In this study,two-dimensional correlation spectroscopy integrated with synchronous fluorescence and infrared absorption spectroscopy was employed to investigate the interaction between humic acids and aluminum coagulant at slightly acidic and neutral pH.Higher fluorescence quenching was produced for fulvic-like and humic-like fractions at pH 5.At pH 5,the humic-like fractions originating from the carboxylic acid,carboxyl and polysaccharide compounds were bound to aluminum first,followed by the fulvic-like fractions originating from the carboxyl and polysaccharide compounds.This finding also demonstrated that the activated functional groups of HA were involved in forming the Al-HA complex,which was accompanied by the removal of other groups by co-precipitation.Meanwhile,at pH 7,almost no fluorescence quenching occurred,and surface complexation was observed to occur,in which the activated functional groups were absorbed on the amorphous Al(OH)3.Two-dimensional FT-IR correlation spectroscopy indicated the sequence of HA structural change during coagulation with aluminum,with IR bands affected in the order of COOH＞COO-＞NH deformation of amide Ⅱ＞aliphatic hydroxyl C-OH at pH 5,and COO-＞aliphatic hydroxyl C-OH at pH 7.This study provides a promising pathway for analysis and insight into the priority of functional groups in the interaction between organic matters and metal coagulants.