Temperature influence on the equilibrium and kinetics of fluoride-ion adsorption by heat-treated Al oxyhydrates

The effect of temperature on the fluoride adsorption by heat-treated samples of aluminum alloy AMG-6 electrochemical machining products has been studied. Kinetic curves and adsorption isotherms at solution temperatures of 20, 30, and 40°C have been received, and the adsorption rate constants, diffusion coefficients, and the values of the activation energy have been determined. The obtained E _a value corresponds to values of the activation energy characteristic of ion exchange processes controlled by internal diffusion. Thermodynamic functions of the adsorption process ΔF₀, ΔH₀, and ΔS₀ have been calculated. The differential free molar energy changes, the isosteric heat, and the entropy of the fluorine adsorption have been analyzed versus the filling degree of the sample surface with fluorine ions. The absence of the process rate dependence on the mixing rates, as well as the character of the dependences of F (equilibrium degree reaching, %) on the √tdemonstrates that the process under consideration is controlled by an internal diffusion step. The mechanism of fluorine ion fixation on the modified surface of aluminum oxyhydrates has been discussed.