19 20The production and discharge of large volumes of wastewater during coal mining activities are one of 21 the major environmental issues in Australia. Therefore, it is crucial to develop and optimise effective 22 treatment processes for the safe disposal of coal mining wastewater (CMWW). In this study, we 23 investigated the performance of a recently developed polytitanium tetrachloride (PTC) coagulant and 24 compared with the performance of titanium tetrachloride (TiCl 4 ) and the commonly used ferric 25 chloride (FeCl 3 ) coagulant for the treatment of CMWW from one of the coal mining sites in Australia. 26The use of Ti-based coagulants is particularly attractive for the CMWW treatment due to the 27 advantage of being able to recycle the sludge to produce functional titanium dioxide (TiO 2 ) 28 photocatalyst; unlike the flocs formed using conventional coagulants, which need to be disposed in 29 landfill sites. The results showed that both PTC and TiCl 4 performed better than FeCl 3 in terms of 30 turbidity, UV 254 and inorganic compounds (e.g. aluminium, copper or zinc) removal, however, PTC 31 performed poorly in terms of dissolved organic carbon removal (i.e. less than 10%). While charge 32 neutralisation and bridging adsorption were the main coagulation mechanisms identified for TiCl 4 33 treatment ; sweep coagulation and bridging adsorption seemed to play a more important role for both 34 PTC and FeCl 3 treatments. The flocs formed by PTC coagulation achieved the largest floc size of 35 around 900 µm with the highest floc growth rate. Both Ti-based coagulants (i.e., PTC and TiCl 4 ) 36 2 showed higher strength factor than FeCl 3 , while TiCl 4 coagulant yielded the flocs with the highest 37 recovery factor. This study indicates that Ti-based coagulants are effective and promising coagulants 38 for the treatment of CMWW. 39

a b s t r a c tPolymeric 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 (TiCl 4 ) and a commonly used coagulant, ferric chloride (FeCl 3 ) in terms of water quality parameters and floc properties. Compared with FeCl 3 coagulant, titanium-based coagulants had broader region of good flocculation in terms of pH and coagulant dose. Further, they achieved higher removal of UV 254 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 TiCl 4 while sweep coagulation and adsorption were found to play a more important role for both FeCl 3 and PTC. The aggregated flocs formed by PTC flocculation had the largest floc size of around 836 lm 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 FeCl 3 , TiCl 4 and PTC respectively) while TiCl 4 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 TiO 2 photocatalyst which is a significant advantage over conventional coagulants.