Clay influence on the crystallization of aluminum hydroxide polymorphs in the presence of citrate, sulfate or chloride

Abstract

The stability of montmorillonite—Al(OH) x interlayer complexes at pH 7–9 increased with lesser amounts of Al precipitate and in the presence of electrolytes. The relative effectiveness of the anions in stabilizing interlayering was in the order, citrate ⪢> sulfate > chloride. Montmorillonite catalyzed the formation of the crystalline Al(OH) 3 polymorphs, nordstrandite and gibbsite, in preference to bayerite. In clay-free systems, particularly in the presence of citrate, stable pseudoboehmite formed readily at pH 8–10. On the contrary, the surface of montmorillonite inhibited the crystallization of pseudoboehmite, probably nucleating gibbsite and/or nordstrandite from amorphous Al(OH) 3. Only in the presence of very high concentrations of electrolyte (2 or 3 N NaCl at pH 9) did pseudoboehmite crystallize as a separate phase with montmorillonite also present. Electron microscopy showed ovoidal crystal morphology of nordstrandite in the presence of clay in citrate systems at 25°C and pH 9; thin, elongated, radial clusters of nordstrandite and/or gibbsite formed at 35°C and pH 9 or higher. In sulfate or chloride systems, nordstrandite occurred as platey, thin ovoid to rectangular particles.