Effect of the Removal of Reactive Surface Material on the Solubility of Synthetic Gibbsites

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Abstract
The solubility of four different gibbsite preparations was measured, including two commercial hydrated aluminas produced by Alcoa, a Fisher ACS-grade Al(OH)3, and a laboratory preparation. The Alcoa samples and the laboratory-prepared sample had been studied previously by other investigators but without using a long-term acid treatment. Scanning electron microscopy showed globular surface material that was removed by a 14-day, 0.1 M HCl treatment. Solubility was determined at pH 4 from both over-and undersaturation with continuous agitation for 228 days. The acid treatment correlated with a decrease in log*Kso[*KSO = (Al3+)/(H+)3] of about 0.5 units. The mean solubility of the three acid-treated commercial gibbsite samples was log*Kso = 7.55 ± 0.055 (*Kso = 3.5 × 107). The solubility of the acid-treated laboratory preparation was log*Kso = 7.86 (*Kso = 7.2 × 107). The greater solubility of the laboratory-prepared gibbsite is attributed to a greater concentration of structural defects in the crystals.