| Coagulation is an important technology used generally in water treatment. The cost of coagulants approximately accounts for 50 percent of the total cost of the water treatment. Therefore, the researches of high efficient,cheap coagulants have been concern. Polyaluminum salt have been widely used as water treatment agent for living, industrial water service and industrial effluent. To improve its abilities of neutralizing electric charge or complexing, it is effective to introduce some other ions into polyaluminum salt to form polyblend of aluminum salt. A series of polyaluminum silicate sulfate (PASiS) with different basicities(B),Al/Si molar ratios and polymerization degree of silicic acid were prepared by means of two kinds of preparation methods: one is to add NaOH into Al2(SO4)3 and polysilicic acid mixed solution at different Al/Si molar ratios and polymerization degree; the other is to add polysilicic acid with different polymerization degree into polyaluminum sulfate(PAS) solution with different basicities(B) at Al/Si molar ratios.The effects of aluminum species distribution and charged characteristic of hydrolysis products of PASiS prouced by various preparation methods and factors were investigated by the Al-Ferron timed complex colorimetric methodand the micro -electrophoresis method. Then PASiS were applied in the wastewater treatment. The experimental results show that the interaction between polysilicic acid and hydrolysis aluminum species in PASiS forms aluminum-silicate polymer composite with larger size polymer. But it is difficult to the hydrolyzed aluminum species and the large polymerization degree of silicic acid to interact with each other. The multinuclear hydroxy aluminum is still the major species in PASiS. Especially the ζ-potential of this PASiS with B=1, Al/Si=2.5,t=150min is higher than that of PAS. B has agreat effect on the distribution of aluminum species andζ-potential and flocculation property. With the B value increasing in the range of B=0.5～1.5, the larger polymer aluminum-silicate composite would form, the better flocculation efficiency would be. But the more B would make silicic acid coagulate. Al/Si molar ratio and polymerization degree of silicic acid also effect on the distribution of aluminum species of PASiS. Through studies of the species and ζ-potential and flocculation property, it was suggested that PASiS has the favorable abilities of charge neutralization and absorption -bridging, and absorption-bridging is the dominate mechanism. The interaction patterns between silicic acid and hydrolysis aluminum has two ways: one is that silicic acid interact directly with hydroxyl of hydrolysis aluminum by hydrogen bond(Si-O...HO-Al) to form the structure such as [Alx(OH)y]H3SiO4(3x-y-1)+; the other is that silicic acid bind the hydrolysis aluminum by oxygen bridge (Si-O-Al). During the microtitration, the structure "SiO4"would partly take the palace of the structure"AlO4" to form the larger polymer molecule such as [SiO4Al12(OH)248+], which is similar to Al12AlO4(OH)247+.PASiS flocculant is not a compound but a mixture, which included polymeric aluminum-silicate composite and polysilicic acid and polymeric hydrolyzed aluminum species. In the wastewater treatment, it is polymeric aluminum-silicate composite and polysilicic acid and polymeric hydrolyzed aluminum species that took effect together. The flocculation properties of PASiS are related to the contents and species of the three parts above. PASiS with different B and Al/Si molar ratio and polymerization degree of silicic acid would show different flocculation properties in the treatment of different water and wastewater.The PASiS could be good flocculating efficiency only at a proper Al/Si molar ratio and polymerization degree of silicic acid. The PASiS has better flocculation property than PAS, and the PASiS prepared by the copolymerization technique has better flocculation property than the PASiS prepared by the composite technique. |
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