Coagulation of silica microspheres with hydrolyzed Al(III)—Significance of Al13 and Al13 aggregates

Coagulation of silica microspheres by Al coagulants with high basicities was investigated at the constant pH 6.5. Aluminum coagulants were consisted of Al13 and [Al13]n aggregates, with OH/Al ratios of 2.46, 2.6 and 2.8, respectively. The species distribution and transformation of the coagulants upon aging were studied by using ferron assay and 27Al NMR. In addition, size distribution and morphology of coagulants were also examined by PCS and SEM. The results showed that the tridecamer Al13 and outer sphere aggregated [Al13]n were the dominant species for coagulants aged weekly, while sol–gels or precipitates were formed upon further aging. Accordingly, charge neutralization, electro-patch coagulation and bridge aggregation were proceeding for particle aggregation at lower aluminum concentration. Besides other three mechanisms, sweep flocculation was also involved at higher dosages. Hence, chemical interaction between particles and coagulants evolved from adsorption to surface precipitation for aluminum polycations by virtue of species transformation.