RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

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 (k^int_f) of 3.3 × 10⁵ L mol⁻¹ s⁻¹ and a desorption rate constant (k^int_r) of 1.8 × 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.