Arsenic immobilization as alunite-type phases: The arsenate substitution in alunite and hydronium alunite

Abstract AsO 4 -for-SO 4 substitution in alunite (KAl 3 (SO 4 ) 2 (OH) 6 ) and hydronium alunite ((H 3 O)Al 3 (SO 4 ) 2 (OH) 6 ) has been investigated by hydrothermal precipitation at 200 °C. Arsenical alunite presented a good precipitation yield and a significant AsO 4 substitution (up to 15% molar). The degree of arsenate substitution depends on the solution composition. It increased as (AsO 4 /(AsO 4  + SO 4 )) alunite  ≅ 0.5 (AsO 4 /(AsO 4  + SO 4 )) L . For (AsO 4 /(AsO 4  + SO 4 )) L 4 ·2H 2 O) co-precipitated. The a unit cell parameter is practically independent of the AsO 4 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 SO 4 /AsO 4 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 H 2 O-for-OH − groups, due to the location of the H-bridges of the H 3 O in the structure. These characteristics make hydronium alunite unsuitable for arsenic immobilization.