Scorodite Crystal Formation on Hematite (Fe₂O₃) Surface in Fe(II) Solution Containing As(V)

Abstract

Scorodite crystal formation on a hematite (Fe₂O₃) surface in excess Fe(II) solution containing As(V) was observed under various conditions using the direct hematite addition method. Gel-like precursors initially formed and covered the entire hematite surface. Scorodite crystal nuclei then appeared preferentially on the grain boundaries of each hematite crystal, rather than on the flat hematite crystal surface. These grew into faceted crystalline scorodite particles through stepwise structure formation. The effects of solution temperature (50, 70, and 95°C), initial solution pH (0.6, 0.9, and 1.6), initial Fe(II) concentration (0, 25, and 55.9 g/L), and initial As concentration (25 and 50 g/L) on gel-like precursor and scorodite crystal formation were comprehensively investigated. The reaction temperature only affected the scorodite crystal formation rate and not the crystal shape in the studied range of 50–95°C. Octahedral faceted scorodite was obtained at all temperatures studied. The solution pH strongly affected scorodite crystal formation, with a higher pH favoring scorodite formation in the studied range of pH 0.6–1.6. Scorodite crystals were not obtained at pH 0.6 because of the lower AsO₄³⁻ concentration in solution. Octahedral faceted scorodite crystals were also obtained at a lower Fe(II) concentration (25 g/L), but the gel-like precursor and scorodite crystals were not obtained in the absence of Fe(II). The As(V) concentration in solution affected the overall scorodite formation rate. Imperfect octahedral-shaped scorodite crystals with larger diameters were observed at lower As(V) concentration (25 g/L), which was attributed to the lower AsO₄³⁻ concentration. This study provides important information for As treatment through scorodite crystal formation using the hematite addition method.