Aluminium sulfate and water treatment

The Group I and Group II metals form carbonates, those of Group II being insoluble in water. Aluminium carbonate, Al2(C03)3, however, cannot be prepared if aluminium sulfate is added to a solution containing carbonate or hydrogen carbonate ions, A1(0H)3 is precipitated and carbon dioxide is evolved  

Reactions 9.16 and 9.17 are of interest because acids liberate CO2 from carbonate and hydrogen carbonate solutions, so here, AP+(aq) plays the part of an acid. This becomes more apparent when you consider that AP+(aq), like Mg +(aq), is shorthand for an octahedral aquo complex (Structure 9.8). Then, Equation 9.17 becomes 

Why can [A1(H20)6] be said to act as a Brpnsted-Lowry acid in this equation  

Three of the six water molecules that were attached to the aluminium have been lost, but the other three have acted as proton donors, leaving aluminium associated with hydroxide ions rather than with water molecules. 

A more obvious sign of the acidic character of [A1(H20)6] is the fact that aqueous solutions of aluminium sulfate, unlike those of, say, sodium sulfate, are acidic  

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