.; Liu, F. B.; Kampf, C. J.; Brune, W. H.; Poschl, U.; Shiraiwa, M., Hydroxyl radicals from secondary organic aerosol decomposition in water. Atmos. Chem. Phys. 2016, 16, (3), 1761-1771. 72. Jia, H. Z; Chen, H.; Nulaji, G.; Li, X.; Wang, C., Effect of low-molecular-weight organic acids on photo-degradation of phenanthrene catalyzed by Fe(III)-smectite under visible light. Chemosphere 2015, 138, 266-271.

713

73. Fu, H.; Liu, H.; Mao, J.; Chu, W.; Li, Q.; Alvarez, P. J.; Qu, X.; Zhu, D., Photochemistry of dissolved black carbon released from biochar: Reactive oxygen species generation and phototransformation. Environ. Sci. Technol. 2016, 50, (3), 1218-1226.

714

74.

712

Duan, X. G.; Sun, H. Q.; Shao, Z. P.; Wang, S. B., Nonradical reactions in environmental 30


Page 30 of 37

Page 31 of 37


715

remediation processes: Uncertainty and challenges. Appl. Catal. B-Environ. 2018, 224, 973-982.

716

75. Xiahong Feng; Ann P. Melander; Klaue, B., Contribution of municipal-waste incineration to trace metal deposition on the vicinity. Water Air Soil Poll. 2000, 119, 295-316.

717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745

76. Wu, S.; Zhou, J.; Pan, Y.; Zhang, J.; Zhang, L.; Ohtsuka, N.; Motegi, M.; Yonemochi, S.; Oh, K.; Hosono, S.; Qian, G., Dioxin distribution characteristics and health risk assessment in different size particles of fly ash from MSWIs in China. Waste Manage. 2016, 50, 113-120. 77. Hsieh, Y.-K.; Chen, W.-S.; Zhu, J.; Huang, Q., Characterization of polychlorinated dibenzo-p-dioxins and dibenzofurans of the flue gases, Fly ash and bottom ash in a municipal solid waste incinerator. Aerosol Air Qual. Res. 2018, 18, (2), 421-432. 78. Poschl, U.; Shiraiwa, M., Multiphase chemistry at the atmosphere-biosphere interface influencing climate and public health in the anthropocene. Chem. Rev. 2015, 115, (10), 4440-4475. 79. Birben, E.; Sahiner, U. M.; Sackesen, C.; Erzurum, S.; Kalayci, O., Oxidative stress and antioxidant defense. World Allergy Organ. J. 2012, 5, (1), 9-19. 80. Charrier, J. G.; McFall, A. S.; Richards-Henderson, N. K.; Anastasio, C., Hydrogen peroxide formation in a surrogate lung fluid by transition metals and quinones present in particulate matter. Environ. Sci. Technol. 2014, 48, (12), 7010-7017. 81. Lakey, P. S. J.; Berkemeier, T.; Tong, H. J.; Arangio, A. M.; Lucas, K.; Poschl, U.; Shiraiwa, M., Chemical exposure-response relationship between air pollutants and reactive oxygen species in the human respiratory tract. Sci. Rep-Uk. 2016, 6, 1-6 82. Lucas, K.; Maes, M., Role of the toll like receptor (TLR) radical cycle in chronic inflammation: Possible treatments targeting the TLR4 pathway. Mol. Neurobiol. 2013, 48, (1), 190204. 83. Balakrishna, S.; Lomnicki, S.; McAvey, K. M.; Cole, R. B.; Dellinger, B.; Cormier, S. A., Environmentally persistent free radicals amplify ultrafine particle mediated cellular oxidative stress and cytotoxicity. Part. Fibre. Toxicol. 2009, 6, (11), 1-14. 84. Verma, V.; Fang, T.; Xu, L.; Peltier, R. E.; Russell, A. G.; Ng, N. L.; Weber, R. J., Organic aerosols associated with the generation of reactive oxygen species (ROS) by water-soluble PM2.5. Environ. Sci. Technol. 2015, 49, (7), 4646-4656. 85. Dou, J.; Lin, P.; Kuang, B. Y.; Yu, J. Z., Reactive oxygen species production mediated by humic-like substances in atmospheric aerosols: Enhancement effects by Pyridine, Imidazole, and their derivatives. Environ. Sci. Technol. 2015, 49, (11), 6457-6465.

31



747

Caption of Figures

748

Figure 1. Relationship of the normalized spin densities, SN (SN = S/[TOC]), on solventextracted and non-extractable EPFRs with metal contents of fly ash samples. (A)-Fe, (B)-Cu, (C)-Zn, (D)-Mn, and (E)-Ti.

749 750 751 752 753 754 755 756 757 758 759

Figure 2. Evolution of EPFRs characteristics of MSW-fly ash samples under visiblelight irradiation. (A)(B)(C) - Original solid samples; (D)(E)(F) - Non-extractable (solvent-extracted) samples. Figure 3. Electron paramagnetic resonance (EPR) spectra of fly ash samples extracted by a mixture of water or DMSO with the spin-trapping agent of DMPO. Figure 4. Concentrations of ROS generated from radical species on fly ash samples. (A) solvent-extracted radicals, and (B) non-extractable radicals. Figure 5. Variation of radicals (·OH and O2-·) with solvent-extracted (A and B) and non-extractable (C and D) EPFRs of fly ash samples.

32


Page 32 of 37

Page 33 of 37


Figure 1

760

Fe

500

(B)

(A)

400

2

r = 0.812, p = 0.001

2

r = 0.667, p = 0.001

15

SN, 10 gNOC

-1

400

Cu

500

300

300 200

200

2

2

r = 0.028, p = 0.873

100

r = 0.023, p = 0.660

100

0

0 0

10

20

30

40

0.4

0.8

-1

(C)

2.0

Mn

500

2

r = 0.628, p = 0.004

(D)

400

2

r = 0.586, p = 0.006

15

SN, 10 gNOC

-1

400

1.6

Cu, mg kg

Zn

500

1.2 -1

Fe, mg kg

300

300

200

200 2

2

r = 0.081, p = 0.381

100

r = 0.082, p = 0.789

100

0

0 4

8

12

16

-1

0.0

Zn, mg kg

0.8

-1

1.2

Mn, mg kg

Ti

500

(E)

2

r = 0.643, p = 0.002

15

SN, 10 gNOC

-1

400

0.4

300

Unfilled Symbols - Solvent-extracted SN Filled Symbols - Non-extractable SN

200 2

r = 0.038, p = 0.852

100 0 0.0

761

1.0

2.0

3.0

4.0

-1

Ti, mg kg

33



Page 34 of 37

Figure 2

762 Original Sample E 2.0035

2.0052

345

2.0034

2.0046

330

g-Factor

(A)

2.0040 2.0034

300 0

50

100

150

200

g-Factor Peak area

(D)

2.0033

315

2.0028

Solvent-extracted samples E

132

2.0031

126

2.0030

120 0

250

50

100

200

250

Original Sample H

Solvent-extracted samples H

325 2.0054

110

(E)

105

320

2.0034

2.0052

100

2.0050

95

315 2.0033

310

90

2.0048 2.0032

85

305 0

50

100

150

200

250

0

50

100

Original Sample I

30 2.006

111

2.0032

108

2.0031 150

200

250

300

40

2.008

114

2.0033

50

(F)

117

100

250

20

2.004 0

350

50

100

Time, min

Time, min

34


150

10 200

Peak area

g-Factor

(C)

50

200

Solvent-extracted samples I

2.010

2.0035

0

150

Time, min

Time, min

2.0034

Peak area

g-Factor

150

Time, min

(B)

763

138

2.0032

Time, min

2.0035

144

Peak area

360

2.0058

Page 35 of 37


Figure 3

764

(A)

3460

3480

3500

3520

3540

3460

Field, G

3480

3500

3520

3540

3460

3540

3480

3500

3520

3540

3460

3480

3500

3520

3540

3500

3520

3540

3460

3500

3520

3540

3460

3480

3500

3520

3540

3520

3540

Field, G

(I)

3480

3500

3520

3540

3460

3480

3500

Field, G

(K)

Field, G

3480

(F)

Field, G

(J)

3480

3460

Field, G

(H)

Field, G

765

3520

Field, G

(G)

3460

3500

(E)

Field, G

3460

3480

Field, G

(D)

3460

(C)

(B)

DMPO in DMSO DMPO in H2O 3480

3500

3520

3540

Field, G

35



Page 36 of 37

Figure 4

766

Solvent-extracted

30

(A)

20

15

Spin  10 , g

-1

10 0 A

B

C

D

E

F

G

H

I

J

K

Non-extractable

30

(B)

20 10 0 A

767

B

C

D

E

F

G

Sample

H

I

J

36


K

Singlet oxygen radical Hydroxyl radical Superoxide radical Oxygen-centered radical Carbon-centered radical

Page 37 of 37


Figure 5

768

15

Concentration, 10 spin g

-1

Solvent-extracted 10.0

8.0

(A)

(B)

·OH

6.0

8.0

4.0

6.0

2.0

4.0

2

r = 0.85 p = 0.0024

0.0 2.5

3.0

-·

O2

3.5

2

r = 0.81

p = 0.001

2.0

4.0

4.5

2.5

5.0

3.0

3.5

4.0

4.5

5.0

4.0

20.0

(C)

·OH

15.0

15

Concentration, 10 spin g

-1

Non-extractable

769

2.0

O2

10.0 5.0

0.0

0.0

2

-2.0 0.0

-·

(D)

r = 0.12 p = 0.388 1.5

3.0 15

4.5

S, 10 spin g

-5.0 0.0

6.0

-1

2

r = 0.16 p = 0.472 1.0

2.0

3.0 15

4.0

S, 10 spin g

37


5.0 -1

6.0

7.0

Formation and Evolution of Solvent-Extracted and Non-Extractable

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