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

Abstract

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 Al_b while the high polymeric species Al_c 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.