Direct electrochemical preparation of solid potassium ferrate

Direct electrochemical preparation of solid potassium ferrate

Franc ßois Lapicque *,G

e rard Valentin Laboratoire des Sciences du G e

nie Chimique CNRS -ENSIC,BP 451,F-54001Nancy,France Received 17July 2002;received in revised form 12August 2002;accepted 12August 2002

Abstract

Mixed KOH–NaOH solutions were shown to allow the production of solid potassium ferrate in an electrochemical cell,due to

the low solubility of the key-salt.Potassium ferrate was produced at 12g l À1with current yields of approximately 20%by disso-lution of a cast iron anode with 2.8%Si,immersed in a 400g l À1KOH,400g l À1NaOH solution.The ferrate produced can be directly separated by conventional solid–liquid techniques,and the recovered solution with a low ferrate content can be reused for further electrochemical runs.Ó2002Elsevier Science B.V.All rights reserved.

Keywords:Potassium ferrate;Anode dissolution;KOH–NaOH solutions;Grey cast iron

1.Introduction

Ferrate has been considered for years for water and wastewater treatment,because of its environmental friendly properties and its high efficiency,as emphasized in two recent papers [1,2].Ferrate species is a very strong oxidant and is capable of disinfecting microor-ganisms,oxidize organic and inorganic impurities,and remove suspended particulate materials.Moreover Fe(III)generated during the oxidation/disinfection by ferrate,exhibits valuable coagulating properties,which improve the global efficiency of the treatment.

Three methods can be used for the preparation of ferrate.

•Wet oxidation methods rely upon the absorption of chlorine in concentrated NaOH solution and the hy-pochlorite formed reacts with ferric ions to form fer-rate whose presence is indicated by the black-purple colour of the solution [3–5].Because of its rapid de-composition in aqueous solutions ferrate has to be separated from the reacting medium in the form of a solid,stable product [3].Replacement of sodium hydroxide by potassium hydroxide leads to the for-mation of the less soluble potassium salt which can be recovered by filtration,as suggested in [6,7].The

oxidation of sodium perchlorite by ferric nitrate can also be considered [8,9].The development of the ex-isting wet processes is hindered by the use of hazard-ous chlorinated compounds.

•The various dry oxidation methods are conducted at high temperature with the use of nitrate,oxides,per-oxides,or hydroxides of alkali metals.Ferrate can be formed by the action of calcium hypochlorite on po-tassium hydroxide,followed by the addition of fer-rous sulfate [10].Other methods rely upon calcination of potassium and sodium peroxides with ferric oxides [11],and in the presence of nitrate as suggested in [12].

•Ferrate can finally be formed by anodic dissolution of iron-containing alloys [6,13,14]and the efficiency of the electrochemical route is highly dependent on the nature of the anode material and the composition of the electrolyte.In particular,the effect of the car-bon content in the iron material has been extensively investigated even though the conclusions reported are not fully consistent.We showed recently the high ef-ficiency of grey cast irons with silicon contents up to 3%.NaOH is the leading electrolyte for the electrolytic preparation of ferrate and the usual concentration is in the range 30–50wt%.The maximal concentration of ferrate obtainable is limited by the low/moderate sta-bility of ferrate species and deactivation of the

anode

Electrochemistry Communications 4(2002)

764–766

*

Corresponding author.Fax:+33-3-83-32-29-75.

E-mail address:lapicque@ensic.inpl-nancy.fr (picque).

1388-2481/02/$-see front matter Ó2002Elsevier Science B.V.All rights reserved.PI I :S 1388-2481(02)00438-1