Emissions of secondary formed ZnO nano-objects from the

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Emissions of secondary formed ZnO nano-objects from the combustion of impregnated wood. An on-line size-resolved elemental investigation. Debora Foppiano, Mohamed Tarik, Elisabeth Müller Gubler, and Christian Ludwig Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03584 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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Environmental Science & Technology

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Emissions of secondary formed ZnO nano-objects

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from the combustion of impregnated wood. An on-

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line size-resolved elemental investigation.

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Debora Foppiano* a,c, Mohamed Tarik a, Elisabeth Müller Gubler b, Christian Ludwig* a,c.

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a

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(ENE), Paul Scherrer Institut (PSI), CH 5232 Villigen PSI, Switzerland

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b

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Scherrer Institut (PSI), CH 5232 Villigen PSI, Switzerland

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c

Bioenergy and Catalysis Laboratory (LBK), Energy and Environment Research Division

Laboratory of Biomolecular Research (LBR), Biology and Chemistry Division (BIO), Paul

Environmental Engineering Institute (IIE), School of Architecture, Civil and Environmental

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Engineering (ENAC), École Polytechnique Fédérale de Lausanne (EPFL), CH 1015 Lausanne,

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Switzerland.

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KEYWORDS

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ZnO nanoparticles, Elemental analysis, On-line measurement, Aerosol, Wood combustion,

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Waste biomass, Particle emission, Thermal desorption, Gas-borne nanoparticles, Redox-sensitive

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Zn, Incineration

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ABSTRACT. The release of secondary nano-objects formed during waste combustion processes

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is becoming a matter of concern, considering their known toxicity and the fact that the 100%

ACS Paragon Plus Environment

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efficiency of filtering systems is not always ensured. An increased cytotoxicity and genotoxicity

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on human peripheral blood lymphocytes is known particularly in the case of ZnO, which is often

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contained in paints and waterproof agents, heading to a relevant quantity present in the waste-

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wood material. In this study, the behavior of ZnO nanoparticles during wood combustion and the

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effect of the reduction potential of generated carbon species on the release of secondarily formed

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ZnO-containing nano-objects were investigated. By hyphenating a modified scanning mobility

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particle sizer (SMPS) and inductively coupled mass spectrometry (ICP-MS), it was possible to

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obtain simultaneously size-resolved and chemical information on the emitted nanoparticles.

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Through the established correlation between SMPS and ICP-MS signals, Zn-containing particles

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were efficiently resolved from the combustion generated particles. Transmission electron

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microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) on size-selected particles

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confirmed the SMPS and ICP-MS data. The use of electron diffraction allowed determining the

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structure of the crystalline materials as hexagonal ZnO. A possible mechanism of reduction of

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ZnO to Zn and further reformation as secondary nano-objects is proposed.

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2. INTRODUCTION

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Engineered nanoparticles (ENPs) and nano-objects are widely used in many applications such as

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motor- or vehicle- related products, electronics, paints, coatings and adhesives, household

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products, and personal care and cosmetic products. Increasingly, with the heightened interest in

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reducing atmospheric particle emissions, the release of such nanoparticles from waste

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incineration and combustion processes has grown in importance. During waste combustion,

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metallic ultrafine particle emissions have been reported (dominated by K, Zn and Ca, Fe, Mg,

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and Na).1


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A recent study carried out by various institutions in Switzerland showed how modern solid-waste

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incinerators can retain nanoparticles through the filtering system.2 However, the occurrence of

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unintentionally produced nanomaterials cannot be excluded. The so-called incidental

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nanoparticles may volatilize in the incinerator during incomplete combustion processes and they

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can re-form after filtration of the flue gas.3 Moreover, new nano-objects can be formed also as

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result of disaggregation of the already present nano-object and re-formation of new nano-sized

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compounds and materials.4

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Depending on their surface area, the evaporation temperature of metal ENPs is significantly

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lower than that of the respective bulk material. However, when heterogeneous reactions and

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agglomeration occur during combustion, a generally decreased surface area of nanoparticles

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allows lower temperatures of evaporation. Moreover, the stability of metal ENPs is enhanced for

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the oxidized form, known to be less soluble and with a higher flash point than the corresponding

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metal. Therefore, certain metal oxide ENPs such as cerium oxide have a higher tendency to

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remain in the bottom ashes without being volatilized during incineration.5 Regarding the matrix

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effect, Jakob et al. showed how thermal treatment of fly ashes, involving thermochemical

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reactions of the heavy metals with other constituents (e.g. alumino-silicates compound, alkali

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metals chlorides and carbon species), provoke their evaporation.6

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Zinc oxide is one of the relevant metal oxide NPs regarding significant exposures

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often contained in paints and impregnation or waterproofing sprays, which are used for wood

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preservation. Particular redox conditions could influence the evaporation of zinc since it can be

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reduced in the presence of carbon from its oxide to its metallic state, which evaporates at a much

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lower temperature.9 Thus, zinc represents an interesting example of unintentionally produced

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nanomaterials during the combustion process, in which a large amount of carbon is present.

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and it is


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Generally, the volatility of Zn decreased in the presence of oxygen and water but is strongly

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enhanced in presence of C contained in the feedstock, leading to the increase in volatility from

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20% at 750 °C to 100% at about 950-1130 °C.10

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Different techniques can be used to determine on-line particle size distribution (PSD) and

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concentration of gas-borne particles. A comprehensive selection of PSD determination

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techniques is reported in several reviews,11,12 but among them, the scanning mobility particle

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sizer (SMPS) is one of the most established techniques in particle emission measurements. On

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the other hand, multi-elemental techniques as Inductively Coupled Plasma Mass Spectrometry

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(ICPMS),13 are often used to determine the chemical composition off-line. However,

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contamination or morphology alteration of the particles for off-line analytical methods must not

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be neglected.

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The presence of Zn in waste wood combustion off-gases was measured by an online elemental

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analysis of process gases, performed with ICP-OES (inductively coupled plasma optical

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emission spectrometer).14 Furthermore, data from several studies have been identifying a Zn-

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enrichment in the fraction of fine particles;15 while in these studies the particle size distribution

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was measured on-line, the chemical composition of the collected fly ashes was determined off-

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line.

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In the current work, the behavior of ZnO during wood incineration has been investigated through

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the use of a recently developed on-line technique, namely SMPS-ICP-MS, using a rotating disk

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diluter (RDD) as introduction system.16,17 The analysis was focused on a size range between 14

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and 300 nm. In the early 2000s, a known survey was published on the health impact of the

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average PM2.5 (the particulate matter or particle pollution with a diameter of 2.5 µm or below),


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showing how particles in this size range have a direct connection with lung cancer and

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respiratory mortality.18 Another study reported that even within this size range the transport of

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NPs to human lung fibroblast can anyway vary a lot for different particle sizes.19 In the case of

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particles in between 25-50 nm, the tendency to rapidly form larger agglomerates limits the

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diffusion, while particles between 250-500 nm are mostly unagglomerated. Therefore, larger NPs

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resulted in a higher uptake of different metal oxides compared to smaller particles, even at a

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concentration of 100 ng g-1. Furthermore, not only the size but also the chemical composition

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and morphology of NPs or nano-objects is affecting the inflammatory response. For example,

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Dandley et al. showed how much an acute pulmonary response is shown for in-vitro human cells

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and mice after exposure to ZnO coated carbon nanotubes, while reduced fibrosis is associated

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with Al2O3 coated carbon nanotubes.20

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ZnO dried particles, beech wood sawdust and, ZnO impregnated sawdust were combusted

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following two different temperature profiles in a lab-scale incinerator. The potential release of

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ZnO nanoparticles in sawdust samples formed in the reducing process gases was of particular

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interest.

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3. EXPERIMENTAL SECTION

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3.1. Chemicals. A commercial zinc oxide nanopowder (5810MR, 1314-13-2, AUER-REMY),

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was used to prepare suspensions of ZnO NPs. The average particle size provided by the

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manufacturer was verified with SEM analysis. Characterizations and further specifications of the

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powder are reported in the supporting information (SI) in Figures S1 and S2 and Table S1. For

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the colloidal stability of ZnO particles dispersion, a solution 0.1% wt of Poly Acrylic Acid


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(PAA, MW2000, 50% wt H2O, electronic grade, 535931-100G, lot # 02115HIV, Sigma-Aldrich)

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was required.

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Nitric acid (HNO3, 65%, Z0346841519, 1.00441.1000, Merck) and hydrochloric acid HCl (30%,

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Z0344318516, 1.00318.1000, Merck), were used to dissolve the deposited material and to clean

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the liner and the quartz tube of the tubular furnace after each experiment. The resulting solutions

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were further diluted and analyzed by ICP-MS. Zn calibration solutions for analysis of the

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collected samples were prepared by the dilution of Zn standard solution 1000 µg/ml (Merck,

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1.19806.0100) with ultrapure water (MilliQ, 18.2 MΩ cm).

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3.2. Samples. ZnO suspensions. A suspension 0.3% wt of zinc oxide (Suspension A) was

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prepared by dispersing 333.3 mg of the commercial powder in 100 g of dispersant solution. The

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amount of nano-powder used was chosen according to the minimum weight Wmin needed to have

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a representative sample of the bulk lot.21 An ultrasonic treatment was applied three times for two

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minutes to homogeneously disperse the suspension; ultrasonic horn was preferred to bath

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immersion since it is reported to be more effective for concentrated dispersions.22 The second

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suspension with a concentration of 600 µg/ml of ZnO (suspension B) was prepared by dilution of

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suspension A with the same dispersant solution, under constant magnetic stirring and ultrasonic

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bath for six minutes. The suspensions were characterized by using a Malvern Zetasizer Nano

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series, a Brookhaven X-ray disc centrifuge, and ICP-MS to verify the effective concentration.

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The results of these analyses are reported in supporting information in Tables S2, S3 and S4.

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Beech wood sawdust. The samples of beech wood were collected in the forest area of Villigen in

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March 2012 and further characterized by Bahrle.23 The multi-element analysis was performed in

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ICP-MS after microwave assisted acid digestion with HNO3. The results are reported in


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supporting info, in Tables S5 and S6. Multi–element calibration solutions were prepared by

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dilution of a commercial multi-elemental standard solution (Bernd Kraft, 10931.2000, batch-no.:

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1530569). Non-treated samples and samples impregnated with ZnO suspension were used for

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incineration experiments.

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Beechwood sawdust impregnated with ZnO suspension. In order to obtain a synthetic nano-waste

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sample, 2 g of beech wood sawdust were impregnated with zinc oxide suspension A (0.3% wt).

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The sawdust samples were kept in contact with 15 ml of ZnO suspension for three hours under

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magnetic stirring. After a drying step of 90 minutes at 105°C in a thermobalance (PM100, LP-

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16, Mettler-Toledo), the samples were carefully removed from the Petri capsule. In every

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experiment, an amount of 400 mg of the prepared batch was transferred into a boat-shaped

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alumina crucible and introduced into the tubular furnace for the thermal treatment experiments.24

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3.3. Instrumental arrangement. The experimental setup consisted of a tubular furnace, used as

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a lab-scale incinerator (Heraeus, RE 1.1), connected to the SMPS-ICP-MS, together with a

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rotating disk diluter (RDD) equipped with a heating tube as introduction system. A detailed

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description of RDD-SMPS-ICP-MS coupling strategy and instruments specifications were

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already presented.17,25

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Several mass flow controllers (MFC) were used to precisely adjust the flows. The different flows

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up- and downstream the tubular furnace were verified by a standard primary flow calibrator

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(Gilibrator-2, Sensidyne) at the beginning of every measurement.


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Figure 1. Flow concept of the thermal treatment and the SMPS-ICPMS setup. The red arrows show the main path of the measurement, while orange arrows show the parallel measurement with a reference SMPS. The different gases provided to the system are instead indicated as light-blue arrows. Legend: MFC = mass flow controller; RDD = rotating disc diluter, ASET =evaporation tube; SMPS = scanning mobility particle sizer, DMA =differential mobility analyzer, CPC =condensation particle counter; ICP-MS= inductively coupled plasma mass spectrometry.

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Figure 1 above shows a drawing of the experimental setup. The argon-borne particles were

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generated starting from a ZnO suspension by the aerosol generator by applying a relative gas

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pressure at the inlet of the nebulizer of approx. 1 bar. The aerosol was then passed through the

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silica gel dryer at ambient pressure, to reduce the relative humidity. A total flow of 1.5 L min-1

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was introduced into the tubular furnace by adding 50 mL min-1 flow of oxygen (or hydrogen) and

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additional argon to the aerosol generated flow. In order to avoid losses due to electrostatic

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deposition in the system, all the instruments were connected using electrically conductive tubes.

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At the outlet of the furnace, the aerosol is diluted with a mixture of Air/Ar and introduced into

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the RDD.


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The RDD is consisting of a dilution head and a control unit as shown in Figure 1. The dilution

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argon flow provided to the dilution head has to be defined by an MFC because it determines also

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the flow rate of the diluted aerosol directed to the DMA. Only the RDD MD193E connected to

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the modified SMPS was also equipped with an evaporation tube, operated at 350°C in all

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experiments. The SMPS is operated such as a size range between 13.1 and 299.6 nm is covered.

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A second SMPS instrument (Electrostatic Classifier 3080, condensation particle counter 3776,

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TSI) was used as a reference, operated with a separate RDD, as shown in Figure 1.The classified

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aerosol leaving the DMA was split between the CPC and the ICP-MS with a ratio 3:7.

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A flow of 4 mL min-1 of a mixture of xenon in argon (Xe 100 ppm), precisely adjusted with an

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MFC, was added to the monodisperse aerosol flow directed into the ICP-MS. This allowed a

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proper tuning of ICP-MS parameters before each measurement to obtain high sensitivity based

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on the recorded signal of

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during the overall analysis.

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3.4. Temperature program of the lab-scale incinerator. In order to determine the optimal

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dilution ratio for the generated emissions and to protect the instrument's detectors, a first

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temperature program was performed with a constant heating rate (5°C min-1) between 200°C and

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500°C. A standard incineration procedure was later applied to simulate the combustion processes

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in an industrial plant (drying step, fast pyrolysis from 300°C to 900°C, combustion at 900°C and

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afterburning from 900°C to 400°C).

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The acquisitions of both SMPS and ICP-MS signals were started simultaneously when a

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temperature of 105°C was reached inside the tubular furnace in the case of the first temperature

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program (Figure S3a in SI). For the incineration program lasting only 24 minutes, the

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Xe, and an eventual plasma drift could, therefore, be monitored


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measurements were started simultaneously with the furnace’s temperature program but the RDD

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channel was opened only for combustion and afterburning steps, 6 minutes after the start (see

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Figure S3b in SI).

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3.5. Size-selected particles collection and sample preparation for TEM. The size-selected

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particles from 13 nm to 340 nm were collected at the outlet of the DMA, using an aspiration EM

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grid sampler26 (detail view and schematic sampling point reported in Figure S15 in SI). The

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particles were filtered through a TEM grid (Quantifoil R 1.2/1.3 Cu 400 mesh) from the tube

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normally directed to the ICP-MS at the outlet of DMA, with a flow rate of 0.3 L min-1 by using a

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membrane pump. The particles were collected in separate replicates of the experiments reported

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in result and discussion for which the ICP-MS detection was not included. The samples were

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later analyzed with a JEOL JEM-2010 transmission electron microscope (200kV, point

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resolution of 0.25nm), to investigate particle size and morphology. The chemical composition

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was clarified through the use of EDS and electron diffraction pattern of the crystalline material.

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4. RESULTS AND DISCUSSION

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4.1. ZnO injected particles during sawdust thermal treatment. Dried ZnO particles generated

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with the use of an aerosol generator were constantly injected into a lab-scale incinerator during

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thermal treatment of beech sawdust under oxidizing conditions (O2 50 mL min-1). The particle

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size distribution and the intensity of 66Zn were measured with SMPS-ICP-MS. The temperature

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program applied is reported below in brown (Figure 2 a, b and d) and gray line (Figure 2c).


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Figure 2. 2D plot of the volume-related PSD of combusted sawdust and injected ZnO particles (a) and the sizerelated ICP-MS intensity of 66Zn (b). Raw ICP-MS intensity for 66Zn and 124Xe (c) showing increasing intensity from the black line; size-related ICP-MS intensity of 66Zn (d) corrected proportionally to the 124Xe signal.

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Figure 2 (b) shows a particle-size-related ICP-MS intensity covering a 100-290 nm size range,

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with intensity more or less constant during the total time of analysis. The contribution of Zn in

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the overall PSD was efficiently resolved through the established correlation between ICP-MS

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and SMPS shown in previous works,17 even though the considerable amount of particles

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generated from wood treatment gives rise to a different PSD plot (Figure 2a). As shown in

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Figure 2c the raw signal of the ICP-MS follows the size selection applied in the SMPS. At a

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specific DMA voltage particles having the same mobility diameter will reach the ICP. By


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increasing the voltage the next size class is selected. This iterative process allows to have a

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corresponding signal between each size class and ICP intensity over a time range of 150 s (up

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scan), followed by a fast down scan from high to low voltage, in order to start the next up scan.

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The size range of particle-size-related ICP-MS intensity is confirmed by experiments performed

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in the same conditions on the ZnO suspension alone, showing a broad distribution ranging from

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50 nm up to 290 nm, with a higher intensity between 100 and 290 nm (Figure S5 and Tables S2

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and S3 in Supporting Information).

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According to TGA studies, it is around 230°-380°C that hemicellulose, cellulose, and lignin

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decompose (forming C, CO, CO2, H2O, etc.), followed by cellulose pyrolysis up to nearly

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480°C.27 Similarly, the experiment with the tubular furnace confirmed the combustion of the

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sawdust within 280°C and 380°C, where a maximum in the particle concentration occur (Figure

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2a). This PSD is confirmed by experiments on the non-treated sawdust in which the same

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temperature profile was applied but no additional stream of particles was injected into the system

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(see SI, Figure S11). In correspondence with this temperature interval, an unexpected increased

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intensity in

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injected ZnO particles (Figure 2b). This behavior may have two possible explanations. First,

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through the combustion, the concentration of C increased resulting in an increased ionization

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efficiency of the plasma, and then in higher sensitivity of the different elements present in the

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plasma, including Zn. Fliegel et al. reported how carbon addition is consistently improving

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sensitivity for all elements from Li to Bi by the use of a mixed gas plasma with methane

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addition.28 This seems to be confirmed by the corresponding increase in the

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particular time frame with respect to the overall analysis (Figure 2c). Second, alterations of the

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ZnO particles occur due to the reducing potential of the carbon compound generated by the

66

Zn ICP-MS signal and a decrease in mobility diameter were detected for the

124

Xe signal, in this


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sawdust combustion, resulting in a decrease of the particle size diameter in comparison to the

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size of the injected particles. This hypothesis is not confirmed by TEM analysis on the size-

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selected particles collected during this time frame (SMPS scan from 30 min to 40 min, Figure

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2c). Figure 3b shows that not only the particle size is comparable but also the morphology is

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consistent with the one analyzed for the ZnO powder (Figure 3a), that is, the carbon generated

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atmosphere is not having a relevant effect on the ZnO nanoparticles but only on the

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corresponding ICP-MS signal intensities, at this specific temperature interval. Therefore, the

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particle-size-related ICP-MS signal was corrected proportionally to the drift registered for the

235

124

Xe signal (Figure 2d).

Figure 3. TEM micrograph on ZnO commercial nano-powder dispersed in ethanol (a); TEM micrograph on injected ZnO particles, which were collected after thermal treatment at about 340°C during the sawdust combustion (b) and after size-selection through the DMA.

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4.2. ZnO impregnated sawdust thermal treatment. As a nano-waste model, a sample of ZnO

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impregnated sawdust was investigated to monitor sub-micron particles release during thermal

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treatment, applying the same conditions of the previously presented experiment (in excess of

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oxygen and with the same temperature profile). For the 66Zn only a few peaks were detectable in

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ICP-MS (Figure 4b), which can be most probably due to physical transport of some ZnO


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particles. Nevertheless, ZnO contained in the sawdust is to great extent retained in the bottom

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ashes. These results might be explained by the fact that ZnO is very stable at the oxidizing

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conditions up to high temperatures.29 Although carbo-thermal reduction of ZnO is a process

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commonly used in metallothermic extraction30, this reaction cannot be taken into consideration

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as a possible scenario, since elemental C and CO (g) generated by decomposition of carbon

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species occur before reaching the temperature at which ZnO can be reduced (at about 500°C in

247

case of excess of H2, as shown in Figure S10 in SI).

Figure 4. Volume-related PSD of ZnO-impregnated sawdust (a) and ICP-MS mass intensity of 66Zn (b), in presence of O2 (50 ml min-1). Temperature profile in brown line with a constant heating rate (5°C min-1) between 200°C and 500°C.

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On the other hand, the investigation of the behavior of the ZnO nano-powder under reducing

249

atmosphere (excess of H2 in an argon atmosphere) confirmed the evaporation of Zn(g) and the

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change in morphology and chemical composition of the reformed Zn-containing particles. At

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nearly 800°C the maximum in intensity is detected by SMPS-ICP-MS (Figure S10 in supporting

252

information). TEM analysis and electron diffraction patterns of the size-selected particles

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collected at the DMA outlet suggest a core-shell structure with inner Zn core and outer


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passivation layer generated by re-oxidation of Zn surface (Figure 5b below and Figure S11d-f in

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SI). In this experiment, a substantial deposition on the quartz tube was observed, either as a

256

white or gray film, in different areas of the tube. Figure 5a shows the presence of Zn in two

257

different morphologies, a lamellar one and a flower-like type, most probably associated with

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ZnO. The Zn deposited in form of lamellar or hexagonal prismatic crystals, probably where the

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temperature in the quartz tube dropped from 600°C to 400°C, as reported in previous

260

experiments by Weidenkaff.31 Further characterizations are reported in SI (Figure S11b-c).

Figure 5. SEM micrograph of ZnO powder (a), after thermal treatment in presence of H2 (50 ml min-1); TEM micrograph of size-selected particles collected at the outlet of the DMA(b), after thermal treatment in presence of H2 (50 ml min-1).

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4.3. ZnO impregnated sawdust incineration. Another sample from the same batch of ZnO

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impregnated sawdust was used for a standard incineration experiment. According to the

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similarity laws described by Wochele et al.,32 the typical 100 min program of a municipal solid

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waste incinerator was reduced to 24 min for a lab-scale incinerator (brown line in Figure 6), in

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order to observe the complete conversion of carbon compound to CO2. The eventual release of

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ZnO particles at these particular conditions is investigated by RDD-SMPS-ICP-MS.


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In opposition to the previous experiments, a clear PSD ranging from 50 nm to 240 nm was

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obtained for Zn-containing particles, as reported in Figure 6b. Additionally, what stands out in

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Figure 6 is the good correlation between the shape of volume-related PSD and

270

mass intensity.

66

Zn ICP-MS

Figure 6. Volume related particle size distribution of ZnO impregnated sawdust (a) and ICP-MS mass intensity of 66

Zn (b), in presence of O2 (50 ml min-1). Temperature profile (in brown line) lasting only 24 minutes: drying step,

fast pyrolysis from 300°C to 900°C, combustion at 900°C and afterburning from 900°C to 400°C. Measurements started simultaneously with the furnace’s temperature program but RDD channel opened only for combustion and afterburning steps, 6 minutes after the start.

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These results indicate that with this temperature program applied, an effective reduction occurs.

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As suggested in the initial paragraph of this section, reduction of ZnO by C or CO is a quite

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known process that leads to the formation of Zn. However, from the above presented SMPS-ICP-

274

MS measurement, it is only possible to relate the sub-micron particles emitted during combustion

275

to generally Zn-containing particles, without having specific information on the molecular

276

composition and morphology. Therefore, the size-selected particles contributing to the intensity

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measured in Figure 6b were collected at the outlet of the DMA through aspiration EM sampler

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and further characterized with TEM.


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First of all, the intercorrelation between PSD measured with TEM and SMPS appears clear. The

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size distribution is confirmed in a range of smaller particles of 20 nm to the statistically

281

significant presence of 50, 70, and 85 nm particles, together with bigger agglomerate with an

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aerodynamic diameter up to around 240 nm probably formed in the cold zone of the quartz tube

283

or during particle collection. In Figure 7, EDX spectra and indexation of electron diffraction

284

pattern of crystalline material interestingly correspond to ZnO with hexagonal crystalline

285

structure. In Figure 7c the intensity of the elements labeled in light blue are attributed to the EM

286

grid itself.

Figure 7. TEM micrograph (a), indexation of electron diffraction pattern (b) and EDX spectrum (c) on size-selected particles collected after the DMA.

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Different experiments confirmed the results shown in the TEM characterization. The bigger

288

agglomerates in Figure 7a and 8a generates powder like electron diffraction pattern (Figure 7b

289

and 8b), while single crystals (Figure 8c) show diffraction only on certain planes indicated by the

290

reported Miller indexes (Figure 8d), which confirm interplanar distance matching hexagonal

291

ZnO structure.

292

In the supporting information we suggested a mechanism of reduction and further re-oxidation to

293

give ZnO as the final product, together with thermochemical calculations in which the input

294

concentrations are based on real ratios between C and Zn in impregnated wood (Figures S12-13).


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Figure 8. TEM micrographs of agglomerated particles (a) and (c); indexation of electron diffraction patterns corresponding to selected area in orange (b) and (d).

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4.5. Wood fly ashes. Elled et al. detected a bi-modal PSD for wood fly ashes with submicron

296

and micrometer range, respectively. The first PSD mode (submicron particles) is attributed to

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inorganic matter while the second one derives from fragmentation of mineral and char.29 The

298

formation of ash particles and aggregates has been previously shown to occur for condensation

299

of organic material or inorganic vapors on the generated ZnO particles. Alkali salts are generally

300

found on the surface of these newly formed particles if efficient combustion occurs.15 Since we

301

focus in our experiments principally on the first peak of this bi-modal distribution only smaller


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inorganic particles were detected, while bigger agglomerates of organic carbon are directed to

303

the excess gas during DMA selection.

304

4.6. Fate of ZnO generated particles - Environmental relevance. The current data highlight

305

the importance of determining the fate of nano-object in fly ashes. Although the work presented

306

by Walser et al. might suggest the entrapment in the bottom ashes of most of the nanoparticles

307

(CeO2) injected in a waste incinerator,2 that case is not considering metal nanoparticles and non-

308

nano particulate metals in waste which may re-form as incidental nanoparticles.3 As explained by

309

Roes et al. secondary nano-objects might be formed depending on the conditions present in the

310

incinerator. Moreover, in the case of PSD below 100 nm, a complete removal from flue gas is

311

not ensured.4

312

This study confirms the presence of incidental ZnO nano-objects in the fine particles fraction

313

upon combustion of a nano-waste model, with dimensions also below 100 nm detected by the

314

use of an on-line analytical technique. Moreover, this finding proves the possibility of formation

315

of secondary nano-objects that can decompose and reform as new species, such as oxide. Due to

316

the demonstrated toxicity of ZnO,33,34,35 a particular attention should be addressed to emissions

317

by small and medium scale plants below 1 MW, in which efficient filtering systems are not

318

always implemented. Therefore, new experiments with larger burners and in particular, test on

319

particle removal are suggested.

320

ASSOCIATED CONTENT

321

Supporting Information. Zinc oxide powder characterization; Zinc oxide suspension

322

characterization; sawdust multi-elemental analysis in ICP-MS; ZnO impregnated sawdust

323

measurements with ICP-MS and calculation on efficiency of the impregnation; description of the


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324

two temperature programs applied for thermal treatment and incineration experiments; data

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processing and contour plots explanation; comparison between PSD of ZnO suspension and PAA

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solution; ZnO suspension thermal treatment in oxidizing and reducing conditions; ZnO powder

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thermal treatment in reducing conditions; HSC thermochemical calculations; beech wood saw

328

dust incineration experiment; sampling point and detailed view of particle collection device, ZnO

329

suggested mechanism of reduction and further re-oxidation. This material is available free of

330

charge via the Internet at http://pubs.acs.org.

331

AUTHOR INFORMATION

332

Corresponding Author:

333

*E-mail: [email protected]

334

*E-mail: [email protected]

335

Author Contributions: The paper was written with contributions from all authors. Debora

336

Foppiano took the lead in compiling the first draft of the manuscript. All authors have given

337

approval to the final version. The incineration experiments and the RDD-SMPS-ICP-MS

338

measurements were carried out equally by Mohamed Tarik and Debora Foppiano. Elisabeth

339

Müller performed the TEM analysis and supervised Debora Foppiano for the indexation of the

340

electron diffraction pattern. Christian Ludwig gave his contribution as the principal investigator

341

of the research project.

342

Notes The authors declare no competing financial interest.

343

ACKNOWLEDGMENT


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Financial support was obtained from the Swiss National Science Foundation (Projects 139136

345

and 136890), the Swiss Nanoscience Institute (Argovia, Project NanoFil), and the Swiss

346

Competence Center for Energy Research – Biomass for Swiss Energy Future (SCCER

347

BIOSWEET) in cooperation with the Commission for Technology and Innovation CTI. The

348

authors would like to thank Marcel Hottiger for the construction of the particle collection cell.

349

Special thanks go to Dr. Andrea Testino, for his help during the ZnO suspension preparation and

350

characterization, and to Dr. Adrian Hess and Albert Schuler for the introductory explanation on

351

the RDD-SMPS-ICP-MS coupling and incineration experiments, respectively.

352

ABBREVIATIONS

353

PSD, particle size distribution; PAA, polyacrylic acid; SMPS, scanning mobility particle sizer;

354

ICP-MS, inductively coupled mass spectrometer; RDD, rotating disk diluter; SEM, scanning

355

electron microscopy; EDS, energy-dispersive X-ray spectroscopy; TEM, transmission electron

356

microscopy; SI, supporting information.

357

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For Table of Contents Only

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Emissions of secondary formed ZnO nano-objects from the

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