The aluminate ion is basically (no pun intended) the hexaaqua aluminium III ion with four protons removed from four of the water molecules...
thus [Al(H2O)6]3+ ---> Al[(H2O)2(OH)4]- +4H+
or if you show then being removed by base (more correct)
[Al(H2O)6]3+ + 4OH- ---> Al[(H2O)2(OH)4]- +4H2O
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