Preparation and characterisation of polyaluminium silicate chloride coagulant.

To improve the coagulation efficiency for water treatment purposes, a composite aluminium-silicon coagulant, the polyaluminium silicate chloride was prepared and characterised. The preparation process included the preparation of polyaluminium chloride and polysilicic acid followed by the compounding of the two solutions. The prepared polyaluminium silicate chloride coagulant solution had an aluminium concentration of 0.10 mole per litre, hydroxyl to aluminium molar ratios between 0.5 and 2.0, and silicon to aluminium molar ratios between 0.075 and 0.075. Aluminium-27 nuclear magnetic resonance spectroscopy, photon correlation spectroscopy, streaming current measurement, jar tests and pilot-scale coagulation tests were employed to study the aluminium speciation, particle size distribution, electrokinetic and coagulation properties of this composite coagulant. In comparison with polyaluminium chloride, polyaluminium silicate chloride contains less polynuclear aluminium [AlO4Al12(OH)24(H2O)12]7+ and shows smaller charge neutralisation capacity. However, its particle size shows a significant increase, which enhances the coagulation efficiency. The results show that polyaluminium silicate chloride is an efficient composite coagulant for water treatment.