Effect of aging condition on species transformation in polymeric Al salt coagulants

Effect of aging time and aging temperature on the transformation process of Al species in hydroxyl Al solutions were analyzed by Ferron assay and 27Al NMR spectroscopy with the help of analysis results of pH, turbidity and morphology of samples. Owing to the high chemical energy provided by elevated temperature, aging temperature exerted greater effect on the transformation of polymers than aging time. Elevation of aging temperature enhanced the transformation rate of various Al species in solution. Polymerization or hydrolysis of monomers in solution was not detected under various aging conditions. Polymeric Al species with non-Keggin structure (e.g., hexameric ring structure) were less stable than Keggin-Al13. The decomposition of Al13 only occurred at higher aging temperature. The coexistence of oligomers affected the transformation of Al13. Morphology and species composition of colloidal Al species transformed from Al13 were different from those from polymeric Al species with non-Keggin structure. Changes in pH, turbidity and morphology of samples agreed well with the distribution of Al species obtained by Ferron assay and 27Al NMR spectroscopy.