Coagulation and Flocculation
The Early Beginnings
Coagulation, Polymerisation, PolyCation Formation and Aluminium Chemistry
So you are adding a Ferric (Ferric Sulphate) or Alum (Aluminium Sulphate) and Lime (Calcium Hydroxide) to water? What exactly is it that happens during the first few seconds of that reaction. The simple answer is is that polycations are formed. This section deals with the formation of these polycations in a process called nucleation. To simplify thing we are assuming for the moment that there are no other particles or molecules present in our imaginary water, just Alum and Lime.
An Alum solution that is delivered into a storage tank is highly acidic and contains Al 3+, SO42- and H+ . In fact the aluminium ion is hydrated as Al(H2O)63+. Once this solution is added to water and lime is added (Ca2+ and OH-), the pH of the water starts to rise and the hydrated water molecules start losing H+ to the solution. We now have a situation where a large number of spiecies can be formed by combining Al3+, OH- and water molecules. Such as:
AlOH(H2O)52+, Al(OH)2(H2O)+, Al(OH)3(H2O)3 which are the monomers (One Al atom). Two of these can collide to form a dimer (two Al atoms). Then you get trimers and so on. A whole range of these ions form, some more stable than others. The stability is an important thing here, it seems that bigger is better in this reaction and that the stability is also affected in how exactly the Al, the O and the H ions are packed together. Is seems that eventually the polycation AL13O4(OH)24 7+ is formed (although it could well be Al13O3(OH)245+ or Al13O4(OH)25 6+, it doesn't really matter). For Iron something similar is formed: FeO4Fe12(OH)24(H2O)24 7+ (although there seems to be some uncertainty of the stability of this polycation)
How exacly these polycations are formed is still open to debate and one of my primary interests. There seem to be a number of routes to the formation of the polycations and if you are interested I have an extra page on this topic here. click here
For some early results on the modelling of the formation of Polycations click here
So what do these things look like:
Figure 1: The Aluminium-13 polycation
Figure 2: The Iron-12 polycation