The redox cycle of chromium compounds in solid and aquatic environment is of great importance in natural phenomena and industrial processes. The oxidation behavior of Cr(III) to Cr(VI), two stable oxidation state of Cr, has been the research focus for Cr(III) compounds are relatively harmless but with low solubility while Cr(VI) compounds are very soluble and mobile with extreme toxicity and carcinogenicity.

Researchers with Institute of Process Engineering (IPE) studied a new technique oxidizing Cr(III) to Cr(VI) which overcame the drawbacks such as low efficiency, complex in storage and transportation, high investment cost. They investigated the indirect electro-oxidation of Cr(III) by in situ generated superoxide at a gold electrode in KOH solutions using cyclic voltammetry and UV–vis spectroscopy.

In experiment, Cr(NO3)3 and KOH stock solution at specific purity were used. Cyclic voltammetry (CV) experiments were conducted in a 150 mL three-electrode electrochemical cell at 25 ± 0.2 °C using a Modulab electrochemical workstation. A gold cylinder electrode was employed as the working electrode and a large-area Pt foil as the counter electrode. All potentials were measured and presented versus a saturated calomel electrode (SCE).

Researchers investigated direct and indirect electro-oxidation of Cr(III) in KOH solutions and recorded the oxygen reduction reaction (ORR) in KOH solution. Analysis given that ORR in KOH solutions was attributed to a quasi-reversible diffusion-controlled reaction. In 0.2 M KOH solution, 4e reduction occured and no reactive oxygen species were generated for the indirect Cr(III) oxidation. Moreover, Cr(III) oxidation was inhibited due to competition for the electrode active sites. In 3.0 M KOH, the protonation of superoxide was greatly suppressed and thus 1e ORR to produce superoxide was dominating. This change in mechanism favored the indirect Cr(III) oxidation through the superoxide as a mediator to oxidize Cr(III) to Cr(IV), which was the rate-determining step of Cr(III) oxidation to Cr(VI).

In summary, it is concluded that the indirect Cr(III) electrochemical oxidation was substantially influenced by KOH concentration. The pH effect, which affected the Cr(III) oxidation mechanism and kinetics, stemed from the in situ reactive oxygen species formation via ORR. For the paper, please refer to the Journal of Physical Chemistry.