Effect of thermal treatment on the formation and transformation of Keggin Al13 and Al30 species in hydrolytic polymeric aluminum solutions

The effect of thermal treatment on the formation and transformation of Al13 and Al30 species in highly concentrated hydrolytic polymeric Al solutions (HPA) (0.2–1.0 mol L−1) were studied quantitatively using high-field 27Al nuclear magnetic resonance (NMR) and time-developed Al-Ferron complex colorimetry. The results indicate that the metastable Al13 species in HPA tends to polymerize and transform into Al30 and other high polymers upon thermal treatment. The polymerization and transformation are speed up by heating at high temperature. Al13 content in HPA decreases upon thermal treatment, and Al30 content increases. Al13 does not convert to Al30 quantitatively due to its aggregation and acidic dissociation reactions. Al13 species is a dominant nanosized Al species in freshly prepared HPA solutions, and its content reaches above 80.0% in HPA with AlT = 0.2 mol L−1 and optimal hydrolysis ratios (B) (ca. 2.3–2.4). But the content of Al30 becomes higher than that of Al13 when the HPA solution is heated at 95 °C for more than 12 h. Al30 becomes the dominant nanosized Al species in HPA, and its content reaches 76.8% in sample with AlT = 0.2 mol L−1 and B = 2.4. Although Al30 is more stable than Al13, it is also a metastable species during the process of Al hydrolysis, polymerization and precipitation. The decomposition and polymerization of Al30 are also speed up by increase the heating temperature. The optimal preparation temperature of Al30 is ca. 95 °C. Al species in HPA solutions transform into more stable and inert species through inter-species polymerization and intra-species structure rearrangement upon thermal treatment. So Alb contents decrease, and Alc contents increase. Al30 cannot react with Ferron within 120 min, and it is classified as Alc species in Al-Ferron analysis. The effect of thermal treatment on Al speciation in HPA solutions with different total Al concentration and B value were also investigated.