Effect of solution temperature and ph on adsorption and density of surface charge at aluminum oxyhydroxide /NaF aqueous solution interface

Abstract

The effect of temperature, pH, and adsorbed fluorine on the density of a surface charge at the aluminum oxyhydroxide /NaF aqueous solution interface is investigated using the method of potentiometric titration. The equilibrium of fluorine ion adsorption in analyzed. Products of electrochemical dimensional machining (ECDM) of an AMG-6 aluminum alloy thermally treated at 200°C are used as an adsorbent. The experiments on the adsorption have been carried out from an acetated buffer with the initial fluorine solution concentrations from 1.0 × 10−3 to 1.5 × 10−1 M/l. The adsorption isotherms were plotted for solution temperatures of 20, 30, and 40°C. Adsorption is shown to obey the Langmuir adsorption model. It is demonstrated that, at all solution temperatures, the pH is 5 for the maximal fluorine adsorption on aluminum oxyhydroxide. It has been revealed that the surface charge at the aluminum oxyhydroxide /NaF aqueous solution interface depends on the solution temperature; i.e., the pH of a point of a zero charge point (pHPZC) of the aluminum oxyhydroxide surface decreases with an increase in temperature. As is the case with temperature, adsorbed fluorine shifts pHPZC toward smaller values, which is typical for specific adsorption. The temperature dependence of pHPZC has made it possible to determine the standard mole enthalpy and entropy of surface-charge formation. The interaction of fluorine with aluminum oxyhydroxide is shown to be the result of the chemical affinity of F with surface centers of AlOH2+ and AlOH and electrostatic attraction.