Polyferric silicate sulphate (PFSiS): Preparation, characterisation and coagulation behaviour

The process of coagulation is a fundamental environmental protection technology, which is mainly used in the treatment of water and wastewater. The relevant research is currently focused on the development of new improved coagulation reagents. A characteristic example is polyferric silicate sulphate (PFSiS), a relatively new prepolymerised inorganic coagulant. In this paper, PFSiS is prepared following two approaches: (a) hydroxylation of the mixture consisting of Fe2(SO4)3 and fresh polysilicic acid (PFSiSc); (b) hydroxylated polyferric sulphate (PFS) combined with fresh polysilicic acid (PFSiSm) under different Fe/Si and OH/Fe molar ratios. The prepared coagulants were characterised in terms of major typical properties, such as the degree of polymerisation, pH value, ζ-potential, conductivity, turbidity etc. Furthermore, the effect of different Fe/Si and OH/Fe molar ratios and of aging time on the values of pH and ζ-potential of products were also investigated. Finally, the coagulation behaviour of PFSiS was assessed by studying the reduction of the turbidity of kaolin (clay) model suspensions. The obtained results suggest that PFSiS exhibits a superior coagulation performance in comparison with other iron-based coagulants.

Fig. 4. Comparison of coagulation efficiency between PFSiS, PFS and FS (ferric sulphate). is worth noting that the amount of cationic charge is important in coagulation process, as it controls the extent of particles destabilisation via the “adsorption and charge neutralisation” mechanisms. However, despite the fact that the cationic charge of new coagulants is partially compromised and thus, the charge neutralisation ability is reduced, this limited drawback is compensated by the enhanced bridge-aggregation capacity. Nevertheless, the amount of silica being introduced into the structure of coagulants is still an issue, as far as the coagulation efficiency is concerned, due to the fact that excessive proportion of silica would virtually neutralise the cationic charge; thus deteriorating the particle destabilisation ability. Therefore, an ideal analogy of iron and silica (expressed as Fe/Si molar ratio) within the composition of the new coagulant must be determined in order to optimise its coagulation efficiency. Additionally, PFSiSc seems to be more efficient in turbidity removal than the PFSiSm samples, which can be attributed to the higher degree of their polymerisation. It has been well established that the presence of polymeric species has an essential role in the coagulation process, resulting in superior coagulation efficiency [15,18,19].