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Chemical Relaxation and Double Layer Model Analysis of Boron Adsorption on Alumina
Author(s) -
Toner C. V.,
Sparks D. L.
Publication year - 1995
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1995.03615995005900020018x
Subject(s) - adsorption , chemistry , desorption , reaction rate constant , equilibrium constant , boron , relaxation (psychology) , analytical chemistry (journal) , thermodynamics , kinetics , chromatography , organic chemistry , physics , psychology , social psychology , quantum mechanics
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‐Al 2 O 3 ) 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 × 10 5 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.