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Molybdate Adsorption on Kaolinite, Montmorillonite, and Illite: Constant Capacitance Modeling

Univ. of Evora, 7000 Evora, Portugal

* Corresponding author.

ABSTRACT

The adsorption of molybdate by kaolinite, montmorillonite, and illite was investigated at 25 and 40 °C, in a 0.01 M NaCl background medium, for equilibrium Mo concentrations ranging from 0.1 to 100 mg L^–1. The constant capacitance model was shown to be able to describe molybdate adsorption on the three minerals and for the two temperatures used. The model comprises five adjustable parameters for each mineral: two surface protonation-deprotonation constants, two molybdate surface complexation constants, and a capacitance density parameter. However, for montmorillonite and illite the number of adjustable parameters was reduced to four since adsorption data could be fitted using only one molybdate complexation constant. The protonation-deprotonation constants were obtained from potentiometric titration data and the molybdate complexation constants from adsorption data. The capacitance density was chosen on the basis of a literature compilation of values used in previous applications of the constant capacitance model to the adsorption of several ions on oxide minerals. Tentative estimations of thermodynamic functions of molybdate adsorption are presented.

Contribution from Univ. of Evora and CECUL (INIC), 1200 Lisbon, Portugal.

Received for publication March 11, 1988.

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