A study of colloidal Al(III)-containing species in fresh/caustic aluminate solutions

The nature of Al(III)-containing solution species as a function of solution NaOH and Al(III) concentrations, prior to Al(OH)3 nuclei formation and growth, has been investigated using light scattering. Three types of solution series were analysed, constant [Al(III)] (0.82 M and 4.10 M) with varying [NaOH] (1.00–6.00 M and 5.00–7.00 M), constant [NaOH] (1.00 M and 5.00 M) with varying [Al (III)] (0.14–0.82 M, 0.40–4.10 M) and constant [NaOH]/[Al] of 1.22 ([NaOH] 0.31–6.00 M). It is found that NaOH concentration determines the nature of the initial speciation. At [NaOH]<1.00 M, a high phase contrast between the scattering species and background solvent is observed. The species appear to be relatively large, Al(III)-polycondensed structures, commensurate with the presence of Keggin ions. A significant decrease in light scattering is observed between 1.00 M and 2.5 M NaOH. This is believed to be due to the presence of species with lower order or density than the Keggin ion. Further increase in NaOH concentration to 7.00 M did not have any significant effect on the light scattering of the species regardless of Al(III) concentration. This indicates that the Al(III) containing species formed have less structure or are small, consistent with predominantly Al(OH)4− monomers, and effectively form part of the solvent.