Hydrothermal Synthesis and Stability Evaluation of Iron (III)-Aluminum (III) Arsenate Solid Solutions

In this work, the hydrothermal synthesis and the chemical stability of several Fe (III)-Al (III) arsenate solid solution materials (Fe_1−x Al _x AsO₄ · 2H₂O) are described. Their synthesis involved hydrothermal precipitation at 160 °C from Fe (III)-Al (III)-As (V) nitrate solutions with fixed [As] = 0.3 M and ([Fe] + [Al]) = 0.33 M over a period of 24 hours. The produced solid solutions are compared to the two end members of the series, namely, scorodite (FeAsO₄ · 2H₂O) and mansfieldite (AlAsO₄ · 2H₂O). The members of the solid solution series were found to exhibit some differences in terms of particle size, shape, morphology, lattice parameters, and stability as a function of the Al fraction in the scorodite structure. It appears that materials with low Al content exhibit characteristics similar to scorodite, whereas materials with high Al content resemble mansfieldite. Extended leachability studies (up to 6 weeks) in the pH range 3 to 7 at 22 °C showed that their stability decreases with increasing Al fraction in their structure.