Thermal aging of heteroatom substituted Keggin type aluminum oxo polycation solutions: Aggregation behavior and impacts on dissolved organic carbon and turbidity removal

Coagulation processes within water treatment plays an important role in contaminant removal and aluminum-oxo Keggin polycations are proved to be an effective coagulating agents. Previous work demonstrated that heteroatom substitution within the Keggin-type polycation ε-Al13 to form ε-GaAl12 and ε-GeAl12 can enhance removal of bacteria, DOC, and turbidity from wastewater. Additional hydrolysis of the ε-Al13 species to form larger Al30 species has also been shown to improve coagulation, but this aspect has not been evaluated for the ε-GaAl12 and ε-GeAl12 systems. In the current study, hydrolysis of ε-Al13, ε-GaAl12 and ε-GeAl12 was promoted through hydrothermal aging to evaluate the overall solution stability/behavior and water treatment efficiency. Turbidity measurement of aged solution indicated that Ga substituted aluminum-oxo Keggin polycations remain stable in solution and DLS studies demonstrated greater diversity in particle sizes within the system. Additional thermogravimetric analyses of metal hydroxide precipitates formed from the aging studies indicate that the GaAl12 system behaves more like an amorphous Al(OH)3 phase, which has higher solubility than other aluminum hydroxide phases. Hydrothermal aging did not significantly change %DOC removal as all solution showed high efficiency for removal across a range of pH values. GaAl12 solutions demonstrated good turbidity removal efficiency in all pH range, with enhanced performance at pH 5. The study suggests that larger, relatively stable oligomers do exist within the aged GaAl12 solutions that may contribute to enhanced contaminant removal in a similar manner to what is observed within the PACl-Al30 coagulant.