Thermal Aging of Heteroatom-Substituted Keggin-Type Aluminum Oxo Polycation Solutions: Aggregation Behavior and Impacts on Humic Acid and Turbidity Removal

Abstract

Keggin-type aluminum-oxo polycations are effective coagulating agents for wastewater treatment. Previous work demonstrated that heteroatom substitution in Keggin-type ϵ-Al13 polycation leads to the formation of ϵ-GaAl12 and ϵ-GeAl12 and can enhance removal of bacteria, DOC, and turbidity from wastewater. Additional hydrolysis of the ϵ-Al13 species creates larger Al30 species and improves coagulation performance, but this aspect has not been evaluated for the heteroatom 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. Thermogravimetric analyses of metal hydroxide formed during aging indicate that the GaAl12 system behaves more like an amorphous Al(OH)3 phase, which has higher solubility than other aluminum hydroxide phases. Aging did not significantly change % humic acid (HA) removal, as all solutions showed high removal efficiency at all studied pH values. GaAl12 solutions demonstrated good turbidity removal efficiency in all pH ranges, with enhanced performance at pH 5. The aged GaAl12 solutions contain relatively stable Keggin oligomers, which may contribute to enhanced contaminant removal.