The influence of Al(III) supersaturation and NaOH concentration on the rate of crystallization of Al(OH)3 precursor particles from sodium aluminate solutions

The growth kinetics of colloidal Al(III)-containing particles (diameter<1000 nm), nucleated in optically clear, supersaturated sodium aluminate solutions as a precursor to Al(OH)3 crystals, has been studied using dynamic light scattering. Two series of solutions were examined at 22 °C to determine the influence of Al(III) supersaturation and NaOH concentration on the initial particle growth behavior. One solution series consisted of solutions with constant Al(III) absolute supersaturation (ΔC) of 1.48 M and [NaOH] range 1.83-4.00 M ([NaOH]/[Al(III)]=1.13-2.15) and Al(III) relative supersaturation (σ)=3.86-10.36. The other solution series had a constant σ of 7.55 and [NaOH] range of 1.50-4.27 M ([NaOH]/[Al(III)]=1.18-1.54) and ΔC=0.86-3.19. The correlation between the initial particle growth rates and supersaturation (ΔC or σ) revealed marked anomalies over the entire supersaturation range studied. The growth rate remained substantially constant in the ΔC range 0.86-2.55 M (for the constant σ solution series), before increasing sharply upon a further increase of ΔC beyond 2.55 M. The variation of the growth rate with σ in the range 3.86-9.00 (for the constant ΔC solution series) was remarkably weak, contrary to expectation. At higher σ (>9.00), however, a marked increase in growth rate with increasing σ was displayed. At constant ΔC or σ, the growth rate showed a strong variation with NaOH concentration, indicating that Na+ and OH− species play a pivotal role in the Al(OH) precursor particles (nuclei) growth process. Furthermore, the kinetics of growth displayed by these nanosized particles are an order of magnitude slower than those observed for macroscopic gibbsite (γ-Al(OH)3) crystals at similar supersaturations and temperature. The difference may be rationalized in terms of particle size and Al(OH)3 dimorphic phase dependent solubility effects. An empirically adequate growth kinetics modeling was achieved when the growth rates were correlated with the Al(III) supersaturation (ΔC or σ) and the excess (free) NaOH concentration, rather than the former alone, as is commonly the case. A critical [NaOH]/[Al(III)] molar ratio of 1.27-1.35, below which the particle growth rate increased markedly and above which the rate was significantly reduced, was observed. This behavior is believed to be linked to solution speciation change that occurs at certain Al(III) and NaOH compositions.