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This article in SSSAJ

  1. Vol. 59 No. 2, p. 395-404
     
    Received: Sept 20, 1993


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doi:10.2136/sssaj1995.03615995005900020018x

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

  1. C. V. Toner and
  2. D. L. Sparks 
  1. Department of Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19717-1303

Abstract

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 (γ-Al2O3) surface from B(OH)3-B (OH)4 solutions. Relaxation times (τ) were measured from pH 7.0 to 9.7 in alumina suspensions with 0.012 mol L−1 total B. A plot of τ−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 (kintf) of 3.3 × 105 L mol−1 s−1 and a desorption rate constant (kintr) of 1.8 × 10−3 L mol−1 s−1. The ratio kintf/kintr yielded an equilibrium constant (log KintKIN) of 8.26, in agreement with the intrinsic equilibrium constant for B(OH)4 adsorption (log KintBIS = 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.

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