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FORMATION OF ACTIVE CHLORINE OXIDANTS IN SALINE-OXONE AEROSOL

By

JAEYOUN JANG

A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ENGINEERING

UNIVERSITY OF FLORIDA

2010

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© 2010 Jaeyoun Jang

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To my grandmother, Jaegeun Jung, for her endless love

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ACKNOWLEDGMENTS

Above all, I am thankful to God for always leading me in the right direction. I would

like to thank my advisor, Dr. Myoseon Jang, for her support and lessons through doing

my master study. I also wish to thank my committee, Dr. Treavor Boyer and Michael

Henley, for their time and helpful comments on this thesis. I want to thank all of my

colleagues in Dr.Jang’s group, Gang Cao, Tianyi Chen, Jiaying Li, Yunseok Lim, Min

Zhong, and Wilton Mui for the mutual learning and valuable time we spent together. I

also would like to thank Air Force and NSF for financially supporting my master study.

Finally, I wish to thank my parents, my brother and friends in Korea for their love and

prayer that can support me throughout my studies.

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TABLE OF CONTENTS

page

ACKNOWLEDGMENTS .................................................................................................. 4

TABLE OF CONTENTS .................................................................................................. 5

LIST OF TABLES ............................................................................................................ 7

LIST OF ABBREVIATIONS ............................................................................................. 9

ABSTRACT ................................................................................................................... 10

CHAPTER

1 INTRODUCTION .................................................................................................... 12

1.1 Oxone ............................................................................................................... 12 1.2 Saline-Oxone Chemistry ................................................................................... 13 1.3 Aerosolized Saline-Oxone ................................................................................ 14

2 EXPERIMENTS ...................................................................................................... 15

2.1 Monitoring of Active Chlorine in Saline-Oxone Aerosol ..................................... 15 2.2 Monitoring of Acidity of Externally Mixed Saline-Oxone Aerosol ....................... 16 2.3 Measurement of Water Amount in Saline-Oxone Particles Using FTIR ............ 17

3 RESULTS AND DISCUSSION ............................................................................... 22

3.1 Active Chlorine Formation and Its Oxidizing Effect ........................................... 22 3.2 Buffer Stability in Saline-Oxone Aerosol ........................................................... 25 3.3 Humidity Effect .................................................................................................. 26 3.4 Dynamics of Aerosol Acidity ............................................................................. 28

4 SUMMARY ............................................................................................................. 35

APPENDIX

A CALIBRATION CURVE FOR WATER CONTENT IN PARTICLES ........................ 36

B CALIBRATION CURVE FOR AEROSOL ACIDITY ................................................ 37

B.1 Experimental Procedures ........................................................................... 37 B.2 Calibration Curve ....................................................................................... 37

LIST OF REFERENCES ............................................................................................... 40

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BIOGRAPHICAL SKETCH ............................................................................................ 43

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LIST OF TABLES

Table page 2-1 Experimental conditions to study aerosol reactivity and acidity using UV-Vis

spectroscopy for the aerosol sample collected on the dyed filter with MY or TBa ..................................................................................................................... 18

2-2 Experimental conditions to monitor the water content in saline, Oxone, and saline-Oxone particles using FTIR and the estimated water amount in various particles in the two different RH at 20% and 70%. .............................................. 19

B-1 Experimental conditions and results to obtain calibration curve for aerosol acidity ................................................................................................................. 39

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LIST OF FIGURES

Figure page 2-1 UV-Visible spectroscopy to monitor the evolution of active chlorine and

aerosol acidity in both internally and externally mixed aerosols.. ....................... 20

2-2 FTIR experimental setup to measure the water amount in oxidant particles. ..... 21

3-1 The ∆A’420,oxi obtained using different sampling systems (F1-F2-F3 and F1-D-F2) for the internally mixed saline-Oxone aerosol after the Oxone was mixed with saline in the aqueous solution. .................................................................... 30

3-2 Time profile of ∆A’420,oxi of the external mixture of Oxone, saline-Oxone, saline-Oxone with phosphate buffer aerosol. ..................................................... 31

3-3 FTIR spectra of the tested buffers (sodium bicarbonate and phosphate buffer) and the subtracted FTIR spectra ([spectrum of Saline/Oxone/Buffer] – [spectrum of Saline/Oxone]). .............................................................................. 32

3-4 FTIR spectra of NaCl particles impacted on silicon FTIR window at different humidity levels. ................................................................................................... 33

3-5 The M’water of NaCl, saline, Oxone, and saline-Oxone aerosols as a function of decreasing humidity. ....................................................................................... 34

B-2 Calibration curve for calculating aerosol acidity .................................................. 39

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LIST OF ABBREVIATIONS

DRH Deliquescent relative humidity

ERH Efflorescent relative humidity

FTIR Fourier transform infrared

MY Metanil yellow

RH Relative humidity

SMPS Scanning mobility particle sizer

TB Thymol blue

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Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the

Requirements for the Degree of Master of Engineering

FORMATION OF ACTIVE CHLORINE OXIDANTS IN SALINE-OXONE AEROSOL

By

Jaeyoun Jang

May 2010

Chair: Myoseon Jang Major: Environmental Engineering Sciences

In this study, the feasibility of producing active chlorine species from saline-

Oxone® aerosols was studied using a 2m3 Teflon film chamber. Aerosol chemistry of

saline-Oxone particles impacted on a silicon window was studied in terms of buffer

stability using a FTIR spectrometer. Between phosphate and bicarbonate buffers used

in this study, phosphate buffer was favorable for oxidant particles due to chemical

stability.

To evaluate the oxidative capability of saline-Oxone aerosol, the aerosol was

collected onto filters impregnated with a dye (e.g., MY) that react with active chlorine in

the aerosol, and monitored by UV-Visible spectroscopy. Spectral data showed that the

dye compound was 90% oxidized by active chlorine within 3 minutes. The presence of

phosphate buffer retarded the formation rate of active chlorine in saline-Oxone aerosol

particles. Only 28% of oxidation in phosphate-buffered aerosol was observed within 10

minutes after the reaction began.

The water content of aerosol particles was estimated using the FTIR absorbance

at 3350 cm-1 and the calibration curve obtained from the NaCl particles that are known

for the inorganic thermodynamic property. Unlike the NaCl particle that has the clear

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phase transition, oxidant particles (e.g., Oxone and saline-Oxone) contain significant

water mass even at a low RH (20%) with no phase transition suggesting that oxidation

in liquid-like particles progresses in the ambient humidity.

Saline-Oxone aerosols were found to be highly acidic as determined by UV-Visible

spectroscopy, indicating that the aerosol produced mostly an undissociated form of

active chlorine species (e.g., HOCl) that was susceptible to partitioning into the gas

phase.

Our study concludes that the aerosolized saline-Oxone is a feasible chlorine

oxidant, but the efficiency of such an approach still needs investigation.

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CHAPTER 1 INTRODUCTION

1.1 Oxone

Oxone® [2KHSO5KHSO4K2SO4, molecular weight (MW) =614.76 g/mol] is a triple

salt compound containing the active oxidant reagent, potassium monopersulfate

(KHSO5-). Oxone is a commonly used for a variety of industrial and consumer

applications such as swimming pool shock oxidizer (Wojtowicz et al. 1983; Gay 1994),

bleach component in denture cleanser (Eoga 1985) and laundry formulations (Eckhardt

et al. 1989).

Oxone has been utilized for efficient oxidation of organic compounds such as the

conversion of alkenes to diols, sulfides to sulfoxides, and ketones to esters (Kennedy

and Stock 1960; Webb and Ruszkay 1998; Jiang 1989). Travis et al. (2003)

demonstrated the oxidation of aldehydes to carboxylic acids or ester products using

Oxone aqueous solution. In addition, Oxone has been used to remove NOx and SO2

through aqueous absorption and oxidation in the wet scrubbing processes (Adewuyi

and Owusu 2003; Martin 2005).

Synergetic reagents have been frequently used to enhance the oxidative capability

of Oxone. For example, Schulze et al. (2006) presented an efficient method for oxidizing

alcohols using a mixture of sodium chloride (NaCl) and Oxone. Delcomyn et al. (2006)

and Wallace et al. (2005) showed that Oxone solutions buffered with sodium

bicarbonate containing a chloride salt or acetone rapidly inactivated viruses, bacteria,

and fungi and proposed that the resulting oxidant solutions appeared to be effective

bleach substitutes. Another interesting application of Oxone was reported (Raber and

McGuire 2002). L-gel was developed by adding of an aqueous solution of Oxone to a

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fumed silica gelling agent. Oxone in L-gel is effective to inactive bacteria (Raber and

McGuire 2002) and L-gel can be applied to surfaces with spray painting equipment.

However, the residual salts and silica increase the mass of waste which can be

hazardous. Also the further studies are needed to test the efficacy of L-gel. In this study,

we focused on the application of Oxone with sea salt (saline).

1.2 Saline-Oxone Chemistry

In the presence of saline aqueous solution, Oxone produces active chlorine

species such as hypochlorous acid (HOCl) and hypochlorite ion (OCl-) through the

reaction between peroxymonosulfate ion (HSO5-) in Oxone and chloride ion as follows

(Delcomyn et al. 2006)

2

5 2 5 3HSO H O SO H O (p 9.4)aK (1-1)

2

5 4HSO Cl H OCl SO (1-2)

HOCl H OCl (p 7.5)aK (1-3)

HOCl(l) HOCl(g) (1-4)

2 2HOCl+H Cl Cl +H O

(1-5)

Free chlorine is defined as the concentration of residual chlorine in water present

as dissolved gas (Cl2), HOCl and OCl-. When pH is between 2 and 7, the equilibrium is

in favor of HOCl. As the pH falls below 2, the predominant form of the chlorine is Cl2

(Wang et al. 2007). The induced active chlorine species (HOCl and OCl-) are more

powerful oxidants than Oxone alone (Delcomyn et al. 2006). The resulting HOCl in the

liquid (l) or particle (p) can evaporate to the gas phase (g) as shown in equation 1-4

thereby reducing the available active chlorine species in the aqueous solution. HOCl

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and OCl- are evenly distributed at pH 7.5 (25 C) (Feng et al. 2007) as shown in

equation 1-3, so a buffer such as sodium bicarbonate has often been used to

manipulate the distribution of HOCl and OCl- (eq.1-3) and HSO5- and SO5

2- (eq.1-1).

1.3 Aerosolized Saline-Oxone

The application of oxidative reactions of Oxone has been mainly utilized through

the aqueous solution. However, little is known about the water uptake properties and

reactivity of Oxone aerosol particles suspended in ambient air. In general, the use of

appropriate aerosolized oxidants would have significant value in remediating chemically

or biologically contaminated surfaces. For example, oxidative aerosols can penetrate

into contaminated interior spaces of structures and buildings where dissemination of

liquid decontaminants would otherwise be difficult.

The understanding of the chemistry of aerosol-phase reaction and reactivity of the

aerosolized Oxone coexisting with chloride is studied herein to determine its potential

value as a decontaminant. The results of this study help to provide an understanding of

the generation and oxidative reactivity of active chlorine from saline-Oxone aerosols.

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CHAPTER 2 EXPERIMENTS

2.1 Monitoring of Active Chlorine in Saline-Oxone Aerosol

The evolution of active chlorine in saline-Oxone aerosol was monitored by UV-

Visible spectroscopy. Figure 2-1 shows the experimental setup to monitor the reactivity

of internally or externally mixed saline-Oxone aerosol using a 2m3 Teflon film indoor

chamber. Internally mixed saline-Oxone aerosols were produced by atomizing a single

solution of saline and Oxone, while externally mixed aerosols were produced by

separately atomizing and separately impacting Oxone and saline solutions. The

chamber was flushed with air purified in house using two clean air generators (Aadco

Model 737, Rockiville, MD; Whatman Model 75-52, Haverhill, MA) prior to experiments.

The humidity of the indoor chamber air was controlled by passing the clean dry air

through a water bubbler. To study the oxidizing capability of saline-Oxone aerosol, MY

(Sigma Aldrich) dye which reacts with active chlorine produced from saline-Oxone

aerosol was used. A 16 mm sampling filter (Gelman Sciences Pallflex, Type TX40HI20-

WW) was dyed prior to particle collection in a MY aqueous solution (1.98 mg/10 mL

water) for 10 minutes and dried in room air.

For the study of the reaction in the internally mixed aerosol, the mixture (saline:

Oxone weight ratio = 1:1) made of 1% saline (sea salt, Sigma-Aldrich) and 2% Oxone

(Potassium monopersulfate triple salt, ≥47% KHSO5 basis, Sigma-Aldrich) aqueous

solutions was atomized into the indoor Teflon chamber using a medical nebulizer (LC

STAR, Pari Respiratory Equipment, Inc., Midlothian, VA). The internally mixed saline-

Oxone aerosol was sampled on two or three MY dyed filters in series with or without a

three channel annular denuder (URG-2000): the filter-filter (F1-F2), or filter-filter-filter

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(F1-F2-F3), or filter-denuder-filter (F1-D-F2) system. The denuder was coated with

K2CO3 to eliminate acidic gases such as HOCl. The sampling time was 2-3 minutes and

the flow rate was 10-11 L/min.

For the study of reaction in the externally mixed aerosol, saline particles or saline

particles buffered with phosphate (Sigma-Aldrich, 0.1 mol/L) were directly impacted on

MY-dyed filters using the nebulizer and weighed using an analytical balance (MX5

Mettler-Toledo Ltd., England) to measure the collected particle mass. Then Oxone

solution (2% W/V) was atomized by a nebulizer into the Teflon chamber and the Oxone

aerosol suspended in the chamber was collected on the dyed filter that contained the

saline or the saline-buffer particles as described above using a pump (Gast, DOA-P704-

AA) at 13 L/min for 5 minutes.

Aerosol particle size distribution and concentration were monitored with a SMPS

(TSI, Model 3080) coupled to a condensation nuclei counter (TSI, Model 3025A and

Model 3022). The aerosol sampling flow rate was 0.3 L/min, and the sheath airflow rate

was 2 L/min. Oxidation of MY-dyed filters from either internally or externally mixed

saline-Oxone particles was measure by reflectance-UV-Visible spectroscopy (Lambda

35, Perkin Elmer). The slit width and wavelength interval of spectral data were each 1

nm with a UV-Visible spectrum scan from 280 to 800 nm.

2.2 Monitoring of Acidity of Externally Mixed Saline-Oxone Aerosol

Acidity of externally mixed saline-Oxone aerosols was measured by collecting

particles on a TB (Sigma Aldrich) dyed filter and analyzing with reflective UV-Visible

spectroscopy (Jang et al. 2008). The filters were dyed with a TB solution prepared by

adding 2 mg of TB dye to a mixture of 5 mL water and 5 mL ethanol. The experimental

conditions and resulting data using UV-Visible spectroscopy are shown in Table 2-1.

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2.3 Measurement of Water Amount in Saline-Oxone Particles Using FTIR

Figure 2-2 shows the FTIR setup (Nicolet Magma 560, Nicolet) to monitor water

content in particles collected on a silicon window. Sodium chloride (Fisher, 99.5%),

saline, Oxone, and saline-Oxone (1:2 weight ratio) with or without buffer solutions were

atomized using a medical nebulizer. Sodium bicarbonate (Sigma-Aldrich, 99.7 %) and

phosphate (Sigma-Aldrich, 0.1mol/L) were used as buffers. The resulting NaCl, saline,

Oxone, and saline-Oxone particles were collected on a silicon FTIR window (13 × 2

mm, Sigma Aldrich) by impaction, dried directly on the window with clean air (Breathing

air, Airgas), and weighed using an analytical balance before and after particle collection

to measure the particle mass.

FTIR spectra of the impacted particles were obtained in the absorption mode

varying RH from 20% to 90% with the particle face of the silicon window exposed to the

air in the small flow chamber (0030-104, Thermo Spectra-Tech). The RH inside the flow

chamber was controlled by combining humid air from a water bubbler and dry air from a

dry air tank (Breathing air, Airgas) with a total air flow rate from 0.3 to 1.0 L/min.

Humidity and temperature were measured with an electronic thermohygrometer (Dwyer

series 485). Table 2-2 shows the experimental conditions and results for the FTIR

studies.

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Table 2-1. Experimental conditions to study aerosol reactivity and acidity using UV-Vis spectroscopy for the aerosol sample collected on the dyed filter with MY or TBa

aThe temperature and RH inside the Teflon chamber ranged from 22.8-27.4 C and 60.8-66.3 %. bThe aerosol composition is described based on the aerosol mass deposited on the filter. The original aqueous solution concentrations are 1% (W/V: weight/volume of water) for saline; 2% for Oxone solution; 2 % for buffer (sodium bicarbonate or phosphate buffer). cAerosol mass deposited on a dyed filter was measured using an analytical balance at the given laboratory RH (For internal mixture, aerosol mass was calculated from SMPS data (sampling flow rate ranged 10.1-11.5 L/min) and density of aerosol was 1.1 g/cm3. dSampled Oxone aerosol mass from Teflon chamber. eProton mass was calculated by equation 11. fN.A.: Not applicable. gThe experiments using the F1-F2 sampling system were carried out to confirm the oxidation ability of either aerosols or the active chlorine gases (e.g., HOCl and Cl2).

Dyed filter and aerosol system

Aerosol compositionb (weight ratio)

Sampling time (min)

Aerosol massc (µg)

Aerosol vol.conc. (nL/m3)

Moxone_aerosol

d

(µg)

HMass

e

(ng/m3)

MY external mixture

saline 5 9.00 331 23.51 N.A.f saline:buffer= 1:2 5 17.00 261 18.40 N.A. Oxone 1.5 - 500 10.60 N.A.

MY internal mixture

saline:Oxone=1:2 (F1-D-F2)

2 9.97 440 - N.A. 2.17 11.00 408 - N.A. 3 12.07 355 - N.A.

saline:Oxone=1:2 (F1-F2-F3)

2.50 10.15 321 - N.A. 3 10.57 317 - N.A.

saline:Oxone=1:2 (F1-F2)g

2 13.83 569 - N.A. 2.17 10.92 402 - N.A. 2.67 10.53 342 - N.A.

TB external mixture

saline 6 11.00 284 21.47 46.86

saline:buffer= 1:2 10.4 14.50 197 28.46 41.54

Table 2-2. Experimental conditions to monitor the water content in saline, Oxone, and saline-Oxone particles using FTIR and the estimated water amount in various particles in the two different RH at 20% and 70%.

Solutiona

(weight ratio) and composition

RH (%,lab)

Temp

( C)

Particle massb (μg)

Possible major aerosol constituents

Phase transition (Figure 3-5)

Mdry-particle at two different RHc (μg)

Mwater at two different RH (μg)

20% 70% 20% 70%

NaCl 52.0 25.5 55 NaCl ERH:55-57 DRH:83-87

36.7 36.7 0 93.0

saline salt- mole%: Na+ (41.94), Cl- (48.84), SO42- (2.53), Mg2+ (4.86), Ca2+ (0.94), K+ (0.89)

26.5 23.8 68 Na+, Cl-, SO42-, Mg2+

ERH:52-59 DRH:76-84

27.7 27.7 20.3 74.4

Oxone (2KHSO5 KHSO4 K2SO4)

19.7 19.4 51 K+ salts of SO5

2-, SO42-,

HSO5-, or HSO4

- No phase transition

30.4 30.4 17.2 34.9

Phosphate buffer (Na2HPO4)

49.3 26.0 65 Na2HPO4 No phase transition

10.5 10.5 N.A.d

45.4

Saline: Oxone = 1: 2 (no buffer)

38.0 22.5 53 Cl-; HOCl; Mg2+, K+ or Na+ salts of SO5

2-, SO42-,

HSO5-, or HSO4

-

No phase transition

17.0 17.0 21.6 30.1

Saline: Oxone: phosphate buffer (Na2HPO4) = 1: 2: 2.4

52.0 25.5 50 Cl-; HPO4

2-; HOCl; Mg2+, K+ or Na+ salts of SO5

2-, SO4

2-, HSO5-, or HSO4

-

No phase transition

27.8 27.8 6.1 26.0

Saline: Oxone: bicarbonate buffer (NaHCO3)= 1: 2: 2.4

53.6 25.0 58 Cl-; HOCl; Mg2+, K+ or Na+ salts of SO5

2-, SO42-,

HSO5-, or HSO4

-; CO2

ERH:51-55 DRH:74-78

32.5 32.5 6.0 84.0

aThe aerosol atomized from the aqueous solution of interest was impacted onto FTIR. The original aqueous solution concentrations are 1% (W/V: weight/volume of water) for saline; 2.5% for NaCl; 2% for Oxone solution; 2.4 % for buffer (sodium bicarbonate or phosphate buffer) bParticle mass deposited on a FTIR silicon window at the laboratory humidity condition. cSolute mass: Actual dry particle mass impacted on a silicon window corrected by relative humidity. dN.A.: Not applicable

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Figure 2-1. UV-Visible spectroscopy to monitor the evolution of active chlorine and

aerosol acidity in both internally and externally mixed aerosols. For externally mixed aerosol, Oxone particles were collected on the dyed filter containing the preexisting saline or saline-buffer particles. For the internally mixed aerosol, the saline-Oxone aerosol is sampled on the dyed filter using the different sampling systems (F1-F2-F3 and F1-D-F2).

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Figure 2-2. FTIR experimental setup to measure the water amount in oxidant particles.

The small flow chamber equipped with silicon and ZnSe FTIR windows was placed in the FTIR optical beam path in the transmission mode.

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CHAPTER 3 RESULTS AND DISCUSSION

3.1 Active Chlorine Formation and Its Oxidizing Effect

To evaluate the feasibility of oxidative reactions on a surface initiated by impacted

Oxone particles, the formation of active chlorine (HOCl) in the oxidant aerosol was

studied. Internally or externally mixed oxidant aerosol was collected on the MY-dyed

filter and monitored with reflective UV-Visible spectroscopy (Jang et al. 2008).

Unfortunately, MY exhibits spectral changes due to both reaction with active chlorine

(Sleiman et al. 2007) and change in proton concentrations ([H+], mol/L). The spectral

changes due to [H+] can be separated because UV absorption maxima of unprotonated-

MY and protonated-MY (HMY+) appear at 420 and 550 nm, respectively.

The absorption cross-section ratio for MY and HMY+ was determined by

measuring spectral shifts when acidic inorganic particles comprising sulfuric acid and

ammonium hydrogen sulfate were collected on a MY-dyed filter. The acidic inorganic

particle causes only spectral shifts as a result of [H+] changes. It is important to note

that the absorption at 550 nm was negligible before collection of acidic particles (i.e.,

MY was initially not protonated). Under the conditions of these experiments, a cross-

section ratio (σ420/ σ550) of 0.21 was measured using a least-squares curve fitting

method described previously (Jang et al. 2008). The absorption spectrum was fit to two

Gaussian functions with peak center frequency, peak absorbance, and peak width as

adjustable parameters.

A420,dye_only is the UV-Visible absorbance of the MY-dyed filter at 420 nm before

collection of particles, and A420,exp is that at 420 nm after collection of particles (acids

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and oxidants). The difference between A420,dye_only and A420,exp can be decoupled into

acidity effect and oxidation effect (∆A420,oxi) as shown in equation 3-1 below.

420420, 420, _ 420,exp 550,exp

550

oxi dye onlyA A A A

(3-1)

420,

420,

_

' oxioxiOxone aerosol

AA

M (3-2)

∆A420,oxi is normalized by the dry Oxone aerosol mass (MassOxone_aerosol). Figure 3-1

and 3-2 shows the ∆A’420,oxi values for the internally and externally mixed oxidant

aerosols, respectively. The gas-particle partitioning of HOCl generated in the internally

mixed saline-Oxone aerosol particles was investigated using the F1-F2-F3 and F1-D-F2

sampling setups (see Figure 2-1). The reaction time for the internally mixed saline-

Oxone aerosol (x-axis in Figure 3-1) is the total elapsed time from solution mixing to UV

scanning (solution mixing: 30-45 s, aerosol atomizing: 30-50 s, stabilizing of aerosol: 9-

10 min, aerosol sampling: 2-3 min, UV scanning: 1min 40 s).

The ∆A’420,oxi values for the F1 filters for both sampling setups (closed symbols)

were the same within the uncertainty of the measurement. Additionally, there was no

significant time dependence of the ∆A’420,oxi values for the F1 filters indicating that the

reaction between saline and Oxone was complete within 10 min and the oxidant

concentration was stable for at least 50 min. Such tendency was reconfirmed by the

additional three data sets using the F1-F2 sampling system.

The oxidation of MY in the F1 filter can be caused by both gas- and condensed-

phase HOCl. To estimate the fraction of MY oxidized by gas-phase HOCl in the F1 filter,

F2 and F3 filters were analyzed in the two different sampling setups. F2 in the F1-D-F2

sampling set-up exhibited negligible ∆A’420,oxi values indicating that oxidation on

24

downstream filters without the denuder is caused primarily by gas-phase oxidants

because the K2CO3 coated denuder removes acidic gases (e.g., HOCl). Together with

the F2 and F3 results without the denuder, this conclusively demonstrates that some of

the oxidation on the F1 filter is caused by gas-phase oxidants.

To estimate the fractional amount of oxidation on F1 from gas-phase HOCl, the F2

and F3 filters were examined. As shown in Figure 3-1, ∆A’420,oxi values for both F2 and

F3 without the denuder are equivalent within the uncertainty of the measurements. This

indicates that the depletion of gas-phase HOCl across the filters was negligible;

otherwise, the oxidation of F3 would be significantly less than F2. We conclude that the

gas-phase HOCl exposure of F1 is equivalent to that of F2 and F3. The observed

oxidation of F2 and F3 was 30% of that for F1, and because the gas-phase HOCl

exposure was the same for all three filters, gas-phase HOCl oxidation of F1 accounts

for 30% of the total oxidation at our sampling and experimental conditions. Conversely,

particle-bound oxidant accounts for 70% of the observed oxidation of MY on the filter.

For the externally mixed particles, active chlorine formed when the Oxone particles

contacted either saline or saline-phosphate buffer particles on the dyed filter. Figure 3-2

shows the ∆A’420,oxi values obtained using equations 3-1 and 3-2 where the x-axis is the

time after collection of the Oxone particles. Overall, higher MY oxidation was observed

for Oxone particles collected in the presence of saline because saline promoted the

formation of active chlorine species. However, even Oxone particles collected in the

absence of saline showed some oxidizing capability. For example, the ∆A’420,oxi value of

the Oxone aerosol 30 min after the reaction began was 26-36% of that for the co-

deposited Oxone and saline or Oxone and saline-phosphate buffer particles.

25

The highest oxidation was observed for the co-collected saline and Oxone

particles. After the Oxone aerosol was collected on the dyed filter containing the

preexisting saline, nearly 90% of the oxidation progressed within 3 minutes compared to

oxidation within 33 minutes for our experimental duration. This result confirms that the

formation of active chlorine from the reaction between saline and Oxone rapidly

progresses. In the presence of phosphate buffer, only 28% of the oxidation in saline-

Oxone aerosol progressed within 10 minutes after the Oxone aerosol contacted the

dyed filter. This result shows that phosphate buffer retards the production rate of active

chlorine without reducing the maximum oxidizing capability for our experimental

duration.

3.2 Buffer Stability in Saline-Oxone Aerosol

Commonly used buffers such as bicarbonate and phosphate were internally mixed

with oxidant particles and monitored using FTIR to evaluate chemical stability in oxidant

aerosols. Figure 3-3 shows the FTIR spectra of each buffer compared to the subtracted

spectra ([spectrum of the saline-Oxone –buffer particles]-[spectrum of saline-Oxone

particles]). The subtracted FTIR spectrum for the particle system containing phosphate

buffer resembles the finger print of phosphate buffer alone, confirming the presence of

buffer in the saline-Oxone particles. However, the subtracted FTIR spectra for saline-

Oxone particles buffered with sodium bicarbonate does not show characteristic peaks at

700, 830, 1000, 1300-1400 cm-1 for bicarbonate buffer (Figure 3-3). This result indicates

that particles buffered with bicarbonate, while potentially at the correct pH, would lose

their buffering capacity since carbonic acid produced from the bicarbonate buffer rapidly

decomposes into carbon dioxide in the presence of water as shown in equations 3-3

and 3-4 (Thomas et al. 2000).

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3 3 2 3 2HCO H O H CO H O

(3-3)

2 3 2 2H CO H O CO (g)

(3-4)

Therefore, the stability of phosphate buffer makes it more favorable for

incorporation into the saline-Oxone aerosol.

3.3 Humidity Effect

Humidity influences kinetics of aerosol-phase reactions through two possible

mechanisms. First, the aerosol-phase water content, which is directly related to the

humidity, affects the reaction of saline-Oxone aerosol through phase transitions in the

particles. Second, the enrichment of inorganic constituents in particles due to the low

water content can influence kinetics of formation of active chlorine and/or

thermodynamic equilibria of inorganic species.

To monitor the humidity effect on saline-Oxone aerosol, FTIR integrated with a

small flow chamber capable of maintaining RH between 20% and 90% was used

(Figure 2-2). The intensity of the broad OH stretching band (centered at 3350 cm-1)

originating from the condensed-phase water in the impacted particles increases with

increasing RH (Criczo and Abbatt 2000; Kreidenweis et al. 2008). Figure 3-4 shows the

FTIR spectra of the NaCl particles at different RH levels: high RH (82%), middle RH

(60%), and low RH (7%).

Water uptake into NaCl particles has been studied previously, so it provides a

convenient reference for these measurements of particles impacted on a silicon

window. In our study, the observed DRH for the impacted NaCl particles was 83-87%

and the ERH was 55-57%. The resulting DRH and ERH values for the NaCl particles of

this study were slightly higher than those known in the literature (75-80% for DRH and

27

40-50 % for ERH) (Gao et al. 2007). A calibration curve (slope: 764.54 and intercept: -

5×10-16) for water content (Mwater, µg) in the impacted NaCl particles was obtained by

relating the FTIR absorbance (Abswater_3350) of the water peak at 3350 cm-1 at different

RH and the water mass fraction predicted from an inorganic thermodynamic model

(Nenes et al. 1998) (see Appendix A). M’water (µg/ µg) represents the particle-phase

water mass [Mwater, (µg)] divided by the sum of Mwater and dry-particle mass (Mdry-particle)

that is estimated from the difference between the total particle mass and the water mass

at a given laboratory RH and a given aerosol particle sample.

waterwater

dry particle water

MM

M M (3-5)

Table 2-2 and Figure 3-5 show the Mwater (µg) and M’water (µg/ µg) obtained for the

various particle compositions. In Figure 3-5, both impacted NaCl particles and impacted

saline particles show step changes in the water mass fraction indicating a phase

transition (ERH~ 55%). However, no dry aerosol was observed for the saline particles

which showed a large water mass fraction (M’water~ 0.4) in the particles even at 20% RH

compared with NaCl particles which showed an insignificant water mass fraction. This

difference is mainly caused by MgCl2 and MgSO4 present in saline particles (Tang et al.

1997; Zhao et al. 2006). MgSO4 promotes gel formation in concentrated liquid solutions

at low RH preventing crystallization of the particles. All other particle of this study exhibit

no apparent phase change (ERH) as RH was reduced to 20%. In fact, all impacted

oxidant particles still have significant water mass fractions suggesting that oxidant

aerosols are liquid and active chlorine species can form in saline-Oxone aerosol over

the entire humidity range of this study. Table 2-2 summarizes possible major aerosol

constituents from the saline-Oxone aerosol with and without buffer.

28

The saline-Oxone aerosol appeared to be least sensitive to changes in RH. The

calculated mole ratio of Cl- to HSO5- in the internally mixed saline-Oxone aerosol is 4.8

at a given composition indicating that the major aerosol constituents is still saline

although potassium and sodium sulfates are present in aerosol as reaction products.

Therefore, it is reasonable to conclude that the aerosol is not crystallized due to gel-like

saline constituents. However, it is unclear why saline-Oxone aerosol is least sensitive to

RH changes among oxidant mixture aerosols of this study. Phosphate buffer aerosol is

very hygroscopic; more than 43% of the total phosphate aerosol mass is water even at

35%. The saline-Oxone aerosol with phosphate buffer also showed no clear phase

transition possibly due to the effects of Na2HPO4 buffer and MgSO4 in saline.

3.4 Dynamics of Aerosol Acidity

Acidity influences the distribution of aqueous-phase chemical species (e.g., SO5-2

vs HSO5- in equation 1-1 and OCl- and HOCl in equation 1-3) which can affect the

oxidizing capability of the resulting aerosol. [H+] (mol/L) of saline-Oxone aerosol was

determined from the proton mass (MH+, ng/m3) measured by UV-Visible spectroscopy

(Jang et al. 2008) and Mwater measured by FTIR. To estimate the MH+ in saline-Oxone

aerosol, TB dye was selected as a pH indicator (Saikia and Dutta 2006) since it is

insensitive to active chlorine oxidants. A color change from yellow (λ= 450 nm) to red

(λ= 550 nm) occurs near pH 1.2-2.8 due to protonation of the TB dye (HTB+). The MH+

of the aerosol filter sample was calculated using the sampler air volume (Vair), UV-

Visible absorbance [A550,aerosol] at 550 nm for the protonated TB, and a calibration curve

for aerosol acidity. The calibration curve was obtained by relating the known proton

mass of inorganic acidic aerosol (MH+

_inorg) (NH4HSO4) (Jang et al. 2008) with A550,aerosol

29

corrected for spectral overlap from the absorbance of unprotonated TB (see Appendix

B).

550,278.63 aerosolH

air

AM

V (3-6)

MH+ calculated from equation 3-6 for the saline-Oxone aerosol (1:2 weight ratio)

and saline-Oxone-phosphate buffer (1:2:2 weight ratio) aerosol are shown in Table 2-1.

The [H+] of the saline-Oxone and the saline-Oxone with phosphate buffer aerosols were

obtained from MH+ (equation 3-6) and Mwater (equation 3-5).

, 55%

55%, 65%

[ ]H RH

RHaerosol RH water

Mass

HV V

(3-7)

where MassH+

,RH=55% represents the MH+ at laboratory RH (55%), Vaerosol,RH=65% is

the collected Oxone particle volume measure with a SMPS at the Teflon chamber RH

(65%), and ∆Vwater is the difference of the aerosol-phase water volume between

laboratory RH and Teflon chamber RH applied to oxidant aerosol. At our experimental

conditions, the obtained [H+] of saline-Oxone and saline-Oxone with phosphate buffer

aerosols using equation 3-7 are 0.17 mol/L and 0.24 mol/L, respectively, showing that

both aerosols are strongly acidic with pH value less than 0. The gas-phase HOCl

formed in aerosol would mostly evaporate to the gas phase due to its high volatility,

although HOCl produces Cl2 in acidic conditions (equation 1-5). However, as shown

above, the negligible oxidation effect on the F2 filter in the F1-D-F2 sampling system

using the K2CO3 coated denuder indicates that the main active chlorine compound

would be HOCl rather than Cl2.

30

0.00

0.01

0.02

0.03

0 10 20 30 40 50 60

A' 4

20

,oxi

Time (min)

F1 filter in F1-F2, F1-F2-F3, and F1-D-F2 sampling system

F2 filter in F1-F2 and F1-F2-F3 sampling system

F2 filter in F1-D-F2 sampling system

F3 filter in F1-F2-F3 sampling system

Figure 3-1. The ∆A’420,oxi obtained using different sampling systems (F1-F2-F3 and F1-

D-F2) for the internally mixed saline-Oxone aerosol after the Oxone was mixed with saline in the aqueous solution. Error bars are estimated from uncertainties in the sampling device, SMPS data, and UV-Visible absorbance.

31

0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0 32.50.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

Saline-Oxone aerosol

Saline-Oxone with phosphate buffer aerosol

Oxone aerosol only

A' 4

20

,oxi

Time (min)

Figure 3-2. Time profile of ∆A’420,oxi of the external mixture of Oxone, saline-Oxone,

saline-Oxone with phosphate buffer aerosol. Error bars were estimated from uncertainties in the sampling device, balance, SMPS data, and UV-Visible absorbance.

32

1800 1600 1400 1200 1000 800

0.0

0.2

0.4

0.6

0.8

1.0A

bso

rban

ce

Wavenumbers(cm-1)

Sodium bicarbonate

(SA+OX) with bicarbonate buffer-(SA+OX)

Phosphate buffer

(SA+OX) with phosphate buffer-(SA+OX)

Figure 3-3. FTIR spectra of the tested buffers (sodium bicarbonate and phosphate

buffer) and the subtracted FTIR spectra ([spectrum of Saline/Oxone/Buffer] – [spectrum of Saline/Oxone]).

33

4000 3000 2000 10000.00

0.05

0.10

0.15

0.20A

bso

rban

ce

wavenumber (cm-1)

82 RH(%)

61 RH(%)

57 RH(%)

7 RH(%)

water peak (3350cm-1)

Figure 3-4. FTIR spectra of NaCl particles impacted on silicon FTIR window at different humidity levels.

34

20 30 40 50 60 70 80 900.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9 Saline:Oxone:Phosphate Buffer (1:2:2.4) Saline:Oxone (1:2)

Phosphate Buffer Only

NaCl (ERH)

Oxone

SalineM' w

ate

r

Relative humidity (%)

Figure 3-5. The M’water of NaCl, saline, Oxone, and saline-Oxone aerosols as a function of decreasing humidity. Error bars were estimated from uncertainties in the FTIR absorbance at O-H stretching and balance.

35

CHAPTER 4 SUMMARY

Our results from the study of oxidizing capability of aerosol-phase active chlorine

species towards the organic dye show that the aerosolized saline-Oxone is a feasible

decontaminant for surfaces. The production rate of active chlorine species varies with

formulation of the oxidizing aerosol (i.e., presence and type of buffer) and aerosol

mixing type (e.g., internally or externally mixed aerosol). For example, when the fast

oxidation is required in the application such as examples shown by Delcomyn et al.

(2006), the externally mixed saline and Oxone particles without phosphate buffer would

be most profitable (Figure 3-2) among oxidant aerosols formulations investigated here.

When slow oxidation is needed, the externally mixed saline-Oxone aerosol with

phosphate buffer is appropriate due to retardation of active chlorine formation in aerosol

(Figure 3-2).

One significant problem with this approach is the partitioning of the aerosol-

originated oxidant to the gas phase. The low pH observed in the particles favors the

formation of HOCl which evaporates from the particles. Future research needs to

quantify the gas-particle portioning of HOCl and investigate formulations that result in

neutral pH.

36

APPENDIX A CALIBRATION CURVE FOR WATER CONTENT IN PARTICLES

NaCl particle was used as a standard particle to estimate the water content in

particles due to known thermodynamic property. The water mass in the NaCl particles

(MNaCl_water, μg) was obtained as follows:

_ _1

wNaCl water NaCl particle

w

fM M

f

Where fw is water mass fraction of the total NaCl particle (Nenes et al. 1998) and

MNaCl_particle (μg) is the NaCl particle mass collected on a silicon window. A calibration

curve for water content in the impacted NaCl particle was obtained using the FTIR

absorbance (Abswater_3350) (x-axis) at different RH (Figure 3-4) and the MNaCl_water (y-

axis). Obtained slope is 764.54 with intercept -5×10-16 and used to calculate the water

content in other particles (equation 3-5).

37

APPENDIX B CALIBRATION CURVE FOR AEROSOL ACIDITY

B.1 Experimental Procedures

A sampling filter (16mm, Gelman Sciences Pallflex, Type TX40HI20-WW) was

dyed with a solution (2mg of TB dye to a mixture of 5mL water and 5mL ethanol) for 10

minutes and dried naturally. A 2m3 Teflon chamber was flushed with clean air using two

clean air generators prior to experiment. The humidity of the chamber was controlled by

passing the clean dry air through a water bubbler. Inorganic aerosol was made from an

aqueous solution of ammonium hydrogen sulfate (0.01M) and sulfuric acid (0.01M) with

different mixing ratio and was atomized into the chamber using an atomizer. The

inorganic aerosol was collected from a Teflon chamber on a TB dyed filter using a pump

for 0.5-3 minutes at 13 L/min. The filter sample color changes associated with proton

concentrations in the aerosol sample were monitored by UV-Visible spectroscopy. Table

B-1 shows the experimental conditions and results.

B.2 Calibration Curve

Proton mass (MH+

_inorg) of inorganic acidic particles was calculated as follows:

_ 1000inorg inorgH HM f M

where Minorg is the total inorganic aerosol mass (µg) and fH+

is the proton mass

fraction of the total aerosol obtained from an inorganic thermodynamic model (Nenes et

al. 1998). Minorg is estimated as follows:

inorg inorg inorgM d V Q t

where dinorg is the density of aerosol (1.4 g/mL for our case), Vinorg is the aerosol

volume concentration obtained from the SMPS, Q is the volumetric air flow from a

sampling system, and t is the sampling time (see Table B-1). The calibration curve

38

(Figure B-1) for calculating available protons in aerosol was obtained by relating

MH+

_inorg (y-axis in Figure B-1) with A550,aerosol corrected for spectral overlap from the

absorbance of unprotonated TB (x-axis in Figure B-1). Obtained coefficient is 278.63

with R2=0.8493 and used to calculate the experimentally observed proton mass of the

aerosol on the filter (equation 3-6).

39

Table B-1. Experimental conditions and results to obtain calibration curve for aerosol acidity

aAS is sulfuric acid and AHS is ammonium hydrogen sulfate. bThe temperature and RH

in the lab ranged from 26-26.9 C and 46-50.6 %.. cSampling flow rate ranged 12.2-12.5 L/min and density of aerosol was 1.4 g/cm3.

y = 278.63x

R² = 0.8493

0

2

4

6

8

10

12

14

16

18

20

0 0.02 0.04 0.06 0.08

Corrected A550,aerosol

MH+_inorg (ng)

Figure B-2. Calibration curve for calculating aerosol acidity

Aerosol composition (volume ratio)

Temp( C)b RH(%) Sampling time (min)

Vinorg (nL/m3)

Minorg c

(ng)

SA:AHS=1:1a 26.3 46.8

3.25 177 10.07 3.28 123 6.95 1.92 126 4.12 0.75 113 1.47

SA:AHS=1:2 26.8 49.1

3.00 191 9.79 2.33 160 6.53

1.42 167 4.12

0.50 135 1.15

40

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43

BIOGRAPHICAL SKETCH

Jaeyoun Jang graduated from Korea University in 2006. Her undergraduate major

was Civil Engineering and came to the University of Florida in 2008 to pursue her

master’s degree. She joined the aerosol research lab in the department of

Environmental Engineering Sciences. She worked on a project, ‘Characterization of and

decontamination by reactive saline aerosols’, and worked as a research assistant under

Dr. Myoseon Jang. She submitted a paper, ‘Formation of active chlorine oxidants in

saline-Oxone aerosol’, to Aerosol Science and Technology in 2010.