The effect of Al speciation on arsenate [As(V)] removal during coagulation process was investigated from the viewpoint of Al hydrolysis species transformation. The As(V) removal efficiency of preformed Al-13 was compared with that of in situ formed Al-13 considering various Al dosages and pH values. The mechanisms of As(V) removal by Al salt coagulation were explored, and adsorption ability of the flocs formed from Al-13 species were evaluated. The results showed that As(V) removal efficiency was positively correlated with the content of Al-13 species during coagulation process. Weak acidic condition (pH 5-7) is the optimal pH range to remove As(V) because Al-13 species, especially for AlCl3, is largely in situ formed from the transformation of monomeric Al species. Through regulating Al speciation, pH adjustment could improve As(V) removal and AlCl3 could benefit most from this. In contrast, the improvement induced by pH control for polyaluminum (PACI) was slighter because Al speciation of PACI was less dependent upon water pH. This present study confirmed that adsorption on formed hydroxide flocs was the most active mechanism for As(V) removal by Al based coagulants. The flocs generated from in situ formed Al-13 presented better As(V) adsorption ability than the flocs generated from preformed Al-13. These findings are important to understand the coagulation mechanisms of arsenic removal by Al based coagulants. (C) 2011 Elsevier B.V. All rights reserved.