The dissolution behavior of scorodite in acidic environments

Abstract

The safe disposal of arsenic-containing waste has been a difficult problem for the mining and metallurgical industry. One of the solutions to the arsenic problem is the precipitation of scorodite, an arsenic-containing mineral. Scorodite is reported to be relatively stable over a wide range of pH, and therefore may be a preferred disposal option. The effect of organic complexing agents on scorodite stability, however, is largely unknown. The present study is a phenomenological investigation into the dissolution kinetics of scorodite in the presence of oxalic acid under varying conditions of pH, oxalic acid concentration and temperature. The effect of scorodite particle size was also investigated. The morphological changes accompanying the dissolution process were examined by SEM and TEM analyses. Dissolution curves were divided into a linear induction period and a post-induction period. Activation energies were determined. Complete dissolution data were fit to the Prout-Tompkins/Austin-Rickett model. Dissolution data are indicative of auto-accelerated processes. The rapid increase in dissolution rate following the induction period is believed to be associated with an increase in the effective surface area. Pitting was observed on the surface of scorodite in the early stages of dissolution. In the later stages of dissolution, these pits were observed to grow and coalesce, in many cases resulting in the formation of dissolution holes.