RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The rate of dissolution of bayerite (γ-Al(OH)₃) in HNO₃-HF solutions of the constant ionic strength 1 M at 298.2°K is given by

R = S(k₁[H⁺] + (k₂C₀K₁^s[H⁺]²[F⁻]³e₊) (1 + (Ka₁^s)⁻¹[H⁺]e₋ + Ka₂^s[H⁺]⁻¹e₊ + K₁^s[F⁻]e₊ + K₂^s[H⁺][F⁻] + K₃^s[H⁺][F⁻]²e₊)⁻¹)

where k₁ = 1.5 (±0.3) × 10⁻⁷ dm³ sec⁻¹ m⁻²; k₂ = 1.11 (±0.1) × 10¹¹ kg dm¹² m⁻² sec⁻¹ mole⁻⁴; S = surface area (m²); C₀ = total concentration of reactive surface groups (=2.66 × 10⁻² mole kg⁻¹). Ka₁^s, Ka₂^s, K₁^s, K₂^s, and K₃^s as defined are equilibrium constants related to the adsorption of H⁺ and F⁻ from the solution to the bayerite surface. Their numerical values were obtained from adsorption studies. The e₊ = exp(Fψ/RT) and e₋ = exp(-Fψ/RT) account for the acting surface potential ψ which was calculated from the surface charge assuming a Helmholtz-type double layer. The results are discussed in terms of the stoichiometry of the precursor of the activated complex and possible reaction mechanisms. It is concluded that the present knowledge on the mechanisms of dissolution reaction of oxides and hydroxides is limited by the present ignorance of the surface structure and the structure of the electrical double layer.