A novel method for synthesizing poly-silicate aluminum sulfate coagulant (PSAS) using a silica-alumina sol was reported. Herein, two modalities (nSiO2/nNa2O: 1.11 and 3.27) of self-made water glasses were used as the silica source for synthesizing the sol precursor. Then, the PSAS1.11 and PSAS3.27 with different basicity were obtained by controlling the Al molar ratio of precursor to aluminum sulfate. The results showed that the PSAS1.11 coagulant prepared with low modulus water glass (LMWS, 1.11) has low turbidity and good stability. Using low modulus water glass, the effect of the Al molar ratio of precursor to aluminum sulfate on the basicity and stability of PSAS1.11 with Al/Si of 20 and the effect of the molar ratio of aluminum to silicon on the basicity and stability of PSAS1.11 were studied, respectively. Based on XRD and Fourier infrared (FTIR) characterization of the sol precursor and PSAS1.11, the synthesis mechanism of PSAS by the silica-alumina sol method was discussed. Al species distribution of PSAS1.11 was determined using the Al-Ferron timed spectrophotometric method. Moreover, the performance of PSAS1.11 coagulant was examined, regarding its efficiency towards color removal of Congo red. The results showed that PSAS1.11 coagulant with Al/Si of 20 and Al molar ratio of 1/12 exhibits excellent performance, and the color removal rate reached 98.6% at an initial pH of 11 and coagulant dosage of 40 mg L-1 (Al mg L-1). Finally, the PSAS coagulant mechanism was discussed in detail through infrared characterization, 27Al NMR, Raman, morphology and mapping of the flocs.
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