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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 AsO 4 AE 2H 2 O) 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 (FeAsO 4 AE 2H 2 O) and mansfieldite (AlAsO 4 AE 2H 2 O). 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.

The hydrolysis of ferric sulfate was studied in a batch reactor at 200 °C with the initial Fe(III) concentration ranging from 0.10 to 0.80 M in the presence/absence of zinc sulfate (1.2 M ZnSO 4 ) with 3 h retention time. Regardless of the presence or not of ZnSO 4 , the hydrolytic precipitation kinetics at 200 °C was determined to be first-order with respect to Fe(III), and expressed by the following rate equation, r hyd ) k hyd (C Fe(III) -C Fe(III),eq ), where the apparent kinetic constant, k hyd , was determined to be 10 -2 min -1 . Two reaction stoichiometries were established: one leading to the formation of kinetically favored basic ferric sulfate at C Fe(III),initial g 0.4 M in the absence of ZnSO 4 or at C Fe(III),initial g 0.7 M in the presence of ZnSO 4 , while the other led to the production of hematite. The hydrothermally produced hematite material possessed very high specific surface area (50-80 m 2 /g) and contained around 1% S (as SO 4 via chemisorption), 0.2% Zn (when ZnSO 4 was present), and about 4.5% H 2 O/OH. After excluding the SO 4 and Zn content, the following stoichiometric formula for hematite was determined: Fe 2 O 3-x (OH) 2x ‚yH 2 O where x ) 0.08 and y ) 0.29.

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