Fluorine adsorption by aluminum oxihydrates subjected to thermal treatment

The adsorptive behavior of fluorine ions by the products of electrochemical machining (AMG-6 aluminum alloy samples) subjected to thermal treatment at temperatures of 100, 200, 400, and 600°C was investigated. The regimes were analyzed, which determine the steady-state distribution of fluorine between the bulk solution and the solid surface. All the samples exhibited good adsorptive properties for fluorine removal; the AMG-200 sample was the most efficient. The kinetic curves and adsorption isotherms were plotted. It was shown that the increase in the solution temperature lead to an increase in the adsorptive capacity for all the samples. This gave evidence of the chemical nature of the investigated process. The following mechanism of fluorine fixation on the modified surface of aluminum oxihydrates was proposed: the anion exchange of F − -ions with surface OH − -groups with formation of covalent bonds, which further transform into ionic bonds.