Zeta potentials and sizes of aluminum salt precipitates – Effect of anions and organics and implications for coagulation mechanisms

Zeta potentials and sizes of aluminum salt precipitates were examined as a function of solution pH at different concentrations, based on laser Doppler velocimetry and dynamic light scattering techniques. A great difference was observed in zeta potential and size of the aluminum hydroxide precipitates from aluminum nitrate and aluminum sulfate (alum) solutions. While the isoelectric point (IEP) of aluminum nitrate precipitate has a fixed value (9), that of alum precipitates decreased with the salt concentration. The size of the early-formed fresh hydroxide precipitates of aluminum nitrate was around a few hundred nm in the acidic pH range, but increased from neutral pH up to 15 μm at the IEP. The particles of precipitates are colloidally stable in the pH range 5–6. In comparison, the size of alum precipitates was comparatively larger in the acidic up to neutral pH, and are colloidally unstable in the whole pH range. Relative influences of anions such as sulfate and phosphate, silicic acid, as well as citrate and oxalate on zeta potential and precipitate size are explicitly revealed experimentally. The results also indicate important roles of van der Waals attraction, as well as hydration force in the aggregation of the primary precipitates. Sulfate ions appear to significantly promote Al3+ precipitation in humic acid water. Overall, this work provides further insight on coagulation mechanisms of clay particles and natural organic materials (NOM) in water treatment, particularly in sweep coagulation.