AsO4-for-SO4 substitution in alunite (KAl3(SO4)(2)(OH)(6)) and hydronium alunite ((H3O)Al-3(SO4)(2)(OH)(6)) has been investigated by hydrothermal precipitation at 200 degrees C. Arsenical alunite presented a good precipitation yield and a significant AsO4 substitution (up to 15% molar). The degree of arsenate substitution depends on the solution composition. It increased as (AsO4/(AsO4 + SO4))(alunite) congruent to 0.5 (AsO4/(AsO4 + SO4))(L). For (AsO4/(AsO4 + SO4))(L) <0.26, arsenical alunite was the unique phase and, above this ratio, mansfieldite (AlAsO4 center dot 2H(2)O) co-precipitated. The a unit cell parameter is practically independent of the AsO4 substitution, but the c unit cell parameter increased consistently with the differences between the As-O1 and S-O1 distances in tetrahedral sites of the structure. The maximum stability of arsenical alunite in short-term tests is between pH 5 and 8, with an As-solubilization of 0.01-0.03 mg/L in 24 h. Long-term tests were performed at some synthesized samples at its natural pH. Arsenical alunite was stabilized at 0.3 mg/L released As in 2.5 weeks. These values were similar to those obtained in pure and largely crystalline natural scorodite (0.4 mg/L released As), but lower than the obtained for synthetic scorodite (1.3 mg/L released As). Thus, arsenical alunite could be effective for arsenic immobilization, especially for effluents or wastes containing large SO4/AsO4 ratio. Hydronium alunite presents a low precipitation yield and a very low arsenate incorporation (up to 1% molar). This may be related by the difficulty of substituting protonated H2O-for-OH- groups, due to the location of the H-bridges of the H3O in the structure. These characteristics make hydronium alunite unsuitable for arsenic immobilization. (C) 2013 Elsevier B.V. All rights reserved.