Reaction mode between Si and Fe and evaluation of optimal species in poly-silicic-ferric coagulant.

A kind of Fe-polysilicate polymer, poly-silicic-ferric (PSF) coagulant was prepared by co-polymerization (hydroxylation of mixture of Fe3+ and fresh polysilicic acid (PS)), in which PSF0.5, PSF1 or PSF3 denotes Si/Fe molar ratio of 0.5, 1 or 3, respectively. The effects of Si/Fe ratio and reaction time (co-polymerization time or aging time) on the reaction mode between Si and Fe were studies, and the optimal species of PSF was evaluated by pH change during the preparation of PSF and coagulation tests. The results showed that the characteristics of PSF are largely affected by both reaction time and Si/Fe ratio. PSF is found to be a essential complex of Si, Fe, and many other ions. The reaction mode between Si and Fe differs with various Si/Fe ratios. The pH of PSF0.5, PSF1 or PSF3 tended to be stable when reaction time is 10, 25 or 55 min, respectively, which is almost consistent with the time reaching the relative stable morphology that is just the optimal species of higher coagulation efficiency. The optimal reaction time reaching optimal species can be evaluated by measuring the pH change during the polymerization process.

[Quantitative investigation on flocculation and destruction of poly-silicic-ferric (PSF) coagulant].

An inorganic polymer, poly-silicic-ferric (PSF) coagulant was prepared using water glass, FeSO4 x 7H2O and NaClO3 by copolymerization. A comparison of coagulation performance, distribution of residual flocs in finished water and influence of turbulent shear force on the flocs between PSF and polyferric aluminum (PFA) was explored by jar tests and particle counter approach. The results indicate that PSF has superior removal of COD at higher dose to PFA. Flocculating speed should not be much higher or lower when using PSF as coagulant in treating surface water, and excellent coagulation efficiency can be obtained by the close cooperation of flocculation speed with flocculation time, that is, higher flocculation speed should be consistent with shorter flocculation time. The PFA flocs are much easier destructed than PSF flocs when increasing the turbulent shear force, and the destructed PSF flocs tend to be more easily re-coupled than PFA flocs. From the order magnitude, the flocculation coefficient KA of PSF is from 10(-2) to 10(-3), in comparison with the destruction coefficient KB from 10(-7) to 10(-8). For particles whose size is smaller than 2 microm, KA of PSF is 4 order magnitude more than that of PFA, and KB of PSF is almost the same to that of PFA; while for the other particles, KA of PSF is 2 order magnitude more than that of PFA, and KB of PSF is almost ten times smaller than that of PFA.