Investigation of the property of kaolin–alum flocs at acidic pH

• There was full re-growth for monomeric Al or in situ polymeric Al13.
• The break-up process displayed irreversibility for unstable small polymeric Al.
• There was a significant increase of Df when the pH changed from 5.2 to 6.0.
• The number of small flocs after breakage and re-growth decreased at acidic pH.
• The strength of flocs is strongly correlated with Df of flocs.

The formation, breakage and re-growth of flocs were investigated using aluminium sulfate (alum) at acidic pH to explore the property of flocs and coagulation mechanism by different Al species formed in situ. The broken flocs can fully re-grow to the size before breakage, when monomeric Al or polymeric Al13 forms dominate the coagulant species. However, for relatively unstable small polymeric Al (Al3–Al8) and alum precipitate, the break-up process displayed a distinct irreversibility.There was a distinct increase of fractal dimension (Df) when the pH changed from 5.2 to 6.0, as well as the strength of flocs (the size of broken flocs), which meant that the flocs structure changed significantly. Also the Df of broken flocs was higher than that of flocs before breakage, and the Df values of re-grown flocs were not greatly different from the values before breakage. The number of small flocs after breakage and re-growth was much lower than that before breakage at acidic pH (pH 5.2) when the average size of flocs was chosen at the same value before breakage and after re-growth. An important finding is that the strength of flocs is strongly correlated with Df of flocs.

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