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Specific Adsorption of Trace Amounts of Calcium and Strontium by Hydrous Oxides of Iron and Aluminum¹

ABSTRACT

Freshly prepared Fe and Al hydrous oxide gels and the amorphous product of heating gibbsite selectively adsorbed traces of Ca and Sr from solutions containing a large excess (1M) of NaNO₃. The fraction of the added Ca (Sr) adsorbed depended principally on the suspension pH, the amount of solid present, and to a lesser extent on the NaNO₃ concentration. Significant Ca and Sr adsorption occurred on the Fe and Al gels, and heated gibbsite, at pH values below the points of zero charge (8.1, 9.4, and 8.3±0.1, respectively), indicating specific adsorption. The pH (± 0.10) at which 50% of the Ca was adsorbed (pH₅₀) occurred at pH 7.15 for the Fe gel (0.093M Fe), 8.35 for the Al gel (0.093M Al), and 6.70 for the heated gibbsite (0.181M Al); for Sr, the pH₅₀ values were 7.10, 9.00, and 6.45, respectively. For the Fe gel and heated gibbsite, an empirical model based on the law of mass action described the pH dependence of adsorption reasonably well and suggested that for each Ca or Sr fraction adsorbed, approximately one proton was released. Failure of the Al gel to fit this model may have resulted from its rapid aging.

¹ Supported in part by the College of Agric. and Life Sci., Univ. of Wisconsin, Madison 53706, under project 1336; and in part by the Ecological Sciences Branch, Division of Biomedical and Environmental Research, US Atomic Energy Commission Contract AT (11-1)-1515-Jackson (Paper COO-1515-38) through an International Consortium for Interinstitutional Cooperation in the Advancement of Learning (ICICAL). Presented in part before Div. S-9, Soil Science Society of America, New York City, N.Y., 17 Aug. 1971.

² Research Assistant; former Associate Professor, currently Professor of Soil Science, Massey Univ., Palmerston North, N.Z.; and Franklin H. King Professor of Soil Science; Univ. of Wisconsin, Madison, respectively.

Received for publication June 19, 1974. Accepted for publication January 3, 1975.