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Basaluminite and Alunite: A Possible Cause of Sulfate Retention by Acid Soils¹

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 Al₄(OH)₁₀SO₄ and KAl₃(OH)₆(SO₄)₂ in soil solutions of sulfate-treated soil. Solution experiments were conducted in which 0.05M Al₂(SO₄)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, Al₄(OH)₁₀SO₄·5H₂O. 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 NaClO₄ 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 K₂SO₄. However, the soil solutions were supersaturated with respect to alunite. When the SO₄-treated soil was subsequently treated with a high rate of KH₂PO₄, 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.

¹ Contribution from the Dep. of Agronomy and Soils, Auburn University, Auburn, AL 36830.

² Professor of Soils and former Research Associate, respectively.

Received for publication December 20, 1976. Accepted for publication April 6, 1977.

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