Aluminum sulfate as a flocculant?

In a lab experiment, a solution of weak acids in water exhibit a pH of 3.7; the solution is known to have a known quantity of dissolved solids in solution.

Paper makers alum (aluminum sulfate = Al2(SO4)3) is supposed to be used to coagulate/flocculate the dissolved solids in order to precipitate those solids.

In the course of the experiment, it appeared that the pH at which the best flocculation occurred using alum was between a pH 4.7 to 5.0 ---- seemingly any higher or lower pH resulted in less preciptable solids?? Why would this be?

Interestingly, much of the literature I could find regarding alum does in fact suggest that aluminum sulfate exhibits optimal flocculation at pH 4.7 ---- however, I cannot understand why this would be.

Why would alum be a better flocculant at pH 4.7 vs. say 3.5 or as high as 6.0??