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Electrochemical synthesis of ferrate in presence of ultrasound using boron dopeddiamond anodes

A. Sánchez-Carretero, M.A. Rodrigo 1, P. Cañizares, C. Sáez ⁎

Department of Chemical Engineering, Facultad de Ciencias Químicas, Universidad de Castilla-La Mancha, Campus Universitario s/n. 13071 Ciudad Real, Spain

a b s t r a c ta r t i c l e i n f o

 Article history:

Received 8 February 2010

Received in revised form 17 February 2010Accepted 19 February 2010

Available online 1 March 2010

Keywords:

Ferrate

Conductive diamond

Electrosynthesis

Ultrasound

The use of an ultrasound processor is proposed to enhance the ef ficiency of the electrosynthesis of ferrates

with boron-doped diamond anodes. The chemical dissolution of the iron powder used as raw material, due

to the extreme pH on the nearness of the anode surface, improves the results significantly. The application of 

ultrasound during electrolyses, besides having a positive effect on the dissolution of the raw material, also

favors the mass transport of iron species to the electrode surface, and thus its used enhances the ef ficiency of 

the process. The hydroxyl ion concentration and the current density also have a clear influence on the

results: high current density favors iron dissolution and hydroxyl radical generation, whereas high hydroxyl

ions concentration contributes to the stability of produced ferrate.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Ferrate ions (FeO4−2) are very powerful oxidizing agents with

standard half-cell reduction potentials ranging from 2.20 V at acidicpHs to 0.72 V vs. NHE at alkaline conditions [1]. They can be used in a

wide range of environmental and industrial applications [2–5].

The ferrate synthesis can be divided into three categories: thermal

chemical synthesis (by heating/melting iron oxides under conditions

of strong alkaline and oxygen flow [6]), wet chemical synthesis (by

oxidizing a Fe(III) salt at a strong alkaline condition and using

hypochlorite or chlorine as the oxidant [7,8]) or electrochemical

techniques (by anodic oxidation using iron as anode and alkaline

electrolyte [9,10]).

In the recent years, electrochemical oxidation with conductive-

diamond anodeshas been converted into a very promisingtechnology

in the electrosynthesis of powerful oxidants [11–13]. The electro-

chemical window of conductive-diamond is large enough to produce

hydroxyl radicals with high ef ficiency [14], and this species seems to

be directly involved in the oxidation mechanisms. In this context,

ferrates have also been generated through the use of these anodes

[15,16], with ef ficiencies even higher than those obtained with iron

electrodes, but with an important limitation in the availability of raw

iron species close to the anode surface ready to be oxidized.

Ultrasound is a cyclic sound pressure with a frequency greater

than the upper limit of human hearing (20,000Hz). It has been widely

used in chemistry (sonochemistry) and it has been observed that

many electrochemical systems are influenced by ultrasound, and themethodology offers considerable practical benefit in a wide range of 

applications. Some applications of sonoelectrochemistry (combina-

tion of ultrasound and electrochemistry) are: sonovoltammetry [17],

sonoelectrosynthesis [18,19], electrodeposition [20], electrode coat-

ing [21,22] or electroanalysis [23,24].

In this work, the enhancement of the electrosynthesis of ferrates

with conductive-diamond electrochemical-oxidation by means of 

ultrasounds is studied. A two-compartment electrochemical cell

equipped with p-Si boron-doped diamond anode and with or without

production of ultrasounds is going to be used to establish the effect of 

ultrasound processor in this process.

2. Experimental

 2.1. Analytical procedures

Theconcentration of ferrate was analyzed by the chromite method

[25]. The soluble iron concentration was measured by Plasma

Emission Spectroscopy (Inductively Coupled Plasma LIBERTY SE-

QUENTIAL VARIAN) [26].

 2.2. Electrochemical cell

The electrosynthesis of ferrate was carried out in a double-

compartment electrochemical flow cell described elsewhere [15,16].

Conductive-diamond was used as anode and stainless steel as

Electrochemistry Communications 12 (2010) 644–646

⁎ Corresponding author. Tel.: +34 902204100x6708; fax: +34 926 295256.

E-mail addresses: manuel.rodrigo@uclm.es (M.A. Rodrigo), cristina.saez@uclm.es

(C. Sáez).1 Tel.: +34 902204100x3413; fax: +34 926 295256.

1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2010.02.020

Contents lists available at ScienceDirect

Electrochemistry Communications

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cathode. The ultrasound generator (UP200S, Hielscher UltrasonicsGmbH, Germany) is equipped with a horn (40 mm diameter, 100 mm

length, 12 Wcm−2, 24 kHz) that is located inside the anolyte

compartment.

 2.3. Experimental procedures

Bench scale electrolyses under galvanostatic conditions were

carried out to determine the influence of the main parameters. The

anolyte and the catholyte consisted of 0.5 dm3 of KOH solutions

(concentration rangesfrom 5 to 14 M). Twotypesof iron-speciesraw-

materials were tested: 4.5 mM of Fe(OH)3 and an iron-powder bed

(200 g) placed close to the anode surface (not directly on the surface

but separated by means of a very thin plastic mesh). The operation

current densities ranged from 1000 to 1250 Am−2.

3. Results and discussion

In previous studies [15,16], the electrosynthesis of ferrate with

conductive diamond electrodes was carried out using oversaturated

suspension of Fe(OH)3 as raw material. The main drawback in this

synthesis was the low solubility of the iron salt, which limited the

ef ficiency of the process. To solve this problem, it was proposed an

enhancement though the use of an iron-powder bed, placed close to

the anode surface and separated by means of a very thin plastic mesh

[16]. This fact favoured the chemical dissolution of the iron-powderdue to the extreme pH on the nearness of the anode surface, and it

allowed improving the process results significantly [15,16]. However,

mass transfer (especially, availability of raw material) remained to be

the limiting step in this process and, for this reason, it is proposed in

this work to enhance mass transfer through the use of ultrasounds.

Fig. 1 compares the concentration of ferrates obtained during their

electrosynthesis with iron hydroxide and with iron-powder beds as

raw materials (KOH 10 M) in a double compartment electrochemical

flow-cell, in the absence and presence of ultrasounds (ultrasonic

power 12 Wcm−2; 24 kHz).

Significant amounts of ferrates are produced during the four

electrolyses. For the range of current charges passed, the concentra-

tion of ferrates achieves an upper limit concentration in thecase of the

electrolyses of iron hydroxide suspensions, with a plateau whichmarks the maximum concentration that can be obtained in a batch

process. On the contrary, concentrations of ferrates do not meet a

constant value in the case of using iron-powder beds as raw materials,

but they increase continuously, almost linearly in the presence of 

ultrasound (US). In this context, an important observation related to

the presence of US in both cases is the improvement in the ef ficiency

of the processes when US are applied, being more significant in the

case of the iron-powder bed (30% improvement with iron hydroxide

suspensions vs. 100%with iron-powder bed). Thisincrease demonstrates

that US deserves an additional positive effect on the ef ficiency of the

electrosynthesis of ferrates.

Fig. 2 shows the iron solubilised during the previous electrolyses

for a particular current charge of 75 Ahdm−3, in both, silent

conditions and within the presence of ultrasounds. Total iron

Fig. 1. Variation of the ferrate concentration with the charge passed, during the

electrolysis of a) saturated Fe(OH)3 solutions and b) iron-powder bed with conductive

diamond anode. (▲) Silent (■) US. Experimental conditions: 10 M KOH; j: 1250 Am−2;

T : 30 °C.

Fig. 2. Iron solubilized during the electrolysis with DBB anodes of Fe(OH)3 and iron-

powder bed with and without the use of ultrasound (electrical charge passed of 

75 Ahdm−3

).

Fig. 3. Variation of the ferrate concentration with the current density in the electrolysis

of iron powder bed with boron doped diamond electrodes. (■) US, (■) Silent.

Experimental conditions: 14 M KOH; T : 30 °C.

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concentration was measured by ICP, after filtering samples with a

0.45 µm filter. As it canbe observed,ultrasounds exert a positive effect

on the release of available iron species in both cases, as they increase

their concentration several times. The effect is more relevant in the

case of the iron bed in which the dissolution process is improved six

times. These results suggest that the improvement in the production

of ferrates can be due to the higher amount of available iron raw

species. In this context, it is known that the application of ultrasounds

during electrolyses increases the mass transport of iron, though the

mechanisms of acoustic streaming and cavitation, and it also has a

positive effect on the dissolution of the raw materials [18].

Fig. 3 shows the effect of the current density on the ef ficiency of 

the synthesis of ferrates with an iron-powder bed as raw material. An

increase of current density of 25% has a positive effect on both the

dissolution of iron species (more than 100% improvement) and in the

production of ferrates (more than 50%). To explain this, it has to betaken into account that iron dissolution is promoted by the extreme

pH on the nearness of the anode surface [27], and also that hydroxyl

radicals production uses to be very important in the production of 

oxidants with conductive-diamond electrodes, and even that there

are clearly identified two different mechanisms in the production of 

oxidants with conductive-diamond electrolysis. In this point, in many

studies [28–30], it was proved the generation of hydroxyl radicals due

to the water sonolysis. This fact could be responsible for the process

improvement when it was used ultrasound. In addition, water

sonolysis is known to lead to the formation of reactive oxygen species

such as ozone and hydrogen peroxide.

Fig. 4 shows the effect of the hydroxyl ions concentration on the

production of ferrates for two particular specific current charges

passed. For a given current charge, the higher the concentration thegreater theproduction of ferrates. This parameteris related to stability

of the produced ferrates, and it was found to be one of the more

significant parameters in previous works about the production of 

these oxidants [2,15,16,31]. As it can be observed, it remains to be a

very relevant parameter in the presence of ultrasounds. This figure

also compares results with the better results reachable in the

electrolysis using iron hydroxide as raw material. The comparison

indicates that this process can be largely enhanced and that the right

used of hydroxyl ions concentration, iron-powder beds and ultra-

sounds can improve greatly the process ef ficiency.

4. Conclusions

Ultrasound processor canbe successfully used to enhance themass

transfer and, thus, the ef ficiency of electrosynthesis of ferrate withdiamond electrodes. Ultrasound processor shows a positive effect on

the release of availableiron species, especially in thecase of using iron

powder bed as raw material. An increase of current density promotes

the iron dissolution and hydroxyl radicals generation which contrib-

ute to ferrate generation. The right hydroxyl ions concentration can

also improve significantly the process ef ficiency.

 Acknowledgements

This work was supported by the JCCM (Junta de Comunidades de

Castilla-La Mancha, Spain) through the project PCI-08-0068-9073.

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Fig. 4. Variation of the ferrate concentration with the hydroxyl anion concentration

during the electrolysis in presence of ultrasound of alkaline and iron-powder bed

solutions. (Δ) 8 Ahdm−3, (♦) 15 Ahdm−3, (■) 30 Ahdm−3. Experimental conditions:

T : 30 °C; j: 1000 Am−2.

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