Nonclassical Two-Stage Pathways for the Formation of Amorphous Ferric Arsenate: Influence of Temperature, pH, and Stoichiometry

Amorphous ferric arsenate (AFA) is a vital sink of arsenic in acidic, As-rich environments. Although its properties and transformation into scorodite are well-documented, the precipitation process of AFA remains understudied. The present study investigates the precipitation kinetics of AFA in relation to pH, As/Fe ratio, and temperature using both in situ and ex situ techniques. In situ small-angle X-ray scattering revealed a two-stage AFA formation process irrespective of the controlled variables. Stage I features the emergence of subnano- to nanoparticles with radii ranging from 8.9 to 10.1 Å without significant growth. Ex situ analyses by X-ray absorption fine structure further indicate these particles may have a similar structure to arsenate-FeO₆ octahedra complexes. Stage II is marked by the growth of these particles into medium-sized particles, likely through a particle-by-particle attachment mode. A positive correlation between temperature and particle sizes was observed in stage I, likely via enhancing FeO₆ octahedra incorporation into Fe-As complexes. Moreover, both pH and As/Fe ratios were found to accelerate precipitation kinetics, possibly by reducing electric repulsive interactions between small particles. These findings enhance our understanding of mineralogical transformations in Fe-As systems and provide insights into developing high-efficiency As remediation strategies for both natural and industrial applications.