Mediated Electron Transfer between FeII Adsorbed onto Hydrous

Aug 26, 2014 - Department of Environmental Management and Ecology (DEME), La Trobe University, Albury-Wodonga, Victoria 3690, Australia. ‡...
0 downloads 0 Views 2MB Size
Subscriber access provided by Aston University Library & Information Services

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

Environmental Science & Technology

1

2

Mediated electron transfer between FeII

3

adsorbed onto hydrous ferric oxide (HFO) and

4

a working electrode

5

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

6

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

7

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

8

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

9

University, Victoria, 3086, AUSTRALIA.

10 11

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

12 13

1 ACS Paragon Plus Environment

Environmental Science & Technology

Page 2 of 31

14

ABSTRACT

15

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

16

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

17

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

18

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

19

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

20

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

21

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

22

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

23

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

24

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

25

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

26

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

27

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

28

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

29

measuring electron-transfer rates in geochemical and environmental systems.

30 31

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

32

sensor.

33

BRIEF A novel electrochemical technique using ferrocene derivatives as molecular

34

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

35

2 ACS Paragon Plus Environment

Page 3 of 31

Environmental Science & Technology

36

Introduction

37

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

38

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

39

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

40

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

41

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

42

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

43

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

44

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

45

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

46

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

47

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

48

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

49

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

50

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

51

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

52

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

53

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

54

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

55

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

56

measurements 19.

57

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

58

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

59

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

60

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

3 ACS Paragon Plus Environment

Environmental Science & Technology

Page 4 of 31

61

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

62

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

63

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

64

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

65

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

66

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

67

cyanide complexes of iron, ruthenium and osmium

68

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

69

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

70

22

71

ideal candidates for use as redox sensors.

20

and the 2,2-bipyridyl –

21

. Ferrocene derivatives are

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

72

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

73

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

74

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

75

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

76

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

77

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

78

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

79

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

80

humic acids 23, structural FeII in clays

81

conceptually similar, the approach presented here is a dynamic electrochemical

82

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

83

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

84

considerable promise for the development of a redox characterization method for

85

environmental and geochemical applications.

24-26

and FeII adsorbed onto goethite

27

. While

4 ACS Paragon Plus Environment

Page 5 of 31

Environmental Science & Technology

86

Theoretical basis

87

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

88

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

89

29

90

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

91

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

92

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

93

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

94

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

95

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

96

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

97

an adsorbed FeII species.

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

98

99 100 101

Figure 1

102

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

103

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

5 ACS Paragon Plus Environment

Environmental Science & Technology

Page 6 of 31

104

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

105

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

106

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

107

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

108

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

109

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

110

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

111

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

112

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

113

cm-3) 29.

114 115

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

(1)

116 117

Experimental section

118

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

119

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

120

previously 6, 30.

121

Ferrocene derivatives. Ferrocene derivatives were obtained from commercial

122

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

123

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

124

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

125

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

126

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

127

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

128

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

Page 7 of 31

Environmental Science & Technology

129

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

130

background electrolyte. Solutions of FcCOOH were prepared by dissolving the

131

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

132

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

133

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

134

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

135

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

136

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

137

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

138

prior to addition to HFO-FeII mixtures.

139

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

140

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

141

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

142

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

143

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

144

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

145

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

146

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

147

1.6 mM

148

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

149

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

150

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

151

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

152

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

7 ACS Paragon Plus Environment

Environmental Science & Technology

Page 8 of 31

153

Electrochemical studies. Cyclic voltammetric studies were carried out using an

154

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

155

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

156

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

157

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

158

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

159

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

160

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

161

measurement of extremely low signals.

162

potential window and resists passivation compared to conventional electrode materials

163

29, 31

164

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

165

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

166

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

167

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

168

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

169

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

170

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

171

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

172

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

173

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

174

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

175

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

176

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

177

suspension constant electrochemical behavior was achieved within 60 seconds (data

8 ACS Paragon Plus Environment

Page 9 of 31

Environmental Science & Technology

178

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

179

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

180

therefore ~10× higher than the mixed suspension concentration.

181

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

182

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

183

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

184

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

185

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

186

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

187

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

188

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

189

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

190

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

191

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

192

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

193

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

194

concentration profiles and voltammetric responses were carried out using the

195

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

196 197

Results and discussion

198

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

199

(hydr)oxides has been described elsewhere

and modeled using a constant

200

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

201

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

202

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

9 ACS Paragon Plus Environment

Environmental Science & Technology

Page 10 of 31

203

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

204

experimental pH range of this study (4.5