Iron-Mediated Oxidation of Methoxyhydroquinone under Dark

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Iron-mediated oxidation of methoxyhydroquinone under dark conditions: kinetic and mechanistic insights Xiu Yuan, James A. Davis, and Peter S. Nico Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03939 • Publication Date (Web): 20 Jan 2016 Downloaded from http://pubs.acs.org on January 26, 2016

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Environmental Science & Technology

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Iron-mediated oxidation of methoxyhydroquinone under dark conditions: kinetic and

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mechanistic insights

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Xiu Yuan, James A. Davis, Peter S. Nico

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Earth Sciences Division, Lawrence Berkeley National Laboratory,

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1 Cyclotron Road, Berkeley, CA 94720, United States

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Resubmitted

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January, 2016

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* Corresponding author. Tel.: +1 (510) 486-7118, Fax: +1 (510) 486-5686

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Email: [email protected]

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1 ACS Paragon Plus Environment


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ABSTRACT

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Despite the biogeochemical significance of the interactions between natural organic matter

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(NOM) and iron species, considerable uncertainty still remains as to the exact processes

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contributing to the rates and extents of complexation and redox reactions between these

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important and complex environmental components. Investigations on the reactivity of low-

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molecular-weight quinones, which are believed to be key redox active compounds within

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NOM, towards iron species, could provide considerable insight into the kinetics and

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mechanisms of reactions involving NOM and iron. In this study, the oxidation of 2-

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methoxyhydroquinone (MH2Q) by ferric iron (Fe(III)) under dark conditions in the absence

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and presence of oxygen was investigated within a pH range of 4 – 6. While Fe(III) was

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capable of stoichiometrically oxidizing MH2Q under anaerobic conditions, catalytic oxidation

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of MH2Q was observed in the presence of O2 due to further cycling between oxygen,

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semiquinone radicals, and iron species. A detailed kinetic model was developed to describe

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the predominant mechanisms, which indicated that both the undissociated and mono-

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dissociated anions of MH2Q were kinetically active species towards Fe(III) reduction, with

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the mono-dissociated anion being the key species accounting for the pH dependence of the

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oxidation. The generated radical intermediates, namely semiquinone and superoxide, are of

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great importance in reaction chain propagation. The kinetic model may provide critical

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insight into the underlying mechanisms of the thermodynamic and kinetic characteristics of

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metal-organic interactions and assist in understanding and predicting factors controlling iron

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and organic matter transformation and bioavailability in aquatic systems.


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1. INTRODUCTION

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Natural organic matter (NOM), a chemically diverse mixture of heterogeneous organic

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compounds with various structural and functional properties, is ubiquitous in terrestrial and

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aquatic environments. Considerable attention has been drawn to the redox properties of NOM

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because of its great significance in a wide range of biogeochemical processes including iron

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bioreduction and contaminant transformation. The electron accepting and donating capacities

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of NOM have been attributed to the presence of redox-active constituents, such as quinone

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and phenolic functional groups.1-4 Numerous studies also suggested that quinone moieties in

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NOM not only contribute to the reduction of organic contaminants, such as alkyl halides and

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nitroaromatics, but also stimulate the reduction of ferric oxides by iron reducing bacteria.3-9

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The term quinone collectively refers to organic structures in three oxidation states with one

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electron reduction of a quinone forming a semiquinone radical that can be further reduced to

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the hydroquinone form.10 These species are common constituents of many biologically

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relevant molecules and serve as a vital link in the movement of electrons through cells and

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tissues.8, 11-13 For example, oxidations of dihydroxybenzenes to benzoquinones and

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corresponding back reactions are often reversible under neutral and acidic conditions, earning

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them a prominent place in organic electron transfer chemistry. Further, the frequently

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observed positive correlation between free radical concentrations in NOM and Fe(III)

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reduction using electron spin resonance (ESR) spectroscopy has been attributed to the

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existence of semiquinone radicals, a radical formed either from one-electron oxidation of

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hydroquinones, or from one-electron reduction of quinones, or from one-electron transfer

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from hydroquinones to quinones (a process termed comproportionation).1, 3, 13-15 Though

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commonly presented in very low concentrations due to its high reactivity, semiquinone 3 ACS Paragon Plus Environment


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radicals are kinetically important in the reversible redox chemistry of quinones.9 Previous

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studies also indicated that quinone moieties are important electron-transfer groups in humic

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substances (HS) for both biotic/abiotic reduction and oxidation of HS.1, 16 Besides, recent

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studies by Garg, et al.17, 18 reported that the occurrence of Fe(III) reduction by Suwanee River

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Fulvic Acid (SRFA, a proxy for terrestrial NOM), in non-irradiated acidic solutions was

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mainly attributed to the reduced hydroquinone-like moieties intrinsically presented in SRFA.2,

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4

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important implications and may provide critical information regarding reaction mechanisms

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and functional groups that participate in the mobilization/immobilization of a variety of

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redox-sensitive metals in terrestrial and aquatic environments. For example, hydroquinone

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mediated reduction of Fe(III) could increase the solubility and bioavailability of this critical

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trace element,6 while quinone mediated oxidation of uranium (IV) could affect its mobility

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and thus the remediation of contaminated sites.5

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Iron, as a micronutrient in both aquatic and terrestrial ecosystems, is essential for virtually all

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life forms and plays a critical role in many biogeochemical processes. In natural waters, iron

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exists in two oxidation states, Fe(II) and Fe(III). Despite being more thermodynamically

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stable, Fe(III) quickly hydrolyzes and precipitates at circumneutral pH and thus becomes

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unavailable for direct biological uptake,19 while the reduced form, Fe(II), is much more

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soluble and thus potentially more abundant in a bioavailable form.17 However, in oxygenated

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waters, Fe(II) is rapidly oxidized to Fe(III) at circumneutral pH, which in turn limits its

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bioavailability. Despite the biogeochemical functioning of the equilibration/cycling of

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Fe(II)/Fe(III) couple, fundamental insights into the control and dynamics of interconversion

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between Fe(II) and Fe(III) in natural waters are strictly limited and how these processes

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restrict or enhance iron bioavailability are still largely unknown.

Therefore, studies involving the redox behaviour of quinones in metal reduction have


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Quinones and iron likely coexist in the majority of the natural ecosystem, ranging from the

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surface water to groundwater, from soils to sediments. Since the highly complex nature of

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NOM could potentially interfere with efforts to explore the reactivity of quinone moieties on

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a molecular level, investigations of low-molecular-weight quinones could provide

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considerable insight into the thermodynamics, kinetics, and mechanisms of reactions

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involving quinone moieties.20 Though there is a range of different quinone moieties presented

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in NOM, including benzoquinone-based compounds, such as sorgoleone, and

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naphthoquinone-based compounds, such as juglone and lawsone, as well as anthraquinone-

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based compounds,20 a number of previous studies have indicated that those compounds are

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kinetically closely related to their redox potentials and functional groups, with kinetic

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understanding of one providing a window to interpretation of associated chemistry and

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biochemistry of the other.8, 11, 14, 20-22 2-Methoxyhydroquinone (MH2Q, see abbreviations of

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different quinone species and detailed structure information in Fig.S1 of the Supporting

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Information), as an intermediate product during lignin degradation, is a common constituent

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of NOM and has the highest oxidoreductase activity among a series of low-molecular-weight

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quinones.23, 24 This study therefore investigated the oxidation of MH2Q by Fe(III) under both

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anaerobic and aerobic conditions over the pH range 4 – 6, with emphasis on development of a

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rigorous kinetic description of the oxidation mechanism and insights on the role of free

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radicals, such as semiquinone and superoxide. Thermodynamic characteristics of this system

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were also discussed by employing the half-cell reduction potentials of different redox couples

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to verify modeled rate constants of the reactions involving quinone species. Acidic conditions

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were employed in this study partially to avoid complications associated with the precipitation

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of iron oxides, but they do reflect conditions of acid mine drainage waters and coastal

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environments with the presence of acid sulfate soils.



2. MATERIALS AND METHODS

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A complete description of the materials and methods used is presented in Section SI2 of the

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Supporting Information (SI).

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2.1. Reagents

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All solutions were prepared using 18 MΩ.cm ultrapure Milli-Q water (Thermo Fisher

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Scientific). Analytical grade chemicals were purchased from VWR International or Sigma-

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Aldrich (or as otherwise stated) and used without further refinement. Stock solutions were

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refrigerated at 4 °C in the dark when not in use. All studies were performed in 10 mM NaCl

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at a controlled room temperature of 22 ± 0.5°C. Experiments were conducted in darkness

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with the reactor covered in foil for the duration of the reaction. Anaerobic data were collected

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in an anaerobic glove bag (Coy Laboratory Products, Grass Lake, MI) filled with 5% H2 and

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95% N2. It is worth mentioning that all the anaerobic experiments conducted in the glove box

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here were completed within 30 min, during which no interference from H2 was observed.

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However, slow reduction of 2-methoxybenzoquinone (MBQ) stock solution to MH2Q during

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an overnight stay in the chamber (more than 17 hr) was observed, suggesting that interference

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reduction from H2 has to be considered for long-term experiments in the glove box.

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2.2. Experimental Measurement

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Measurement of MBQ: Concentrations of MBQ were determined spectrophotometrically

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using a UV-Vis spectrophotometer (Thermo Fisher Scientific) with ε257 = 1.48 × 104 M−1

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cm−1.


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Fe(II) Determination: Concentrations of total Fe(II) were determined using a modified FZ

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method.17

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H2O2 Determination: Previous studies21, 22 indicated the potential interference from

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hydroquinones on H2O2 detection using the fluorescence method,25 therefore, a

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chemiluminescence-based method (the AE-CL method)26 was employed for H2O2

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measurement in this study.

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2.3. Kinetic Modeling

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The kinetic model was fitted to the experimental data over a range of experimental conditions

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using the program Kintek Explorer.27 The sensitivity of the model to changes in individual

140

rate constant values was also assessed by examining the change in the relative difference

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between the experimental data and the kinetic model (defined as the relative residual, r) when

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one rate constant was varied while holding the others fixed at their optimal values. The

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program Kintecus28 was employed to evaluate the kinetic model using a VBA program to

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calculate r (see detailed description and Fig.S3 in SI3).

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3. RESULTS AND DISCUSSION

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3.1 Anaerobic Oxidation of MH2Q by Fe(III)

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Previous studies have investigated the redox chemistry of hydroquinones and metal species in

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oxic environments,17, 18, 21, 22, 29 but interaction involving these active components under dark

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anaerobic conditions is also of great interest, not only because such matrix composition better

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features the redox circumstances in anoxic environments (groundwater, deep ocean,

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sediments, et al.), but also the absence of O2 allows full expression of the redox functions of

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metals and quinones and thus enables a thorough estimation of the generation of semiquinone

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radicals under dark anaerobic conditions. Further, the reaction mechanism under anaerobic

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condition is an essential component for the complete kinetic description of the oxidation

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mechanism occurring in the presence of O2. Therefore, in this study, the oxidation of MH2Q

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by Fe(III) was firstly investigated under anaerobic conditions, with results shown in Figures 1

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– 2 (and Fig.S4 and S5). Precipitation of Fe(III) species in the system can be characterized as

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insignificant in these experiments for several reasons: 1) the acidic conditions which enable

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the existence of more soluble Fe(III) species, 2) the very low concentrations of Fe(III)

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employed here, and 3) evidence that > 90% of added Fe(III) was quickly reduced to Fe(II) by

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MH2Q before any significant nucleation/precipitation occurred (Figures 2, 4, and Fig.S8, also

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see detailed explanation in section SI4). Therefore, no further consideration was given to

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precipitation of Fe(III) in this study. Also, Fe(III) and Fe(II) were used as single entities to

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represent all ferric and ferrous species respectively.

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Figure 1 illustrates results for anaerobic oxidation of 500 nM MH2Q by different

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concentrations of Fe(III) at pH 5. The steady-state concentrations of generated MBQ were

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proportional to added [Fe(III)]0, with a specific ratio of 1:2. This is in accordance with


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previous studies30-32 reporting the overall stoichiometry for the oxidation of hydroquinone by

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Fe(III) is: +

( − H ), kanaerobic MH 2 Q + 2Fe(III)  → MBQ + 2Fe(II)

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(1)

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which could be further confirmed by Fe(II) generation profiles shown in Fig.S6. At pH 5,

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while 500 nM Fe(III) was completely reduced to Fe(II) at t = 20 min, only ~250nM of MBQ

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was generated. pH dependence of MH2Q oxidation by Fe(III) was shown in Figure 2, as well

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as in Fig.S4 and S5 of the SI. The generation rate of Fe(II), as can be seen from Figure 2,

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increased significantly with increasing pH. For example, in the presence of 500 nM [MH2Q]0,

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500 nM Fe(III) was completely reduced to Fe(II) within 5 min at pH 6, while less than 50%

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of Fe(III) was reduced within the same period at pH 4. Similar trends could also be observed

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in the generation rate of MBQ in Fig.S4 which showed that 250 nM [Fe(III)]0 was capable of

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oxidizing ~120 nM MH2Q in the first 2 min at pH 6 but less than 20 nM MH2Q was oxidized

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in the same duration at pH 4 (Fig.S4, and similar behaviour in Fig.S5). The pH dependence of

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redox reactions between hydroquinones and ferric or cupric species has been previously

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observed and was interpreted in terms of reactions via different ionized forms of

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hydroquinones in various reaction routes.21, 22, 33-37 While the mono-dissociated hydroquinone

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was postulated as the most kinetically active species towards the reduction of metal species,8,

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21, 38

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could also contribute to the reduction of cytochrome c by hydroquinone at acidic pH, which

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is consistent with the conclusion from a recent study on copper-catalyzed oxidation of

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naphthohydroquinone.22

studies by Rich and Bendall35, 36 reported that the undissociated hydroquinone species



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3.2 Aerobic Oxidation of MH2Q by Fe(III)

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The autoxidation of MH2Q (autoxidation is oxidation in the absence of metal catalysts, see

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results in section SI6) was found to be negligibly slow under the conditions of this study,

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principally because of the lower pHs employed here and the high redox potential/pKa's

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possessed by MH2Q,8, 22 making the first electron transfer from MH2Q to O2 highly

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endergonic,4 thus this reaction was not further considered in this study. Fe(III)-mediated

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oxidation of MH2Q in the presence of O2 was inspected, with emphasis on the effects of pH,

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[Fe(III)]0 and [MH2Q]0, as shown in Figure 3. First and foremost, unlike the results obtained

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under anaerobic conditions, non-stoichiometric oxidation, or in other words, catalytic

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oxidation of MH2Q by Fe(III) was observed, especially in Figure 3(D), which demonstrated

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that at pH 6, 500 nM Fe(III) was capable of completely oxidising 10 µM MH2Q within 2 hr.

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Similar tests were also conducted on 1,4-benzohydroquinone and no significant catalytic

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oxidation was found, presumably because the methoxy group substitution lowers the redox

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potential of the hydroquinone and makes it more facile for oxidation by Fe(III).8 Since we

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already confirmed that direct oxidation of MH2Q by O2 is negligible in this system, the rapid

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oxidation of additional MH2Q must occur, at least partially, with mediation of iron.

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It is also noteworthy that Figure 3 showed that Fe(III) oxidizes MH2Q to MBQ in a biphasic

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manner, with initial rapid formation of MBQ followed by a much slower increase in MBQ

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concentration over time. The initial concentration of MBQ that was rapidly formed was

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approximately linear-proportional to the added Fe(III) concentration (similar to anaerobic

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oxidation, where stoichiometric amount of MH2Q was quickly oxidized by added Fe(III)),

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and thus was ascribed to direct oxidation of MH2Q by added Fe(III). This could be further

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justified by Figure 4 and Fig.S8 showing Fe(III) was nearly completely reduced to Fe(II) by

216

the end of the initial rapid phase of oxidation, indicating the direct oxidation of MH2Q by


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Fe(III) is rapid and not rate limiting. Instead, the following much slower process, involving

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multiple O2-mediated reactions, such as regeneration of Fe(III) by oxidizing Fe(II) at such

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acidic pH,39, 40 might be more rate-determining in the overall oxidation.

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On the other hand, it is interesting to notice that in Figure 3, the rate of oxidation is not only

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correlated with the Fe(III) concentration, but also increased with increasing the initial

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concentration of the hydroquinone ([MH2Q]0, Fig.S9), similar to behaviour observed in a

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previous study on copper-mediated oxidation of naphthohydroquinone.22 More specifically,

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as shown in Figure 3(D), at pH 6, more MH2Q was oxidized per unit of time when [MH2Q]0

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= 10 µM than that with [MH2Q]0 = 5 µM; and similarly, comparison between Figures 3(A)-

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(C) and Figure 3(D) indicates that at each pH with [Fe(III)]0 =500 nM, much more MH2Q

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was oxidized per unit of time when [MH2Q]0 = 5 µM than that with [MH2Q]0 = 500 nM. This

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was especially observed within the second oxidation phase, indicating that iron-mediated

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reactions might not be the only factor dominating the “slow oxidation phase”, and instead

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MH2Q-involved processes must also play a role. Nevertheless, as we declared that the direct

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interaction between MH2Q and Fe(III) is rapid and thus not likely to be rate limiting, other

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reactions involving intermediates generated from MH2Q-dependent activities, most likely the

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semiquinone radicals,21, 22 might be more important in determining the slow oxidation rate.

234

Similar to anaerobic oxidation, there is also apparent pH dependence of the aerobic oxidation

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of MH2Q by Fe(III) (as shown in Figure 3 and Fig.S7 in the SI), i.e., the overall oxidation

236

rate increased with increasing pH from 4 to 6, and such dependence was observed in both

237

oxidation phases (the initial rapid phase and the later slow phase). Apparently, the

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mechanism accounting for the anaerobic pH dependence (involvement of different ionized

239

forms of MH2Q in reactions with ferric species), is inevitably contributing here, but other pH



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sensitive processes, such as the oxidation of Fe(II) by dioxygen and superoxide,19, 39, 40 could

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also impact the overall pH dependence.

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Besides the generation of MBQ, the transformation of iron in the system is also of great

243

interest. Direct measurement of Fe(II) concentrations over a range of conditions was

244

undertaken in order to assist in determining the mechanism and constraining the fitting

245

parameters. The corresponding results are shown in Figure 4 and Fig.S8. Within the pH range

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4 – 6, all of Fe(III) added to 500 nM MH2Q solution was quickly reduced to Fe(II), with the

247

reduction rate increasing with increase in pH. As for pH 4 (Fig.S8), the concentration of Fe(II)

248

reached its maximum level after more than 30 min of the reaction initiation, while at pH 6,

249

the highest concentration was quickly achieved within 7 min (Figure 4). Though the re-

250

oxidation of Fe(II) to Fe(III) was happening concurrently,19, 39, 40 this reaction was too slow to

251

compete against the rapid reduction of Fe(III) to Fe(II) by MH2Q. This is to say, all of the

252

Fe(III) regenerated from the oxidation of Fe(II) by O2 and other oxidants (such as superoxide

253

and semiquinone radicals, see detailed mechanism in next section and in Table 1) was

254

immediately re-reduced to Fe(II) by MH2Q, thus almost all of the iron was observed as Fe(II)

255

in the system in spite of the predominating oxygenated conditions. The cycling between

256

Fe(III) and Fe(II) kept proceeding until MH2Q was totally consumed, and subsequently the

257

concentration of Fe(II) started to decline simply because of the oxygenation of Fe(II) to

258

Fe(III). This is clearly the case demonstrated in Figure 4 with the top curve representing the

259

reduction of 500 nM Fe(III) by 500 nM MH2Q at pH 6. After the pseudo-equilibrium

260

between Fe(II) and Fe(III) (8 – 60 min), the obvious decrease of [Fe(II)] began at ~ t = 60

261

min, at which point the added 500 nM of MH2Q was nearly completely transformed to MBQ

262

(Figure 3(C)).


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In order to gain some insight into the Fe(III)/Fe(II) transformation in the system, the cycling

264

rate and the turnover frequency of iron have been calculated using a modified version of the

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kinetic model reported in Table 1, with detailed description and results provided in Section

266

SI7 (Fig.S12 and S13). As an example, for the case of 500 nM Fe(III) and 10 µM MH2Q at

267

pH 6, cycling between Fe(III) and Fe(II) occurs at the very beginning of the reaction, with a

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turnover frequency of ~6 h−1. By examining the contribution of different oxidants on the

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oxidation of MH2Q (Fig. S13), it is clear that besides Fe(III)/Fe(II), the O2/O2•− couple, which

270

oxidized almost 40% of MH2Q, is another significant force driving the overall oxidation. This

271

also explains the occurrence of non-stoichiometric oxidation of MH2Q by Fe(III) in the

272

presence of O2. The additional MH2Q was oxidized not only by the regenerated Fe(III) that

273

resulted from oxidation of Fe(II), but also by direct reaction with O2•−, generated either from

274

reduction of O2 by semiquinone or by Fe(II) (reactions 6 and 8, respectively, Table 1).

275

Previous studies also suggested that during the metal-catalyzed oxidation of hydroquinones,

276

stoichiometric amounts of hydrogen peroxide (H2O2) would be generated.8, 22, 41-44 H2O2 is of

277

particular interest because of its ubiquitousness both in the cells of organisms as a by-product

278

of oxygen metabolism, and in surface seawaters primarily due to photochemical processes

279

mediated by NOM.45 A recent study by Murphy, et al.46 also reported rapid production of

280

H2O2 (with maximum concentrations of 3.85 µM) when anoxic groundwater mixed with

281

oxidized, iron rich sediments, indicating a metal redox cycling mechanism under dark

282

conditions. More importantly, in the presence of iron, H2O2 may lead to the formation of

283

highly reactive and damaging oxidants such as hydroxyl radical (HO•), a radical which

284

always present in vanishingly small concentration but will in turn induce the oxidation of

285

both NOM and anthropogenic contaminants.46-48 Therefore the generation of H2O2 during the

286

oxidation of MH2Q by Fe(III) at different pH values was also investigated, with results



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shown in Figure 5. Apparently, the rate of H2O2 generation, similar to the oxidation of MH2Q,

288

also exhibited strong pH dependence, with more rapid H2O2 generated at pH 6 than at pH 4.

289

3.3 Kinetic Modelling of MH2Q Oxidation by Fe(III)

290

Since the dissociation states of a hydroquinone is of key importance for its electron-donating

291

capability, the protonation/deprotonation reactions of MH2Q were included in the model as

292

reactions 1 and 2 in Table 1, with previously reported ionization constants.20, 49 Under

293

anaerobic conditions, the oxidation of MH2Q by Fe(III) occurs via direct interactions between

294

Fe(III) and hydroquinone species, and both the undissociated and mono-dissociated

295

hydroquinones have been reported to be active towards reduction of ferric species, as

296

expressed by reactions 3 and 4 in Table 1.34, 36 Although the doubly dissociated species MQ2–

297

is thermodynamically more reactive, its reaction with Fe(III) is negligible in this study

298

considering its extremely low concentration (10–16 – 10–20 M, from pH 6 to 4 under

299

conditions of this study), and thus was not included in the model. The intermediate generated

300

from one electron oxidation of the hydroquinone, semiquinone radical, has been widely

301

proposed to be inevitably involved in the pool with both oxidizing and reducing capibilities.8,

302

18, 21, 34-37

303

included in the model as reaction 5, as well as the oxidation of Fe(II) by MSQ•− (back

304

reaction of reactions 3 and 4) with a reported rate of 1 × 105 M−1·s−1 in recent studies by Garg,

305

et al.17, 18, 29 on iron redox transformations in the presence of natural organic matter.

306

When O2 is present, in addition to those reactions occurring anaerobically, reactions

307

describing the electron transfers between oxygen and semiquinone radical, between

308

superoxide and quinone species, as well as reactions depicting the oxidation of Fe(II), must

309

also be included. The additional reactions included in the model due to the presence of O2

Therefore, the reduction of Fe(III) by 2-methoxysemiquinone radical (MSQ•−) was


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310

with corresponding rate constants are listed as reactions 6 – 9 in Table 1. As previously stated,

311

the autoxidation of MH2Q is negligibly slow under conditions of this study, and thus was not

312

required in the model. Because O2 has no charge, Marcus theory of electron transfer predicts

313

that rate constants for reaction of semiquinone radical with O2 will be a function of change in

314

the Gibbs free energy.8, 15 It has also been reported that for quinones with a standard mid-

315

point potential of E0(Quinone/Semiquinone) ≥ − 150 mV, rate constants for the forward

316

reactions follow well the Marcus theory for electron transfer and can be estimated by a

317

reported linear relationship.8, 36, 50 Based on this and E0(MBQ/MSQ•−) = − 0.017 V (see

318

detailed calculations in SI8), the rate constant for the forward reaction can be estimated as k6

319

= 1.5 × 106 M−1·s−1. The equilibrium constant (K) of reaction 6 can also be determined by the

320

standard reduction potential (E0) of both redox couples (MBQ/MSQ•− and O2/O2•−) using the

321

Nernst equation:55

322

0 E(O

- E0

 -(MBQ/MSQ ) 2 /O 2 )

=

RT ln K nF

(2)

323

where R is the gas constant in appropriate units, 8.314 J K−1 mol−1. T is the temperature in

324

Kelvin, F is the Faraday constant 9.6485 × 104 C mol−1 and n is the number of electrons

325

involved in the reaction. Thus, when n = 1, at 25 °C, Equation (4) can be written as:

326

log K =

0 E(O

- E0

 -(MBQ/MSQ ) 2 /O 2 )

(3)

0.059

327

Here we employed the values of E0(O2/O2•−) = − 0.155 V56-58 and E0(MBQ/MSQ•−) = − 0.017

328

V in equation (3) to calculate K6 = 4.58 × 10−3, thus k−6 = k6/K6 = 3.3 × 108 M−1·s−1. These

329

values agree very well with previous reported rate constants of methyl-hydroquinone (k6 =

330

1.2 × 106 M−1·s−1 and k−6 = 7.3 × 108 M−1·s−1),50 which has close redox potentials to MH2Q.

331

Further, because of the substituent effect, the redox potential of MH2Q lies between that of

332

1,4-benzohydroquinone and 1,4-naphthohydroquinone, both the forward and reverse rate 15 ACS Paragon Plus Environment


333

constants of reaction 6 are also between these two analogous aromatics,21, 22 which again

334

confirms that the redox potential of the quinone/semiquinone couple governs the reaction of

335

semiquinone with dioxygen to form superoxide.8

336

The comproportionation reaction between MH2Q and MBQ was not important in the system,

337

as a result of the extremely low concentrations of the doubly dissociated hydroquinone

338

species (MQ2−, the well-established dominant active species towards comproportionation

339

reaction,15, 34, 36, 49 having a concentration of ~10–16 – 10–20 M from pH 6 to 4 under

340

conditions of this study). Since MSQ•− can be both oxidizing and reducing, it will readily

341

disproportionate to corresponding hydroquinone and quinone. However, due to its low

342

concentration and its rapid removal by dioxygen and iron species (reactions 3 – 6, Table 1),

343

the disproportionation of MSQ•− plays no significant role here. This could be further verified

344

by the result that inclusion of these two reactions in the kinetic model with varying rate

345

constants from 10 – 1010 M−1·s−1 has no observable effect on the overall model fitting

346

(Fig.S3(G) and (H) in SI). Therefore, these two reactions were not included in the model.

347

The oxidation of MH2Q by superoxide, which was expected to have a relatively low reaction

348

rate,58 was included in the model as reaction 7. Previous studies58-60 also suggested that

349

reactions involving H2O2 in this system (such as the backward reaction of reaction 7, Table 1)

350

were not important, presumably because the reaction rate between H2O2 and semiquinones

351

(~104 − 105 M−1·s−1) is low.61, 62 Therefore, this reaction is unlikely to compete effectively

352

with the rapid removal of semiquinones by dioxygen (reaction 6, Table 1). Further evidence

353

for this was given by the sensitivity analysis, which demonstrated that inclusion of this

354

reaction in the kinetic model and varying the rate constant from 10 – 105 M−1·s−1 had

355

negligible effect on the model fitting (see Fig. S3(I) in SI). As such, this reaction was

356

excluded from the model. The disproportionation of O2•−, with rates of 6 × 106 and 6 × 108 16 ACS Paragon Plus Environment

Page 16 of 34

Page 17 of 34


357

M−1·s−1 at pH 6 and 4 respectively,63, 64 was not important here because of its low

358

concentration and rapid reaction with iron and quinone species (reactions 6 – 9 in Table 1).

359

This was also confirmed by the sensitivity analysis (see Fig.S3(J) in SI). Cross-dismutation

360

between MSQ•− and O2•− might also occur, but little information on this reaction has been

361

previously reported and the rate constant is not known. Inclusion of this reaction in

362

simulations and varying the rate constant over a reasonable range revealed minimal effect on

363

the fitting (Fig. S3(K) in SI), justifying exclusion of this reaction from the model. The

364

oxidation of Fe(II) (reactions 8 and 9), as the key route for regeneration of Fe(III) in the

365

presence of O2, has been extensively investigated at acidic and circumneutral pHs. Therefore,

366

here we employed the well-established pH-dependent rates from previous studies19, 39, 40, 52-54

367

for interactions between O2, Fe(II)/Fe(III) and O2•−.

368

Due to the limited information available in the literature, rate constants for the interactions of

369

Fe(III) and quinone species (reactions 3, 4 and 5, Table 1) and the reaction between MH2Q

370

and O2•− (reaction 7, Table 1) were determined empirically by best fit of the model to the

371

experimental data. Results of model fitting over the pH range 4 – 6 using reactions 1 – 9

372

(Table 1) are illustrated in Figures 1 – 5 (and Fig.S4-S9 in the SI). In general, the model

373

provides reasonable fits to the experimental data over a range of conditions. The good

374

agreement between the model output and the experimental data validates our assumptions of

375

the proposed mechanism and the assignment of individual rate constants.

376

In accordance with previous studies,34-36 both the undissociated and singly dissociated MH2Q

377

species (MH2Q0 and MHQ−) were found to be kinetically active towards Fe(III) reduction

378

with intrinsic rate constants of 2.4 × 103 and 3.0 × 108 M−1·s−1, respectively. Since the

379

standard one-electron reduction potential of the MSQ•−/MH2Q0 couple is much higher than

380

that of the MSQ•−/MHQ− couple (see detailed calculations in SI8), reaction 3 is less 17 ACS Paragon Plus Environment


381

favourable in a thermodynamic sense and occurs via a rather slower rate than that of reaction

382

4. This again confirms that the mono-dissociated hydroquinone anion is a substantially more

383

facile electron donor to ferric species.8, 34, 36 Although MHQ− only occupies a small fraction

384

of the total hydroquinone species at acidic pH, the protonation/deprotonation equilibrium is

385

faster than that of redox reactions and thus is not rate-limiting.35, 65 It is also worth

386

mentioning that, although thermodynamically less favourable, reaction 3 is still feasible. We

387

can conclude here that the back reaction is more rapid than the forward reaction, since the

388

rate of reaction between Fe(II) and semiquinone radical is almost two orders of magnitude

389

higher than that of the forward reaction. Moreover, the rapid removal of the product, MSQ•−,

390

by Fe(III) and O2 (reactions 5 and 6, respectively) is another significant force driving the

391

oxidation reaction to the right.8, 35 Thus, the presence of O2 in the system not only promotes

392

Fe(III) re-supply by continuously oxidizing Fe(II), but also induces the rapid removal of

393

MSQ•− with resultant generation of O2•−. The model also indicates that the rate of MSQ•−

394

removal by O2 > rate of removal by Fe(III) > rate of removal by Fe(II) > rate of removal by

395

disproportionation, consistent with previously reported conclusion.36 However, unlike the

396

study by Uchimiya and Stone,11 there is no back reduction of MBQ by Fe(II) observed in this

397

study, presumably due to the acidic pH and low concentrations of both reactants employed in

398

this study.

399

The model predicted rate of 1.2 × 107 M−1·s−1 for reduction of Fe(III) by MSQ•− (reaction 5,

400

Table 1) is higher than the value of 2 × 106 M−1·s−1 reported by Yamazaki and Ohnishi34 on

401

the reduction of ferricytochrome c by semiquinones, but lower than the value of 2 × 107

402

M−1·s−1 reported in a recent study on the reduction of cupric species by 1,4-semiquinone.21

403

Furthermore, MSQ•−, with a standard one-electron reduction potential of E0(MBQ/MSQ•−) =

404

− 0.017 V, is more reducing than oxidizing. This is to say, in a system with both Fe(III) and


Page 18 of 34

Page 19 of 34


405

Fe(II) present, instead of shifting back to MH2Q by oxidizing Fe(II), MSQ•− is more likely to

406

transfer to MBQ by reducing Fe(III), consolidating the model predicted k5 > k-3. As indicated

407

in the study by Eyer,58 the oxidation of MH2Q by superoxide, was expected to have a

408

relatively low reaction rate of ~104 – 105 M−1·s−1.66, 67, 68 Our model predicted rate of 7 × 105

409

M−1·s−1 is also close to the value of 2 × 105 M−1·s−1 reported in recent studies by Garg, et al.17

410

on iron redox transformations in the presence of NOM under acid conditions.

411

Despite the capability of the kinetic model in providing a good description of the various

412

reactions between the quinone triad (MBQ, MSQ•− and MH2Q), Fe(III)/Fe(II), and O2 over a

413

range of experimental conditions with a relatively well-constrained set of rate constants, the

414

model is limited by the assumption that the composite terms Fe(II) and Fe(III) adequately

415

represent all inorganic iron in their +2 and +3 oxidation states, respectively. Therefore, care

416

must be taken when interpreting the model results, since the model is condition-specific by

417

default. For example, the presence of complexing organic ligands would influence the

418

reactivity of various iron species and as such the composite “intrinsic” rate constants of

419

various reactions with iron will need to be re-evaluated.

420

3.4 Implication of Findings

421

In this study, we investigated the oxidation of MH2Q by Fe(III) under dark conditions in the

422

absence and presence of oxygen. While Fe(III) was capable of stoichiometrically oxidizing

423

MH2Q under anaerobic conditions, catalytic oxidation of MH2Q was observed in the presence

424

of O2 due to further cycling between iron, oxygen and semiquinone radicals. Based on the

425

experimental observations, a detailed kinetic model was developed to describe the

426

predominant mechanisms operating in this system and to provide insights into the observed

427

pH dependence and the role of generated radical intermediates. Our results also showed that



428

the direct oxidation of MH2Q by O2 is spin-restricted and very slow under conditions of this

429

study, but the Fe(III)/Fe(II) couple could overcome such restriction and perform as an

430

electron transfer agent between the hydroquinone and O2 (similar to previous observed

431

behaviours of copper species21, 22), leading to the existence of pseudo-stable but high

432

concentrations of reduced Fe(II) under oxic and dark conditions that could potentially

433

promote the generation of ROS. One particular example in this study is the generation of O2•−

434

and H2O2 from the redox cycling of iron between its +3 and +2 oxidation states in the

435

presence of MH2Q and O2 confirms the essential role of NOM and reduced metals in non-

436

photochemical pathway for ROS formation in natural aquatic environments,69 which, at the

437

same time, are particularly important for the oxidative degradation of NOM itself and any

438

further cascade of radical-promoted reactions with other constituents in natural waters.

439

Though the presence of O2 is imperative for iron cycling and chain propagating to reach

440

catalytic oxidation of MH2Q in this study, in deep ocean/lakes and groundwater, where O2

441

deficiency prevails, other components presented in such anoxic/anaerobic environments

442

could serve to bridge Fe(II) and Fe(III), and thus facilitate iron cycling and further redox of

443

quinones and NOM. For instance, Uchimiya and Stone11 reported the oxidation of Fe(II) by

444

various quinones at circumneutral pH could serve as a vital link in networks of electron

445

transport in suboxic and anoxic environments. Studies also showed chromium(VI) and

446

manganese(III, IV) (hydr)oxides were capable of oxidising Fe(II) under anoxic conditions.70,

447

71

448

communities may also contribute, especially in low pH environments.72 Therefore, the

449

coexistence of quinones and iron could potentially facilitate the redox cycling of both species

450

in allowable environments, and separately or in concert, iron species (both dissolved and

At the same time, the presence of fluctuating redox conditions and (micro)biological


Page 20 of 34

Page 21 of 34


451

precipitated) and quinones are believed to be pivotal participants in a wide range of

452

environmental redox reactions.

453

Due to the close correlations between kinetic/thermodynamic characteristics of quinone triads

454

and their structures and functional groups, the kinetic model and associated rate constants

455

reported in this study could provide insight into the rate and extent of interactions between

456

other quinone members and metals that have different electrophilicity and redox potentials.

457

On the other hand, while quinone moieties are recognized to dominate the redox properties of

458

NOM, care must be taken when interpreting the results observed in pure quinone solutions to

459

the natural aquatic environment, where the presence of an extended network of complexing

460

organic ligands would influence the reactivity of various iron species and the stability of

461

radicals. For example, similarities and differences in iron redox transformations between a

462

simple quinone and the quinone moieties presented in SRFA were identified in recent studies

463

by Jiang et al.73 and Garg, et al.,18 where increased stability of semiquinone radicals were

464

found in SRFA solution compared to that in pure hydroquinone solution, indicating quinone

465

moieties presented in highly conjugated and substituted environment of NOM will play a

466

much more important role in the rate and extent of metal redox transformations.



467

4. ACKNOWLEDGEMENTS

468

The work reported here is supported as part of the Sustainable Systems (SS) Scientific Focus

469

Area (SFA) program at LBNL, funded by the U.S. Department of Energy, Office of Science,

470

Office of Biological and Environmental Research, Subsurface Biogeochemical Research

471

Program, under Award Number DE-AC02-05CH11231. The authors gratefully acknowledge

472

the assistance from Dr. Christopher Miller at the University of New South Wales (Australia)

473

for the model sensitivity calculations.

474

ASSOCIATED CONTENT

475

Supporting Information

476

Additional detailed information on experimental methods, manipulative experiments,

477

sensitivity analysis and extra experimental data and calculation. This material is available

478

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

479 480 481 482 483


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2. Aeschbacher, M.; Graf, C.; Schwarzenbach, R. P.; Sander, M., Antioxidant properties of humic substances. Environ. Sci. Technol. 2012, 46, (9), 4916-4925.

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63. Zafiriou, O. C., Chemistry of superoxide ion-radical (O2-) in seawater. I. pKasw* (HOO) and uncatalyzed dismutation kinetics studied by pulse radiolysis. Mar. Chem. 1990, 30, 31-43.

653 654 655

64. Yuan, X.; Pham, A. N.; Xing, G.; Rose, A. L.; Waite, T. D., Effects of pH, chloride, and bicarbonate on Cu(I) oxidation kinetics at circumneutral pH. Environ. Sci. Technol. 2012, 46, (3), 1527-1535.

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65. Adams, G.; Michael, B., Pulse radiolysis of benzoquinone and hydroquinone. Semiquinone formation by water elimination from trihydroxy-cyclohexadienyl radicals. Trans. Faraday Soc. 1967, 63, 1171-1180.

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66. Roginsky, V.; Barsukova, T., Kinetics of oxidation of hydroquinones by molecular oxygen. Effect of superoxide dismutase. J. Chem. Soc., Perkin Trans. 2 2000, (7), 1575-1582.

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67. Kitagawa, S.; Fujisawa, H.; Sakurai, H., Scavenging effects of dihydric and polyhydric phenols on superoxide anion radicals, studied by electron-spin-resonance spectrometry. Chem. Pharm. Bull. (Tokyo) 1992, 40, (2), 304-307. 27 ACS Paragon Plus Environment


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68. Deeble, D. J.; Parsons, B. J.; Phillips, G. O.; Schuchmann, H.-P.; Von Sonntag, C., Superoxide radical reactions in aqueous solutions of pyrogallol and N-propyl gallate: The involvement of phenoxyl radicals. A pulse radiolysis study. Int. J. Radiat. Biol. 1988, 54, (2), 179-193.

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69. Page, S. E.; Kling, G. W.; Sander, M.; Harrold, K. H.; Logan, J. R.; McNeill, K.; Cory, R. M., Dark Formation of Hydroxyl Radical in Arctic Soil and Surface Waters. Environ. Sci. Technol. 2013, 47, (22), 12860-12867.

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70. Buerge, I. J.; Hug, S. J., Influence of Organic Ligands on Chromium(VI) Reduction by Iron(II). Environ. Sci. Technol. 1998, 32, (14), 2092-2099.

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71. Postma, D.; Appelo, C. A. J., Reduction of Mn-oxides by ferrous iron in a flow system: column experiment and reactive transport modeling. Geochim. Cosmochim. Acta 2000, 64, (7), 1237-1247.

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73. Jiang, C.; Garg, S.; Waite, T. D., Hydroquinone-mediated redox cycling of iron and concomitant oxidation of hydroquinone in oxic waters under acidic conditions: Comparison with iron-natural organic matter interactions. Environ. Sci. Technol. 2015.

682 683 684 685 686 687 688 689 690


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Page 29 of 34


Table 1. Modeled reactions and rate constants of MH2Q oxidation by Fe(III)

691

692 693

a

Unless noted otherwise.

694 695 696 697 698 699 29 ACS Paragon Plus Environment


Page 30 of 34

700

Figure Captions

701

Figure 1. Anaerobic oxidation of 500 nM MH2Q by Fe(III) at pH 5, () [Fe(III)]0 = 100 nM,

702

() [Fe(III)]0 = 250 nM, and () [Fe(III)]0 = 500 nM. Error bars (some of which are too

703

small to be visible) are the standard errors from triplicate measurements, and solid lines

704

represent the model fit using reactions 1 – 5 (Table 1).

705

Figure 2. Generation of Fe(II) from anaerobic oxidation of MH2Q by Fe(III) at different pH,

706

[MH2Q]0 = 500 nM, () [Fe(III)]0 = 250 nM at pH 4, () [Fe(III)]0 = 500 nM at pH 4, ()

707

[Fe(III)]0 = 500 nM at pH 5, and () [Fe(III)]0 = 500 nM at pH 6. Error bars (some of which

708

are too small to be visible) are the standard errors from triplicate measurements, and solid

709

lines represent the model fit using reactions 1 – 5 (Table 1).

710

Figure 3. Aerobic oxidation of MH2Q by Fe(III) at different pH. (A) – (C): [MH2Q]0 = 500

711

nM at (A) pH4, (B) pH5 and (C) pH6 with () [Fe(III)]0 = 100 nM, () [Fe(III)]0 = 250 nM

712

and () [Fe(III)]0 = 500 nM; (D): [Fe(III)]0 = 500 nM at (◇) pH 4 with [MH2Q]0 = 5 µM,

713

() pH 5 with [MH2Q]0 = 5 µM, () pH 6 with [MH2Q]0 = 5 µM, and () pH6 with

714

[MH2Q]0 = 10 µM. Error bars (some of which are too small to be visible) are the standard

715

errors from triplicate measurements, and solid lines represent the model fit using reactions 1

716

– 9 (Table 1).

717

Figure 4. Generation of Fe(II) from aerobic oxidation of MH2Q by Fe(III) at pH 6, [MH2Q]0

718

= 500 nM with () [Fe(III)]0 = 100 nM, () [Fe(III)]0 = 250 nM and () [Fe(III)]0 = 500

719

nM. Error bars (some of which are too small to be visible) are the standard errors from

720

triplicate measurements, and solid lines represent the model fit using reactions 1 – 9 (Table 1).

721

Figure 5. Generation of H2O2 from aerobic oxidation of MH2Q by Fe(III) at different pH,


Page 31 of 34


722

[MH2Q]0 = 500 nM and [Fe(III)]0 = 500 nM at () pH 4, () pH 5 and () pH 6. Error bars

723

(some of which are too small to be visible) are the standard errors from triplicate

724

measurements, and solid lines represent the model fit using reactions 1 – 9 (Table 1).

725



726

Page 32 of 34

Figure 1

250

[M B Q ], n M

200 150 100 50 0 0

5

10

15

20

25

30

T im e , m in

727 728 729

Figure 2

500

[ F e ( II) ] , n M

400 300 200 100 0 0

730

5

10

15

20

T im e , m in

731 732 733 32 ACS Paragon Plus Environment

25

Page 33 of 34

734


Figure 3

500

500

(A )

(B )

400

[M B Q ], n M

[M B Q ], n M

400 300 200 100

300 200 100

0

0 0

30

60

90

120

150

180

0

30

60

90

T im e , m in

500

10000

(C )

300 200 100

180

(D )

6000 4000 2000

0

0 0

30

60

90

120

150

180

0

20

T im e , m in

40

60

Figure 4

500

[ F e ( II) ] , n M

400 300 200 100 0 0

80

T im e , m in

735

737

150

8000

[M B Q ], n M

[M B Q ], n M

400

736

120

T im e , m in

30

60

90

120

150

T im e , m in

738 739 33 ACS Paragon Plus Environment

180

100

120


740

Page 34 of 34

Figure 5

300

[ H 2 O 2 ], n M

250 200 150 100 50 0 0

10

20

30

40

50

T im e , m in

741 742 743 744 745

Table of Contents Graphic

746


60

Iron-Mediated Oxidation of Methoxyhydroquinone under Dark

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