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Formation of Light Absorbing Soluble Secondary Organics and Insoluble Polymeric Particles from the Dark Reaction of Catechol and Guaiacol with Fe(III) Samantha Slikboer, Lindsay Grandy, Sandra L. Blair, Sergey A. Nizkorodov, Richard Smith, and Hind A. Al-Abadleh Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 03 Jun 2015 Downloaded from http://pubs.acs.org on June 5, 2015

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Environmental Science & Technology

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Formation of Light Absorbing Soluble Secondary Organics and Insoluble Polymeric

13

Particles from the Dark Reaction of Catechol and Guaiacol with Fe(III)

14 15

Samantha Slikboer,†¶ Lindsay Grandy, †‡ Sandra L. Blair, §‡ Sergey A. Nizkorodov,§

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Richard W. Smith,# and Hind A. Al-Abadleh†*

17 18

†

Department of Chemistry and Biochemistry, Wilfrid Laurier University, Waterloo, ON

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

N2L 3C5, Canada §

Department of Chemistry, University of California, Irvine, CA 92697, United States #

University of Waterloo Mass Spectrometry Facility, Department of Chemistry, University of Waterloo, Waterloo, ON, N2L 3G1, Canada

_________________ * Corresponding author phone: (519)884-0710, ext.2873; fax: (519)746-0677; e-mail: [email protected]. ¶ Current address: Department of Chemistry and Chemical Biology, McMaster University ‡ Contributed equally to the work

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ACS Paragon Plus Environment


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Abstract

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Transition metals such as iron are reactive components of environmentally-relevant

37

surfaces. Here, dark reaction of Fe(III) with catechol and guaiacol was investigated in an

38

aqueous solution at pH 3 under experimental conditions that mimic reactions in the adsorbed

39

phase of water.

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elemental analysis, dynamic light scattering and electron microscopy techniques, the reactants,

41

intermediates and products were characterized as a function of reaction time. The reactions of

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Fe(III) with catechol and guaiacol produced significant changes in the optical spectra of the

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solutions due to the formation of light absorbing secondary organics and colloidal organic

44

particles. The primary steps in the reaction mechanism were shown to include oxidation of

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catechol and guaiacol to hydroxyl- and methoxy- quinones. The particles formed within a few

46

minutes of reaction and grew to micron-size aggregates after half an hour reaction. The mass-

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normalized absorption coefficients of the particles were comparable to those of strongly-

48

absorbing brown carbon compounds produced by biomass burning. These results could account

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for new pathways that lead to atmospheric secondary organic aerosol formation and abiotic

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polymer formation on environmental surfaces mediated by transition metals.

Using UV-vis spectroscopy, liquid chromatography, mass spectrometry,

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1.

Introduction

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Despite decades of progress in research on atmospheric aerosols, secondary pathways

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for their formation,1,2 their role in absorbing and scattering radiation (direct effect on climate)

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acting as cloud and ice condensation nuclei (indirect effect on the climate)3, and contributing to

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heterogeneous chemistry of air pollutants remain active areas of investigation.4 Given their

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ubiquitous presence in field-collected aerosols from different origins,5,6 organic compounds are

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involved in a number of reactions and processes that transform chemical composition,

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hygroscopicity, phase, and optical properties of atmospheric aerosols.7,8 For example, single

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particle analysis of sea spray particles generated from ‘ocean-in-a-lab’ experiments revealed

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that under acidic conditions, organic material concentrates at the surface,9 and that these

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particles contain transition metals such as iron.10 The role of transition metals in the aging and

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transformation of the organic content on atmospherically-relevant surfaces that include aerosols,

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buildings, and surface water is still unclear and warrants further investigation.6,11,12

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Aging of organics in aerosols due to heterogeneous chemistry with gas phase oxidants

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such as O3, OH, and NO3 was studied extensively.2 Dark and photochemical processes driven

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by dissolved redox active species such as Fe, OH, H2O2, and excited triplet states of certain

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organics were also investigated in detail with respect to their chemistry in large cloud/fog

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droplets.13-15 However, our understanding of oxidation processes relevant to multicomponent

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atmospheric systems containing sparse amounts of water such as environmental films and wet

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aerosols is still limited. For example, fluctuations in the water content and acidity of aerosols

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due to changes in ambient temperature, relative humidity and reactions with acidic/basic gases

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affect the solvent:solute ratios and the relative importance of bulk versus surface reactions.6

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Moreover, for some aerosols systems such as mineral dust particles and other atmospherically-

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important surfaces such as buildings, water exist in small pockets of aqueous phase or ‘adsorbed

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phase’ in the form of islands or films depending on the underlying substrate and environmental

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conditions.11 The high concentrations of solutes in these systems results in reactions that are

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uniquely different from the corresponding reactions in more dilute bulk aqueous solutions.16

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The redox chemistry and strong chelating abilities of iron (Fe) in the bulk aqueous phase

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are well established at the fundamental level12 and recently were utilized in functionalizing

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surfaces and nanoparticles for applications in green chemistry17 and in the development of

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biomedical and sensing devices.18-20 In contrast, the role of transition metals such as Fe in

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driving secondary organic aerosols (SOA) formation from aliphatic and aromatic precursors in

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heterogeneous/multiphase reactions is not well understood.1,2,12 This study demonstrates that

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dark reaction of Fe with catechol and guaiacol under high solute:solvent ratio that mimic

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reactions in the adsorbed phase of water leads to an efficient redox reaction resulting in

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complex polymeric products that strongly absorb visible radiation. Catechol and guaiacol

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(Scheme S1), are semi-volatile phenolic compounds emitted from biomass burning. They are

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moderately soluble in water with Henry’s law constants of 4×103 and 900 M atm-1,

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respectively.21 These compounds are well known aromatic SOA precursors22 and are simple

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models for the aromatic fraction of humic like substances (HULIS) in aerosols.22 Catechol,

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guaiacol and other catecholates are capable of chelating Fe19,23 (Scheme S1), and they easily

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oxidize to the corresponding quinones (e.g., o-quinone in Scheme S1). Our observation of the

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formation of polymeric products, in addition to the expected quinone, could account for new

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pathways that lead to SOA formation mediated by transition metals, and explain observations

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from single particle analysis of field-collected ‘aged’ mineral dust aerosol,11 sea spray from

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coastal regions impacted by Fe,10 and abiotic polymer formation on surfaces24 such as those

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imaged in sea spray aerosols.9 Furthermore, this reaction may serve as a source of atmospheric

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brown carbon – organic aerosol capable of strongly absorbing near-UV and visible radiation

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translating into direct effect on climate.25,26

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2.

Experimental

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2.1

Chemicals. All chemicals were used as received without further purification: catechol

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(1,2-benzendiol, >99%, CAS: 120-80-9, Sigma-Aldrich), guaiacol (2-methoxyphenol, ≥98%,

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CAS: 95-05-1, Sigma-Aldrich), iron(III) chloride hexahydrate (FeCl3⋅6H2O, CAS: 10025-77-1,

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Sigma-Aldrich). Chemicals used as reference compounds and for the hematite dissolution

111

experiments are listed in the Supporting Information (SI). Aqueous phase solutions were

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prepared by dissolving the chemicals in Milli-Q water (18.5 MΩ cm) or 18O-labeled water (97

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atom%

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chloride (KCl powder, 99.5%, EM Science) to stabilize the pH reading. The pH was adjusted

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using solutions of hydrochloric acid (HCl 6 N, Ricca Chemical Company) and sodium

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hydroxide (NaOH pellets, 99-100%, EMD).

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2.2

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using either a fiber optic UV-vis spectrometer (Ocean Optics USB 4000) or a Shimadzu UV-

119

1800 spectrophotometer with a 1 cm quartz cuvette. Chromatograms were collected using a

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Waters Delta 600 instrument equipped with a Waters 2487 dual wavelength absorbance

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detector. An in-line degasser was used for sparging at 20 mL/min throughout the experiment to

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avoid air bubbles. The 4.6 x 250 mm column had Hypersil GOLD C8 stationary phase, with 5

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µm particle size, and 175 Å pore size. The mobile phase was flown isocratically using 95%

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water, with 0.05% TFA and 5% acetonitrile at a flow rate of 1 mL/min. The injection volume

18

O, Sigma-Aldrich) with an ionic strength adjusted to 0.01 M by adding potassium

UV-visible spectroscopy and HPLC experiments. UV-vis spectra were collected

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of the sample was 30 µL. Further details are provided in the SI.

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2.3

127

to liquid chromatography separation (LC-ESI-MS) experiments were performed with a

128

ThermoFisher Scientific Q-Exactive hybrid Orbitrap mass spectrometer. Typical operating

129

conditions are provided in the SI. Isocratic elution was achieved with 5/95 v/v mixture of

130

acetonitrile/H2O + 0.1% formic acid. TFA was not used in these experiments because it causes

131

ion suppression in ESI and must be avoided, not just for these studies but also for all subsequent

132

LC/MS analyses. The reaction of chemicals prior to injection was done in a manner similar to

133

that described above for the HPLC experiments.

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2.4

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catechol/guaiacol reaction were collected on nylon membrane filters (0.2 µm pore size, 25 mm

136

dia., EMD) and images of the filters were taken using a digital camera. Mass yield experiments

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were performed after 2 hours reaction using 1:2 molar ratio organic reactant:Fe by weighing the

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filters before and after filtration followed by overnight drying. For these experiments, the

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reaction volumes were considerably scaled up (70 mL for catechol:Fe, and 140 mL for

140

guaiacol:Fe) in order to achieve easily measurable mass yields of particles, keeping the starting

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concentrations the same in all experiments. The particles were washed with Milli-Q water

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several times prior to drying. Also, collected filtrates were subjected to filtration four times to

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ensure minimum loss of particles.

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inductively coupled plasma mass spectrometry (ICP-MS) using method EPA 3050, ALS Global

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Labs. The preparation of samples for scanning electron microscopy-energy dispersive X-ray

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spectroscopy (SEM-EDS) experiments is described in detail in the SI. Dynamic light scattering

147

(DLS) experiments were performed on FeCl3 with either catechol or guaiacol reaction solutions

Mass spectrometric experiments. Negative ion mode electrospray ionization coupled

Particles characterization. Particles that formed in the solution as a result of FeCl3 +

The iron content in the particles was analyzed using

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in a 1 cm Teflon capped-quartz cuvette with a Malvern Zetasizer Nano (ZEN3600) with settings

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for each measurement to average 10 runs with a duration of 10 s. For FTIR measurements, the

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particles were deposited on a ZnSe attenuated total internal reflectance (ATR-FTIR) cell from a

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water/ethanol slurry followed by drying overnight. Absorbance spectra of these particles were

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obtained by referencing to the clean and dry ZnSe crystal.

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2.5

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nanoparticles were used a model for iron (oxyhydr)oxides in mineral dust aerosols. A slurry of

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hematite nanoparticles was prepared in 0.01 M KCl adjusted to pH 1 using HCl, and allowed to

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mix for 10 days at medium speed in a vortex mixer followed by filtration and determination of

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total dissolved iron concentration (see details in the SI including Figure S1). After adjusting the

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pH of the filtrate to 3, catechol or guaiacol was added at half the total iron concentration

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determined by UV-vis spectroscopy. Samples were allowed to mix in the dark for 60 min prior

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to filtration and photographing.

Acid-dissolution of hematite followed by reaction with organics.

Hematite

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3.

Results and Discussion

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3.1

Optical properties and time profile of reactants and products. Upon reaction of

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FeCl3 with organics, color development and colloid formation was observed overtime. Figure 1

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(a-d) and Figure S2 show time-dependent images of solutions and filters resulting from the dark

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reaction of catechol and guaiacol with FeCl3 at pH 3 in the absence of added oxidants such as

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H2O2. It is important to emphasize here that aqueous solutions are exposed to air, and hence

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dissolved O2 is the oxidant and Fe(III) plays a role as a catalyst.

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spectra after filtration (i.e., with particles taken out of the solution) are also shown (Figure 1e-f)

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and compared to those of the initial spectra of catechol/guaiacol prior to adding FeCl3. While

7


The corresponding UV-vis


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the concentrations are relatively high, they mimic interfacial regions on surfaces where aromatic

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compounds are enriched at the interface.27 Both catechol and guaiacol solutions are transparent

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in the visible range and show a UV band around 274 nm due to π→π* transitions. The UV-vis

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absorbance spectrum of FeCl3 solution (pale yellow) shows a band around 295 nm from the

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ligand-to-metal-charge

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[Fe(H2O)5OH]2+.28 Spectra collected at pH=1-5 exhibit a shift in this peak because the iron

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speciation is strongly pH-dependent.29 The pH used in our studies was chosen because a

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number of studies29 reported maximum photoreactivity of iron species towards organic

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degradation at pH 3. These photochemical studies are currently underway in our lab. This pH is

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also environmentally-relevant since aerosols are generally acidic (e.g., pH 3 is possible for fog

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droplets in highly-polluted areas).13 Studies at pH values lower than 3, which are characteristic

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of wet aerosols, are currently ongoing in our lab for comparison. In the case of catechol, the

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initial green color was observed for 1:2, 1:1 and 2:1 organic reactant:Fe molar ratios, attributed

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to the formation of a bidentate mononuclear catechol-Fe complex (Scheme S1) with a LMCT

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band around 700 nm.30 The intensity of this feature varies with the amount of Fe in the solution

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mixture, being the least intense for organic reactant:Fe 2:1 solution mixtures. The Fe(III)/Fe(II)

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cycling can be followed by flow injection analysis and also online by a continuous flow

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analysis31,32 where the concentration of Fe(II) and total Fe after the reduction of Fe(III) can be

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determined on the basis of the color of the reaction as shown in the SI. This work is currently

190

underway in our labs.

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of o-quinone species formed from oxidation of catechol-Fe complexes.33 We observed this

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peak at lower concentrations of catechol and iron under acidic conditions, and also for other

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catecholates such as gallic acid.28 The intensity of this feature decreases with less Fe in the

transfer

(LMCT)

of

the

prevailing

species

in

solution,

The intense spectral feature at 390 nm is attributed to n→π* transitions

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solution mixture (a)

(b)

30 min

3 min

60 min

30 min

3 min

(c)

(d)

0.63 cm

0.63 cm

30 min 2.5 (e) 2.0 Absorbance

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1.5 390

30 min 2.5 (f)

Mixing time (min) 120 90 60 30 1 0.98 mM catechol 1.96 mM FeCl3 , pH 3

Mixing time (min) 60 45 30 15 3 1 0.5 mM guaiacol 1 mM FeCl3, pH 3

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2.0 1.5 1.0

1.0 0.5

412 470

0.5

715

0

0 300

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60 min

400 500 600 700 Wavelength (nm)

300

800

400 500 600 700 Wavelength (nm)

800

196 197 198 199

Figure 1: Dark reaction of catechol and guaiacol with FeCl3 at pH 3: digital images of 1:2 organic reactant:Fe molar ratio of unfiltered solutions as a function of time (a,b); particles on filter after 30 min (c,d); the corresponding UV-vis spectra after filtration (e,f).

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(Figure S2a-b versus Figure1e). The presence of –OCH3 group in the case of guaiacol inhibits

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the formation of the iron complex evident by the absence of the characteristic LMCT band in

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Figure 1f.

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oxidation products (compounds V-VIb, Scheme S1) due to iron redox chemistry. These spectra

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are identical to those observed by Hwang et al.34 who identified products from the biochemical

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oxidation of guaiacol by manganese peroxidase (MnP) in the presence of H2O2 by a suite of

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analytical techniques. In our study, in-situ reduction of Fe(III) to Fe(II) leads to the formation of

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phenoxy radicals, which proceeds through C-C radical coupling form compound V.

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Compounds VIa and VIb (Scheme S1) give rise to the spectral features at 412 and 470 nm

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(Figure 1f), which were observed to decrease in intensity upon overnight storage of

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solutions.34,35 Schmalzl et al. assigned the 470 nm peak to an unstable 4,4’-diphenoquinone

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intermediate.36 Further characterization of guaiacol oxidation products was not carried out

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herein given the similarities between our data in Figure 1f and published product identification

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studies.34,35 Because studies on the oxidation of catechol by Fe(III) are limited under our

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experimental conditions, the following paragraphs describe in detail the time profile of reactants

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and products from HPLC studies.

Instead, spectra in Figure 1f suggest the formation of soluble amber-colored

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Figure 2 shows HPLC chromatograms collected before and after the addition of FeCl3 to

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a catechol solution at pH 3, which were recorded at λ = 271 and 388 nm. These wavelengths

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were chosen based on UV-vis spectra shown in Figure 1e to separate compounds contributing to

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the reactant and product peaks. The chromatograms at λ = 271 show two major peaks at 7 and

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17 min retention time (RT), whereas only one major peak is present in the chromatograms at λ

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= 388 nm (RT 7 min).

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respectively, from the comparison with the chromatogram of catechol standard solution. The

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integrated areas of these peaks are plotted as a function of time in Figures 2c-d for solution

These peaks are assigned to a product and catechol (reactant),

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mixtures with 1:2 molar ratio of catechol:Fe. Similar experiments were conducted by varying

HPLC signal (a.u.)

0.6

(a) λ = 271 nm 7

17

0.25

Mixing time

7

120

0.20

0.5 90

0.4

0.15 60

0.3

0.10

30

0.2 1

0.05

0.1 initial

0 0

3.0x10

6

5

10 15 Time (min)

20

0 0

25

6

3.0x10

(c)

2.0 17 min

1.5 1.0

Peak Area, 388 nm

Peak Area, 271 nm

5

10 15 Time (min)

20

25

(d)

2.5

2.5

0.5

2.0 1.5 1.0 0.5

7 min 0

20

40

60 80 Time (min)

100

7 min

0.0

0.0

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(b) λ = 388 nm

(min)

0

120

20

40

60 80 Time (min)

100

120

226 227 228 229 230

Figure 2: HPLC chromatograms collected at 271 nm (a) and 388 nm (b) for the initial catechol solutions (0.98 mM) and after reaction with FeCl3 at pH 3 as a function of reaction time. Panels (c) and (d) show the resulting kinetic curves from the integrated areas of the peaks at 7 and 17 min. The solution mixture contains 1:2 molar ratio of catechol:Fe.

231

the molar ratios, and the data are presented in Figure S3 for 1:1 and 2:1 molar ratios of

232

catechol:Fe. Despite the drop in the performance of the detector at 700 nm, where the catechol-

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Fe complex absorbs, experiments were conducted to examine the kinetic behavior of the

234

product peak at this wavelength. Figure S4 shows results from collecting chromatograms at λ =

235

700 nm and the resulting kinetic curves for the product peak at 7 min compared with data

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generated from the chromatograms collected at 271 nm. These kinetic curves track well with

237

each other supporting the assignment of the peak at 7 min to a catechol-Fe complex.

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The time profiles of the peak at RT=7 min clearly show that it is an intermediate species

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that reaches its maximum concentration at 3 min. Because this product peak has absorptions at

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271, 388, and 700 nm, it is consistent with the formation of the catechol-Fe complex and an

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unstable quinone species that undergoes further oxidation in the presence of dissolved O2 and

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excess Fe(III).30 It is likely that they both exist in equilibrium on the time scale of the collection

243

time. This assignment is also similar to earlier observations of the time profile of peaks in the

244

range 400-500 nm from guaiacol oxidation (see Figure 1). For solution mixtures containing less

245

Fe as in the 1:1 and 2:1 molar ratios shown in Figure S3, a smaller amount of this intermediate

246

is formed, reduced by 50 and 23%, respectively, relative to that in 1:2 catechol:Fe molar ratio

247

solutions. This result suggests that iron plays an important catalytic role in forming this

248

oxidation product of catechol.

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In addition, the kinetic curves of the RT=17 min peak show that it reaches a minimum

250

after 3 min of reaction (~83% decay in catechol, Figure 2c), and then the signal starts to go up

251

due to the contribution of products to the absorbance at λ = 271. These products appear to co-

252

elute at the same RT as catechol suggesting similar polarity. Attempts to resolve these peaks

253

using gradient elution were not successful. Lower amounts of Fe in the solution mixtures with

254

1:1 and 2:1 molar ratio catechol:Fe resulted in lower decay rates of catechol within the first 3

255

min (39% and 22%, respectively, Figure S3c). Scheme S2 shows a suggested mechanism for

256

the catechol + Fe(III) reaction based on the data presented herein.

257

explains the formation of polycatechol particles that form over time, which are examined more

12


This mechanism also

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closely in the last section. To further explore the identity of these products, the section below

259

describes results from LC-ESI-MS/MS experiments.

260

3.2

261

chromatograms (TIC) of a solution mixture containing 1:2 molar ratio of catechol:Fe at pH 3

262

after reaction for 3 min. The reaction was carried out in normal water (H216O, (a)-(b)) and in

263

18

264

containing an added O-atom. Similar data were collected for catechol standard solutions as

265

shown in Figure S5. These chromatograms are similar to those collected using HPLC on a

266

separate instrument with a UV-vis detector (Figure 2) with slightly different elution times for

267

the reactant peak (RT=11 min) and product peak (RT=4.5 min). The insets in Figures 3a,c and

268

S5a,c show the mass spectra of these major peaks. While ESI-MS/MS detection is not directly

269

quantitative, peak ratios from spectra collected for samples under identical ionization conditions

270

provide insight into the relative amount of a given species. In the absence of FeCl3, the MS

271

spectra of catechol standard solutions (Figure S5) show the expected [M-H]– ion peak at m/z 109

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(C6H5O2) and a peak at m/z 123 ([M-H]–, C6H3O3) that co-elutes with catechol with a RT of ~11

273

min (the RT changes slightly in the 18O-labeled water but we are going to refer to this peak as

274

the 11 min peak regardless of the solvent). Note that the 11 min peak elutes later in 18O-labeled

275

water but the eluted compound is the same in both normal and isotopically labeled solvents.

276

The relative ratio of the m/z 109 and m/z 123 is ~100:15 in normal water and 100:6 in

277

labeled water. As detailed below, the addition of FeCl3 increases the intensity of the m/z 123

278

peak confirming that it originates from a more oxidized species than catechol.

279

fragmentation pattern of the m/z 123 from MS/MS spectra is shown in Figures S5b,d, and it is

280

identical regardless of the isotopic state of water solvent. In general terms, the MS/MS of the

Product identification using LC-ESI-MS/MS.

Figure 3 shows total ion

O-labeled water (H218O, (c)-(d)) to investigate the source of the oxygen atoms in products

13


18

O-

The


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m/z 123 fragmentation is identical in both intensity, accuracy, ions observed and assigned

282

elemental composition. The absence of mass shifts confirms that the added oxygen atom in the 100

16

Relative Intensity

Relative Intensity

(a) 1mM Catechol:2mM FeCl3, pH3 in H 2 O 109.02982

100

80

100

123.00923

80 60

60

60

40

40

11

20 100

120 m/z

123.00933

20

0

40

109.02985

80

0

140

100

4.5

20

0 5 10 16 120 0 (b) 1mM Catechol:2mM FeCl3, pH 3 in H 2 O 100 11 4.5 m/z=123 69 100 MS2 80 60

15

20

15

20

95.01

40

40

53

20 0 40

60

80 100 m/z

120

0 100 0 5 18 10 (c) 6mM Catechol:12mM FeCl3 in H2 O 11 Relative Intensity

140

80

60

20

80 100

109.02977

80

60 40

60

40

40

20

20

0

0

120 m/z

109.02962

80

60

100

20

100

123.00923

140

123.00906

100

4.5

Relative Intensity

120 m/z

0 120 0 5 18 10 (d) 6mM Catechol:12mM FeCl3 in H2 O 100 m/z=123 80 69 MS2

120 m/z

140

15

20

15

20

11

100

80

60

60

95.01

40

40

4.5

53

20 0

20

40

60

80 100 m/z

120

0 0

283 284 285

5

10 Time (min)

Figure 3: Total ion and m/z 123 ESI-LC-MS/MS negative ion mode chromatograms for the reaction of catechol with FeCl3(aq) under acidic conditions after 3 min dark reaction in normal 14


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286 287 288 289

water (H216O, (a)-(b)) and in 18O-labeled water (H218O, (c)-(d)). The insets in (a) and (c) show the mass spectra for the major peaks, and those in (b) and (d) show the MS/MS spectra for the m/z 123 ion.

290

oxidized catechol originates from dissolved O2 and not water solvent, as suggested earlier.30

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Catecholates and guaiacol are susceptible to oxidation in the presence of dissolved O2.37

292

Scheme S3 shows a likely mechanism that explains the formation of the observed species with

293

m/z 123. The relatively high voltage and ionization conditions under ambient conditions in the

294

ESI chamber may catalyze the formation of radical anions, which, in the presence of O2(aq), can

295

form hydroxylated quinone species, in the ortho- and para- positions. Artifacts associated with

296

oxidation and reduction of analytes in ESI sources were not sufficiently studied. Even though

297

the source was polarized for negative ion production, both oxidation and reduction reactions

298

may still occur on the tip of the electrospray capillary. Two reference compounds, 1,2,4-

299

benzenetriol (VII) and pyrogallol (VIII), with the chemical formula C6H6O3 were tested to

300

examine the fragmentation pattern of their oxidation products in the ESI chamber. Figure S5

301

shows the chromatograms and MS/MS spectra at pH 3 for these compounds. In Figure S5a,

302

1,2,4-benzenetriol shows two peaks with the same fragmentation pattern because it is known to

303

be autooxidizible38 with inter-conversion between reduced and oxidized occurring over the

304

separation time scale. The likely oxidation steps of 1,2,4-benzenetriol are presented in Scheme

305

S3b.

306

chromatograms were collected for 1 mM standard solution of 1,2,4-benzentriol at pH 3. Figure

307

S7 shows these results where one chromatographic peak is observed, which is the oxidized form

308

of this molecule. Greenlee et al.38 developed a method for separation and quantification of

309

phenolic compounds by adding antioxidants and running experiments under oxygen-free

310

conditions using N2 gas purging.

To explain the two chromatographic peaks with the same MS ion, HPLC/UV-vis

The MS/MS spectra shown in the inset of Figure S6a is

15



Page 16 of 30

311

identical to that in the insets of Figures 3b,d and S5b,d. On the other hand, pyrogallol shows

312

one peak in the TIC and in the selected ion chromatogram corresponding to m/z 123 (Figure

313

S6b). The fragmentation pattern of this peak (inset) is different than that shown in Figures 4a,

314

3b,d and S5b,d. Hence, the main conclusion is that autoxidation of catechol takes place to some

315

extent in the ESI source, and the main product is 1,2,4-benzentriol. To the best of the authors’

316

knowledge, the oxidation of catecholates under electrospray ionization conditions was not

317

previously reported.

318

As detailed above, the addition of FeCl3 to catechol solutions results in complex

319

formation and quinone production. The TICs in Figures 3a,c show a product peak that elutes at

320

RT~4.5 min, with mass spectra containing m/z 109 and 123 with 4x and 13x increase in

321

intensity for the latter peak relative to RT~11 min, respectively. Table S1 shows that this

322

enhancement in intensity is consistently observed for variable amounts of Fe in solution. These

323

results can be explained in light of the UV-vis and HPLC data presented in the previous section:

324

catechol-Fe complex is more polar (and therefore elutes earlier) than catechol. The complex is

325

not observed mass spectrometrically as it is not stable in the mass spectrometric time frame. As

326

a result, we only see the anionic portion of this complex, that is, the m/z 109, which upon

327

formation, is readily oxidized to 1,2,4-benzentriol with m/z 123 (identical fragmentation pattern

328

with product peak at RT~4.5 min). Moreover, HPLC/MS experiments were conducted in the

329

positive ion mode under the same elution conditions as negative mode. The resulting spectra

330

yielded no meaningful data because these species do not yield [M+H]+, [M+Na]+, etc under

331

these solution conditions. Also, the presence of excess Fe(III) in these solutions leads to cluster

332

formation and ion suppression.

16


Page 17 of 30


333

Previous studies34-36 on guaiacol reaction with transition metals including iron

334

characterized some of the soluble products of the oxidation process. Using a combination of

335

HPLC, 1H NMR, fast atom bombardment, and chemical ionization mass spectrometry,

336

Schmalzl et al.36 studied the reaction of guaiacol with FeCl3 and reported elemental

337

composition, retention times, characteristic chemical shifts and masses of guaiacol oligomers

338

ranging from dimers to pentamers at 246 (compound V), 368, 490 and 612 mass units. These

339

oligomers formed a precipitate soluble in organic solvents and were found to be mainly organic

340

in composition. Similar oligomers up to trimers were observed in the mass spectra of reaction

341

products from enzymatic oxidation of guaiacol.34,35 The established mechanism that explains

342

these results is mainly carbon-carbon coupling of guaiacoxy radicals, with little evidence for

343

carbon-oxygen coupling.34,36 As detailed in the last section, formation of these polymeric

344

species has implications on the overall optical properties and chemical reactivity of the surfaces

345

these products coat.

346

3.3

347

digital photographs of particles collected on a filter from the dark reaction of catechol and

348

guaiacol with FeCl3 at pH 3. Figure 4 shows SEM images of these particles, which clearly

349

display their amorphous nature as micron-sized conglomerates of nanometer-sized primary

350

particles. EDS experiments showed that polycatechol and polyguaiacol particles are organic

351

and had less than 0.5 atomic %Fe which was most likely due to contamination from its salt

352

remaining after washing the particles with water. The iron content in these particles analyzed

353

by ICP-MS was also below the detection limit, similar to the results of the control filter with no

354

particles. Moreover, changes in particle size with reaction time was monitored using DLS

355

experiments, which showed that particles appeared within 3 min of the reaction of FeCl3 with

Formation of polymeric catechol and guaiacol particles. Figures 1c and 1d show

17



Page 18 of 30

356

either catechol or guaiacol solutions (Figure 5). Particles grew to average diameters on the

357

order of a micron, which is consistent with SEM images, after 30 min of reaction time when

358

sedimentation becomes important. Also at this time, polycatechol particles became larger in

359 360 361 362

Figure 4: SEM images for (a) polycatechol and (b) polyguaiacol collected on copper grids after a 90 min dark reaction of catechol with FeCl3 at pH 3 in a 1:2 molar ratio.

363

average diameter than polyguaiacol particles because of the 1.8x faster growth rate of

364

polycatechol (36 nm/min) compared to that of polyguaiacol particles (20 nm/min). These

365

results suggest that these particles are organic in nature and not a precipitate of iron

366

(oxyhydr)oxides. Mass yield experiments were also performed where particles were collected

367

and extensively washed with Milli-Q water after 120 min reaction of FeCl3 with either catechol

18


Page 19 of 30


368

or guaiacol (1:2 organic reactant:Fe molar ratio) at pH 3. Polycatechol mass yield was found to

369

be 47±4% and that of polyguaiacol was 49±14%. This mass yield is comparable or larger than

370

the typical mass yields of SOA obtained by photooxidation of common volatile organic

Average diameter,

4

polycatechol 3

2

polyguaiacol

1

0 0

20

40

60

80

100

120

371 372 373 374 375

Reaction time (min) Figure 5: DLS measurements of the average particle size of polycatechol and polyguaiacol as a function of reaction time during the dark reaction of catechol with FeCl3 at pH 3 in a 1:2 molar ratio.

376

compounds, such as terpenes,39 and it is also larger than the yields associated with aqueous

377

SOA (aqSOA) photochemical production.40

378

catecholates definitively have the potential to produce SOA with high efficiency.

Therefore, the iron-catalyzed reactions of

379

Moreover, Figure S8a shows ATR-FTIR spectra of solid polycatechol and polyguaiacol

380

formed in reactions of Fe(III) with catechol and guaiacol. These spectra are compared with

381

those recorded for aqueous phase catechol28 and guaiacol (Figure S8b).

382

differences between the spectra of monomers and polymers in the 2000-1000 cm-1 spectral

383

range containing vibrations of aromatic (1640-1400 cm-1), C-O and C-C stretching (1400-1200

384

cm-1) and C-H bending (1200-1000 cm-1) modes.

19

There are clear

This is inline with earlier reports on



Page 20 of 30

385

polycatechol using transmission FTIR using KBr pellets,41,42 where broadening and shift in peak

386

frequencies was observed due to the rigid structure of polymers. The high intensity of features

387

in the 1400-1200 cm-1 is characteristic of phenylene (C-C) and oxyphenylene (C-O-C) linkages.

388

These spectra show no absorbance around 1700 cm-1 indicative of carbonyl (C=O) groups,

389

which were reported for catechol and guaiacol SOA due to reaction with ozone.22 This clearly

390

shows that particle formation catalyzed by iron proceeds via a different reaction pathway than

391

SOA formation via metal-free atmospheric photo-oxidation reactions.

392

Oxidative enzymatic polymerization of guaiacol and catechol is well studied in

393

biochemical and polymer synthesis fields.17,24,43 Brick-red colored polyguaiacol prepared from

394

the enzymatic oxidation of monomer guaiacol using peroxidase-H2O2 as catalyst was previously

395

characterized because of its usefulness a model polymer for lignin biodegradation research.44 In

396

another study,36 the solid material was collected from the reaction of guaiacol with Fe(III)

397

chloride and Cr(VI) oxide, which was then solubilized for analysis using UV-vis, HPLC and

398

mass spectrometry. In the case of polycatechol formation, enzymes free of metal centers such

399

as peroxidase45 and laccase42 were shown to catalyze polymer synthesis in solutions containing

400

hydrogen peroxide and dissolved oxygen, respectively.

401

catalyze C-C coupling and formation of ether (C-O-C) linkages between catechol monomers (as

402

shown in Scheme S2). Because of their unique thermal, structural properties, and ability to

403

form strong charge transfer complexes with metal oxides, polycatechols are exploited in surface

404

modifications as adhesives and coatings over a wide range of organic and inorganic

405

materials.24,46 For example, chelating abilities of anachelin (produced by cyanobacteria)19 and

406

mussel adhesive proteins20,46-49 containing catechol moieties were exploited in modifying TiO2

407

surfaces via poly(ethylene glycol) to form stable, protein resistant adlayers (i.e. antifouling

20

Mechanistically, these enzymes


Page 21 of 30


408

polymer) desirable in biomedical devices and marine technology. More related to the work

409

presented herein is the use of the catechol derivative dopamine in anchoring functional

410

molecules to iron oxide shell of magnetic nanoparticles.18,50 To our knowledge, oxidative

411

polymerization of catechol in the presence of Fe(III) and in the absence of any added oxidants

412

has not received attention.

413

Recently, Shi et al.51 investigated the dominant mechanism that leads to iron dissolution

414

in dust. Iron-containing dust samples were collected from two sites that represent sources of

415

Saharan and Asian desert dust.

416

conditions) and cloud droplets (i.e., more neutral pH and lower ionic strength conditions) on

417

dissolved iron concentration was simulated over about 3 hours in the dark. The results showed

418

that insoluble iron dissolves readily under the acidic conditions relevant to wet aerosols,

419

whereas under more neutral pH, the dissolved iron precipitates as poorly crystalline

420

nanoparticles. The relative amount of time mineral dust particles spend as either a wet aerosol

421

or in cloud droplets will affect the amount of bioavailable iron upon deposition after long range

422

transport. Also, uptake of acids within clouds can also enhance iron dissolution in the droplets

423

or in the residual aerosol formed after droplet evaporation.

424

simulate acid-driven dissolution of iron (oxyhydr)oxides and followed the reaction of either

425

catechol or guaiacol with the dissolved iron from these samples. Figure 6 shows digital images

426

of the slurry and filtrates before and after the addition of the organics, in addition to the filters

427

after 1 hr reaction. Clearly, the chemistry described above using FeCl3 as a source of dissolved

428

iron is similar to that from simulated aged iron-containing mineral dust particles. As

429

emphasized in the introduction, pathways for particle formation from phenol derivatives are of

430

increased interest to atmospheric chemists. The implications of these findings are summarized

The effect of cycling between wet aerosols (i.e., acidic

21


Herein, we used hematite to


431 432 433

Page 22 of 30

in the following section. To help fill the gap in our understanding of the surface chemistry driven by iron on atmospherically-relevant surfaces, we showed that the dark reaction of catechol and guaiacol Adding catechol pH 3, 0.02 mL 25.5 mM

filtration

polycatechol

filtration Hematite slurry After 10 days mixing at pH 1 in KCl

Filtrate of hematite pH 1 ca. 1.3 mM [Fe]tot

Filtrate of hematite (1 mL) adjusted to pH 3, ca. 1 mM [Fe]tot

Adding guaiacol pH 3, 0.02 mL 25.5 mM

polyguaiacol After 1 hr dark reaction

434 435 436 437

Figure 6: Formation of polycatechol and polyguaiacol from reaction with dissolved iron from acid-promoted hematite dissolution.

438

with Fe(III) under acidic conditions and in the absence of any added oxidants results in the

439

formation of soluble and reactive secondary organics and insoluble polymeric particles. The

440

strong visible light absorption by the resulting soluble products and insoluble polymeric organic

441

material has implications for the climate. More quantitatively, the significance of these results

442

can be assessed with the help of bulk mass-normalized absorption coefficient (MAC) of the

443

organics, which can be calculated from the base-10 absorbance (A), cuvette width (l), and the

444

initial mass concentration of dissolved catechol (Cmass) as follows:25

445

446

MAC(λ ) =

A( λ ) × ln(10) l × Cmass

(1)

Figure S9 shows a MAC plot for the 3 min dark reaction of 1 mM catechol with 2 mM FeCl3 at

22


Page 23 of 30


447

pH 3 for an unfiltered solution, i.e. with both particles and soluble species contributing to the

448

absorption. Even though we have not accounted for scattering by the particles in the solution,

449

which contributes to the background, it is clear that the MAC values at near- UV and visible (λ

450

> 290 nm) wavelengths can be as high as 2×104 cm2 g-1. These values are comparable to those

451

from biomass burning aerosols (103-104 cm2 g-1).52

452

between phenols and Fe has the potential to produce “brown carbon” aerosol by a secondary

453

mechanism, as opposed to the direct production in biomass burning.

Therefore, the efficient dark reaction

454

Moreover, the colored polymeric particles are colloidal in solution due to their high

455

hydrophobicity. Depending on the amount of ‘adsorbed water’, these polymers could partition

456

to the surface, thus affecting the aerosols’ cloud condensation ability,53,54 and chemical

457

reactivity of surfaces particularly towards reactive gas phase oxidants such as ozone,55 OH,56,57

458

N2O5 and NO3,58 in addition to light-initiated4 and redox reactions.59 The dark formation of

459

these particles driven by Fe(III) under acidic conditions represent a potentially important abiotic

460

pathway for in situ polymer and HULIS formation in atmospheric aerosols and surface water.

461

This secondary pathway is poorly understood relative to formation pathways of SOA,2

462

biopolymeric component of dissolved organic carbon,60 and HULIS.61,62 HULIS in particular

463

were shown to generate reactive oxygen species such as hydrogen peroxide and superoxide

464

radicals.63 The chemistry presented herein provides molecular level details to processes that

465

could take place in acidic multicomponent systems containing organics, iron, and chloride.

466 467

Acknowledgments

468

HAA acknowledges funding from NSERC Discovery Program and Early Researcher Award by

469

Ontario Ministry of Research and Innovation. SAN and SLB acknowledge support from US

23



470

NSF grant AGS-1227579. RWS acknowledges funding from NSERC Research Tools and

471

Instruments and the University of Waterloo.

472 473

Supporting Information Available

474

Figures and tables showing spectra and additional data analysis. This material is available free

475

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

476

24


Page 24 of 30

Page 25 of 30


477

5.

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Zhou, W.-H.; Lu, C.-H.; Guo, X.-C.; Chen, F.-R.; Yang, H.-H.; Wang, X.-R., Musselinspired molecularly imprinted polymer coating superparamagnetic nanoparticles for protein recognition. J. Mater. Chem. 2010, 20, 880-883. Shi, Z.; Krom, M.D.; Bonneville, S.; Benning, L.G., Atmospheric processing outside clouds increases soluble iron in mineral dust. Environ. Sci. Technol. 2015, 49, 14721477. Chen, Y.; Bond, T.C., Light absorption by organic carbon from wood combustion. Atmos. Chem. Phys. 2010, 10, 1773-1787. Taraniuk, I.; Graber, E.R.; Kostinski, A.; Rudich, Y., Surfactant properties of atmospheric and model humic-like substances (HULIS). Geophys. Res. Lett. 2007, 34, L16807, DOI: 10.1029/2007GL029576. Dinar, E.; Taraniuk, I.; Graber, E.R.; Anttila, T.; Mentel, T.F.; Rudich, Y., Hygroscopic growth of atmospheric and model humic-like substances. J. Geophys. Res-Atmos. 2007, 112, D05211, DOI:10.1029/2006JD007442. Nieto-Gligorovski, L.I.; Net, S.; Gligorovski, S.; Wortham, H.; Grothe, H.; Zetzsch, C., Spectroscopic study of organic coatings on fine particles, exposed to ozone and simulated sunlight. Atmos. Environ. 2010, 44, 5451-5459. Bertram, A.K.; Ivanov, A.V.; Hunter, M.; Molina, L.T.; Molina, M.J., The reaction probability of OH on organic surfaces of tropospheric interest. J. Phys. Chem. A 2001, 105, 9415-9421. George, I.J.; Slowik, J.; Abbatt, J.P.D., Chemical aging of ambient organic aerosol from heterogeneous reaction with hydroxyl radicals. Geophys. Res. Lett. 2008, 35, L13811, DOI: 10.1029/2008GL033884. Gross, S.; Bertram, A.K., Reactive uptake of NO3, N2O5, NO2, HNO3, and O3 on three types of polycyclic aromatic hydrocarbon surfaces. J. Phys. Chem. A 2008, 112, 31043113. Duesterberg, C.K.; Waite, T.D., Kinetic modeling of the oxidation of p-hydroxybenzoic acid by Fenton's reagent: Implications of the role of quinones in the redox cycling of iron. Environ. Sci. Technol. 2007, 41, 4103-4110. Aluwihare, L.I.; Repeta, D.J.; Chen, R.F., A major biopolymeric component to dissolved organic carbon in surface sea water. Nature 1997, 387, 166-169. Graber, E.R.; Rudich, Y., Atmospheric HULIS: How humic-like are they? A comprehensive and critical review. Atmos. Chem. Phys. 2006, 6, 729-753. Noziere, B.; Dziedzic, P.; Cordova, A., Formation of secondary light-absorbing "fulviclike" oligomers: A common process in aqueous and ionic atmospheric particles? Geophys. Res. Lett. 2007, 34, L21812, DOI:10.1029/2007GL031300. Lin, P.; Yu, J.Z., Generation of reactive oxygen species mediated by humic-like substances in atmospheric aerosols. Environ. Sci. Technol. 2011, 45, 10362-10368.

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TOC Graphic

OH

+ Fe(III) OH

OCH 3

+ Fe(III) OH

654 655

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OH Environmental Science & Technology Page 30 of 30 OH

+ Fe(III)

OCH ACS Paragon Plus Environment 3 OH

+ Fe(III)

Formation of Light Absorbing Soluble Secondary ... - ACS Publications

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