Electrochemical investigation of scorodite synthesis for arsenic fixation using hematite as an iron source: Elucidation of reaction acceleration by Fe2+ using a local-cell model

Abstract

Owing to the chemical stability of scorodite, the synthesis of scorodite for arsenic fixation has gained interest as a treatment method for arsenic-containing wastewater generated at nonferrous metal smelting plants and other facilities. Diverse synthesis processes have been investigated such as hydrothermal, oxidation, and solid iron oxide methods. The solid iron oxide method using hematite as an iron source is particularly promising owing to its many advantages. Though the overall reaction equation for this process does not include Fe2+, the reaction is influenced by Fe2+ in the solution, significantly increasing the reaction rate. In the present study, we examine the mechanism of this reaction electrochemically to elucidate the role of Fe2+. By analyzing the open-circuit potentials and the polarization behaviors of each local reaction and conducting short-circuiting experiments based on the predicted local-cell model, we demonstrate the electrochemical nature of the reaction, which involves the reduction of hematite and the oxidation of Fe2+. In addition, the multistep reaction involved in the formation of scorodite is discussed using potential–pH diagrams based on thermodynamic reference data.