The alum used in water and wastewater treatment is Al2(SO4)3 • 14H2O. (The "14" actually varies from 13 to 18.) For brevity, this will simply be written without the water of hydration as Al2(SO4)3. When alum is dissolved in water, it dissociates according to the following equation (Sincero, 1968):
By rapid mix, the ions must be rapidly dispersed throughout the tank in order to effect the complete coagulation process.
Because the water molecule is polar, it attracts Al3+ forming a complex ion according to the following:
In the complex ion Al(H2O)6+, Al is called the central atom and the molecules of H2O are called ligands. The subscript 6 is the coordination number, the number of ligands attached to the central atom; the superscript 3+ is the charge of the complex ion. The whole assembly of the complex forms what is called a coordination sphere.
As indicated in Equation (12.4), aluminum has a coordination number of 6 with the water molecule. This means that no more water molecules can bind with the central atom but that any interaction would not be a mere insertion into the coordination sphere. In fact, further reaction with the water molecule involves hydrolysis of the water molecule and exchanging of the resulting OH- ion with the H2O ligand inside the coordination sphere. This type of reaction is called ligand exchange reaction.
Some of the hydrolysis products of the ligand exchange reaction are mononu-clear, which means that only one central atom of aluminum is in the complex; and some are polynuclear, which means that more than one central atom of aluminum exists in the complex. Because the water molecule is not charged, Al(H2O)36+ may simply be written as Al3+. This is the symbol to be used in the complex reactions that follow. Without going into details, we will simply write at once all the complex ligand exchange equilibrium reactions.
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