The slow dissolution of oxide minerals controls a variety of processes in terrestrial systems. Dislocation of iron and aluminum oxides during podzolization and buffering of acid deposition by aluminium oxides are among the examples for such processes. This study focuses on the dissolution kinetics of aluminum oxide for a range of geochemically relevant conditions. The influence of solution saturation state on dissolution kinetics has been investigated as well as competitive and synergistic effects of multiple adsorbed species. The results are discussed in the framework of the surface complexation / dissolution model.
Dissolution studies of d-aluminum oxide suspensions were conducted in batch experiments and under steady state conditions using continuous flow stirred tank reactors (CFSTR). The experiments have been designed to investigate
a) dissolution kinetics in the absence of organic ligands (pH range 3 to 9)
b) adsorption of organic ligands (pH range 3 to 9)
c) influence of pH on ligand enhanced dissolution (pH range 3 to 9)
d) influence of solution saturation state on ligand enhanced dissolution (pH 7.5)
e) influence of competitive adsorption of ligands on dissolution kinetics (pH 7.5).
The organic ligands used in this study were 8-hydroxyquinoline-5-sulfonate (HQS), EDTA and catechol. These ligands adsorb to the aluminum oxide surface over a wide pH range.
The dissolution rates in the absence of an organic ligand are highest at the lowest and highest pH within the studied range. In the presence of an organic ligand (HQS) a similar trend is observed with lowest dissolution rates at pH 5.5. The study indicates that significant ligand enhanced dissolution may take place even at neutral pH.
The saturation state of the solution with regard to aluminum oxide influences the net dissolution rates. Far from equilibrium the dissolution reaction proceeds with a constant rate. The net dissolution rate decreases near equilibrium. As equilibrium is approached, the concentration of weakly adsorbed aluminum ligand complexes increases.
Dissolution kinetics in the presence of two organic ligands (HQS and EDTA) indicate that both ligands compete for adsorption at potential sites for dissolution.