Characteristics of poly-silicate aluminum sulfate prepared by sol method and its application in Congo red dye wastewater treatment

Abstract

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 PSAS_1.11 and PSAS_3.27 with different basicity were obtained by controlling the Al molar ratio of precursor to aluminum sulfate. The results showed that the PSAS_1.11 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 PSAS_1.11 with Al/Si of 20 and the effect of the molar ratio of aluminum to silicon on the basicity and stability of PSAS_1.11 were studied, respectively. Based on XRD and Fourier infrared (FTIR) characterization of the sol precursor and PSAS_1.11, the synthesis mechanism of PSAS by the silica-alumina sol method was discussed. Al species distribution of PSAS_1.11 was determined using the Al-Ferron timed spectrophotometric method. Moreover, the performance of PSAS_1.11 coagulant was examined, regarding its efficiency towards color removal of Congo red. The results showed that PSAS_1.11 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.