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Retention of Boron by Layer Silicates, Sesquoxides, and Soil Materials: II. Sesquioxides¹

ABSTRACT

Boron retention by hydroxy iron and aluminum materials was found to be pH dependent with maximum retention occurring in the alkaline range. The hydroxy aluminum materials retained B in amounts that were an order of magnitude greater than the amounts retained by the hydroxy iron materials. Boron retention by these materials was significantly reduced by aging on a steam bath prior to being treated with B.

Iron and aluminum were precipitated from solutions in the presence of B in application of the "mole ratio" method of determining the formulas of complexes. Evidence was obtained indicating Fe(III) or Al(III) and B were precipitated in stoichiometric proportions. Considering these precipitated forms as "solid complexes" with the general formula, MX_n, a greater variety of complexes was found in the Al(III) systems than in the Fe(III) systems. In the Fe(III)-B systems, values of n for the most part were 1.0; in the Al(III)-B systems, values of n were 1.0.

Samples of similar constitution to those used in the "mole ratio" study were prepared for each metal at three pH levels. The amount of B combined in the precipitate was determined as a function of aging. The amount of B combined in the precipitate of the pH 6 samples remained relatively unchanged throughout the course of the 42-day aging period whereas the amount of B combined in the pH 9, 10, and 11 samples decreased with time. Since the observed decreases in the amounts of B combined in the precipitates were limited to the pH 9, 10, and 11 samples, it was concluded that the ready source of hydroxyl ion in these suspensions promoted hydrolysis of the hydroxy iron and aluminum precipitates, resulting in expulsion of B from the precipitates probably as borate ions.

¹ Contribution of the Department of Soils & Plant Nutrition, University of California, Citrus Research Center & Agr. Exp. Sta. Riverside. Part of a dissertation submitted by the senior author in partial satisfaction of the requirements of a graduate program in soil science Ph.D degree. Presented in part before Div. S-2, Soil Science Society of America, August 1966, at Stillwater, Okla.

² Former Graduate Student and Professor of Soil Science, respectively. Present address of senior author: Assistant Professor of Soils, Montana State University, Bozeman.

Received for publication October 17, 1967. Accepted for publication January 15, 1968.