A study of the crystallization kinetics of scorodite via the transformation of poorly crystalline ferric arsenate in weakly acidic solution

In this work, the transformation of poorly crystalline ferric arsenate nanoparticle aggregates to polycrystalline scorodite particles (FeAsO4·2H2O) in an acidic nitrate medium (pH 2–4) and elevated temperature (40–80 °C) is described. Monitoring of the transformation process via different parameters such as arsenic and iron concentration in solution, iron to arsenic molar ratio in solid phase, or crystalline fraction in product, exhibited “S”-shape or sigmoidal kinetic curves, characteristic of an autocatalytic-type reaction. The lengths of the induction period and self-acceleration stage were found to depend strongly on pH and reaction temperature with the fastest transformation achieved at pH 2 and 80 °C. The transformation kinetics have been described by the Avrami–Erofeev nucleation-growth model. The selected nucleation-growth model along with FEG-SEM and FEG-TEM observations supplemented with SAED and microanalysis suggested that the mechanism of transformation involved crystallization within the amorphous precursor phase aggregates. The poorly crystalline ferric arsenate was found to contain an excess of iron, which was attributed to ferrihydrite co-precipitation. The minor ferrihydrite phase was found to convert to crystalline scorodite via a postulated mechanism that involves AsO4 ion adsorption, surface build-up of ferric arsenate and crystallization. TGA and XRD analysis revealed the final crystallization stage to be rather slow not leading to full development of crystalline structure even after 24 h ageing at pH 2 and 80 °C.