Chemical Relaxation and Double Layer Model Analysis of Boron Adsorption on Alumina
- C. V. Toner and
- D. L. Sparks
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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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