Electrochemical Aspects in Scorodite Synthesis Process Using Magnetite for Arsenic Immobilization

Scorodite is of industrial significance for the immobilization of hazardous arsenic. In the present study, electrochemical reactions involved in aqueous synthesis of scorodite were investigated using magnetite. In an acidic solution, arsenic acid reacted with magnetite to produce scorodite and Fe2+. Adding Fe2+ before the reaction accelerated the reaction rate and promoted the formation of well-faceted crystalline scorodite. To elucidate the mechanism of this seemingly paradoxical behavior, in which the addition of reaction products promoting the reaction, isotope tracer experiments were conducted. A large portion of Fe(III) in the scorodite (≥ 60%) was derived from Fe(II) in solution rather than from magnetite. This finding indicates that redox reactions involving electron transfer contribute to the formation of scorodite. Electrochemical analysis using magnetite electrodes further demonstrated that a local cell reaction occurred that involved reductive decomposition of magnetite and oxidation of Fe2+ to precipitate scorodite. The local anodic reaction evidentially determined the overall rate, and thus the addition of Fe2+ enhanced the kinetics. To optimize the synthesis of scorodite through this process, it is essential to understand that the reaction is driven by redox reactions in which magnetite acts as an oxidizing agent.