Electrochemical Cr(III) Oxidation and Mobilization by In Situ Generated Reactive Oxygen Species in Alkaline Solution

Abstract

A novel electrochemical chromium oxidation strategy by in situ electro-generated reactive oxygen species was developed in this study. Activated multi-walled carbon nanotubes modified electrode was employed to produce H2O2 via two-electron-pathway oxygen reduction reaction with onset potential of −0.06 V (vs Hg/HgO) and peak current density of 1.54 mA·cm−2. The H2O2 formation was confirmed by selective luminescent probes with UV-vis spectroscopy, and the Cr(III) can be efficiently oxidized by the as formed H2O2. Interestingly, ·OH can also be obtained by the Cr(III)-induced Fenton-like reaction, and offer further oxidative stress to enhance the Cr(III) oxidation. Consequently, a significantly enhanced Cr(III) conversion of 98.5 ± 1.0% was achieved in this indirect oxidation as compared to 29.8 ± 1.3% in the corresponding direct electrochemical oxidation. Besides, indirect pathway exhibited a considerable improvement for the Cr(III) oligomers oxidation (from 17.6 ± 1.0% to 93.5 ± 1.5%), indicating a potential solution to overcome the polymerization bottleneck of Cr(III) oxidation. Moreover, the electrolysis current well maintained the original value after stability test, suggesting favorable alkaline tolerance. The proposed method provided a cleaner approach for Cr(III) oxidative mobilization in alkaline media, and new insight on the redox cycle of electro-generated reactive oxygen species.