The Nature of Polynuclear OH-Al Complexes in Laboratory-Hydrolyzed and Commercial Hydroxyaluminum Solutions

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Abstract
Laboratory-hydrolyzed and commercial OH-AI solutions were characterized using kinetics of Al-ferron color development, kinetics of structural OH neutralization with H +, 27A) NMR spectroscopy, and sulfate precipitation. The results showed that the All3 complexes having the Keggin structure were dominant only in fresh, laboratory-hydrolyzed OH-A1 solutions of OH/Al molar ratio = 1.8 and above. These species gradually converted to other polynuclear forms that reacted with ferron slowly, were not detectable by 27A1 NMR spectroscopy, and yielded different basic A) sulfates following Na2SO4 addition. These more stable complexes can best be interpreted to have a Al(OH)3-fragrnent structure. In the three commercial aluminum chlorohydrate (ACH) solutions studied, A)I3 complexes accounted for a small portion of the total AI present. More than 80% of the AI was present as species that were not detectable with NMR spectroscopy and resembled the slow-reacting complexes in aged, laboratory-hydrolyzed OH- AI solutions. Small portions of the slow-reacting complexes appeared to be submicron particulates that acted as nuclei for gibbsite formation or aggregates of All3 complexes that dispersed to Al 13 upon dilution. Polyaluminum chloride (PA) solution resembled the moderately aged laboratory-hydrolyzed OH-AI so- lutions.