Aluminum Hydroxide Formation

Could you please help me better understand the formation of aluminum hydroxide --- for instance:

Again, lets assume we have a generally weak acid solution to which we add aluminum sulfate. The objective is to precipitate aluminum hydroxide per the following reaction which I believe to be correct.

Al2(SO4)3 + HCO3- = 2Al(OH)3 + SO4--

I do not want Al2(SO4)3 + H2O = 2Al(OH)3 + H2SO4

I want to ensure that the reaction prefers or goes in the direction of the first equation.

However, pH has something to do as to which reaction is favored ---- if pH is too low I believe the second equation is favored. If pH is closer to neutral then I believe the first equation is favored.

But why specifically does aluminum hydroxide formation seem to be maximum at a pH between 4.5 and 5.5 per literature?? Why not pH 6 or even 7? What is so special about pH 5 when it comes to Al(OH)3 formation?