Coagulation behavior of polyaluminum-titanium chloride composite coagulant with humic acid: A mechanism analysis

•The binding behavior between HA and Al/Ti species at different pH was illustrated.•Al hydrolysates complexed with OH and COOH to form COAl coordinate bonds.•Protonation of HA affected binding behaviors due to the competition of H+ for Al3+.•PATC performed well due to the combined effects of various hydrolysates.•Dominant mechanism shifted from charge neutralizing to sweep with pH increased.