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Temperature Dependence of the Photochemical Formation of Hydroxyl Radical from Dissolved Organic Matter Garrett McKay, and Fernando L. Rosario-Ortiz Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 26 Feb 2015 Downloaded from http://pubs.acs.org on February 26, 2015
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Temperature Dependence of the Photochemical Formation of
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Hydroxyl Radical from Dissolved Organic Matter
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Garrett McKay and Fernando L. Rosario-Ortiz*
4
Department of Civil, Environmental and Architectural Engineering, 428 UCB, University
5
of Colorado, Boulder, CO 80309, USA
6 7 8 9 10
• Corresponding author: [email protected]; Phone: 303-492-7607; Fax: 303-492-7317
Abstract
11
In this paper, the temperature dependence of the photochemical production of the
12
hydroxyl radical (•OH) from dissolved organic matter (DOM) was investigated by
13
measuring the apparent temperature dependence of the quantum yield (Φa) for this
14
process. Temperature dependent Φa values were analyzed using the Arrhenius equation.
15
Apparent activation energies obtained for DOM isolates purchased from the International
16
Humic Substances Society ranged from 16 to 32 kJ mol-1. Addition of 40 units mL-1
17
catalase, used to hinder the hydrogen peroxide (H2O2)-dependent pathway to •OH, did
18
not impact the observed activation energy. However, an increase in activation energy was
19
observed in lower molecular weight DOM obtained by size fractionation. We also
20
measured the temperature dependence of p-benzoquionone photolysis as a model
21
compound for DOM and observed no temperature dependence (slope p = 0.41) for the
22
formation of phenol from oxidation of benzene (the •OH probe used), but a value of
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about 10 kJ mol-1 for p-benzoquinone loss, which is consistent with formation of a water-
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quinone exciplex. These data provide insight into DOM photochemistry as well as
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provide parameters useful for modeling steady state •OH concentrations in natural
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systems.
27 28
Introduction
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Reactive oxygen species (ROS) play an important role in the photochemistry of
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natural aquatic systems. These ROS include the hydroxyl radical (•OH), singlet oxygen
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(1O2), superoxide (O2•-), hydrogen peroxide (H2O2) and triplet excited states. From an
32
engineering standpoint, degradation of many organic contaminants is increased by
33
ROS.1,2 Ecologically, ROS are involved in the mineralization of organic carbon,3
34
contributing to the global carbon cycle. The presence of H2O2 was first detected in
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marine waters in 1966 and was shown later to result from photochemical reactions.4
36
Since then, other ROS such as •OH and 1O2 have been shown to be produced
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photochemically in natural waters.3,5,6
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Multiple chemical species undergo photochemical reactions to produce ROS, such
39
as nitrate, nitrite, and dissolved organic matter (DOM).3,5,7 In particular, DOM photolysis
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produces •OH and 1O2, as well as H2O2, triplet states (here abbreviated as 3DOM*), and
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O2•-. The generally accepted model is that 1DOM* is formed following absorption of a
42
photon by ground state DOM, which can subsequently undergo internal conversion,
43
vibrational relaxation, or intersystem crossing to 3DOM*. Early work by Zepp et al.
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established an average energy of around 250 kJ mol-1 for 3DOM* molecules,8 sufficiently
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high to form
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disproportionation of O2•-, which is formed by reaction of dissolved oxygen with some
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photo produced reductant.9 The •OH radical is believed to be formed by multiple
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pathways, however, steady state concentrations are limited to around 10-18 to 10-16 M in
1
O2 from reaction with dissolved oxygen. H2O2 results from the
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natural waters due to its high reactivity.3,10 1O2 is a weaker and more selective oxidant
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than •OH and thus reaches steady state concentrations a few orders of magnitude higher.7
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Additionally, steady state concentrations of •OH and 1O2 are generally higher for effluent
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organic matter compared to DOM isolates.11,12 Reported apparent quantum yields (Φa) for
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•OH radical are on the order of 10-4 to 10-5, again being generally higher for wastewater
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samples.11
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While it is clear that DOM is a photochemical source of •OH, the underlying
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mechanisms are not completely understood. Some of the formation pathways are
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summarized in Scheme 1. There likely exist two pathways to •OH: an H2O2-dependent
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and H2O2-independent pathway.5,7,11 The H2O2-dependent pathway involves the photo
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Fenton reaction13,14 and possibly direct photolysis of photochemically produced H2O2.15
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The importance of this pathway has been demonstrated by using catalase to decompose
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H2O2, which decreases •OH production by up to 50 % for some samples.16,17 Super-
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oxidized iron species (e.g. ferryl, FeO2+), which are operational at neutral pH,18 may also
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account for some •OH-like activity by reacting with various organic compounds.19 The
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H2O2-independent pathway involves species that lead to free •OH as well as so-called
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low energy hydroxylating species.17,20,21 H-atom abstraction from water by some DOM
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excited state (3DOM*, 1DOM*, or charge transfer species-DOM•+/•-) is one mechanism
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suggested to produce free •OH, but this has not been confirmed.7,22 Generation of low
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energy hydroxylators has been argued largely on the quenching of •OH-like activity with
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addition of methane. For example, Page et al. (2011) showed that some DOM isolates
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exhibited less than calculated decreases in photochemical •OH production with addition
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of known concentrations of methane.17 Though uncertain, these species could be modeled
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by the quinone-water exciplex formed during irradiation of 2-methyl-p-benzoquinone.
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For example, Pochon et al. (2002)20 and Gan et al. (2008)21 demonstrated no methyl
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radical production upon irradiation of methyl-p-benzoquinone containing methane as an
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•OH probe.
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This paper presents data on the apparent temperature dependence of Φa for bulk
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DOM isolates, with and without catalase present, as well as apparent molecular weight
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fractions. The temperature dependence of the photolysis of p-benzoquinone was also
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investigated as a model compound for comparison. These data provide insight into DOM
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characterization and photochemistry by comparing the temperature dependence of
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different DOM samples to a model compound. In addition, the measured activation
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energies can be used for kinetic modeling of steady state •OH concentrations in natural
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and engineered systems exhibiting temperature fluctuations or large deviations from
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room temperature.
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Materials and Methods
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DOM isolates including Suwannee River Fulvic Acid I (SRFA, 1S101F),
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Suwannee River Humic Acid II (SRHA, 2S101H), Suwannee River NOM (SRNOM,
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2R101N) Pony Lake Fulvic Acid (PLFA, 1R109F), and Elliot Soil Humic Acid (ESHA,
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1S102H) were purchased from the International Humic Substance Society. All other
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chemicals were obtained from commercial sources. The solution conditions, irradiation
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conditions, and analytical methods used in this study are presented in detail elsewhere.11
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Briefly, solutions used in irradiation experiments were buffered at pH 7.2 in equimolar
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KH2PO4/K2HPO4 (total [phosphate] = 10 mM). Samples were irradiated in clear, glass
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vials free of headspace. Irradiations were ran in duplicate for 2.5 to 8 hours (depending
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on sample to achieve measurable •OH concentrations) with an Oriel 1429A Solar
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Simulator equipped with a 1000 W Xenon lamp (see Figure S1 for lamp spectrum). Over
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the irradiation period, samples were collected at 5 time intervals. Temperature control
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within ± 2 °C was accomplished by laying the vials flat in a water-jacketed petri dish.
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Benzene was used as an •OH probe by following the formation of phenol through the
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Dorfman reaction.23,24 Though not described in detail here, this mechanism involves
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reaction of the cyclohexadienyl radical with dissolved oxygen, which facilitates the
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formation of phenol. Based on the experimental conditions used in this study (gas tight
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vials, ~ µM phenol formation) no oxygen depletion due either to temperature changes or
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phenol formation during the course of experiments was expected. A benzene
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concentration of 3 mM was used to quantitatively scavenge •OH radicals. Phenol
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concentrations were measured by HPLC utilizing a gradient mobile phase of pH 2.8, 10
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mM H3PO4 and methanol with a flow rate of 1.0 mL min-1. The retention time of phenol
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under these conditions was around 4.7 minutes. Using the same HPLC method, p-
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benzoquinone eluted at 2.2 minutes (column dead time ~ 1.6 minutes). The column used
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was a Zorbax Eclipse XBD (4.6 x 150 mm) C18 column. Bovine liver catalase was used
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in some experiments to quench H2O2.
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Apparent molecular weight fractions were collected using < 5 kDa and < 1 kDa
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ultrafiltration membranes (Millipore, USA). Membranes were soaked three times in fresh
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1 L batches of Milli-Q water before use. To remove any remaining organic material, 200
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mL of Milli-Q water was passed through the membrane before fractionating DOM
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samples. Approximately 200 mL of DOM solution was added to the ultrafiltration cell.
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The first 100 mL of permeate was collected.
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The apparent quantum yield of •OH from DOM photolysis is given by
R•OH Φa = kDOM−a [DOM]
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(1)
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where R•OH is the rate of •OH radical formation in M s-1 and kDOM-a is the specific rate of
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light absorption by DOM in s-1. The specific rate of light absorption was calculated by the
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following equation 400nm
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kDOM−a = ∑ λ =290nm
− ε DOM ( λ )[DOM]z
E p0(λ )ε DOM ( λ )(1−10
)
ε DOM (λ )[DOM]z
(2)
εDOM(λ) is
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where ܧ (λ) is the photon irradiance of the solar simulator in Einstein-1 cm-2,
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the molar absorptivity of DOM in MC-1 cm-1, [DOM] is the DOM concentration in MC,
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and z is the cell depth in cm. Absolute irradiance was measured with a spectroradiometer.
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Absorbance was measured with a Cary 100 UV/Vis spectrophotometer. Molar
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absorptivity data for p-benzoquinone were taken from the National Institute of Standards
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and Technology Chemistry WebBook.25
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The rate of phenol formation in this system is given by
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d[C6H5OH] = kC6H6-OH[ OH][C6H5 ]= Yphenol R OH dt
(3)
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where kC6H6-OH is the second-order rate constant between benzene and •OH radical and
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Yphenol is the yield of phenol from this reaction. Making the steady state assumption for
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•OH radical gives
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d[ OH] = kDOM−a Φ a [DOM]-∑ ks [S][ OH] ≈ 0 dt
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[ OH]ss =
kDOM−aΦ a [DOM]
∑ k [S]
(5)
s
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where kS is the second order rate constant between •OH and scavenger S. Since the
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experimental conditions are such that Σks[S][•OH] ≈ kC6H6-OH[C6H6][•OH], that is,
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benzene is the dominant •OH scavenger equation (3) becomes
140 141
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d[C6H5OH] = kDOM−a Φ a[DOM] dt
(6)
and solving for Φa gives
Yphenol Φ a = R OH kDOM−a[DOM]
(7)
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Clearly, Φa depends on the yield of phenol from hydroxylation of benzene. We discuss
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below the temperature dependence of Yphenol.
145 146 147
Activation energies were determined by plotting the measured temperature dependent Φa according to the Arrhenius equation:
E 1 ln(Φ a ) = − a + ln(A) RT
(8)
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Quantum yields were measured at 4 to 5 temperatures over a range of around 10 to 40 °C.
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Temperature dependent quantum yields have been analyzed in this way before26-28 and
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the present data generally fit the Arrhenius model, with the average relative error (based
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on a linear fit of the Arrhenius plots) in activation energies being 15 %.
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Results and Discussion
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Temperature Dependence of Phenol Yield from Benzene
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In order to use equation (7) at different temperatures, the temperature dependence of
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Yphenol needed to be assessed. Work by Chu and Anastasio (2003) has demonstrated that 7
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the •OH probe benzoic acid is not sensitive to temperature,27 suggesting that this may be
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the case for benzene.
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Measuring Yphenol involves knowing the actual rate of phenol produced (Rphenol)
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and the theoretical or calculated amount of •OH resulting from photolysis of H2O2
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(R•OH,calc) – i.e. Yphenol = Rphenol / R•OH,calc. R•OH,calc depends on the rate constant k for the
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photochemical transformation, which is calculated in general by
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R•OH ,calc = k [H 2 O 2 ]
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k = 2.303
(9)
400nm
∫ (Φλε λ I λl)dλ
λ =290nm
(10)
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where Φλ, ελ, and Iλ represent the quantum yield of •OH from H2O2, molar absorptivity,
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and irradiance at a given wavelength and l is the pathlength. Note that the (small)
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temperature dependence of Φλ and ελ measured by Chu and Anastasio was used in
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equation (9).29 Solutions of 90 to 100 µM H2O2 were spiked with 3 mM benzene and
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irradiated for periods of 100 to 220 min. Rphenol was found from linear fits of the resulting
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data (not shown). Experiments were conducted at temperatures ranging from 11.6 to 38.4
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°C and resulted in Yphenol values of 0.63 ± 0.07 (see Figure S2) with no apparent
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temperature dependence (slope p = 0.14). In addition, an activation energy of 16.6 ± 5.0
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kJ mol-1 for the production of •OH from nitrate photolysis was measured using benzene
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as the •OH probe. This value is in good agreement with that measured by Chu and
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Anastasio (2003)27 of 19 ± 4 kJ mol-1 and Zellner et al. (1990)26 of 16 ± 4 kJ mol-1.
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The Yphenol determined here is within the range of values in the literature (0.43 to
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0.95 depending on pH)23,30-32 and agrees well with the recently determined value of 0.69
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measured by Sun et al. (2014).33 In previous publications by our group11,22,34 and others,35
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a yield of 0.85 was used. In this work, the determined yield was 26 % lower. This
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highlights the importance authors reporting •OH formation rates should report the value
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of Yphenol to allow for comparison to other studies.
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One limitation to this study is that irradiations were performed using a solar
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simulator (opposed to a monochromatic source) as the quantum yield for •OH radical are
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strongly wavelength-dependent.5,36 Subsequently, the quantum yields reported here
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should be interpreted as an average over the spectral output of the solar simulator (λ >
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290 nm). Nevertheless, this paper provides the first temperature dependent measurements
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of Φa for •OH; also, previous studies have used polychromatic irradiation to investigate
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DOM photochemistry.
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Temperature Dependence for Bulk DOM Isolates
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DOM isolates were chosen to give a range of bulk chemical functionalities, type (e.g.
190
humic acid vs. fulvic acid), and source (e.g. Pony Lake vs. Suwannee River). A typical
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data set is shown in Figure 1 for ESHA. Apparent quantum yields were plotted according
192
to the Arrhenius equation (eq. 8) in order to obtain the apparent activation energy
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(hereafter called activation energy). The term apparent activation energy is used to
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acknowledge the complexity of the system in that DOM is a heterogeneous, complex
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material and that the activation energies are obtained using a polychromatic source.
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Activation energies for all samples are summarized in Table 1. There is surprisingly little
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variation in the values for bulk DOM isolates, besides SRFA (34.3 ± 7.2 kJ mol-1), which
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suggests a consistent mechanism for •OH production. In addition, data are plotted
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according to the Eyring equation (see Table S1) and some discussion is provided in the
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Supplemental Information.
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Values for bulk samples are in the range of reported activation energies for other
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photochemical processes (see Table 2), though there is some variation depending on the
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reaction. For example, aqueous photolysis of H2O2 and NO3- results in primary
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photochemical processes characterized by different photofragments as shown in
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equations 11-13.
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hν •OH H2O2 →
(11)
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[NO3–]* → NO2- + O(3P)
(12)
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2O [NO3–]* → NO2• + O•- H → NO2• + •OH + OH–
(13)
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The lower activation energies of ≈ 16 to 18 kJ mol-1 for ESHA, SRHA, and SRNOM are
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closer to values characterizing diffusion-controlled reactions (≈15 kJ mol-1),37 while
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those for PLFA and SRFA, greater than 20 kJ mol-1, are slightly higher. An explanation
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for the higher activation energy for nitrate photolysis (relative to H2O2) is that the
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primary photoproduct of H2O2 photolysis (•OH radical) diffuses more easily from the
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solvent cage than that of nitrate photolysis (O•-).26,29,38,39 Therefore the activation energies
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for Φa from DOM could reflect either secondary thermal chemistry or diffusion of
216
different photoproducts from the solvent cage.
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Considering the activation energies for photochemical reactions of organic
218
molecules, values for H-atom abstraction by ketones can vary from 9.2 to 29.3 kJ mol-1
219
depending on the reaction conditions and organic substrate.40 These Norrish Type
220
reactions involve H-atom abstraction from the triplet state, subsequently producing a
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carbon-centered diradical. It is interesting that the activation energies in Table 1 match
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these values. However, the above activation energies are for production of a carbon-
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centered radical in organic solvents, while H-atom abstraction from water by 3DOM*
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would lead to •OH in aqueous solution.
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Catalase Experiments
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The activation energies shown in Table 1 are for DOM samples not containing catalase,
227
which is used to quench photochemically produced H2O2 in order to minimize the
228
contribution of direct photolysis and photo-Fenton reactions to •OH production.16,17
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Activation energies for the photochemical production of H2O2 from DOM have recently
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been reported by Kieber et al. (2014) for seawater samples, ranging from about 8 to 53 kJ
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mol-1 depending on the irradiation wavelength.28 Their analysis revealed that when taken
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as an average across all samples, activation energies ranged from 15 to 30 kJ mol-1 over
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290 to 400 nm.
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To examine the H2O2-independent pathway, Φa values were measured for ESHA
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solutions containing increasing concentrations of catalase. A similar experiment
236
including the same DOM sample is described by Page et al. for the terephthalate probe.17
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These data for ESHA are shown in Figure S3 and demonstrate an almost identical
238
quenching of •OH production for the same sample (ESHA) determined by Page et al.
239
(2011) - about 50 %, at similar concentrations of catalase (≈ 40 units mL-1).17 Using a
240
catalase concentration of 40 unit mL-1, an activation energy of 16.6 ± 3.4 kJ mol-1 was
241
measured for ESHA, which matches well with the non-catalase value of 17.6 ± 1.3. This
242
result indicates that the H2O2-independent pathway to •OH (H-atom abstraction by
243
3
244
for ESHA. The addition of catalase also limits photo-Fenton reactions from occurring.
245
Again the similarity in activation energies with addition of catalase, which essentially
DOM* and/or low-energy hydroxylator) has an activation energy of about 17 kJ mol-1
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shuts down any Fenton chemistry, suggests that the H2O2-dependent pathway has a
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similar or lower activation energy than the H2O2-independent pathway.
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Temperature dependence of p-Benzoquinone photolysis
249
To gain further insight into the H2O2-independent pathway, the temperature-dependent
250
photolysis of p-benzoquinone was investigated using benzene as an •OH probe. Others
251
have investigated this system at room temperature using dimethylsulfoxide (DMSO) to
252
trap •OH, leading to the mechanism proposed in Scheme 2.20,21
253
In our experiments, solutions of 92.5 µM p-benzoquinone produced about 500 nM
254
phenol over 12 minute irradiations (Figure S4). The fact that phenol from irradiation of p-
255
benzoquinone shows that benzene may be susceptible to hydroxylation by species other
256
than free •OH, e.g. low energy hydroxylators. Though there is some debate as to whether
257
irradiation of quinones produces free •OH, there is evidence that hydroxylation occurs
258
through a quinone-water exciplex.20,21 As quinone moieties are believed to be present in
259
DOM, this may explain the presence of low energy hydroxylators in irradiated DOM
260
observed previously.17,20
261
As seen in Figure 2a, the Φa of phenol from p-benzoquinone irradiation was
262
essentially temperature independent with an activation energy of ≈ 0 kJ mol-1 (slope p =
263
0.41). The loss of p-benzoquinone, however, shows more complex behavior (Figure 2b).
264
At temperatures lower than ≈ 23 °C, p-benzoquinone loss is temperature dependent (9.7
265
kJ mol-1), while above 23 °C, there is almost no temperature dependence. These data are
266
consistent with formation of a water-quinone exciplex that can hydroxylate benzene or
267
proceed by other reactions to form hydroxy-p-benzoquinone or hydroquinone. The loss of
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p-benzoquinone, and production of hydroxy-p-benzoquinone and hydroquinone was
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confirmed in separate experiments by measuring the spectral changes in irradiated
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solutions containing 75 µM p-benzoquinone with and without 3 mM benzene (with
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benzene experiment shown in Figure S5). The presence of 3 mM benzene did not
272
eliminate the production of hydroxy-p-benzoquinone, suggesting that the intermediate
273
species formed is not quantitatively quenched by benzene at a concentration of 3 mM.
274
Conversely, others have reported quenching of this intermediate by DMSO at
275
concentrations of 100 mM.21
276
The reactions shown in Scheme 2 are likely affected by temperature in two ways.
277
First, forward reactions to form phenol, hydroxy-p-benzoquinone, and the hydroquinone
278
are expected to increase with increasing temperature. Additionally, an increase in
279
temperature likely encourages dissociation of the exciplex to re-form ground state p-
280
benzoquinone. Based on these data, it seems that an increase in temperature favors
281
production of hydroxy-p-benzoquinone and the hydroquinone up to a certain point
282
(around 23 °C), while at higher temperatures, dissociation of the exciplex dominates.
283
This explanation also accounts for the net zero temperature dependence of Φa for phenol.
284
Negative and near zero activation energies have been taken as evidence for exciplex
285
formation in other chemical systems,41-43 albeit better characterized systems than DOM.
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Taken as a whole, these experiments demonstrate that the activation energies observed
287
for •OH production from DOM cannot be attributed solely to a simple quinone model,
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even though the experimental approach depended on the measurement of the formation of
289
phenol from the hydroxylation of benzene.
290 291
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Effect of Molecular Size
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Additional observation of Table 1 reveals differences in activation energies between
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humic (average of 17.1 ± 0.9 kJ mol-1) and fulvic (average of 27.9 ± 9.0 kJ mol-1) acids.
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It is difficult to specify what could account for this phenomenon because there are many
296
differences between these samples. For example, although SRFA and PLFA are both
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fulvic acids, they differ widely in geographic source (Antarctica vs. Georgia, USA) and
298
type (microbial vs. terrestrial influence). However, fulvic acids in general have lower
299
average molecular weights.44
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To further discern the effect of molecular size on the temperature dependence of
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Φa, SRNOM was size-fractionated with
Article
Temperature Dependence of the Photochemical Formation of Hydroxyl Radical from Dissolved Organic Matter Garrett McKay, and Fernando L. Rosario-Ortiz Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 26 Feb 2015 Downloaded from http://pubs.acs.org on February 26, 2015
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Environmental Science & Technology
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Temperature Dependence of the Photochemical Formation of
2
Hydroxyl Radical from Dissolved Organic Matter
3
Garrett McKay and Fernando L. Rosario-Ortiz*
4
Department of Civil, Environmental and Architectural Engineering, 428 UCB, University
5
of Colorado, Boulder, CO 80309, USA
6 7 8 9 10
• Corresponding author: [email protected]; Phone: 303-492-7607; Fax: 303-492-7317
Abstract
11
In this paper, the temperature dependence of the photochemical production of the
12
hydroxyl radical (•OH) from dissolved organic matter (DOM) was investigated by
13
measuring the apparent temperature dependence of the quantum yield (Φa) for this
14
process. Temperature dependent Φa values were analyzed using the Arrhenius equation.
15
Apparent activation energies obtained for DOM isolates purchased from the International
16
Humic Substances Society ranged from 16 to 32 kJ mol-1. Addition of 40 units mL-1
17
catalase, used to hinder the hydrogen peroxide (H2O2)-dependent pathway to •OH, did
18
not impact the observed activation energy. However, an increase in activation energy was
19
observed in lower molecular weight DOM obtained by size fractionation. We also
20
measured the temperature dependence of p-benzoquionone photolysis as a model
21
compound for DOM and observed no temperature dependence (slope p = 0.41) for the
22
formation of phenol from oxidation of benzene (the •OH probe used), but a value of
23
about 10 kJ mol-1 for p-benzoquinone loss, which is consistent with formation of a water-
24
quinone exciplex. These data provide insight into DOM photochemistry as well as
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provide parameters useful for modeling steady state •OH concentrations in natural
26
systems.
27 28
Introduction
29
Reactive oxygen species (ROS) play an important role in the photochemistry of
30
natural aquatic systems. These ROS include the hydroxyl radical (•OH), singlet oxygen
31
(1O2), superoxide (O2•-), hydrogen peroxide (H2O2) and triplet excited states. From an
32
engineering standpoint, degradation of many organic contaminants is increased by
33
ROS.1,2 Ecologically, ROS are involved in the mineralization of organic carbon,3
34
contributing to the global carbon cycle. The presence of H2O2 was first detected in
35
marine waters in 1966 and was shown later to result from photochemical reactions.4
36
Since then, other ROS such as •OH and 1O2 have been shown to be produced
37
photochemically in natural waters.3,5,6
38
Multiple chemical species undergo photochemical reactions to produce ROS, such
39
as nitrate, nitrite, and dissolved organic matter (DOM).3,5,7 In particular, DOM photolysis
40
produces •OH and 1O2, as well as H2O2, triplet states (here abbreviated as 3DOM*), and
41
O2•-. The generally accepted model is that 1DOM* is formed following absorption of a
42
photon by ground state DOM, which can subsequently undergo internal conversion,
43
vibrational relaxation, or intersystem crossing to 3DOM*. Early work by Zepp et al.
44
established an average energy of around 250 kJ mol-1 for 3DOM* molecules,8 sufficiently
45
high to form
46
disproportionation of O2•-, which is formed by reaction of dissolved oxygen with some
47
photo produced reductant.9 The •OH radical is believed to be formed by multiple
48
pathways, however, steady state concentrations are limited to around 10-18 to 10-16 M in
1
O2 from reaction with dissolved oxygen. H2O2 results from the
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natural waters due to its high reactivity.3,10 1O2 is a weaker and more selective oxidant
50
than •OH and thus reaches steady state concentrations a few orders of magnitude higher.7
51
Additionally, steady state concentrations of •OH and 1O2 are generally higher for effluent
52
organic matter compared to DOM isolates.11,12 Reported apparent quantum yields (Φa) for
53
•OH radical are on the order of 10-4 to 10-5, again being generally higher for wastewater
54
samples.11
55
While it is clear that DOM is a photochemical source of •OH, the underlying
56
mechanisms are not completely understood. Some of the formation pathways are
57
summarized in Scheme 1. There likely exist two pathways to •OH: an H2O2-dependent
58
and H2O2-independent pathway.5,7,11 The H2O2-dependent pathway involves the photo
59
Fenton reaction13,14 and possibly direct photolysis of photochemically produced H2O2.15
60
The importance of this pathway has been demonstrated by using catalase to decompose
61
H2O2, which decreases •OH production by up to 50 % for some samples.16,17 Super-
62
oxidized iron species (e.g. ferryl, FeO2+), which are operational at neutral pH,18 may also
63
account for some •OH-like activity by reacting with various organic compounds.19 The
64
H2O2-independent pathway involves species that lead to free •OH as well as so-called
65
low energy hydroxylating species.17,20,21 H-atom abstraction from water by some DOM
66
excited state (3DOM*, 1DOM*, or charge transfer species-DOM•+/•-) is one mechanism
67
suggested to produce free •OH, but this has not been confirmed.7,22 Generation of low
68
energy hydroxylators has been argued largely on the quenching of •OH-like activity with
69
addition of methane. For example, Page et al. (2011) showed that some DOM isolates
70
exhibited less than calculated decreases in photochemical •OH production with addition
71
of known concentrations of methane.17 Though uncertain, these species could be modeled
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by the quinone-water exciplex formed during irradiation of 2-methyl-p-benzoquinone.
73
For example, Pochon et al. (2002)20 and Gan et al. (2008)21 demonstrated no methyl
74
radical production upon irradiation of methyl-p-benzoquinone containing methane as an
75
•OH probe.
76
This paper presents data on the apparent temperature dependence of Φa for bulk
77
DOM isolates, with and without catalase present, as well as apparent molecular weight
78
fractions. The temperature dependence of the photolysis of p-benzoquinone was also
79
investigated as a model compound for comparison. These data provide insight into DOM
80
characterization and photochemistry by comparing the temperature dependence of
81
different DOM samples to a model compound. In addition, the measured activation
82
energies can be used for kinetic modeling of steady state •OH concentrations in natural
83
and engineered systems exhibiting temperature fluctuations or large deviations from
84
room temperature.
85
Materials and Methods
86
DOM isolates including Suwannee River Fulvic Acid I (SRFA, 1S101F),
87
Suwannee River Humic Acid II (SRHA, 2S101H), Suwannee River NOM (SRNOM,
88
2R101N) Pony Lake Fulvic Acid (PLFA, 1R109F), and Elliot Soil Humic Acid (ESHA,
89
1S102H) were purchased from the International Humic Substance Society. All other
90
chemicals were obtained from commercial sources. The solution conditions, irradiation
91
conditions, and analytical methods used in this study are presented in detail elsewhere.11
92
Briefly, solutions used in irradiation experiments were buffered at pH 7.2 in equimolar
93
KH2PO4/K2HPO4 (total [phosphate] = 10 mM). Samples were irradiated in clear, glass
94
vials free of headspace. Irradiations were ran in duplicate for 2.5 to 8 hours (depending
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on sample to achieve measurable •OH concentrations) with an Oriel 1429A Solar
96
Simulator equipped with a 1000 W Xenon lamp (see Figure S1 for lamp spectrum). Over
97
the irradiation period, samples were collected at 5 time intervals. Temperature control
98
within ± 2 °C was accomplished by laying the vials flat in a water-jacketed petri dish.
99
Benzene was used as an •OH probe by following the formation of phenol through the
100
Dorfman reaction.23,24 Though not described in detail here, this mechanism involves
101
reaction of the cyclohexadienyl radical with dissolved oxygen, which facilitates the
102
formation of phenol. Based on the experimental conditions used in this study (gas tight
103
vials, ~ µM phenol formation) no oxygen depletion due either to temperature changes or
104
phenol formation during the course of experiments was expected. A benzene
105
concentration of 3 mM was used to quantitatively scavenge •OH radicals. Phenol
106
concentrations were measured by HPLC utilizing a gradient mobile phase of pH 2.8, 10
107
mM H3PO4 and methanol with a flow rate of 1.0 mL min-1. The retention time of phenol
108
under these conditions was around 4.7 minutes. Using the same HPLC method, p-
109
benzoquinone eluted at 2.2 minutes (column dead time ~ 1.6 minutes). The column used
110
was a Zorbax Eclipse XBD (4.6 x 150 mm) C18 column. Bovine liver catalase was used
111
in some experiments to quench H2O2.
112
Apparent molecular weight fractions were collected using < 5 kDa and < 1 kDa
113
ultrafiltration membranes (Millipore, USA). Membranes were soaked three times in fresh
114
1 L batches of Milli-Q water before use. To remove any remaining organic material, 200
115
mL of Milli-Q water was passed through the membrane before fractionating DOM
116
samples. Approximately 200 mL of DOM solution was added to the ultrafiltration cell.
117
The first 100 mL of permeate was collected.
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The apparent quantum yield of •OH from DOM photolysis is given by
R•OH Φa = kDOM−a [DOM]
119
(1)
120
where R•OH is the rate of •OH radical formation in M s-1 and kDOM-a is the specific rate of
121
light absorption by DOM in s-1. The specific rate of light absorption was calculated by the
122
following equation 400nm
123
kDOM−a = ∑ λ =290nm
− ε DOM ( λ )[DOM]z
E p0(λ )ε DOM ( λ )(1−10
)
ε DOM (λ )[DOM]z
(2)
εDOM(λ) is
124
where ܧ (λ) is the photon irradiance of the solar simulator in Einstein-1 cm-2,
125
the molar absorptivity of DOM in MC-1 cm-1, [DOM] is the DOM concentration in MC,
126
and z is the cell depth in cm. Absolute irradiance was measured with a spectroradiometer.
127
Absorbance was measured with a Cary 100 UV/Vis spectrophotometer. Molar
128
absorptivity data for p-benzoquinone were taken from the National Institute of Standards
129
and Technology Chemistry WebBook.25
130
The rate of phenol formation in this system is given by
131
d[C6H5OH] = kC6H6-OH[ OH][C6H5 ]= Yphenol R OH dt
(3)
132
where kC6H6-OH is the second-order rate constant between benzene and •OH radical and
133
Yphenol is the yield of phenol from this reaction. Making the steady state assumption for
134
•OH radical gives
135
d[ OH] = kDOM−a Φ a [DOM]-∑ ks [S][ OH] ≈ 0 dt
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[ OH]ss =
kDOM−aΦ a [DOM]
∑ k [S]
(5)
s
137
where kS is the second order rate constant between •OH and scavenger S. Since the
138
experimental conditions are such that Σks[S][•OH] ≈ kC6H6-OH[C6H6][•OH], that is,
139
benzene is the dominant •OH scavenger equation (3) becomes
140 141
142
d[C6H5OH] = kDOM−a Φ a[DOM] dt
(6)
and solving for Φa gives
Yphenol Φ a = R OH kDOM−a[DOM]
(7)
143
Clearly, Φa depends on the yield of phenol from hydroxylation of benzene. We discuss
144
below the temperature dependence of Yphenol.
145 146 147
Activation energies were determined by plotting the measured temperature dependent Φa according to the Arrhenius equation:
E 1 ln(Φ a ) = − a + ln(A) RT
(8)
148
Quantum yields were measured at 4 to 5 temperatures over a range of around 10 to 40 °C.
149
Temperature dependent quantum yields have been analyzed in this way before26-28 and
150
the present data generally fit the Arrhenius model, with the average relative error (based
151
on a linear fit of the Arrhenius plots) in activation energies being 15 %.
152
Results and Discussion
153
Temperature Dependence of Phenol Yield from Benzene
154
In order to use equation (7) at different temperatures, the temperature dependence of
155
Yphenol needed to be assessed. Work by Chu and Anastasio (2003) has demonstrated that 7
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the •OH probe benzoic acid is not sensitive to temperature,27 suggesting that this may be
157
the case for benzene.
158
Measuring Yphenol involves knowing the actual rate of phenol produced (Rphenol)
159
and the theoretical or calculated amount of •OH resulting from photolysis of H2O2
160
(R•OH,calc) – i.e. Yphenol = Rphenol / R•OH,calc. R•OH,calc depends on the rate constant k for the
161
photochemical transformation, which is calculated in general by
162
R•OH ,calc = k [H 2 O 2 ]
163
k = 2.303
(9)
400nm
∫ (Φλε λ I λl)dλ
λ =290nm
(10)
164
where Φλ, ελ, and Iλ represent the quantum yield of •OH from H2O2, molar absorptivity,
165
and irradiance at a given wavelength and l is the pathlength. Note that the (small)
166
temperature dependence of Φλ and ελ measured by Chu and Anastasio was used in
167
equation (9).29 Solutions of 90 to 100 µM H2O2 were spiked with 3 mM benzene and
168
irradiated for periods of 100 to 220 min. Rphenol was found from linear fits of the resulting
169
data (not shown). Experiments were conducted at temperatures ranging from 11.6 to 38.4
170
°C and resulted in Yphenol values of 0.63 ± 0.07 (see Figure S2) with no apparent
171
temperature dependence (slope p = 0.14). In addition, an activation energy of 16.6 ± 5.0
172
kJ mol-1 for the production of •OH from nitrate photolysis was measured using benzene
173
as the •OH probe. This value is in good agreement with that measured by Chu and
174
Anastasio (2003)27 of 19 ± 4 kJ mol-1 and Zellner et al. (1990)26 of 16 ± 4 kJ mol-1.
175
The Yphenol determined here is within the range of values in the literature (0.43 to
176
0.95 depending on pH)23,30-32 and agrees well with the recently determined value of 0.69
177
measured by Sun et al. (2014).33 In previous publications by our group11,22,34 and others,35
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a yield of 0.85 was used. In this work, the determined yield was 26 % lower. This
179
highlights the importance authors reporting •OH formation rates should report the value
180
of Yphenol to allow for comparison to other studies.
181
One limitation to this study is that irradiations were performed using a solar
182
simulator (opposed to a monochromatic source) as the quantum yield for •OH radical are
183
strongly wavelength-dependent.5,36 Subsequently, the quantum yields reported here
184
should be interpreted as an average over the spectral output of the solar simulator (λ >
185
290 nm). Nevertheless, this paper provides the first temperature dependent measurements
186
of Φa for •OH; also, previous studies have used polychromatic irradiation to investigate
187
DOM photochemistry.
188
Temperature Dependence for Bulk DOM Isolates
189
DOM isolates were chosen to give a range of bulk chemical functionalities, type (e.g.
190
humic acid vs. fulvic acid), and source (e.g. Pony Lake vs. Suwannee River). A typical
191
data set is shown in Figure 1 for ESHA. Apparent quantum yields were plotted according
192
to the Arrhenius equation (eq. 8) in order to obtain the apparent activation energy
193
(hereafter called activation energy). The term apparent activation energy is used to
194
acknowledge the complexity of the system in that DOM is a heterogeneous, complex
195
material and that the activation energies are obtained using a polychromatic source.
196
Activation energies for all samples are summarized in Table 1. There is surprisingly little
197
variation in the values for bulk DOM isolates, besides SRFA (34.3 ± 7.2 kJ mol-1), which
198
suggests a consistent mechanism for •OH production. In addition, data are plotted
199
according to the Eyring equation (see Table S1) and some discussion is provided in the
200
Supplemental Information.
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201
Values for bulk samples are in the range of reported activation energies for other
202
photochemical processes (see Table 2), though there is some variation depending on the
203
reaction. For example, aqueous photolysis of H2O2 and NO3- results in primary
204
photochemical processes characterized by different photofragments as shown in
205
equations 11-13.
206
hν •OH H2O2 →
(11)
207
[NO3–]* → NO2- + O(3P)
(12)
208
2O [NO3–]* → NO2• + O•- H → NO2• + •OH + OH–
(13)
209
The lower activation energies of ≈ 16 to 18 kJ mol-1 for ESHA, SRHA, and SRNOM are
210
closer to values characterizing diffusion-controlled reactions (≈15 kJ mol-1),37 while
211
those for PLFA and SRFA, greater than 20 kJ mol-1, are slightly higher. An explanation
212
for the higher activation energy for nitrate photolysis (relative to H2O2) is that the
213
primary photoproduct of H2O2 photolysis (•OH radical) diffuses more easily from the
214
solvent cage than that of nitrate photolysis (O•-).26,29,38,39 Therefore the activation energies
215
for Φa from DOM could reflect either secondary thermal chemistry or diffusion of
216
different photoproducts from the solvent cage.
217
Considering the activation energies for photochemical reactions of organic
218
molecules, values for H-atom abstraction by ketones can vary from 9.2 to 29.3 kJ mol-1
219
depending on the reaction conditions and organic substrate.40 These Norrish Type
220
reactions involve H-atom abstraction from the triplet state, subsequently producing a
221
carbon-centered diradical. It is interesting that the activation energies in Table 1 match
222
these values. However, the above activation energies are for production of a carbon-
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centered radical in organic solvents, while H-atom abstraction from water by 3DOM*
224
would lead to •OH in aqueous solution.
225
Catalase Experiments
226
The activation energies shown in Table 1 are for DOM samples not containing catalase,
227
which is used to quench photochemically produced H2O2 in order to minimize the
228
contribution of direct photolysis and photo-Fenton reactions to •OH production.16,17
229
Activation energies for the photochemical production of H2O2 from DOM have recently
230
been reported by Kieber et al. (2014) for seawater samples, ranging from about 8 to 53 kJ
231
mol-1 depending on the irradiation wavelength.28 Their analysis revealed that when taken
232
as an average across all samples, activation energies ranged from 15 to 30 kJ mol-1 over
233
290 to 400 nm.
234
To examine the H2O2-independent pathway, Φa values were measured for ESHA
235
solutions containing increasing concentrations of catalase. A similar experiment
236
including the same DOM sample is described by Page et al. for the terephthalate probe.17
237
These data for ESHA are shown in Figure S3 and demonstrate an almost identical
238
quenching of •OH production for the same sample (ESHA) determined by Page et al.
239
(2011) - about 50 %, at similar concentrations of catalase (≈ 40 units mL-1).17 Using a
240
catalase concentration of 40 unit mL-1, an activation energy of 16.6 ± 3.4 kJ mol-1 was
241
measured for ESHA, which matches well with the non-catalase value of 17.6 ± 1.3. This
242
result indicates that the H2O2-independent pathway to •OH (H-atom abstraction by
243
3
244
for ESHA. The addition of catalase also limits photo-Fenton reactions from occurring.
245
Again the similarity in activation energies with addition of catalase, which essentially
DOM* and/or low-energy hydroxylator) has an activation energy of about 17 kJ mol-1
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shuts down any Fenton chemistry, suggests that the H2O2-dependent pathway has a
247
similar or lower activation energy than the H2O2-independent pathway.
248
Temperature dependence of p-Benzoquinone photolysis
249
To gain further insight into the H2O2-independent pathway, the temperature-dependent
250
photolysis of p-benzoquinone was investigated using benzene as an •OH probe. Others
251
have investigated this system at room temperature using dimethylsulfoxide (DMSO) to
252
trap •OH, leading to the mechanism proposed in Scheme 2.20,21
253
In our experiments, solutions of 92.5 µM p-benzoquinone produced about 500 nM
254
phenol over 12 minute irradiations (Figure S4). The fact that phenol from irradiation of p-
255
benzoquinone shows that benzene may be susceptible to hydroxylation by species other
256
than free •OH, e.g. low energy hydroxylators. Though there is some debate as to whether
257
irradiation of quinones produces free •OH, there is evidence that hydroxylation occurs
258
through a quinone-water exciplex.20,21 As quinone moieties are believed to be present in
259
DOM, this may explain the presence of low energy hydroxylators in irradiated DOM
260
observed previously.17,20
261
As seen in Figure 2a, the Φa of phenol from p-benzoquinone irradiation was
262
essentially temperature independent with an activation energy of ≈ 0 kJ mol-1 (slope p =
263
0.41). The loss of p-benzoquinone, however, shows more complex behavior (Figure 2b).
264
At temperatures lower than ≈ 23 °C, p-benzoquinone loss is temperature dependent (9.7
265
kJ mol-1), while above 23 °C, there is almost no temperature dependence. These data are
266
consistent with formation of a water-quinone exciplex that can hydroxylate benzene or
267
proceed by other reactions to form hydroxy-p-benzoquinone or hydroquinone. The loss of
268
p-benzoquinone, and production of hydroxy-p-benzoquinone and hydroquinone was
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confirmed in separate experiments by measuring the spectral changes in irradiated
270
solutions containing 75 µM p-benzoquinone with and without 3 mM benzene (with
271
benzene experiment shown in Figure S5). The presence of 3 mM benzene did not
272
eliminate the production of hydroxy-p-benzoquinone, suggesting that the intermediate
273
species formed is not quantitatively quenched by benzene at a concentration of 3 mM.
274
Conversely, others have reported quenching of this intermediate by DMSO at
275
concentrations of 100 mM.21
276
The reactions shown in Scheme 2 are likely affected by temperature in two ways.
277
First, forward reactions to form phenol, hydroxy-p-benzoquinone, and the hydroquinone
278
are expected to increase with increasing temperature. Additionally, an increase in
279
temperature likely encourages dissociation of the exciplex to re-form ground state p-
280
benzoquinone. Based on these data, it seems that an increase in temperature favors
281
production of hydroxy-p-benzoquinone and the hydroquinone up to a certain point
282
(around 23 °C), while at higher temperatures, dissociation of the exciplex dominates.
283
This explanation also accounts for the net zero temperature dependence of Φa for phenol.
284
Negative and near zero activation energies have been taken as evidence for exciplex
285
formation in other chemical systems,41-43 albeit better characterized systems than DOM.
286
Taken as a whole, these experiments demonstrate that the activation energies observed
287
for •OH production from DOM cannot be attributed solely to a simple quinone model,
288
even though the experimental approach depended on the measurement of the formation of
289
phenol from the hydroxylation of benzene.
290 291
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Effect of Molecular Size
293
Additional observation of Table 1 reveals differences in activation energies between
294
humic (average of 17.1 ± 0.9 kJ mol-1) and fulvic (average of 27.9 ± 9.0 kJ mol-1) acids.
295
It is difficult to specify what could account for this phenomenon because there are many
296
differences between these samples. For example, although SRFA and PLFA are both
297
fulvic acids, they differ widely in geographic source (Antarctica vs. Georgia, USA) and
298
type (microbial vs. terrestrial influence). However, fulvic acids in general have lower
299
average molecular weights.44
300
To further discern the effect of molecular size on the temperature dependence of
301
Φa, SRNOM was size-fractionated with
