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Kinetics and Mechanisms of Aluminum Adsorption on Kaolinite Using a Two-site Nonequilibrium Transport Model¹

ABSTRACT

Kinetics of Al adsorption on Ca-kaolinite at selected concentrations, pH, and column lengths were studied using a miscible displacement technique. Observed breakthrough curves (BTC) for continuous injection of pH 4.3, 0.75 to 7.75 µg mL^–1 Al solution were well-described by a two-site nonequilibrium transport model which assumes that sorption sites can be divided into two fractions: (1) instantaneous equilibrium sites and (2) sites following first-order reversible kinetics. Independent verification of the fitted parameter ß, related to the fraction of type-1 equilibrium sites, was possible by assuming negative surface charge sites of kaolinite were in local equilibrium with the solution phase. Confirmation of the fitted parameter R, which is a measure of the total Al adsorbed, was not possible due to the nonsingular nature of the equilibrium adsorption isotherm. First-order rate coefficients () for Al adsorption on kaolinite were unaffected by column length. Effects of influent Al concentration on were small but suggested that kinetic reaction of Al with kaolinite was not strictly first-order. Decreasing the pH of influent solution from 4.3 to 4.0, eliminated type-2 kinetic sites. These effects indicate that kinetic reactions involve Al polymerization on kaolinite while equilibrium reactions reflect Ca-Al exchange. This scenario is supported by Ca desorption data of the effluent and by potentiometric titrations of effluent solutions.

¹ Contribution from the Dep. of Agronomy, Virginia Polytechnic Institute and State Univ., Blacksburg VA 24061. Work was funded in part by U. S. Department of Energy under contract number DE-AS05-83ER60179.

² Graduate Research Assistant, Assistant Professor, and Professor, respectively.

Received for publication September 10, 1984. Accepted for publication February 1, 1985.