A comparative study on the characteristics and coagulation mechanism of PAC-Al13 and PAC-Al30

Polyaluminum chlorides with a high Al13 content (PAC-Al13) and a high Al30 content (PAC-Al30) were prepared in a laboratory. The crystalline morphology, SEM images, XRD patterns, aluminum species distribution, and charge neutralization capability of PAC-Al13 and PAC-Al30 with a basic ratio (B = [OH−]/[Al3+]) of 2.4 were investigated comparatively. The branching morphology and prismatic crystal images of PAC-Al13 and PAC-Al30 were obtained firstly by a simple concentration and crystallization method, and interestingly it was found that these branches appeared to be growing along the diagonal of the tetragonal crystals. The aluminum hydrolytic species distributions of PAC-Al13 and PAC-Al30 were quite different: PAC-Al13 mostly contained the medium polymeric species Alb while the high polymeric species Alc was the main ingredient in PAC-Al30. PAC-Al13 and PAC-Al30 exhibited almost the same charge neutralization capability throughout the investigated dosage range, which demonstrated that the coagulation differences of PAC-Al13 and PAC-Al30 investigated in our previous study were probably caused by the stronger bridge-aggregation and sweep-flocculation actions of PAC-Al30 in the coagulation process. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were also used to characterize the powder PAC-Al13 and PAC-Al30 samples. Moreover, the monomolecular aluminum salt of AlCl3 was investigated in this study as a control.