Direct and indirect electrochemical oxidation of ethanethiol on grey cast iron anode in alkaline solution

Abstract

In this study, the electrochemical generation of ferrate (Fe(VI)) from grey cast iron anode in sodium hydroxide (NaOH) solution was utilized in situ to degrade the malodorous pollutant of ethanethiol (C2H5SH). During electrolysis, C2H5SH was oxidized by the combination of direct anodic oxidation and Fe(VI) oxidation (indirect oxidation). The direct anodic oxidation process, which played an important role in the decontamination process but was often disregarded in related studies, was investigated in detail. The electrochemical generation of Fe(VI) was investigated, and the maximum current efficiency of Fe(VI) generation was obtained in 14 M NaOH solution. The voltammetry studies of the grey cast iron anode in NaOH solutions containing and without C2H5SH revealed that the dissolution of anode was improved in the presence of C2H5SH. A decrease in NaOH concentration increased the contribution of direct anodic oxidation but decreased the electrochemical generation of Fe(VI). C2H5SH was degraded on the grey cast iron anode at a higher rate in less concentrated NaOH solution (k = 4.15 × 10−2 min−1 in 2 M NaOH solution), which could be ascribed to the enhancement in direct anodic oxidation as well as in the oxidizing power of Fe(VI). Higher current density promoted the degradation rate of C2H5SH in both direct and indirect oxidation processes, but it also decreased the current efficiency due to the improvement of the side reactions. Additionally, the in-situ and on-line applications of the electrochemical generated Fe(VI) in recent studies were presented and discussed. This study provides a comprehensive understanding of C2H5SH oxidation on grey cast iron anode, and offers a feasible electrochemical scrubbing approach using grey cast iron as the anode for C2H5SH odors control.