Hydration of γ-Alumina in Water and Its Effects on Surface Reactivity

Abstract

The stability of γ-alumina suspensions in water has been investigated by long-duration experiments (1 day to 6 months). Several complementary methods were used to characterize the solid (acid-base titration and dissolution rate measurements, X-ray photoelectron spectroscopy, X-ray diffraction, gravimetric/ differential thermal analysis, infrared spectroscopy, and scanning electron microscopy). It appears that γ-alumina, which is widely used as a model oxide in sorption experiments, thanks mainly to its high specific surface area and to the presence of aluminol groups on its surface, is progressively transformed to bayerite (β-Al(OH)3). This transformation was characterized by an induction period of about 4 days, corresponding to the formation of a transient amorphous hydrated phase, followed by an increase in the bayerite concentration, which levels off after about 2 months. This transformation results in a dramatic decrease in the surface density of sites reactive to protons. Thermodynamical calculations predict the hydration reaction of γ-alumina leading to a more stable phase (bayerite, gibbsite, or boehmite). Since the surface reactivity and sorption properties of solids are factors controlling the transport of elements in water, it is necessary to take into account the stability of solids when modeling sorption experiments. These factors are important for the long-term prediction of the effectiveness of barriers placed around radioactive waste depositories.

Effect of pH and hydration on the normal and lateral interaction forces between alumina surfaces

“…Staszczuk et al [42] report the presence of chemisorbed water 1-2 layers thick that could only be removed from the surface at higher temperatures. Lefevre et al [43] state that γ -alumina samples hydrated for 2 days show water loss at temperatures between 200 and 400 • C. All these results agree with Alwitt [44], who observed a dehydration peak for alumina between 280 and 330 • C.…”

Surface acid–base properties and hydration/dehydration mechanisms of aluminum (hydr)oxides

“…The surface charge, which is due to the protonation/deprotonation reaction of surface sites, is mainly a function of pH and the ionic strength of the aqueous solution, and consequently potentiometric titration techniques are the most commonly used method to evaluate the surface acid-base properties [7][8][9][10][11]. However, due to the slow surface hydration process, phase transformations at the surface and the solubility of oxides [4,7,[12][13][14][15], the titration results should be carefully evaluated in order to obtain consistent interpretation of various macroscopic parameters, such as PZC values and the acidity constants.…”

“…The surface morphologies keep changing during the hydration process. For instance, the surface of γ -Al 2 O 3 transforms to bayerite with prolonged hydration [12][13][14] and a mixture of α-Al(OH) 3 (bayerite) and γ -Al(OH) 3 (gibbsite) appears when the surface of α-Al 2 O 3 is exposed to water vapor for an hour [31]. Due to the chemical transformations and the complexity of the surface morphology during the hydration process of alumina, we can understand the aforementioned variation of surface properties.…”

Seeded growth of monodisperse gibbsite platelets to adjustable sizes

“…Secondly, gibbsite is studied for applications as a product as such: as polishing agent in toothpaste [5,6], as a fire retardant [5], as coating and filler in paper manufacturing [6], or as rheology enhancer in drilling muds [7]. And thirdly, since gibbsite is an important part of soils, people have studied its surface chemistry extensively [8][9][10][11].…”