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The Solubility of Mixed Magnesium-Aluminum Hydroxide in Various Aqueous Solutions¹

ABSTRACT

The object of this study was to investigate the release of Mg to water and various aqueous solutions by synthetic crystalline Mg-Al hydroxide and the same material freshly precipitated on hydroxy Al-montmorillonite. Washing in centrifuge tubes and leaching columns was used to determine the amount of Mg released by 50-mg samples of the synthetic mineral in water, in solutions of NaCl, and in solutions of NH₄NO₃ and NH₄OAc at pH 6.0 and 9.0. Mixed Mg-Al hydroxide, precipitated on hydroxy Al-coated montmorillonite, was also leached with similar solutions. NH₄OAc buffered at pH 6.0 released nearly 25% of the Mg present in crystalline mixed hydroxide, while NH₄NO₃ at pH 5.7 and NaCl at pH 6.3 released 11.5% and 4.2$, respectively. The dissociation of the mixed Mg-Al hydroxide, either well crystallized or freshly precipitated, is strongly dependent on the pH of the solution and, therefore, the weakly acidic NH₄ solutions were very effective in releasing Mg. The mineral is partially soluble in water and only slightly more soluble in NaCl.

¹ Contribution of the Dept. of Soils & Plant Nutrition, Univ. of Calif., Riverside.

² Former Graduate Student and Professor of Soils, respectively.

Received for publication November 13, 1969. Accepted for publication June 25, 1970.