Investigation of electrochemical synthesis of ferrate - Part II: Optimization of the process parameters of ferrate(VI) electrochemical synthesis

Abstract

In part I [1] of the investigation the behavior of iron and selected low carbon steels in concentrated (10M - 15 M) water solution of NaOH and KOH in wide range of electrode potentials, between hydrogen evolution reaction and oxygen evolution reaction, is investigated and discussed. On the base of experimental data obtained by LSV and galvanostatic pulse methods, it is concluded [1] that efficient synthesis of ferrate (VI) can be expected in the region of anodic potentials between + 0,55 V and + 0,75 V against Hg|HgO reference electrode. In this paper optimization of electrolysis parameters of the electrochemical synthesis of ferrate(VI) is elaborated. The most important parameters chosen for optimization process were: anode material, alkaline electrolyte concentration, regime of electrical potential control, current density and electrolyte temperature. The best current efficiency and yield of ferrate(VI) synthesis of the explored anode materials (electrical steel, low carbon cold rolled steel plate, and structural steel) is obtained when electrical steel with 3 wt% of silicon is applied. The worst current efficiency is obtained with anodes made of structural steel with higher concentration of manganese, chromium and nickel. The influence of alloying elements on the process of electrochemical synthesis of ferrate(VI) is discussed in terms of their influence on formation and stability of anodic passive layer and oxygen evolution reaction. The increase of electrolyte concentration from 10M to 15M NaOH and KOH provided the increase of current efficiency with maximum obtained for 14M NaOH. The yield of ferrate(VI) synthesis increases with temperature raise, having maximal value at about +50?C, and after that, at higher temperatures, instability of ferrate(VI) increases and the yield of synthesis lessens. Considering the influence of electrical regime control it is concluded that the biggest yield of ferrate(VI) can be expected with constant anodic potential of about +0,65 V against Hg|HgO reference electrode and with pulsating potential with ratio Emax : Emin = +0,8 V : +0,5 V against Hg|HgO electrode and pulse duration of tmax : tmin = 2s : 1s, figure 1. Circulation of electrolyte has shown relatively small, but positive, effect on the synthesis efficiency, particularly in cases when potassium hydroxide is present in the electrolyte.