Investigation of the Scorodite Formation Mechanism in As(V) Solution Containing Fe(II) with Hematite Addition Using a Stable Iron Isotope

Abstract

Arsenic is a toxic element, and development of effective methods for As removal and stabilization are necessary. Removal and stabilization of pentavalent As as crystalline scorodite (FeAsO₄·2H₂O) is a promising method for As treatment of industrial byproducts. When hematite (Fe(III)₂O₃) powder is added to As(V) solution containing Fe(II) under appropriate conditions, scorodite crystals form from the gel-like precursor. When Fe(II) is not contained in the solution, the formation reaction does not proceed, even if hematite is added. Therefore, it is considered that the Fe(II) in the solution is heavily involved in the formation mechanism. Here, to elucidate the scorodite crystal formation mechanism in the hematite addition method, the origin of the iron in scorodite was investigated through scorodite synthesis experiments with addition of a stable iron isotope (⁵⁴Fe) to the reaction solution. It was estimated that the Fe(III) constituting the scorodite crystals was mainly (more than about 80 atom%) derived from the Fe(II) in the solution. From this result, scorodite formation from the reaction solution with solid hematite can be considered to occur as follows. The gel-like precursor is composed of Fe(II) from the reaction solution together with Fe(III) from hematite. During conversion of the gel-like precursor to scorodite crystals, Fe(II) in the precursor is oxidized by Fe(III), and it then combines with the arsenate ion to form scorodite. With conversion to scorodite crystals, the Fe(II) generated by this electron exchange (originally from hematite) dissolves in the reaction solution. It is speculated that electron exchange between solution-derived Fe(II) and hematite-derived Fe(III) plays an important role in formation of scorodite crystals in this synthetic method.