Oxidation of a Dimethoxyhydroquinone by Ferrihydrite and Goethite

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Oxidation of a dimethoxyhydroquinone by ferrihydrite and goethite nanoparticles: iron reduction versus surface catalysis Lelde Krumina, Gry Lyngsie, Anders Tunlid, and Per Persson Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02292 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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Environmental Science & Technology

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Oxidation of a dimethoxyhydroquinone by ferrihydrite and goethite nanoparticles: iron

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reduction versus surface catalysis

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Lelde Krumina,1,2 Gry Lyngsie,1 Anders Tunlid,2 Per Persson1,2*

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Centre of Environmental and Climate Research, Lund University, SE-223 62, Lund, Sweden

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Department of Biology, Lund University, SE-223 62, Lund, Sweden

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*Corresponding author:

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E-mail: [email protected]

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Phone: +46 46-222 17 96

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ACS Paragon Plus Environment

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Abstract

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Hydroquinones are important mediators of electron transfer reactions in soils with a capability

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to reduce Fe(III) minerals and molecular oxygen, and thereby generating Fenton chemistry

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reagents. This study focused on 2,6-dimethoxy hydroquinone (2,6-DMHQ), an analogue to a

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common fungal metabolite, and its reaction with ferrihydrite and goethite under variable pH

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and oxygen concentrations. Combined wet-chemical and spectroscopic analyses showed that

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both minerals effectively oxidized 2,6-DMHQ in the presence of oxygen. Under anaerobic

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conditions the first-order oxidation rate constants decreased by one to several orders of

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magnitude depending on pH and mineral. Comparison between aerobic and anaerobic results

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showed that ferrihydrite promoted 2,6-DMHQ oxidation both via reductive dissolution and

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heterogeneous catalysis while goethite mainly caused catalytic oxidation. These results were

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in agreement with changes in the reduction potential (EH) of the Fe(III) oxide/Fe(II)aq redox

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couple as a function of dissolved Fe(II) where EH of goethite was lower than ferrihydrite at

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any given Fe(II) concentration, which makes ferrihydrite more prone to reductive dissolution

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by the 2,6-DMBQ/2,6-DMHQ redox couple. This study showed that reactions between

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hydroquinones and iron oxides could produce favorable conditions for formation of reactive

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oxygen species, which are required for non-enzymatic Fenton-based decomposition of soil

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organic matter.

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Table of content (TOC):

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Introduction

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Quinones are redox-active compounds that occur in three different oxidation states, which are

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coupled via one-electron transfer reactions. These redox states determine whether the quinone

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acts as an electron acceptor (quinone, Q), electron donor (hydroquinone, H2Q) or as an

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intermediate semiquinone radical (SQ-). These unique redox properties make quinones

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effective electron shuttles in a range of biological and soil processes.1–4 Quinones have been

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shown to be present in both soil and aquatic natural organic matter as a result of the

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decomposition of organic litter material.5 They are also biosynthesized by a number of

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microbes, plants and insects.6–8 The occurrence and redox chemistry of quinones have been

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thoroughly reviewed by Uchimiya and Stone.9,10

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Quinones take part in the degradation of wood via the so-called brown-rot mechanism

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mediated by several different fungi.11–14 This mechanism involves hydroxyl radicals (OH)

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generated by a reaction between Fe(II) and H2O2 (the Fenton reaction), and in this context the

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hydroquinone plays a dual role. It acts as a reductant towards Fe(III) producing Fe(II), and

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produces a superoxide radical (OOH) when oxidized by O2. This radical is also generated

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when the intermediate semiquinone or Fe(II) reacts with O2. The superoxide may

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subsequently dismutate and form H2O2,13,15 or react with Fe(III) to generate Fe(II).16,17 Hence,

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the hydroquinone has the potential to produce both Fenton reagents.

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The hydroxyl radicals generated by the Fenton reaction can degrade a wide range of organic

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compounds, including cellulose and partially also lignin. In brown-rot wood degradation the

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fungal metabolite identified for this purpose, so far, is the 2,5-dimethoxyhydroquinone (2,5-

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DMHQ). 2,5-DMHQ has been found in three distantly related species of brown-rot fungi, i.e.

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Gloeophyllum trabeum,12 Postia placenta13 and recently Serpula lacrymans.14 These findings

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suggest that the pathways for synthesizing 2,5-DMHQ were present in a common ancestor; in

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turn suggesting that 2,5-DMHQ is a rather widespread fungal metabolite. While the


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degradation of wood by OH has been thoroughly studied less is known about these processes

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in soil and their potential contribution to decomposition of soil organic matter (SOM). It has

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been shown that SOM is decomposed by an ectomycorrhizal fungus (Paxillus involutus) via

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non-enzymatic radical reactions.18 Since ectomycorrhizal fungi are common in boreal soils

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this suggests that radical-based reactions triggered by extracellular metabolites can have a

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substantial influence on the turnover of SOM. In order to assess the potential contribution

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from radical-based SOM degradation in complex soil environments, the reactions between

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extracellular metabolites and soil components have to be investigated and characterized. In

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the case of hydroquinones three main oxidation pathways has to be considered: (1)

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autoxidation to quinones by O2 present in the system; (2) catalytic oxidation by transition

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metal catalysts, similar to autoxidation but promoted by catalysts; (3) coupled hydroquinone

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oxidation and metal reduction. These are represented by the following overall reactions:

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(1) Autoxidation: H2Q + O2 → Q + H2O2

O2-dependent ௖௔௧௔௟௬௦௧

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(2) Redox metal as catalyst: H2Q + O2 ሱۛۛۛۛሮ Q + H2O2

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(3) Reductive dissolution: H2Q + 2FeOOH(s) → Q + 2Fe(II)aq + 4OH-

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Oxygen is the electron acceptor in the first two reactions hence these are O2-dependent while

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the third reaction involves a metal ion as electron acceptor.19 The latter reaction will thus be

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independent of the oxygen concentration.

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Earlier soil-relevant studies have primarily focused on the reactions between Fe(III)-bearing

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soil minerals and the simplest of hydroquinones, 1,4- and 1,2-hydroquinone (catechol).20–24 In

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buffered systems and anaerobic conditions these indicated a reaction stoichiometry of 1 H2Q:

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2 Fe(II): 1 Q; i.e. one molecule of H2Q produced two Fe(II) and one Q.20–23 The reaction


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mechanism proposed involves rapid hydroquinone adsorption to the mineral surface followed

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by electron transfer and formation of a semiquinone radical and an Fe(II) ion. A second

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reaction cycle is repeated with the semiquinone and another Fe(II) is released and the

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benzoquinone is formed. Comparison between ferrihydrite and goethite particle reactivity

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towards 1,4-hydroquinone showed that ferrihydrite was more reactive than goethite.

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Moreover, ferrihydrite reduction rates increased with decreasing particle size and increasing

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specific surface area.20,21

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In comparison with 1,4-hydroquinone and catechol the fungal metabolite 2,5-DMHQ has two

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additional –OCH3 groups, which increase the electron density of the aromatic system and

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thereby lowers the reduction potential. Indeed, fungi producing this extracellular metabolite

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have been shown to reduce soluble Fe(III) as well as initiating the Fenton reaction.11–13 In this

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study we explore 2,6-DMHQ, a commercially available isomer of 2,5-DMHQ, and the effect

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of the electron-donating substituents on hydroquinone reactivity towards the common Fe soil

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minerals ferrihydrite and goethite. The main objectives of the study were to determine how

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the 2,6-DMHQ oxidation pathways were affected by pH, oxygen concentrations and mineral

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property. This was accomplished by combining solution chemical analysis with in-situ IR

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spectroscopy, which probes the reactions at the water-mineral interfaces.

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Materials and methods

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Chemicals.

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dimethoxybenzoquinone, 97%) were purchased from Sigma-Aldrich (see S1.1 for quinone

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chemical properties). All quinone solutions were prepared fresh before each experiment using

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deionized water that was boiled and bubbled with N2 to remove CO2 from the starting

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solutions.

2,6-DMHQ

(2,6-dimethoxyhydroquinone,

97%),


and

2,6-DMBQ

(2,6-

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Ferrihydrite and goethite were synthesized according to procedures previously described.25

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Typical spherical ferrihydrite and needle-shaped goethite particles were identified by

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transmission electron microscopy (Figure S1) and the structural purity was confirmed by X-

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ray diffraction. The particles used in the batch experiments were not dried at any stage but

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stored under nitrogen at pH 6-7 in a fridge at 4 °C as 1.9 g/L ferrihydrite and 10.6 g/L

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goethite stock suspensions. For characterization purposes only the iron oxide particles were

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freeze-dried. The surface areas of the goethite and the ferrihydrite were estimated to 66 and

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300 m2/g, respectively (see S1.1).

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Batch experiments. The 2,6-DMHQ oxidation and reductive iron oxide dissolution as a

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function of pH and dissolved oxygen concentrations were studied by means of batch

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experiments (see S1.2). These were performed at two different approximate oxygen

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concentrations denoted aerobic and anaerobic, respectively, in the following text. The aerobic

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condition corresponds to experiments at ambient atmospheric pressure and [O2] = 270-310

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µM whereas the anaerobic experiments were performed in a glove bag filled with N2 (g) with

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[O2] = 0-20 µM. These concentrations were measured in the stock solution and initial

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representative batch samples using either an oxygen optode system (UniSense) or a

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polarographic probe (Thermo scientific Orion Star A213); note that O2 was not monitored

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during the experiments. The total 2,6-DMHQ concentrations were normalized with respect to

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the iron oxide surface area in order to obtain the same concentrations in units of µmol/m2 for

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ferrihydrite and goethite. After reaction the quinone species were analyzed by means of

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HPLC and Fe(II) in solution was determined by the Ferrozine assay. The batch experiments

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were performed in triplicates and Fe(II) was measured in each replicate, while HPLC was

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performed on either duplicates or on one of these replicates only (see S1.2).

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IR spectroscopy. IR spectra as a function of time at fixed pH values were collected in-situ

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using a modified version of the simultaneous infrared and potentiometric method previously


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reported by Loring et al.26 and Krumina et al.25 (see S1.3). The IR spectral data sets were

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analyzed by means of a multivariate curve resolution with alternating least squares (MCR-

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ALS) Matlab script following the procedures described by Jaumot et al.27 This procedure

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decomposed the data sets into component spectra contributing to the spectral variation and

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their corresponding concentration profiles. The data sets were prepared for MCR-ALS

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analysis using the script of Felten et al.,28 which included asymmetric least squares smoothing

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baseline correction, and the final analysis was carried out with the script by Jaumot et al.27

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Density functional theory (DFT) calculations. Geometry optimization and frequency

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calculations were performed for 2,6-DMHQ and 2,6-DMBQ as well as the corresponding

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semiquinone both in a protonated and unprotonated state. We employed density functional

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theory (DFT) using the hybrid functionals B3LYP and the standard 6-31++G(d,p) basis set.

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Solvation effects were modeled using different numbers of explicit water molecules. The

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calculations and visualizations were performed with the program Spartan ‘14 by

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Wavefunction Inc.

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Results

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2,6-DMHQ oxidation by ferrihydrite and goethite. The batch experiments showed that

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ferrihydrite and goethite particles significantly enhanced the 2,6-DMHQ oxidation rates as

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compared to autoxidation under aerobic conditions in solution (Figure 1). Moreover, the pH

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dependence of the oxidation was similar for both minerals, and displayed increased aerobic

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oxidation rates when pH was raised from 4.5 to 7.0. The initial rates of 2,6-DMHQ oxidation

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approximately followed first-order kinetics (Figure S4). The first-order rate constants

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highlighted the large effects of the iron oxide surfaces at aerobic conditions, increasing the

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rate constants by ca. 1-2 orders of magnitude as compared to autoxidation (Table 1). The


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initial ferrihydrite rates were faster than the corresponding goethite ones at all conditions

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investigated. This indicated a higher reactivity of ferrihydrite, but at pH 4.0 and 4.5 there

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might also be an effect from a greater pH drift in presence of ferrihydrite (see Table S2) that

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will speed up the 2,6-DMHQ oxidation. The 2,6-DMHQ oxidation was significantly slower at

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anaerobic conditions, and this effect was most pronounced for goethite. At pH 4.5 both

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minerals accomplished complete aerobic oxidation after 4 h, whereas only ca. 80% or 20%

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was oxidized by ferrihydrite and goethite, respectively, during the same time at anaerobic

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conditions.

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We also observed reduced 2,6-DMHQ oxidation efficiency as a function of ferrihydrite age

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(Figure S5). This was consistent with previously detected aging and phase transformation

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effects in ferrihydrite suspensions.29 These effects were not observed in the goethite

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experiments (Figure S5), which is consistent with the thermodynamic stability of this phase.

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Figure 1. Oxidation of 2,6-DMHQ (520 µM, 1.5 µmol/m2, corresponding to 1.9 g/L and 10.6

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g/L for ferrihydrite and goethite, respectively) as a function of time in presence and absence


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of iron oxide particles. pH 7.0 aerobic,

pH 4.0 aerobic,

pH 4.5 aerobic,

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pH 4.5 anaerobic,

pH 7.0 anaerobic. At pH 4.0 and 4.5 in the presence ferrihydrite

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pH was continuously adjusted during the experiment with 40 mM HCl or 40 mM NaOH in

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0.1 M NaCl. No significant pH drift was observed in any experiment at pH 7.0 or in the

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presence of goethite. Ferrihydrite experiments were performed with particles aged for 2

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weeks. Experimental uncertainties are exemplified by duplicates analyzed in some

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experimental series.

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Table 1. First order rate constants, k (in min-1), of 2,6-DMHQ oxidation in presence and

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absence of iron oxides.* Ferrihydrite

Goethite

Autoxidation

kpH4, aerobic

1.5 ×10-1

2.0 ×10-2

6.8 ×10-4

kpH4.5, aerobic

1.2 ×10-1

3.0 ×10-2

7.2 ×10-4

kpH4.5, anaerobic

2.2 ×10-2

1.6 ×10-3

kpH7, aerobic

3.2 ×10-1**

1.8 ×10-1

kpH7, anaerobic

8.5 ×10-4

3.3 ×10-4

2.0 ×10-2

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*The rate constants were calculated from the initial points in Figure 1 obeying first-order kinetics.

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**Calculated from an experiment with ferrihydrite aged for 4 months in order to capture the fast 2,6-DMHQ oxidation (see Figure S4).

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Reaction products of 2,6-DMHQ oxidation. In presence of both 2,6-DMHQ and 2,6-

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DMBQ HPLC detected low concentrations of a third species that were assigned to the

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semiquinone (Figure S6). We base our assignment on the transient nature of this component,

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which is correlated to the co-existence of the hydroquinone and the quinone, and the presence

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in pure aqueous solution during autoxidation (Figure S6). This behavior of semiquinones is

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well documented in the literature and from previous results semiquinone concentrations in the

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micro-molar range are not unrealistic under our experimental conditions.30 When all

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hydroquinone was oxidized only the benzoquinone was detected. However, the complete

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aerobic oxidation of 2,6-DMHQ by the iron oxides (Figure 1) did not always result in a 100%

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recovery of 2,6-DMBQ (Figure 2). Separate IR experiments showed that adsorption of 2,6-

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DMBQ onto the iron oxide surfaces was negligible (data not shown), and accordingly the

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incomplete recovery of 2,6-DMBQ indicated side reactions. The fact that these products were

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not detected by HPLC suggested strong affinities for the iron oxide surfaces, and thus

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enrichment at the water-mineral interface. The side reactions were most significant at low pH


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whereas the 2,6-DMBQ recovery was almost complete at pH 7.0 in presence of both iron

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oxides (Figure 2). The results supported previous suggestions that a range of different reaction

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products, such as dimers, trimers and other polymers, aldehydes and even CO2, can form as a

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result of the reactions between hydroquinones or phenolics and mineral surfaces at aerobic

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reaction conditions.24,31–41

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Figure 2. Sum of the 2,6-DMHQ and 2,6-DMBQ concentrations after 240 minutes, expressed

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as the percentage of the total 2,6-DMHQ concentrations added in the batch experiments.

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Iron reduction. Under aerobic conditions Fe(II) concentrations generated by reductive

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dissolution were significantly lower than the amounts of 2,6-DMHQ oxidized (Figure 1 and

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3), and thus far from the theoretical 2 Fe:1 hydroquinone limit of iron oxide reduction by 2,6-

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DMHQ (Reaction 3 and Figure S7). Merely 30-40 µM Fe(II) was produced aerobically from

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goethite by 520 µM 2,6-DMHQ at pH 4.5, this was increased to ca. 70 µM at pH 4.0. A 70

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µM Fe(II) concentration was also obtained under anaerobic conditions at pH 4.5. This pattern

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of increasing Fe(II) concentration at low O2 was also observed when a lower total

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concentration of 2,6-DMHQ was employed (90 µM, 0.4 µmol/m2) (Figure 3). In general, the

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low Fe(II) concentrations together with the strong dependence of 2,6-DMHQ oxidation on O2


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concentration (Figure 1) indicated minor contribution from reductive dissolution (Reaction 3)

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and a predominance of catalytic oxidation (Reaction 2) in the presence of goethite. Note that

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the Fe(II) concentration may underestimate the reductive dissolution somewhat due to re-

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adsorption and/or re-oxidation (Figure S8). At pH 7.0 no Fe(II) was detected in solution in

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presence of goethite.

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Reduction of ferrihydrite at pH 4.0 and 4.5 yielded substantially higher Fe(II) concentrations

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as compared to goethite, but again these concentrations were close to or below the detection

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limit at pH 7.0 (Figure 3). The adsorption of Fe(II) onto ferrihydrite at pH 4.0 and 4.5 has

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been reported to be very low and correlated to the re-oxidation.42 Under our experimental

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conditions in absence and presence of ferrihydrite at pH 4.5 and absence of buffers the loss of

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Fe(II), either via adsorption or re-oxidation, was slow (Figure S8). Thus, at pH 4.0 and 4.5 the

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dissolved Fe(II) concentration was a quantitative measure of the extent of reductive

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dissolution. As shown by the ratios between Fe(II) produced and 2,6-DMHQ consumed

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(Figure S7) and the decrease in dissolved Fe(II) at increasing O2 (Figure 3), the reductive

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process was predominating at anaerobic conditions whereas the competition from catalytic

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oxidation of 2,6-DMHQ increased in the presence of O2. Furthermore, comparison between

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the rates of 2,6-DMHQ consumption and Fe(II) production showed that catalytic oxidation is

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faster than reductive dissolution (Figure S9).

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Figure 3. Fe reductive dissolution by 2,6-DMHQ in 0.1 M NaCl from ferrihydrite (left) and

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goethite (right).

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aerobic,

520 µM 2,6-DMHQ at pH 4.5 anaerobic,

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aerobic,

90 uM 2,6-DMHQ at pH 4.5 anaerobic,

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and anaerobic conditions. At pH 4.0 and 4.5 in the presence ferrihydrite pH was continuously

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adjusted during the experiment with 40 mM HCl or 40 mM NaOH in 0.1 M NaCl. No

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significant pH drift was observed in any experiment at pH 7.0 or in the presence of goethite.

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IR spectroscopy. The IR spectra of ferrihydrite reacted with 2,6-DMHQ at pH 4.5 and 7.0

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showed marked changes with time indicating at least two predominant surface species with

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different reaction kinetics (Figure S10). MCR analyses of these data sets resolved two main

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components (Figure 4). One component (C1), characterized by the bands at 1385 and 1532

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cm-1, displayed a steady increase and was completely dominating at the end of the

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experiment. At this point 2,6-DMBQ is the dominating quinone of the triad under aerobic

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conditions (Figure 1 and 2). Additional experiments performed with 2,6-DMBQ at identical

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total concentration produced IR data with no discernible bands implying that 2,6-DMBQ has

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very low affinity for the iron oxide surfaces. Therefore, the 1385 and 1532 cm-1 bands likely

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originated from the products of the side-reactions indicated by the incomplete 2,6-DMBQ

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recovery (Figure 2). Note that although the positions and relative intensities of these bands

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were very similar at all investigated conditions the C1 spectra contained bands that varied

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between the experiments e.g. at 1595 cm-1 and between 1000-1250 cm-1. This can either be

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due to incomplete separation from the C2 component discussed below or that range of

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additional side-products are formed depending on the experimental conditions. In any case,

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the strong contribution of the 1385 and 1532 cm-1 bands to the IR spectra showed that this

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side-product has a high affinity for the iron oxide surfaces.

520 µM 2,6-DMHQ at pH 4 aerobic,

520 µM 2,6-DMHQ at pH 4.5 90 µM 2,6-DMHQ at pH 4.5 520 µM 2,6-DMHQ at pH 7.0


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The second MCR component (C2) was the predominant surface species during the first 20-30

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minutes, and the C2 spectra varied as a function of the experimental conditions (Figure 4). At

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pH 7 and aerobic conditions the C2 spectrum closely resembled that of 2,6-DMHQ(aq) as well

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as the DFT calculated IR spectrum of hydrated 2,6-DMHQ (Figure S11). On the other hand,

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the C2 spectrum was distinctly different from the DFT-calculated IR spectra of the

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semiquinone and quinone species (cf. Figure S11 and S12). Accordingly, these results

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indicated adsorption of intact 2,6-DMHQ. The small band shifts as compared to 2,6-

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DMHQ(aq) mainly concerned complex vibrational motions involving hydrogens (Table S3)

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and we attribute these shifts to interactions between the OH groups of the neutral

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hydroquinone and the positively charged hydrated surface. At pH 4.5 and aerobic conditions

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the C2 spectrum lost the band at 1595 cm-1 and the main in this region band now appeared

276

around 1500 cm-1 (Figure 4). The computed IR spectra of the quinone triad indicated that this

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shift was caused by the presence of semiquinones (Figure S12), and the overall spectral

278

features of the C2 spectrum resembled those of the deprotonated semiquinone, or a mixture

279

with the protonated form (Figure S13). The detection of the semiquinone was also in

280

agreement with the HPLC results that suggested the existence of the semiquinone at pH 4.5

281

and aerobic conditions during the first part of the experiment (Figure S6). Under anaerobic

282

conditions at pH 4.5 the C2 spectrum was more similar to hydrated 2,6-DMHQ but with some

283

contributions from the semiquinone. Hence, indicating a shift in distribution between the

284

hydroquinone and the semiquinone induced by the low oxygen concentration.

285

IR spectra collected at low 2,6-DMHQ concentration identical to batch conditions (1.5

286

µmol/m2 ferrihydrite) yielded similar results but with different MCR concentration profiles

287

(Figure S14). Under these conditions it was notable that C2 decayed faster (in agreement with

288

Figure 1) whereas the behavior of C1, assigned to the side-reaction products, was in all

289

respects very similar to that observed at the high 2,6-DMHQ concentration.


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IR spectra of 2,6-DMHQ reacted with goethite were of poor quality and allowed only

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qualitative identification of the adsorbed hydroquinone and the bands at 1385 and 1532 cm-1

292

originating from the oxidation side-products (Figure S15). The low signal-to-noise was most

293

likely a result of the larger particle size of goethite as compared to ferrihydrite. This resulted

294

in a situation where a smaller number of adsorbed molecules in the ATR overlayers were

295

exposed to the IR light in the case of goethite at similar surface coverage.

296 297

Figure 4. Multivariate curve resolution analysis of IR spectra of ferrihydrite during reaction

298

with 2,6-DMHQ (total concentration = 9.9 µmol/m2). The IR data sets contained spectra were

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collected every minute for approximately 140 minutes. The estimated MCR concentration

300

profiles and the corresponding spectra are coded using the same color and line style.

301

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Discussion


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Oxygen effects. A striking result from the present study was the strong oxygen-dependence

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of the iron oxide-mediated 2,6-DMHQ oxidation (Figure 1). The consumption of 2,6-DMHQ

305

is due to oxidation by either Fe(III) or O2. Thus, under anaerobic conditions the Fe(III)-driven

306

reaction should predominate, and comparison between anaerobic and aerobic conditions

307

facilitates distinction between oxidation via iron reductive dissolution and surface catalysis.

308

Under all anaerobic conditions the first order oxidation rate constant decreased by 1 to 3

309

orders of magnitude as compared to aerobic oxidation, with the largest relative effects at pH

310

7.0 (Figure 1 and Table 1). It follows that catalytic oxidation is significant in the presence of

311

oxygen. These results contrast those in a recent study on oxidation of 2-methoxyhydroquinone

312

by low concentrations of soluble Fe(III) in aqueous solution that clearly showed similar and

313

rapid oxidation rates both at aerobic and anaerobic conditions.43 This comparison emphasizes

314

the importance of catalytic reactions in the presence of surfaces.

315

At pH 4.5 the difference in extent of 2,6-DMHQ aerobic and anaerobic oxidation was larger

316

for goethite than for ferrihydrite; both iron oxides caused complete aerobic oxidation after 4 h

317

while ca. 20% and 80% 2,6-DMHQ was remaining in presence of ferrihdyrite and goethite,

318

respectively, under aerobic conditions (Figure 1). Ferrihydrite mediated both Fe(III) reduction

319

and surface catalysis whereas Fe(III) reduction only played a minor role at goethite surfaces.

320

At pH 7.0 both iron oxides displayed an even more dramatic decrease in 2,6-DMHQ

321

oxidation rates when oxygen was excluded (Table 1). These results corroborated the minor

322

contribution at pH 7.0 from a mechanism involving Fe(III) reduction followed by rapid Fe(II)

323

adsorption and/or re-oxidation because in this case 2,6-DMHQ oxidation should have been

324

substantial also under anaerobic conditions. Moreover, these findings once more emphasized

325

the difference between oxidation in homogeneous solution and at iron oxide surfaces. Yuan et

326

al.43 showed that molecular oxygen did not contribute to hydroquinone oxidation in the

327

presence of aqueous Fe(III), but played an indirect role via the formation of OOH, by Fe(II)


17


Page 18 of 31

328

or semiquinone oxidation. Subsequently, the superoxide oxidized the hydroquinone. This

329

mechanism requires the formation Fe(II) and the semiquinone through Fe(III) reduction,

330

which was occurring only at a slow rate under our anaerobic conditions, if it occurred at all

331

(note that our experimental conditions were not strictly anaerobic and the slow oxidation

332

could be due to remaining low levels of dioxygen). Hence, Fe(III) reduction would be the

333

rate-limiting step also under aerobic conditions if oxidation is driven by the superoxide and

334

therefore cannot explain the very rapid reaction at pH 7.0 in the presence of both iron oxides.

335

This has to involve a heterogeneous catalytic reaction between 2,6-DMHQ and molecular

336

oxygen.

337

Reaction mechanisms. At pH 7.0 IR spectra indicated a rapid initial adsorption of intact 2,6-

338

DMHQ onto ferrihydrite (Figure 4 and S11), and at the same time 2,6-DMHQ was aerobically

339

oxidized via a surface catalytic reaction. This catalyzed oxidation of 2,6-DMHQ is in many

340

ways similar to autoxidation involving a reaction between the hydroquinone and molecular

341

oxygen. In the case of autoxidation, the rate has been correlated to the pKa values of different

342

hydroquinones i.e. the rate of oxidation increases with decreasing pKa values, and from this

343

follows that oxidation rates also increases with increasing pH.19 This was also shown by our

344

results where the rate constant of autoxidation increased from 6.8×10-4 min-1 at pH 4 to

345

2.0×10-2 min-1 at pH 7.0 (Table 1). It follows that any interaction that lowers the pKa values

346

and thereby promotes deprotonation of the hydroquinone potentially will increase the rate of

347

oxidation. Previous studies have shown that deprotonated forms of organic acids adsorbed to

348

iron oxides are stabilized, and the pKa values of the interfacial species are lowered

349

substantially compared to bulk solution.44–47 Accordingly, we propose that a contributing

350

factor to the catalytic oxidation at the iron oxide surfaces is a downward shift of the 2,6-

351

DMHQ pKa values resulting from the hydroquinone accumulation at the positively charged

352

interface. The oxidation rate may be further increased by electronic structure changes of


18

Page 19 of 31


353

hydroquinone surface species and/or molecular oxygen induced by interactions with Fe(III).48

354

The predicted high pKa values of 2,6-DMHQ (10.80 and 12.79) implies that increased

355

oxidation rates are achieved without predominance of the deprotonated forms at pH 7.0; i.e.

356

pKa values can shift and oxidation rates increase and still the deprotonated forms will only be

357

small fractions of the total hydroquinone concentration. It is therefore not surprising that our

358

IR spectra indicated the predominance of protonated 2,6-DMHQ on ferrihydrite (Figure 4, C2

359

dotted green).

360

At pH 4.5 the aerobic oxidation rates on ferrihydrite and goethite were slower than at pH 7.0,

361

thus following the same pH trend as autoxidation, whereras the relative effect by the iron

362

oxide surfaces as compared to autoxidation was greater at pH 4.5 than at pH 7 (Table 1).

363

Under these mildly acidic conditions ferrihydrite caused both catalytic oxidation and

364

oxidation via reductive dissolution while goethite mainly acted as a catalyst (Figure 1). IR

365

spectra of ferrihydrite indicated initial predominance of an adsorbed semiquinone when both

366

catalytic and reductive dissolution processes were active (Figure 4, C2 dotted blue). Thus,

367

adsorption to the iron oxide surface could play a role in further stabilizing the semiquinone.

368

Under anaerobic conditions at pH 4.5 the surface speciation on ferrihydrite changed and intact

369

2,6-DMHQ predominated, similar to the observations at pH 7.0 (Figure 4, C2 dotted red).

370

This difference in surface speciation between aerobic and anaerobic conditions suggested that

371

surface-catalyzed oxidation of adsorbed 2,6-DMHQ is a rapid process maintaining the surface

372

concentration of 2,6-DMHQ at a low level in presence of oxygen. The fact that we detected

373

adsorbed semiquinone also indicated that oxidation of this species is comparatively slow.

374

Furthermore, the shift to predominance of adsorbed 2,6-DMHQ under anaerobic conditions is

375

in agreement with a slower 2,6-DMHQ oxidation via reductive dissolution than via surface

376

catalysis (Table 1). This explains why the initial oxidation of 2,6-DMHQ is faster than the


19


Page 20 of 31

377

release of dissolved Fe(II) from both ferrihydrite and goethite under aerobic conditions at pH

378

4.5 (Figure S9).

379

Generation of dissolved Fe(II). A consequence of the parallel catalytic and reductive

380

dissolution pathways of aerobic 2,6-DMHQ oxidation was that Fe(II) concentrations

381

generated via the latter process were far from the limiting value of one oxidized 2,6-DMHQ

382

per 2 Fe(II) produced, predicted by reaction (3) (Figure 1 and 3, S7). Under anaerobic

383

conditions and low pH the concentrations were closer to this ratio, but the reaction slowed

384

down or even stopped at substantial concentrations of 2,6-DMHQ remaining in the

385

suspensions (Figure 1). In order to reconcile these Fe(II) results as well as differences

386

between ferrihydrite and goethite we need to consider the reduction potentials (EH) of the

387

reactants and their variations with experimental conditions. In a recent study Gorski et al.49

388

convincingly showed how EH of the Fe(III) oxide/Fe(II)aq redox couple was decreased as a

389

function of increasing Fe(II)aq concentration. At the same time EH of the 2,6-DMBQ/2,6-

390

DMHQ redox couple will increase as a result of the oxidation according to the Nernst

391

equation, and at some point EH(2,6-DMBQ/2,6-DMHQ) will be higher than EH(Fe(III)

392

oxide/Fe(II)aq) and the reductive dissolution will stop. In Figure 5 we have compared the

393

change in EH(Fe(III) oxide/Fe(II)aq) to EH(2,6-DMBQ/2,6-DMHQ) calculated from the

394

concentrations of these quinone species after 240 minute reaction time under anaerobic

395

conditions (Figure 1). At pH 4.5 in the presence of goethite EH(2,6-DMBQ/2,6-DMHQ) is

396

indicated to cause reductive dissolution when the Fe(II) concentration is below ca. 100 µM.

397

This explains why the Fe(II) concentration reaches a limiting value around 80 µM, despite the

398

fact that ca. 400 µM 2,6-DMHQ still remains in solution, because the difference in EH values

399

between the redox couples does not produce the thermodynamic driving force needed for

400

reductive dissolution. In contrast, in presence of ferrihydrite EH(Fe(III) oxide/Fe(II)aq) is

401

sufficiently positive for reductive dissolution to occur also at milli-molar Fe(II)


20

Page 21 of 31


402

concentrations, which is indicated also by the continued increase of the Fe(II) concentration

403

(Figure 3). Here, the reaction rate is slowed down as a result of the low remaining 2,6-DMHQ

404

concentration, and the dissolution will eventually stop since this concentration will become

405

too low.

406

At pH 7.0 under anaerobic conditions the same general trends are observed, but in this case,

407

EH(Fe(III) oxide/Fe(II)aq) drops below that of EH(2,6-DMBQ/2,6-DMHQ) at lower Fe(II)

408

concentrations (Figure 5B) Therefore reductive dissolution is less efficient at pH 7.0 as

409

compared to pH 4.5 under anaerobic conditions, and it is likely that in the presence of oxygen

410

the contribution from iron oxide reduction to the overall 2,6-DMHQ oxidation is small. We

411

conclude from this discussion that the relative importance of the reductive dissolution and

412

catalytic oxidation pathways at a given 2,6-DMHQ concentration will largely be determined

413

by EH of the iron oxides and the O2 concentration.

414

415 416

417

Figure 5. Reduction potentials (EH) of the Fe(III) oxide/Fe(II)aq redox couple as a function of

418

dissolved Fe(II) concentrations in the presence of ferrihydrite or goethite at (A) pH 4.5 and

419

(B) pH 7.0. The solid lines were calculated according to Ref. 49. The dashed lines represent

420

EH of the 2,6-DMBQ/2,6-DMHQ redox couple at the 2,6-DMHQ and 2,6-DMBQ


21


Page 22 of 31

421

concentrations after 240 minutes under anaerobic conditions calculated using the Nernst

422

equation and the EH0 value from Huynh et. al.50 (see S2.3).

423

Quinone decomposition, generation of radicals and implications to soil processes.

424

Previous studies have demonstrated that efficient degradation of aromatic molecules can be

425

accomplished by generating ROS through H2O2 decomposition using Fe-containing materials

426

as catalysts.51–53 Some of these catalytic systems are capable of ring cleavage thus converting

427

aromatic molecules into aliphatic products. A key species in these reactions is the OH, which

428

is formed via Haber-Weiss or Fenton-like processes. We conclude from this previous

429

literature that the 2,6-DMHQ oxidation side-products detected in our experiments (Figure 2)

430

probably are caused by reactions involving radicals. According to Reaction (2) the catalytic

431

oxidation of 2,6-DMHQ by the iron oxides has the potential of generating one H2O2 molecule

432

for every oxidized hydroquinone, and this reaction was most efficient in presence O2 at pH

433

7.0. It has been shown that Fe-containing solids can catalyze H2O2 decomposition and

434

generate hydroxyl radicals,54,55 but at the same time at pH 7.0 H2O2 is unstable and

435

disproportionate into O2 and H2O, a process which also is catalyzed by surfaces.54,56 Our

436

results from the pH 7.0 experiments showed that most 2,6-DMHQ was converted into 2,6-

437

DMBQ with little formation of side products, which indicated that generation of radicals at

438

this pH was inefficient. Still, the IR experiment performed at pH 7.0 and high 2,6-DMHQ

439

concentration showed small amounts of side products adsorbed to ferrihydrite indicating non-

440

negligible radical production.

441

At acidic pH values the thermodynamic stability of H2O2 increases. In addition, reductive

442

dissolution by 2,6-DMHQ is more efficient, which generates dissolved Fe(II) especially in

443

presence of ferrihydrite (Figure 3). This Fe(II) has a dual role. Together with H2O2 it will

444

generate OH via Fenton reaction and it may also contribute to the production of H2O2


22

Page 23 of 31


445

through re-oxidation and the formation of superoxide which subsequently dismutate to

446

peroxide:

447

(4) Fe(II)aq + O2 + H+ → Fe(III)s + HOO·

448

(5) 2HOO· → H2O2 + O2

449

Thus, at pH 4.5 under aerobic conditions the reactions between 2,6-DMHQ and the iron

450

oxide, produce all necessary ingredients to generate OH; H2O2 primarily via the catalytic

451

oxidation and Fe(II) via the reductive dissolution. Indeed, around 25% of 2,6-DMHQ was

452

converted into side products under these conditions in the presence of ferrihdyrite and

453

goethite (Figure 2). A decrease of O2 will lead to lower production of H2O2, and therefore the

454

lower production of oxidation side products in our anaerobic experiments (Figure 2) was

455

likely caused by H2O2 limitation.

456

Previous studies on soil remediation have demonstrated that belowground injection of H2O2

457

can effectively degrade organic contaminants, in particular in presence of iron

458

minerals.41,51,57,58 Similarly, the aggregated results from this study have shown that redox

459

reactions between dimethoxyhydroquinones, exuded by several brown-rot fungi, and iron

460

oxide minerals can produce conditions at the water-mineral interfaces that degrade organic

461

compounds. This implies that such non-enzymatic radical-based processes have the potential

462

to contribute to decomposition of soil organic matter without an external supply of H2O2. In

463

this respect, the possibility to generate Fenton chemistry at surfaces that also accumulate

464

organic matter will make the decomposition process efficient because the hydroxyl radical

465

operates at very short length-scales.

466

Acknowledgments

467

This work was supported by grants from the Swedish Research Council (VR) and the Knut

468

and Alice Wallenberg Foundation.


23


469

Supporting Information. Additional information on chemicals, experimental methods,

470

results from quantitative chemical analysis and IR spectroscopy.


Page 24 of 31

24

Page 25 of 31


471

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