Mediated Electron Transfer between FeII Adsorbed onto Hydrous

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Mediated electron transfer between FeII adsorbed onto hydrous ferric oxide (HFO) and a working electrode Annaleise R. Klein, Ewen Silvester, and Conor F. Hogan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es501561d • Publication Date (Web): 26 Aug 2014 Downloaded from http://pubs.acs.org on August 27, 2014

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Environmental Science & Technology

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Mediated electron transfer between FeII

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adsorbed onto hydrous ferric oxide (HFO) and

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a working electrode

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Annaleise R. Klein1, Ewen Silvester1* and Conor F. Hogan2

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1. Department of Environmental Management and Ecology (DEME), La Trobe

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University, Albury-Wodonga, Victoria, 3690, AUSTRALIA.

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2. Department of Chemistry, La Trobe Institute for Molecular Science, La Trobe

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University, Victoria, 3086, AUSTRALIA.

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*Corresponding Author: [email protected]; +61 2 6024 9878

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1 ACS Paragon Plus Environment


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ABSTRACT

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The redox properties of FeII adsorbed onto mineral surfaces have been highly studied

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over recent years due to the wide range of environmental contaminants that react with

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this species via abiotic processes. In this work the reactivity of FeII adsorbed onto

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hydrous ferric oxide (HFO) has been studied using ferrocene (bis-cyclopentadiene

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iron(II); Fc) derivatives as electron shuttles in cyclic voltammetry (CV) experiments.

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The observed amplification of the ferrocene oxidation peak in CV is attributed to

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reaction between the electrochemically generated ferrocenium (Fc+) ion and adsorbed

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FeII species in a catalytic process (EC’ mechanism). pH dependence studies show that

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the reaction rate increases with FeII adsorption, and is maintained in the absence of

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aqueous Fe2+, providing strong evidence that the electron transfer process involves the

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adsorbed species. The rate of reaction between Fc+ and adsorbed FeII increases with

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the redox potential of the ferrocene derivative, as expected, with bimolecular rate

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constants in the range 103 – 105 M-1 s-1. The ferrocene-mediated electrochemical

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method described has considerable promise in the development of a technique for

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measuring electron-transfer rates in geochemical and environmental systems.

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KEYWORDS Ferrocene, electron transfer, adsorbed iron(II), molecular shuttle, redox

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sensor.

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BRIEF A novel electrochemical technique using ferrocene derivatives as molecular

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shuttles probes the redox reactivity of FeII adsorbed onto hydrous ferric oxide.

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Introduction

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Over the past 20 years there has been considerable interest in the redox reactivity of

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FeII adsorbed onto oxide mineral substrates 1-3 and FeII incorporated into clay mineral

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structures 4, 5. The adsorption of FeII onto iron (oxyhydr)oxides, in particular, has been

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well studied, with the adsorbed FeII observed to participate in an electron transfer

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process with the host oxide in a process that can induce recrystallization of the

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substrate 6-10. Such processes are likely to be important in iron (oxyhydr)oxide phase

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changes in natural systems 11, 12. Adsorbed FeII retains its reactivity (although possibly

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in a different crystallographic location) and the adsorption behavior can be modelled

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as a surface complexation process 3. The high reactivity of FeII adsorbed onto FeIII

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(oxyhydr)oxides has been demonstrated for a range of oxidants, including

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nitroaromatics 1, 13, 14, chlorinated alkanes, 14-17 and uranyl (UO22+) 3, 18.

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Rest potentials for FeII adsorbed onto hydrous ferric oxide (HFO), and nano-sized

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ferric oxide can be measured using an inert metallic (Pt) electrode, and match closely

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with that predicted from surface complexation models 6. The same is not true for FeII

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adsorbed onto goethite (α-FeOOH) and lepidocrocite (γ-FeOOH); this has attributed

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to the larger particle size, and the corresponding poor electrical contact with the Pt

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electrode, leading to low electrode exchange currents. The issue of sensing the redox

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potentials of adsorbed and solid species in geochemical systems is a generic problem

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and is one of the factors that have historically prevented useful field and laboratory

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measurements 19.

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One way in which the reactivity of redox surface species can be usefully

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characterized is through reaction rate data within “families” of redox probes which

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react by a similar mechanism, such as substituted aromatic compounds 1. The use of

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solution phase redox probes to measure the redox potential of heterogeneous systems



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overcomes the issue of electrical contact with solid and adsorbed redox species. The

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ideal type of solution phase redox probes are molecules that react without surface

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coordination (i.e. outer-sphere), have reversible electrochemical properties, and have

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redox potentials that are independent of solution pH. A wide variety of outer sphere

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electron transfer reagents have been reported, with among the best known being

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ferrocene (bis-cyclopentadiene iron(II)) and its derivatives

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cyanide complexes of iron, ruthenium and osmium

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particularly attractive as they typically exhibit fast reversible one-electron transfer

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characteristics. Furthermore, there are more than 30 reported derivatives of ferrocene

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ideal candidates for use as redox sensors.

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and the 2,2-bipyridyl –

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. Ferrocene derivatives are

, with a range of solvation properties and electrochemical potentials, making them

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In this work we present an electrochemical approach for the characterization of

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surface redox species based on ferrocene (Fc) derivatives, where the oxidized form of

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ferrocene (ferrocenium; Fc+) is generated at the working electrode (WE) in a 3-

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electrode configuration and then reacts with reduced surface species (adsorbed FeII)

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adjacent to the electrode. The model suspension used in this work is the hydrous ferric

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oxide – FeII system (HFO-FeII) 6. The use of electron transfer mediators to enhance

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electron transfer between colloidal materials and a solid electrode has recently been

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used in determining electron donor (or acceptor) capacities and redox potentials of

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humic acids 23, structural FeII in clays

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conceptually similar, the approach presented here is a dynamic electrochemical

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technique, based on cyclic voltammetry that measures electron transfer rates. This

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technique is also sensitive to the surface potential of the substrate and has

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considerable promise for the development of a redox characterization method for

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environmental and geochemical applications.

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and FeII adsorbed onto goethite

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. While


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Theoretical basis

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The cyclic voltammetry (CV) approach used in this work is inspired by the glucose

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sensor electrode which operates by a similar mediated electron transfer mechanism 28,

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transfer reagents, such as ferrocenes (fast ET), and redox-active particulate materials

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(slow ET) at a working electrode (WE). It was reasoned that systems containing both

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a ferrocene compound and a redox active mineral substrate could exhibit a similar

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electrocatalytic effect (so-called EC’ mechanism) whereby ferrocenium (Fc+) is

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constantly re-generated at the WE surface whilst being converted to ferrocene (Fc) by

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the reduced surface species giving an amplification of the ferrocene oxidation peak.

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The processes involved in the EC’ mechanism are shown in Figure 1 for reaction with

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an adsorbed FeII species.

. Our approach exploits the differences in electron transfer (ET) rates of one-electron

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Figure 1

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The EC’ amplification effect can be used to extract a rate constant for the reaction

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between the ferrocenium and the particulate material, provided that the scan rate is



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sufficiently slow so that the limiting condition is achieved where the rate of electron

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transfer is not controlled by the diffusion processes. Under these conditions the shape

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of the voltammogram assumes a more sigmoidal (wave rather than peak) shape and

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the bimolecular rate constant for the ET process can be extracted from equation 1,

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where; iL is the limiting current, n is number of electrons transferred, F is Faraday

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constant (96485 C mol-1), A is the WE area (cm2), C*O is the bulk concentration of

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ferrocene mediator (mol cm-3), D is the ferrocene diffusion coefficient (cm2 s-1), k’ is

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the second order rate constant for the reaction between ferrocenium (Fc+) and the

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adsorbed species (s-1), and C*Z is the bulk concentration of the adsorbed species (mol

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cm-3) 29.

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i L = nFAC *O (Dk ' C*Z )

(1)

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Experimental section

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Synthesis of hydrous ferrous oxide (HFO). The preparation of HFO suspensions

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(10 g dm-3) with reproducible surface charging properties has been described

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previously 6, 30.

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Ferrocene derivatives. Ferrocene derivatives were obtained from commercial

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sources, including: ferrocene carboxylic acid (FcCOOH; Sigma-Aldrich), ferrocene

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methyl trimethyl ammonium iodide (FctertAm+; STREM), dimethyl amino methyl

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ferrocene (FcDA; Sigma-Aldrich), ferrocene carboxaldehdye (FcCHO; Sigma-

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Aldrich), α-hydroxy ethyl ferrocene (alphaFc; STREM), and hydroxy methyl

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ferrocene (FcCH2OH; STREM). The redox potentials (E½) of these derivatives are

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given in Table 1. E½ is defined as (Ep,ox +Ep,red)/2, where Ep,ox and Ep,red are the peak

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oxidation and reduction potentials from cyclic voltammetry. 6 ACS Paragon Plus Environment

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All ferrocene derivatives were prepared as 4 mM stock solutions in 0.1 M NaNO3

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background electrolyte. Solutions of FcCOOH were prepared by dissolving the

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compound in an equal molar amount of 0.04 M NaOH and then made to volume with

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0.1 M NaNO3. FctertAm+ solutions were prepared from the iodide salt in 0.1 M

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NaNO3, slurried with AG 1-X8 resin (chloride form) to remove iodide and filtered.

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FcCH2OH and alphaFc were dissolved in small amounts (~1g) of ethanol and added

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dropwise to 0.1 M NaNO3. FcDA was dissolved in a small amount of acetone (~1g)

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and then made to volume with 0.1 M NaNO3. FcCHO was dissolved directly in 0.1 M

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NaNO3 and shielded from light. All stock solutions were degassed under vacuum

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prior to addition to HFO-FeII mixtures.

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Preparation of HFO-FeII suspensions. HFO-FeII mixtures were prepared in 100

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mL reactor vessels with a multi-port cap allowing for electrode placement (pH:

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Metrohm® Aquatrode; EH: combined Pt-ring electrode), gas sparging, and sample

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removal. HFO-FeII mixtures were prepared in a fixed background medium of 0.1 M

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NaNO3 and 0.05 M 2-[N-Morpholino]ethanesulfonic acid (MES) buffer. Suspensions

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of 100 mL were prepared by combining: 24 mL of 0.1 M NaNO3, 30 mL of 0.2 M

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NaNO3, 10 mL of 0.5 M MES, 20 mL of HFO (10 g/l) and 16 mL 10 mM FeSO4 (in

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0.1 M NaNO3), giving a surface site (≡ FeIIIOHtot) concentration of 2 mM and FeIItot =

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1.6 mM

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addition of 0.2 M NaOH in 0.1 M NaNO3. Suspensions were stirred with a

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mechanical stirrer and bubbled with ultra-high purity argon preconditioned in 10%

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H2SO4, ~1% pyrogallol in 10% NaOH, and 0.1 M NaNO3. Solutions containing

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aqueous Fe2+ alone were prepared in the same way as that described for HFO-FeII

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suspensions except that HFO in the above procedure was replaced by 0.1 M NaNO3.

3, 6

. Adsorption of FeII onto the HFO surface was carried out by the gradual



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Electrochemical studies. Cyclic voltammetric studies were carried out using an

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Autolab PGSTAT12 potentiostat (Eco Chemie) with GPES 3.9 software. A 3-

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electrode configuration was used consisting of a 3 mm diameter boron-doped

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diamond (BDD) working electrode (WE; Windsor scientific), a Pt rod counter

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(auxiliary) electrode, and a Ag/AgCl (3M KCl) reference electrode; (E½ = 0.210 V vs

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SHE). Electrochemical measurements were conducted in a Metrohm 1 – 50 mL

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reaction vessel, modified as described below. Working electrodes made of boron-

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doped diamond are well known to give lower background currents, and allow the

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measurement of extremely low signals.

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potential window and resists passivation compared to conventional electrode materials

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at the beginning of the study. The experiments reported here were carried out as pairs

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of scans, with the electrode polished between each pair (0.3 µm, alumina). All

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measurements were carried out at ambient temperature (20±2 ˚C).

In addition BDD provides an extended

. The BDD electrode was subject to a cathodic pre-treatment in 1 M HNO3 once

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In order to achieve EC’ conditions, the CZ* (adsorbed FeII) concentration needs to

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be sufficiently high so as to not be depleted during the electrochemical measurement.

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Balanced against this is the need to conduct the FeII adsorption under controlled

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conditions and avoiding surface precipitation; the conditions chosen in this work were

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a compromise between these two competing requirements, with HFO and FeII

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concentrations 10× higher than that used in previous (and related) work 3, 6. In order to

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further increase the CZ* concentration the electrochemical vessel was modified with

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the working BDD electrode mounted in an inverted configuration from the bottom of

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the cell (see Figure 1). This allowed the HFO suspension to settle adjacent to the WE,

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further increasing CZ*. A study of the settling behavior showed with the HFO-FeII

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suspension constant electrochemical behavior was achieved within 60 seconds (data


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not shown), and that the volume occupied by the settled suspension was ~1/10 of the

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solution volume; the CZ* concentration during the electrochemical measurements was

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therefore ~10× higher than the mixed suspension concentration.

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Aliquots (25 mL) of the equilibrated HFO-FeII mixture were transferred to the

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electrochemical cell by syringe under anaerobic conditions (not in a glovebox;

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electrochemical cell pre-purged with high purity argon). The suspension was then

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allowed to settle (settled volume approximately 2.5 mL). Two background scans

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(without ferrocene) were recorded at each scan rate, with the BDD electrode polished

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between each scan rate pair. 100 µL of the selected ferrocene derivative stock solution

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(at 4 mM) was then added to the suspension (final [Fc] =16 µM) and the cyclic

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voltammograms recorded over the same range of scan rates, again polishing the BDD

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electrode between each scan rate pair. For the polishing procedure the electrochemical

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vessel was placed on an angle, the BDD electrode removed (maintaining an Ar purge

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in the reactor), polished and then re-positioned. The suspension was then re-mixed

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and allowed to settle again. Wave height was measured by linear extrapolation of pre-

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edge current using the data analysis module of GPES software (v 3.9). Simulations of

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concentration profiles and voltammetric responses were carried out using the

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electrochemical simulation package: DigiElchTM version 6.F (build 3.005) 32.

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Results and discussion

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Adsorption of FeII on HFO. The adsorption of FeII onto HFO and other FeIII 3, 6

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(hydr)oxides has been described elsewhere

and modeled using a constant

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capacitance model for the electrical double layer. In this work the concentration of

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both HFO and total FeII were increased by a factor of 10 compared to these previous

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studies in order to increase the CZ* (adsorbed FeII) concentration. The observed



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adsorption of FeII on HFO at this higher concentration is shown in Figure 2a over the

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experimental pH range of this study (4.5

Mediated Electron Transfer between FeII Adsorbed onto Hydrous

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