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Effects of Ca on the distribution, structure and morphology of aluminum species in polyaluminum chloride

The flocculation efficiency of polyaluminum chloride (PAC) is closely related to the distribution and structure of its Al species, and Al13 is the optimal species in PAC for flocculation. A series of PAC containing Ca was prepared by adding Ca before and after the basifying reaction. The effects of Ca on the Al species were studied by 27 Al nuclear magnetic resonance (NMR) and atomic force microscope (AFM) techniques. The experimental results show that the introduction of Ca increases the content of Alm and Al13 in PAC and decreases their chemical shifts in NMR spectra due to the electric repulsion between the positive Ca species and Al species and the formation of Al-O-Ca complexes. With the rise of Ca/Al molar ratio, the Al species in PAC tend to scatter. It is observed that the formerly branch-aggregated clusters are tending to form granule-aggregated ones whose diameter gets smaller and smaller, and the floccule aggregates are formed at a higher Ca/Al molar ratio. The introduction of Ca to PAC, which increases the Al13 content, is certainly to enhance the flocculation efficiency of PAC in water treatment.