Characterization of poorly-crystalline ferric arsenate precipitated from equimolar Fe(III)-As(V) solutions in the pH range 2 to 8

The neutralization of equimolar (0.1 M) Fe(III)-As(V) acidic sulfate or nitrate solutions at 22 °C (295 K) over the pH range 2 to 8 yielded a predominantly poorly-crystalline ferric arsenate that resembles its scorodite precursor: FeAsO^sub 4^ * (2 + x)H^sub 2^O (where 0 < x < 1). The X-ray powder diffraction (XRD) pattern of it consists of two broad peaks similar to those of two-line ferrihydrite, but clearly different. In addition to ferric arsenate, a small fraction of twoline ferrihydrite was found to be present in the precipitate, increasing in significance with the pH, from around 5 pct at pH 2 to 4 to around 30 pct at pH 8. A field emission gun-transmission electron microscope (FEG-TEM) analysis and a collection of X-ray chemical maps revealed a nanocrystalline structure that is relatively chemically homogeneous in the acidic domain (a constant iron-to-arsenic ratio at 0.98) but becomes progressively disordered and nonuniform at the high pH end. An aqueous phase arsenic concentration was found to increase with the pH and to vary at a fixed pH with the type of solution (SO^sub 4^ vs NO^sub 3^) and initial arsenic concentration used. Such variation in solubility appears to be linked to nanodomain structural differences. [PUBLICATION ABSTRACT]