Enhanced Coagulation-Flocculation Performance of Iron-Based Coagulants: Effects of PO(4) (3-) and SiO(3) (2-) Modifiers

PO(4) (3-) and SiO(3) (2-) are often used as modifier to improve stability and aggregating ability of the iron-base coagulants, however, there are few reports about their detailed comparison between the coagulation performance and mechanisms. In this study, three coagulants—polyferric phosphoric sulfate (PFPS), polysilicon ferric sulfate (PFSS), and polyferric sulfate (PFS) were synthesized; their structure and morphology were characterized by Fourier transformed infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) and Scanning electron microscope (SEM). Alkali titration and Ferron species analysis were employed to investigate the hydrolysis performance and species distribution. Jar test was conducted to measure their coagulation behaviors at different dosage, pH, and temperatures in which the flocs properties were measured. The results showed that a number of new compounds were formed due to the presence of PO(4) (3-) and SiO(3) (2-). Moreover, PFPS and PFSS had similar level in Fe(a) as well as Fe(b). Among them, PFPS produced more multi-core iron atoms polymer and content of Fe(b), and the formed flocs were larger and denser. It exhibited superior coagulation performance in terms of turbidity reduction, UV(254) removal and residual ferric concentration. Jar test and floc breakage/regrowth experiments indicated other than charge neutrality, the dominated mechanism involved in PFSS was the adsorption between polysilicic acid and solution particle, while PFPS was sweeping, entrapment/adsorption resulting from larger polymer colloid of Fe-P chemistry bond.