Multinuclear studies of aluminium compounds

With the largest molecule size, most surface charge and highest polymerization degree among Al species with known structure, Al30 ([(AlO4)2Al28(OH)56(H2O)26]18+) has been attached increasing importance to various fields, including water treatment. Based on the relevant literatures in the past decades, this review tries to comprehensively elaborate the historical development of Al30, ranging from its discovery and formation, structure and physicochemical property, to potential application. Different from Al13 ([AlO4Al12(OH)24(H2O)12]7+), there is semi-pore geometry structure in Al30 central beltway, and hence the uneven distribution of surface electrostatic potential, which was more positive in beltway than that at cap. Meanwhile, Al30 exposed different-type oxygen functional groups, and the bound H2O located near the beltway was most acidic, making it most easy to deprotonate. The deprotonation and aggregation of Al30 occurred in similarly successive pattern, but the formed Al30 aggregate maintained positive charge within a wider pH range than Al13 aggregate. Moreover, Al30 is advantageous in high structural stability, which makes it intact under acidic or high-temperature condition. These unique characteristics facilitate the coagulation performance of Al30 towards organic matter, arsenic and microorganism by enhanced adsorption, charge neutralization and complexation. Meanwhile, with Al30, the effective dosage range was broader than that with Al13, and the coagulation process was more resistant to pH variation, which was beneficial to overcome the water quality fluctuation in actual water treatment. Another application of Al30 is the usage as pillaring agent to improve the adsorption and catalysis performance of clay. Finally, this review identifies the challenge and direction in the future development of Al30, including the improvement of characterization and quantitative methods, and pilot-scale investigation in actual water treatment.