Electrochemical preparation and application of the ferrate (VI) ion for wastewater treatment

Abstract

The electrochemical preparation of ferrate by the anodic dissolution of iron and its alloys in concentrated sodium hydroxide has been reinvestigated. It has been shown that the current efficiency for ferrate formation can be improved significantly by (a) the use of a three dimensional iron anode and (b) the choice of an iron alloy with a high carbon content. Ferrate (VI) was produced by anodic dissolution of an iron wool anode in 10 M NaOH in a membrane flow cell. It was shown that using a current density of 18 mA cnY2 , the current efficiency could be as high as 60 % but it usually dropped to z 25 % during an extended electrolysis. The reason for this change in current efficiency was investigated using voltammetric techniques and it is proposed that the ratio of oxygen evolution/ferrate production is sensitive to the composition, structure or thickness of the iron oxide/hydroxide layer on the anode surface. Periodic current reversal overcomes this loss in current efficiency to some extent. Ile influence of sodium hydroxide concentration and temperature on the electrolyses were also investigated. Voltammetric methods have also been used to define the influence of the composition of the anode on the transpassive dissolution of the iron and its alloys. It was concluded that the carbon content of the steel was critical and this was also demonstrated by preparative electrolyses at flat plate anodes. Ferrate may have applications as a water treatment chemical and experiments were carried out to define the ways in which it could be employed. Hence, the kinetics of the reaction of ferrate (VI) with water, aliphatic alcohols and phenol have been investigated. It has been shown that the ferrate (VI) ion exhibits good stability at 298 K in aqueous 10 M NaOH solution, having a half life of approximately one month. The stability of the ion was found to be strongly dependent on temperature with the decomposition occurring 1000 times faster when the temperature was raised to 333 K The NaOH concentration also determines the life time of the ion in solution; in IM NaOK the half life of the ion was only 7 minutes and the mechanism of decomposition had changed. Ferrate can, however, be added as a solid to polluted water whereupon it will react selectively with the organics. The kinetics of the reactions of ferrate (VI) were also studied in 10 M NaOH; commonly, the reactions are very fast compared to that with water. Moreover, with Jt was shown that some organics (methanol, ethanediol and phenol) undergo complete oxidation to C02 and H20. Other organics appear to undergo only partial oxidation although these results may be distorted by adsorption of the organics on the Fe(OH)3 precipitate formed during the reaction.

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