Abstract:
Ligand-controlled dissolution of δ-Al2O3 and goethite in the presence of 8-hydroxyquinoline-5-sulfonate (HQS) has been observed as a function of the solution saturation state at constant pH and temperature. A continuous flow stirred tank reactor (CFSTR) was designed to conduct the experiments under steady-state conditions. Far from equilibrium, the dissolution kinetics of goethite are independent of the solution saturation state. Net dissolution rates decrease as equilibrium is approached. As the solution becomes oversaturated, precipitation is observed. Two factors may influence the goethite dissolution kinetics as a function of solution saturation state: the increase of the back reaction (precipitation) as equilibrium is approached and changes of the metal-organic surface speciation. Both effects can be accounted for using a general rate law for ligand-controlled oxide dissolution kinetics. The effect of the solution composition on the dissolution of δ-Al2O3 is characterized by a ligand-promoted recrystallization of the unstable parent mineral phase to yield a (meta) stable product.