The interactions between Al (III) and Ti (IV) in the composite coagulant polyaluminum-titanium chloride

Safe and effective coagulants are necessary for surface water treatment. The polyaluminum-titanium chloride (PATC) composite coagulant has promise but its application is limited due to the lack of fundamental theoretical research. We prepared a series of novel PATCs and investigated the effects of the interaction between Al (III) and Ti (IV) on the chemical species distribution, physicochemical properties and coagulation efficiencies. The Al-O-Ti bonds observed in the PATC infrared spectra were predicted by density functional theory computation to appear in 4-atom rings in the structure. The differences in Al species distribution of PATC and PAC illustrated that the introduction of Ti retarded the hydrolysis of monomeric Al species (Ala) and medium polymerized Al species (Alb) to colloidal Al species (Alc) at low B levels, but the transformation was promoted at high basicities (B). PATCs exhibited higher charge densities and larger molecular weights than PAC at the same B levels, which contributed to their excellent coagulation efficiencies. PATC with low B values but high Al/Ti molar ratios mainly provided charge neutralization in removal of humic acid kaolin, while the dominant coagulation mechanism converted from charge neutralization to sweeping and bridging effects at high B values or low Al/Ti molar ratios.

Automated summary

In this study, a series of novel polyaluminum-titanium chloride (PATC) coagulants were prepared and the effects of Al (III)-Ti (IV) interaction on chemical species distribution, physicochemical properties, and coagulation efficiencies were investigated. The results revealed that PATCs exhibited higher charge densities and larger molecular weights, leading to excellent coagulation efficiencies.