Study on the hydrolysis/precipitation behavior of Keggin Al13 and Al30 polymers in polyaluminum solutions

The hydrolysis/precipitation behaviors of Al(3+), Al(13) and Al(30) under conditions typical for flocculation in water treatment were investigated by studying the particulates' size development, charge characteristics, chemical species and speciation transformation of coagulant hydrolysis precipitates. The optimal pH conditions for hydrolysis precipitates formation for AlCl(3), PAC(Al13) and PAC(Al30) were 6.5-7.5, 8.5-9.5, and 7.5-9.5, respectively. The precipitates' formation rate increased with the increase in dosage, and the relative rates were AlCl(3)>>PAC(Al30)>PAC(Al13). The precipitates' size increased when the dosage increased from 50 microM to 200 microM, but it decreased when the dosage increased to 800 microM. The Zeta potential of coagulant hydrolysis precipitates decreased with the increase in pH for the three coagulants. The iso-electric points of the freshly formed precipitates for AlCl(3), PAC(Al13) and PAC(Al30) were 7.3, 9.6 and 9.2, respectively. The Zeta potentials of AlCl(3) hydrolysis precipitates were lower than those of PAC(Al13) and PAC(Al30) when pH>5.0. The Zeta potential of PAC(Al30) hydrolysis precipitates was higher than that of PAC(Al13) at the acidic side, but lower at the alkaline side. The dosage had no obvious effect on the Zeta potential of hydrolysis precipitates under fixed pH conditions. The increase in Zeta potential with the increase in dosage under uncontrolled pH conditions was due to the pH depression caused by coagulant addition. Al-Ferron research indicated that the hydrolysis precipitates of AlCl(3) were composed of amorphous Al(OH)(3) precipitates, but those of PAC(Al13) and PAC(Al30) were composed of aggregates of Al(13) and Al(30), respectively. Al(3+) was the most un-stable species in coagulants, and its hydrolysis was remarkably influenced by solution pH. Al(13) and Al(30) species were very stable, and solution pH and aging had little effect on the chemical species of their hydrolysis products. The research method involving coagulant hydrolysis precipitates based on Al-Ferron reaction kinetics was studied in detail. The Al species classification based on complex reaction kinetic of hydrolysis precipitates and Ferron reagent was different from that measured in a conventional coagulant assay using the Al-Ferron method. The chemical composition of Al(a), Al(b) and Al(c) depended on coagulant and solution pH. The Al(b) measured in the current case was different from Keggin Al(13), and the high Al(b) content in the AlCl(3) hydrolysis precipitates could not used as testimony that most of the Al(3+) was converted to highly charged Al(13) species during AlCl(3) coagulation.