RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Synthetic gibbsite was prepared and thoroughly characterized using X-ray diffraction, high-resolution transmission electron microscopy, atomic force microscopy, and Fourier transform infrared techniques. The protonation and charging of the particle surface(s) in 20 and 100 mM (Na)Cl ionic media were investigated using potentiometric titrations and zeta potential measurements. These measurements indicated a pH_pznpc = 9.0 ± 0.2 and pH_IEP = 10.0 ± 0.1. Furthermore, it was concluded that the concentration of singly coordinated sites (≡AlOH) located at the edges of the particles is too low (less than 7% of the total number of crystallographic sites) to explain the proton uptake. Therefore, the doubly coordinated sites (≡Al₂OH) on the basal planes must be proton reactive. Both proton data and zeta potentials could be described using a three-plane model where adsorbed Na⁺ at the basal planes was placed in the 1-plane and all other adsorbed medium ions were placed in the 2-plane. The following results were obtained:

Basal planes:
≡Al₂OH + H⁺ + Cl⁻ ↔ ≡Al₂OH₂⁺Cl⁻   lgK = 8.49 ± 0.01   (1)
≡Al₂OH + Na⁺ ↔ ≡Al₂O⁻Na⁺ + H⁺   lgK = -9.62 ± 0.01   (2)

Edges:
≡AlOH^0.5− + H⁺ ↔ ≡AlOH₂^0.5+   lgK = 10.0   (3)
≡AlOH^0.5− + Na⁺ ↔ ≡AlOH^0.5−Na⁺   lgK = 0.1   (4)
/ ≡AlOH₂^0.5+ + Cl⁻ ↔ ≡AlOH₂^0.5+−   lgK = 0.1   (5)

The protonation constant for the singly coordinated sites (reaction 3) was set equal to pH_IEP. The values of reactions 4 and 5 were taken from the literature. The effect of different reaction times on proton uptake and zeta potential is highlighted. It is clear that increased “equilibrium times” will result in an increased proton uptake, whereas the zeta potential remains constant. This is also reflected in the divergent models presented in the literature with respect to the protonation and charging properties of gibbsite particles.