Facile synthesis of composite poly-ferric-magnesium-silicate-sulfate coagulant with enhanced performance in water and wastewater

Coagulation process is a core technology that has been widely applied in water and wastewater treatment. In this paper, novel composite coagulants poly-ferric-magnesium-silicate-sulfate (PFMS) were synthesized by using Na₂SiO₃·9H₂O, Fe₂(SO₄)₃ and MgSO₄ as the raw materials. The effects of aging time, Fe:Si:Mg and OH:M molar ratios (M represents the metal ions) on the coagulation performance of the as-prepared PFMS were systematically investigated to obtain optimum coagulants. The results showed that PFMS coagulant exhibited good coagulation properties in treating simulated humic acid-kaolin surface water and reactive dye wastewater. When the molar ratios were controlled at Fe:Si:Mg=2:2:1 and OH:M=0.32, the obtained PFMS presented excellent stability along with high coagulation efficiency (the removal efficiency of UV₂₅₄ was 99.81%, and the residual turbidity of the surface water was 0.56 NTU at a dosage of 30 mg/L). After standing the coagulant for 120 days in the laboratory, the removal efficiency of UV₂₅₄ and residual turbidity of the surface water still maintained 88.12% and 0.68 NTU, respectively, meeting the surface water treatment requirements. In addition, the coagulation performance in treating reactive dye wastewater was greatly improved by combining the advantages of magnesium and iron salts. Compared with poly-ferric-silicate-sulfate (PFS) and poly-magnesium-silicate-sulfate (PMS), PFMS coagulant played a good decolorization role within the pH range of 7-13.