Effect of Particulate Matter Mineral Composition on Environmentally

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The Effect of Particulate Matter Mineral Composition on Environmentally Persistent Free Radical (EPFR) Formation Elisabeth Feld-Cook, Gudrun Lisa Bovenkamp-Langlois, and Slawomir M Lomnicki Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01521 • Publication Date (Web): 17 Aug 2017 Downloaded from http://pubs.acs.org on August 20, 2017

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Environmental Science & Technology

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The Effect of Particulate Matter Mineral

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Composition on Environmentally Persistent Free

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Radical (EPFR) Formation

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Elisabeth E. Feld-Cook#‡, Lisa Bovenkamp-Langlois+‡, Slawo M. Lomnicki&‡* #

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+

Center for Advanced Microstructures & Devices (CAMD), Louisiana State University, Baton

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Department of Chemistry, Louisiana State University, Baton Rouge, LA

Rouge, LA &

Department of Environmental Sciences, Louisiana State University, Baton Rouge, LA

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KEYWORDS: Environmentally Persistent Free Radicals, EPFRs, Fly Ash, sulfur dioxides, iron

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oxide, zinc oxide, XANES, synchrotron, XPS, EPR

12 13 14 15 16 17

ACS Paragon Plus Environment

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Abstract

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Environmentally Persistent Free Radicals (EPFRs) are newly discovered, long-lived

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surface bound radicals that form on particulate matter and combustion borne particulates, such as

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fly ash. Human exposure to such particulates lead to translocation in to the lungs and heart resulting

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in cardio-vascular and respiratory disease through the production of reactive oxygen species.

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Analysis of some waste incinerators fly ashes revealed a significant difference between their EPFR

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content. Though EPFRs formation occurs on the metal domains, these differences were correlated

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with the altering concentration of calcium and sulfur. To analyze these phenomena, surrogate fly

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ashes were synthesized to mimic the presence of their major mineral components, including metal

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oxides, calcium and sulfur. The results of this studies led to the conclusion that the presence of

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sulfates limit formation of EPFRs due to inhibition or poisoning of the transition metal active sites

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necessary for their formation. These findings provide a pathway towards understanding differences

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in EPFR presence on particulate matter and uncovers the possibility of remediating EPFRs from

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incineration and hazardous waste sites.

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Introduction

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Particulate emission from combustion systems is typically a carrier of many condensable

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pollutants including but not limited to heavy and transition metals, polycyclic aromatic

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hydrocarbons (PAHs), polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs), polybrominated

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dibenzo-p-dioxins and furans (PBDD/Fs), and other halogenated compounds. Particulate matter

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(PM) and fly ash (FA) are the largest transporters of these pollutants through the air. Within the


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last 10 years a new type of pollutant associated with PM has been discovered: Environmentally

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Persistent Free Radicals (EPFRs).

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EPFRs are long-lived surface bound radicals known to form on particulate matter1. Their

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formation typically occurs in the cool zone of combustion systems, where the temperature drops

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below 600ºC2, through a series of surface mediated reactions2-4 between transition metals (iron,

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copper, nickel, and zinc5-7) and organic precursors such as (but not limited to) chlorinated benzenes

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and phenols. The organic aromatic precursor adsorbs to the metal oxide on the surface of the

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particulate (Scheme 1)1.

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The lifetime of EPFRs in

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atmospheric conditions ranges

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from 1-5.2 days5-7. This extreme

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and unusual stability of such

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surface-bound

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currently not fully understood.

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However, it is believed to be result of the following factors: the resonance between the carbon and

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oxygen centered radicals, metal center charge transfer, surface stabilization effects and changing

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degrees of freedom.

radicals

is

Scheme 1. EPFR formation between organics and transition metals (figure adapted from Balakrishna, S. et. al., 2009)1.

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EPFRs are a risk to human health through both acute2, 8-12 and chronic exposure11, 13.

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Further, they are intermediates in the formation of polychlorinated dibenzo-p-dioxins and

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polychlorinated furans (PCDD/F)14-15. The major route of human exposure is through PM

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inhalation; once in the lungs, EPFRs undergo a cyclic process with the net output of reactive

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oxygen species (ROS). The overproduction of ROS results in oxidative stress increasing


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susceptibility to respiratory and cardiovascular diseases1, 4, 12-13, 16-21. The condensation reactions that

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EPFRs undergo to form PCDD/Fs constitute another significant environmental and health risk that

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calls for EPFR prevention.

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Recent EPFR studies have primarily focused on the mechanistic understanding of EPFR

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formation, fate, and properties on single metal-oxide systems1, 14, 22-23. While important, single-

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metal oxide systems are not an accurate depiction of real world combustion systems because many

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of the condensable pollutants (i.e. metals) are present at the EPFR inception conditions. Therefore,

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studying mixed metal-oxide complexes with additional components (i.e. calcium and sulfur) will

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give a more accurate representation of PM & FA produced during the incineration process. In the

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presented research study, we were particularly interested in whether EPFR formation can be

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affected by chemical activity of the PM surface composition in the cool zone of combustion

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systems.

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To investigate the potential effects of the surface chemistry as well as calcium and sulfur

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presence on the formation of EPFRs, a set of model particles was constructed. The model

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composition was based on incinerators fly ashes analysis in respect to their major inorganic

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elemental components and subjected to EPFR formation tests. A combinatorial bottom-up

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approach was taken with respect to introducing calcium and sulfur into the system, where both of

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the elements were either co-introduced or subsequently added into the surrogate ash, in both the

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solid formation and post-formation stages. These two approaches correspond to different off-gas

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treatment methods, either in the post-flame zone or cool zone2. In the post-flame zone, all of the

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initial waste is atomized and the particles start to condense; by adding the calcium and sulfur at

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this stage, it can change the particle chemistry and alter the chemical composition of solids.

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Alternatively, adding calcium and sulfur in the cool zone as a post-treatment agent, where EPFRs


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form, can potentially affect the surface chemistry of the formed particles24-25. In addition to the

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elemental composition determined by Inductively Coupled Plasma- Atomic Emission

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Spectroscopy (ICP-AES) and X-ray Fluorescence (XRF), different techniques were applied to

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characterize the atomic/molecular structure of the EPFRs in real-world and surrogate FA. The

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surface composition of the real-world and surrogate FA was examined using X-ray Photoelectron

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Spectroscopy (XPS). X-ray Absorption Near Edge Structure (XANES) spectroscopy was used to

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determine the chemical environment and coordination of sulfur and iron in both, real world and

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surrogate FA within the bulk of the samples.

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Materials and Methods

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Real World Fly Ashes. These samples were collected from bag houses from the United States

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Environmental Protection Agency medical waste incinerators (MEDWI) and from multiple

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municipal waste incineration sites (MWI) in the following eastern and southeastern cities of the

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People’s Republic of China: Bingjiang, Ding Hu, Ning Bo, Xiao Shan, and Yuhang. The samples

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were collected by local researchers and were used immediately upon receipt to minimize radical

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decay.

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Surrogate Fly Ash Synthesis. Cabosil (Cabot, 99+%, specific surface area 380 m2/gram, Cab-o-

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sil EH-5) was impregnated using the incipient wetness method5 with metal nitrates water solution Table 1. The order of introduction of elemental components into surrogate fly ashes and respective naming notation of the samples. Surrogate

BeforeCalcination PostCalcination

Mixed Metals (MM)

BMMCaS

PMMCaS

BMMCa

PMMCa

BMMS

PMMS

MM

MM, Ca, S

MM

MM, Ca

MM

MM, S

MM

-

-

Ca, S

-

Ca

-

S


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to obtain particles with final metal concentrations of 0.1% Cu, 5% Fe, 2% Mg, 2% Zn (Table 1).

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Some ashes were additionally impregnated with calcium nitrate and/or ammonium sulfate to

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achieve final concentrations of Ca and S of 30% and 10%, respectively. The order of introduction

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of the elemental components to the surrogate ashes is presented in Table 1. In general, two types

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of samples were synthesized: (I) with Ca and/or S introduced into the surrogate ashes together with

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metal components (before-calcination or B samples) and (II) with Ca and/or S introduced into the

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surrogate ashes in a second stage, once metal oxides were already formed (post-calcination or P

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samples).

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Following the introduction of the precursors onto the matrix, samples were left to adsorb

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the components for 24 hours. The surrogates were then dried in an oven for 12 hours at 120 °C.

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Lastly, the surrogates were calcined for 5 hours at 450 °C and sieved to the 80 mesh size. For B

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samples, ashes were impregnated simultaneously with metal nitrates, Ca and/or S precursors using

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the incipient wetness method. For P samples, ashes went through incipient wetness method twice,

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with the first time containing only metal nitrates and the second time with Ca and/or S precursors.

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EPFR Formation on Surrogate Fly Ash. The procedure for EPFR formation was adapted from

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Dellinger et. al. 20073. In short, the ash surface, after initial activation under vacuum for 1 hour at

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450ºC, was exposed at 230ºC to 2-monochlorophenol (2-MCP, Aldrich 99%) vapors for 5

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minutes3, 6. Following the exposure, samples were evacuated for 1 hour, cooled to room

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temperature and the EPR measurement was taken by the procedure described in the next section.

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Electron Paramagnetic Resonance (EPR). EPFRs were measured on a Bruker EMX EPR 10/2.7

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using the following parameters: 9.8 GHz, 2.0 mW power, centerfield 3460 Gauss, time constant

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40.96 msec and 3.56x104 gain. Collected spectra were the average of 3 scans. 2,2-diphenyl-1-

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picrylhydrazyl (DPPH) was used as a standard for EPFR concentration calibration and


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calculation26. The EPFR concentration was calculated by directly comparing the radical

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concentration in the ashes to the standard, DPPH, and normalizing for experimental parameters.

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Radical species determination was based on g-value, a mathematical parameter derived from the

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energy difference during the excitation and relaxation of the radical electron.

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Inductively-Coupled Plasma- Atomic Emission Spectroscopy (ICP-AES). Ashes were

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digested in a 1:1:3:5 mixture of HCl to HNO3 to HF to H2O using a CEM Microwave Accelerated

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Reaction System (MARS) 5 at 200°C for 60 minutes. Spectra were taken on a Perkin Elmer ICP-

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AES.

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X-ray Photoelectron Spectroscopy (XPS). All XPS experiments were performed on a Kratos

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Axis 165 Auger/XPS using a mono-Al beam. Ashes were pressed into pellets (~1mm) and

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mounted on carbon tape. A single survey scan was taken over the range of 0-1200 eV, at 80 eV

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pass energy, with 0.5 eV step size and 100 ms dwell time. Individual elements were scanned twice

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over their specific energy range (based on the Binding Energy (BE) values from the NIST

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database) for a high-resolution scan at 40 eV pass energy, with 0.1 eV step and 100-500 ms dwell

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time27-30.

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X-ray Absorption Near Edge Structure (XANES). For XANES, sample preparation was similar

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to Hormes et. al.31 and analyzed at the Low Energy X-ray Absorbance Spectroscopy (LEXAS)

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beamline in fluorescence mode at the Center for Advanced Microstructures and Devices (CAMD)

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in Baton Rouge, LA. XANES measurements at S K-edge (2470 eV) were taken at a reduced

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pressure of 42 Torr inside the experimental setup over the following ranges and step sizes: pre-

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edge (2449-2469 eV, 1 eV), main edge (2069-2499 eV, 0.2 eV), and post-edge (2499-2520 eV,

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0.5 eV). Due to high sulfur content, the self-absorption correction in the ATHENA program was


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used before data analysis. XANES measurements at the Fe K-edge (7113 eV) were taken over the

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following ranges and step sizes: pre-edge (6913-7083 eV, 5 eV), main edge (7083-7143 eV, 0.5

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eV), post-edge 1 (7143-7213 eV, 1 eV) and post-edge 2 (7213-7662 eV, 0.05k). Composition

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analysis was performed using ATHENA of the IFFEFIT32 package to normalize, analyze and

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perform linear combination fitting (LCF) of all spectra32-33.

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Results and Discussion

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Real World Fly Ash Radical Composition Characterization. Previous studies of combustion

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derived samples such as diesel particulates, PM

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samples and different soot materials have

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indicated a ubiquitous presence of EPFRs14, 25-26.

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In contrast to previous studies, a significant

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difference in radical concentration was detected

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for fly ashes collected at US and China

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incineration facilities (Figure 1).

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Samples collected at medical waste

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incinerators in the U.S. (MEDWI) have a radical

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concentration of 1017spins/gram (Figure 1). The

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detected radicals are characterized by an average

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g value of 2.0028 and average ΔHp-p of 3.567.

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The broad radical signal (> 6 Gauss) indicates the multi-component makeup of the EPFRs present

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on these samples. These EPFRs are characteristic of primarily carbon-centered radicals based on

Figure 1. EPFR species formation difference for MWI and MEDWI of varying ages.


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the g-value and low ΔHp-p values. The ΔHp-p corresponds to the width between the peaks which

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gives insight to the number of radicals present in a sample34.

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Contrary to MEDWI ashes, MWI ashes do not have a detectable radical concentration. This

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is unusual, as EPFRs have been commonly found in combustion-emitted particulates. One of the

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reasons for the lack of EPFRs in the MWI ashes could be an analytical artifact. The high

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concentration of paramagnetic metal species in the MWI ashes produced a very strong EPR signal,

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which caused tuning problems and required EPR measurements to be performed on ashes with a

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mass 10 times lower than normal. Lower sample mass may have caused the EPFR concentration

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to fall below the detection limit. Therefore, the EPFRs, while potentially on the surface, are not

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detected by the EPR. Nevertheless, even in such event, the overall concentration of EPFRs would

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be much smaller compared to other samples (1-2 orders of magnitude). Low concentrations of

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EPFRs (or complete absence) on MWI ashes provides an interesting opportunity to investigate the

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factors inhibiting their formation on the surface of the ashes.

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Real World Fly Ash Elemental Composition Characterization.

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It is well documented that EPFRs are associated with the presence of transition metals,

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which act as the reaction/formation center and contribute to radical stabilization on the surface5-7,

182

35

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and changes to their persistency in ambient air14. Among the studied metals, copper and iron have

184

shown the highest activity of EPFR formation, while zinc and nickel indicated extremely long

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EPFRs lifetimes in ambient air (in

. Previous studies have also shown that different metal speciation result in varying radical yields

5-7, 35

186

months)

. The elemental analysis

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of the bulk ashes by ICP-AES is

Table 2. ICP data of real world ashes in % w/w. Ca

Mg

P

S

Ti

Zn

MWI 8.6-25 0.9-4.0 1.3-2.0 0.3-1.3 1.3-3.4 0.3-0.8 0.3-0.7 MEDWI 10-11

188

Fe

0.2

1.0

2.3

0.3

0.0 4.3-6.5

presented in Table 2.


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Distinct differences can be observed between the MWI and MEDWI samples: the major

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metal component on MWI ashes is Fe with 1-4%, while the Fe content of MEDWI ashes is only

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0.2%. On the other hand, MEDWI contain higher quantities of Zn (4-6%) compared to MWI.

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Among other elements, sulfur shows distinctly higher concentrations in MWI samples. Also, for

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the majority of MWI samples, calcium is more abundant than for the MEDWI samples.

Table 3. XPS based surface composition of MEDWI and MWI fly ashes in atomic %. Mg

Si

P

S

MWI

0-3.7

0-5.3

0

0-8.7

MEDWI

0

0-6.6

0-0.4

0

Cl 7.135.3 0

C 15.633.7 71.582.1

K 0-8.5 0

Ca 10.116.5 0

O 30.741.9 16.621.4

Zn

Na

0

0-1.8

0.5-0.7

0

194

It is important to understand the difference of the elemental composition between surface

195

vs. bulk of the ashes since EPFRs are surface bound radicals. If the metal sites responsible for

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EPFR formation were unavailable at the surface this could impact the observed differences in

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EPFR concentration in the real-world ashes. The elemental composition of the surface for MEDWI

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and MWI investigated by XPS (Data Table 3 and SI Figure 1) further emphasizes the differences

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between these two types of ashes. The surface of MEDWI samples contain significant amounts of

200

carbon, which is expected for the combustion borne particulate. MEDWI samples also contain Zn

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both on the surface and in bulk (see ICP analysis above). For MWI ashes, no iron, copper or zinc

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was detected on the surface by XPS; however, high amounts of calcium and sulfur were observed

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(not detected at MEDWI surface). At the same time the carbon content on the surface was

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significantly reduced for MWI, compared to MEDWI. While the bulk elemental composition of

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fly ash samples can be related with the composition of waste burned, the enrichment of the surface

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with calcium and sulfur may be related to the post treatment of the combustion exhaust. Limestone

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and lime injection at high temperature zone of combustors are the cheapest and most common


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practices for SO2 emission control36-37, and result in the formation of calcium sulfate. XANES

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analysis of the MWI samples has indeed confirmed that sulfur is mostly present in the form of

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sulfate on those samples (SI Figure 2).

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The formation of large amounts of calcium sulfate can have a significant impact on the

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detected concentration of EPFRs on the samples. Firstly, precipitation and deposition in the

213

collection devices of large quantities of calcium sulfate will result in a “dilution effect” of

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combustion borne particulates – though the total output of fly ashes and EPFRs would not be

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changed; introduction of calcium sulfate will decrease per unit mass concentration of EPFRs.

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Secondly, precipitating calcium sulfate can deposit on the surface of already formed PM, blocking

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access to the surface metal centers, which are active down the exhaust stream in the formation of

218

EPFRs. Thirdly, introduction of large lime or limestone quantities indicates high concentrations of

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SO2 in the systems - it is possible that SO2 interacts with metal centers and prevents the formation

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of EPFRs. With that said, there is evidence to show that sulfur compounds have an effect on

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particulate formation from incineration processes38-39.

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It is unclear which scenario is applicable to MWI ashes, but the data may provide vital

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information on ways to control EPFR formation/emission from combustion sources. Indeed zinc,

224

identified earlier as an EPFR active metal, is the prominent surface element in MEDWI samples,

225

which also contained EPFRs. The detection of a significant amount of iron deep in the bulk of

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MWI ashes further suggests its inability to bind to the organics on the surface, therefore hindering

227

EPFR formation (iron was not detected on the surface of those samples – Table 2).

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Surrogate Fly Ash Radical Formation.

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To do further investigate these effects, we have designed a model fly ash composed of the

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common metals present in the incinerator fly ashes and supplemented them, with calcium and


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sulfur. The inclusion of Ca and S mimics their introduction in the post-flame and cool zone of the

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incinerators: lime injections into combustion processes (dry or wet) and reactivity of gaseous SO2

233

with the surface.

234

The introduction of just calcium ions to ashes, for both sample types, before and after

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calcination, did not significantly affect the EPFR formation as the mixed metal (MM) surrogates

236

produced a similar concentration of EPFRs after exposure to the radical precursor (BMMCa and

237

PMMCa - Figure 2). The addition of both calcium and sulfur to the metal bearing surrogate

238

unexpectedly increased the propensity of the samples to generate EPFRs. While this result is

239

unexpected, it is consistent with both BMMCaS and PMMCaS. However, it is beyond the scope

240

of this study to understand on the chemical nature of this phenomenon. On the other hand,

241

introduction of only sulfur to the samples strongly suppressed the ability for EPFRs to be formed

Figure 2. EPR spectra (top) and resulting plot of EPFR concentration (bottom) for all surrogates. Note the significant difference between surrogates with only sulfur (* - p100 when compared to BMMCa, PMMCa and MM.


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The effects of sulfur on the emission of organic pollutants have been previously reported.

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Studies have shown that PCDD/F emissions are suppressed by sulfates40. There is a close

246

relationship between EPFR and PCDD/F emissions as EPFRs are direct intermediates to PCDD/Fs

247

in the surface mediated formation in the cool zone of incinerators41-43. Considering the mechanism

248

of the EPFR formation, where the organic precursor chemisorbs on the metal site, one can

249

speculate that the presence of sulfur dioxide can result in adsorption of SO2 on the metal centers

250

forming surface sulfate species. Indeed, SO2 is a known catalytic poison for metal oxide catalysts

251

by surface sulfidation44-45.

252

To analyze the role of sulfur in the EPFR suppression, XANES studies were performed at

253

the S K-edge (Figure 3). The XANES spectra could not be analyzed by LCF due to the high

254

disorder in the non-crystalline phase of the

255

amorphous surrogates46 which could not be

256

represented by linear combinations of the

257

available crystalline structure reference

258

compounds.

Disorder

259

amorphous

materials

260

broadening of the peak shapes i.e. shape

261

resonances in the case of sulfur, used to

262

distinguish between sulfate species. The S

263

K-XANES spectra of BMMCaS and

264

PMMCaS (Figure 3) show the following

265

features: the sulfate white line maximum at

266

2481.4 eV, a post-edge peak at 2485 eV,

is

higher and

in

causes

Figure 3. S K-edge XANES spectra at the for surrogates containing sulfur.


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and shape resonances with maxima at 2492 and 2498 eV that are characteristic of the S K-XANES

268

spectrum for anhydrite calcium sulfate (sulfur references are shown in SI Figure 4). On the

269

contrary, the XANES spectra of BMMS and PMMS do not have a post-edge peak at 2485 eV and

270

only a very broad shape resonance with maxima at 2497 eV, indicating the absence of anhydrite

271

calcium sulfate. The shape of this shape resonance has more similarity with zinc sulfate or iron

272

sulfate. Based on the composition and S K-XANES spectra of BMMS and PMMS, iron and/or

273

zinc sulfate are the most probable sulfate species present in these samples. Shape resonances

274

resembling iron and/or zinc sulfate are not present in BMMCaS and PMMCaS, which indicates

275

the potential availability of Fe and Zn ions to form EPFRs on those samples.

276

Iron sulfate formation on BMMS and PMMS samples was further confirmed using

277

XANES studies at the iron K-edge (Figure 4, iron references shown in SI Figure 5). The dominant

278

species determined using LCF analysis for

279

BMMS and PMMS is iron (III) sulfate

280

(Table 4, Figure 4), while iron hydroxide is

281

the

282

surrogates (Table 4, Figure 4). All

283

surrogates except MM contained similar

284

amounts of iron oxide (20-30%); the

285

remaining

286

hydroxide (

Effect of Particulate Matter Mineral Composition on Environmentally

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