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Chemical Relaxation and Double Layer Model Analysis of Boron Adsorption on Alumina

Department of Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19717-1303

* Corresponding author ( disparks{at}brahms.udel.edu).

ABSTRACT

Boron is a plant nutrient essential for adequate plant growth, yet the range between B deficiency and toxicity levels is rather narrow. Boron adsorption reactions with soil components, particularly sesquioxides, most often regulate the amount of B in the soil solution. The reaction mechanisms of B adsorption on oxides have not been fully characterized, however. Pressure-jump relaxation experiments were conducted to measure the rates and determine the reaction mechanism for B adsorption on an alumina (-Al₂O₃) surface from B(OH)₃-B (OH)^–₄ solutions. Relaxation times () were measured from pH 7.0 to 9.7 in alumina suspensions with 0.012 mol L^–1 total B. A plot of ^–1 vs. B(OH)^–₄ plus surface site concentration obtained from the triple layer model (TLM) assuming inner sphere B(OH)^–₄ adsorption yielded an adsorption rate constant (k^int_f) of 3.3 x 10⁵ L mol^–1 s^–1 and a desorption rate constant (k^int_r) of 1.8 x 10^–3 L mol^–1 s^–1. The ratio k^int_f/k^int_r yielded an equilibrium constant (log K^int_KIN) of 8.26, in agreement with the intrinsic equilibrium constant for B(OH)^–₄ adsorption (log K^int_BIS = 7.69) obtained from adsorption isotherms. Four additional surface complexation models were tested for their ability to model both the equilibrium and kinetic data simultaneously: the constant capacitance model, the diffuse layer model, a Stern model variant, and the TLM assuming outer sphere B(OH)^–₄ adsorption. Only the TLM, assuming both B(OH)₃ and B(OH)^–₄ were adsorbed via ligand exchange on neutral surface sites, was successful. The TLM indicated that B(OH)^–₄ is the predominant adsorbed species throughout the pH range 7.0 to 10.8.

The authors appreciate the partial support of this research from the U.S. Borax Corporation. The senior author also acknowledges the receipt of a graduate research fellowship from the Univ. of Delaware.

Received for publication September 20, 1993.

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