Coagulation performance and floc characteristics of polytitanium tetrachloride (PTC) compared with titanium tetrachloride (TiCl4) and iron salts in humic acid–kaolin synthetic water treatment

[Display omitted] •PTC and TiCl4 had broader flocculation region in terms of pH and coagulant dose.•Titanium-based coagulants achieved higher turbidity and UV254 removal.•PTC showed the fastest growth rate and the largest floc size.•Sweep coagulation and adsorption were the main mechanisms for PTC. Polymeric metal coagulants are increasingly used to improve the coagulation/flocculation process efficiency, yet the research on the development of titanium and particularly polytitanium salts remains very limited. In this study, the performance of recently developed polytitanium tetrachloride (PTC) coagulant was compared with both titanium tetrachloride (TiCl4) and a commonly used coagulant, ferric chloride (FeCl3) in terms of water quality parameters and floc properties. Compared with FeCl3 coagulant, titanium-based coagulants had broader region of good flocculation in terms of pH and coagulant dose. Further, they achieved higher removal of UV254 and turbidity but lower dissolved organic carbon (DOC) removal. Charge neutralisation, physical entrapment of colloids within coagulant precipitates and adsorption were found to be the main coagulation mechanisms for TiCl4 while sweep coagulation and adsorption were found to play a more important role for both FeCl3 and PTC. The aggregated flocs formed by PTC flocculation had the largest floc size of around 836μm with the highest floc growth rate. A little distinction of the floc strength factor was found among the coagulants tested (i.e. 44.8%, 44.2% and 38.9% for FeCl3, TiCl4 and PTC respectively) while TiCl4 coagulant yielded the flocs with the highest floc recovery factor. This study indicates that Ti-based coagulants are effective and promising coagulants for water purification. Besides, the resulted flocculated sludge can be recycled and produce functional TiO2 photocatalyst which is a significant advantage over conventional coagulants.