Characterization of ferrate ion electrogeneration in acidic media by voltammetry and scanning electrochemical microscopy. Assessment of its reactivity on 2,4-dichlorophenoxyacetic acid degradation

The electrogeneration of ferrate (FeO42−) ion at a boron-doped diamond (BDD) anode from the oxidation of Fe3+ ion formed from Fe2+ ion as precursor in 0.1 mol dm−3 HClO4 has been studied by cyclic voltammetry and rotating disc electrode (RDE) voltammetry. The cyclic voltammograms showed that the anodic peak related to FeO42− ion generation, appearing just before water discharge with hydroxyl radical production, was controlled by a mixed process of diffusion and charge. The height of the corresponding RDE voltammetric wave was strongly inhibited with rising rotation speed. This anomalous behaviour was studied by scanning electrochemical microscopy (SECM), which confirmed that the electrogenerated FeO42− ion suffers a decomposition process to give Fe3+ and O2. From the SECM analysis, it was also found a clear inhibition effect on ferrate ion generation due to the presence of high Fe3+ concentrations in solution. The reactivity of the electrogenerated FeO42− ion for the removal of organic pollutants was assessed by treating the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) in 0.1 mol dm−3 HClO4 solution. Its direct oxidation at the BDD anode produced a slow disappearance of the herbicide. The destruction of 2,4-D was accelerated under the additional oxidative action of generated FeO42− ion, but yielding a low mineralization degree. HPLC measurements of the electrolyzed solutions revealed the formation of 2,4-dichlorophenol as aromatic by-product and maleic, malic, oxalic and formic acids as short chain carboxylic acids, most of which were formed more largely when FeO42− ion was generated.

► Ferrate ion is electrogenerated from Fe2+ oxidation at a BDD anode in acidic medium.
► A complex process is found in cyclic voltammetry, inhibited with rising rotation speed in RDE voltammetry.
► Decomposition of ferrate ion into Fe3+ and O2 is confirmed by scanning electrochemical microscopy.
► Anodic oxidation of 2,4-dichlorophenoxyacetic acid is accelerated by generated ferrate ion.
► 2,4-Dichlorophenol and maleic, malic, oxalic and formic acids are formed as by-products.