Coagulation is the best available method for removing intracellular organic matter (IOM),which is released from algae cells and is an important precursor to disinfection by-products in drinking water treatment.To gain insight into the best strategy to optimize IOM removal,the coagulation performance of two Al salts,i.e.,aluminum chloride (AlCl3) and polyaluminum chloride (PACl,containing 81.2％ Al13),was investigated to illuminate the effect of Al species distribution on IOM removal.PACl showed better removal efficiency than AlCl3 with regard to the removal of turbidity and dissolved organic carbon (DOC),owing to the higher charge neutralization effect and greater stability of pre-formed Al13 species.High pressure size exclusion chromatography analysis indicated that the superiority of PACl in DOC removal could be ascribed to the higher binding affinity between Al13 polymer and the low and medium molecular weight (MW) fractions of IOM.The results of differential log-transformed absorbance at 254 and 350 nm indicated more significant formation of complexes between AlCl3 and IOM,which benefits the removal of tryptophan-like proteins thereafter.Additionally,PACl showed more significant superiority compared to AlCl3 in the removal of ＜5 kDa and hydrophilic fractions,which are widely viewed as the most difficult to remove by coagulation.This study provides insight into the interactions between Al species and IOM,and advances the optimization of coagulation for the removal of IOM in eutrophic water.