Evaluation of aluminum-silicate polymer composite as a coagulant for water treatment

Aluminum-silicate polymer composite (PASiC), a new kind of inorganic coagulant, was produced by two approaches: (1) hydroxylation of the mixture of AlCl3 and fresh polysilicate (PASiCc); (2) hydroxylated polyaluminum-chloride (PAC) combined with fresh polysilicate (PASiCm). The PASiC products had the following properties: Al2O3 content = 6.40-7.30%, SiO2 content = 0.40-0.82%, Al/Si ratios = 10-20, basicity (OH/Al molar ratio, denoted B) = 1.2-2.0. The coagulation behaviour of PASiC and PAC under conditions typical for coagulation and flocculation in water treatment were investigated by studying the rate of floc size development, the variety of streaming current value, the efficiency of turbidity removal and the effect of pH on the turbidity removal efficiency, and the stability of PASiC. The results detailed in this study suggest that, compared with PAC, PASiC may enhance aggregating efficiency and give better coagulating effects, but weaken charge effectiveness in coagulation process or become unstable when stored for longer time, especially at higher B value and lower Al/Si ratio. The coagulating effect of PASiC is closely linked to the preparation procedure. With the increase of B value and the decrease of Al/Si ratio, the coagulation efficiency of PASiC increases, and at the same B value and Al/Si ratio, PASiCc seems to give a little better coagulation effect than PASiCm but less stability. The Al/Si ratio should not be too low or the B value should not be too high, otherwise, the PASiC products tend to become cloudy or partly gelatinous, which will make them loss some coagulation efficiency.