Basaluminite and Alunite: A Possible Cause of Sulfate Retention by Acid Soils

Abstract Pure‐solution and soil‐solution experiments were conducted in which the solubility products of synthetic basaluminite and alunite in pure solutions were compared with the ion‐activity products of Al 4 (OH) 10 SO 4 and KAl 3 (OH) 6 (SO 4 ) 2 in soil solutions of sulfate‐treated soil. Solution experiments were conducted in which 0.05 M Al 2 (SO 4 ) 3 was titrated with NaOH, KOH, or Ca(OH) 2 to an OH/AI mole ratio of 2.0. The initial precipitate was amorphous, with the chemical composition of basaluminite, Al 4 (OH) 10 SO 4 ·5H 2 O. Aging the amorphous precipitates under different conditions transformed them to compounds of different composition and crystallinity: amorphous precipitate remained unchanged when aged in their mother solutions at 25°C; crystalline basaluminite formed in Ca mother solutions when aged at 50°C; crystalline alunite formed in Na and K mother solutions when aged at 50°C; crystalline alunite formed in K mother solutions that had been seeded with alunite or bentonite when aged at 25°C. The solubilities of the amorphous and crystalline precipitates were determined in dilute NaClO 4 solutions and gave the following average calculated pK sp values: 116.0 for amorphous basaluminite; 117.7 for crystalline basaluminite; 79.7 for Na‐alunite; and 85.4 for K‐alunite. Similar K sp values for basaluminite were found for soil solutions from a sandy loam soil that had been treated with different rates of K 2 SO 4 . However, the soil solutions were supersaturated with respect to alunite. When the SO 4 ‐treated soil was subsequently treated with a high rate of KH 2 PO 4 , the soil solution remained supersaturated with respect to alunite, but became undersaturated with respect to basaluminite. It is proposed that sulfate retention by acid soils is a consequence of the solubility of basaluminite and/or alunite.