The effect of precursor speciation on the growth of scorodite in an atmospheric scorodite synthesis

Abstract

In this study, we propose a growth pathway of scorodite in an atmospheric scorodite synthesis. Scorodite is a non-direct product, which is derived from the transformation of its precursor. Different precursor speciation leads to different crystallinity and morphology of synthesized scorodite. At 10 and 20 g l^-1 initial arsenic concentration, the precursor of scorodite is identified as ferrihydrite. At 10 g l^-1 initial arsenic concentration, low arsenic concentration is unfavourable to the complex between arsenate and ferrihydrite, inhibiting the transformation of ferrihydrite into scorodite. The synthesized scorodite is 1-3 µm in size. At 20 g l^-1 initial arsenic concentration, higher arsenic concentration favours the complex between arsenate and ferrihydrite. The transformation process is accessible. Large scorodite in the particle size of 5-20 µm with excellent crystallinity is obtained. However, the increasing initial arsenic concentration is not always a positive force for the growth of scorodite. When initial arsenic concentration increases to 30 g l^-1, Fe(O,OH)₆ octahedron preferentially connects to As(O,OH)₄ tetrahedron to form $\text{Fe}{\text{H}}_2\text{As}{\text{O}}_4^{2+}$ or $\text{FeHAs}{\text{O}}_4^{+}$ ion. Fe-As complex ions accumulate in solution. Once the supersaturation exceeds the critical value, the Fe-As complex ions deprotonate and form poorly crystalline ferric arsenate. Even poorly crystalline ferric arsenate can also transform to crystalline scorodite, its transformation process is much slower than ferrihydrite. Therefore, incomplete developed scorodite with poor crystallinity is obtained.

Keywords: atmospheric scorodite synthesis; ferrihydrite; growth of scorodite; poorly crystalline ferric arsenate; precursor speciation.