The composition of hydrolyzed Al species is essential for the understanding of coagulation with Al-based coagulants. Surface characteristics of flocs formed by coagulation with two distinct polyaluminum chloride (PACl) coagulants were identified. One commercial coagulant (PACl-C) with voluminous monomeric Al and colloidal Al(OH)(3) and a custom-made PACl (PACl-Al-13) containing high Al-13 content were applied to destabilize kaolin particles. The flocs formed by PACl-C and PACl-Al-13 at neutral and alkaline pH ranges, respectively, were observed by FE-SEM and HR-TEM. In addition, the Al composition of these flocs was characterized by XPS and HR-XRD, and the imaging of Al(OH)(3) precipitates and Al-13 aggregates were conducted by SEM as well as tapping mode AFM in liquid system. The observations of flocs indicate that the morphology of Al(OH)(3)-rich flocs are fluffy and porous around the edge of flocs, while the Al-13-aggregate flocs have a glossy contour and irregular structure. Both Al(OH)(3)-rich and Al-13-aggregate flocs do not possess well-formed crystalline structure except for the Al-13-like crystal exists in the Al-13-aggregate flocs. Among Al(OH)(3) precipitates, colloidal Al(OH)(3) is micro-scale in size, while amorphous Al(OH)(3) is nano-scale. During the formation of Al-13 aggregates, some coiled and clustered Al-13 aggregates with smoother surface were observed. The XPS study on floc surface showed that tetrahedral (Al-IV)/octahedral (Al-VI) Al ratio on the surfaces of PACl-C and PACl-Al-13 flocs is 1:1.6 and 1:9.9, respectively. Of the in situ formed Al-13, almost half of Al-hydroxide precipitates on the surface of Al(OH)(3)-rich flocs possess the Al-IV center. It also found that the irregularly aggregated Al-13 with a similar Al-13 Crystalline structure subsists on the surface of Al-13-aggregate flocs. (C) 2009 Elsevier Ltd. All rights reserved.