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Relationships among Adsorbed Phosphate, Silica, and Hydroxyl during Drying and Rewetting of Kaolinite Suspension¹

ABSTRACT

Kaolinite suspensions after 1 week of equilibration with tagged phosphate solution were dried isothermally (24C) from an initial water: clay weight ratio of 24:1 to 0.89:1. At eight points along the drying process triplicate subsamples were removed and kept sealed until the final point of the drying stage was reached. The concentrations of ³¹P, ³²P, and silica were determined in the supernatant. The equilibrium concentration of phosphate in the solution remained unchanged along the drying process. The silica concentration increased while drying, but to a lesser extent than would have occurred in the absence of the clay. The pH changed irregularly within the limits of 6.1 ± 0.4. Kaolinite at a moisture content of 0.89 ml water per 1 g clay was rewetted to the initial water content and equilibrated for seven days. Part of the air-dried kaolinite was oven-dried (110C) for 48 hours and then rewetted to its original moisture content. The amount of adsorbed P released into the solution when the pH increased from 6.5 to 11.0 was smaller in the oven-dried clay than in the air-dried sample. This is attributed to an increase in the negative charge of the surface of the oven-dried kaolinite. Increasing the pH, the silica was readsorbed to a maximum at pH 9.2, as expected, and released at higher pH values in accordance with the negative surface charge.

¹ This work was done in partial fulfillment of the requirements for the degree of M.Sc. by the senior author at the Faculty of Agriculture, Rehovot, of the Hebrew University of Jerusalem. It was supported by a grant from the Ford Foundation, Project no. A-2. Contribution from The Volcani Institute of Agricultural Research, Bet Dagan, Israel, 1968 Series; no. 1442-E.

² Research assistant, Dept. of Soil Science, Faculty of Agriculture, Rehovot, now at the Division of Soil Chemistry, The Volcani Institute of Agricultural Research, Rehovot; Senior soil scientist, Division of Soil Chemistry, the Volcani Institute of Agricultural Research, Rehovot; and Visiting Assoc. Prof., Agronomy Department, Colorado State University, Fort Collins, Colo. 80521.

Received for publication December 9, 1968. Accepted for publication June 19, 1969.