Properties and Evaluation of Polyferric-Silicate-Sulfate (PFSS) Coagulant as a Coagulant for Water Treatment

Abstract

Polyferric-Silicate-Sulfate (PFSS). as a new type of coagulant, was prepared by using sodium silicate, sulfuric acid and ferric sulfate as materials. Three topics were investigated: (I) the zeta potential of hydrolyzate of PFSS under different pH values, (2) the effects of Fe/Si molar ratio and dosage of PFSS on turbidity removal, and (3) the relationship between the stability of PFSS and Fe/Si molar ratio in PFSS solutions. There is a relationship between the optimum coagulation pH range and Fe/Si molar ratio, and the coagulation mechanism of PFSS was discussed. The experimental results show that Fe/Si molar ratio has an effect on the zeta potential of hydrolyzate, the stability of PFSS, the coagulation performance and the optimum coagulation pH range of PFSS. If the Fe/Si ratio is too high or too low. it will not be beneficial to the stability of PFSS solutions. A Fe/Si ratio of about 1.0 gives the most stable PFSS solution. It was also found that PFSS gave the best turbidity removal effect when its Fe/Si molar ratio was 1.5. The results suggest that the composite of ferric sulfate and polysilicic acid can enhance aggregating efficiency due to higher molecular weight, but is weaker in tenns of charge effectiveness in the coagulation process due to charge ncutralization occurring when hydrolyzed ferric species of different charge and polysilicic acid are combined. Therefore, it is important to consider the aggregating efficiency and charge effectiveness when developing a PFSS coagulant.