Coagulation and Flocculation
Neutralise and Wrap
Particle Removal
What would happen if we had particles floating around in the solution? First of all it is important to remember that all water coming into a water treatment plant contains particles which are negativly charged. This is such an important thing, that we will make it rule 2.
RULE 2: All particulate to be removed is negativly charged
This is a bit too easy though, because a particle has something called an electrical double layer. A particle is negativly charged it will act as a magnet for positive ions that will sit close to its surface and make it look like it has far less charge than it has in actuality. This is a very complicated theory that I am not going to discuss in detail here but what it comes down to is that monovalent positive ions, such as H+, are not very good at this, divalent positive ions, such as Ca+ and Mg+, are much better (about 10x better) and trivalent positive ions are just very good (about 1000 times better) at what is calles"depressing the double layer". Now our iron and aluminium ions are trivalent and will depress the double layer to such an extend that negative particles will start colliding with each other. There is a name for this as well, it is called "destabilising the sollution" or "destabilising the colloid" or "coagulation"
This is where I have a little bit of a problem because I have two available scenarieos that may both be valid. My guess is that the iron ions that are essentially fixed in position close to the surface of our particle can very easily form a Fe(12) or Al(13) polycation. This polycation will then adsorb to the surface, decreasing the overall surface charge of the particle (reducing its negative charge). This process will happen a number of times until the surface is completely full of polycations. Polycations that have formed in the solution in the meantime can then attach to the surface-polycations and start to form their usual chains/metworks.
Alternativly, it is possible that (chains of) polycations formed in the solution will be attracted to the surface and attach to it. This seems less likely because of the perculuarities of the electrical double layer but cannot really be excluded. It doesn't really matter though because the effect of both scenarios is the same. Put it down as something a scientis would worry about and move on.
To recap, there are two processes operating at the same time:
1. The solution of particles is destabilised and particles start colliding
2. Polycations start to form networks and eventually round structures, some of which become attached to the surface of the particles
The final outcome of both processes is that the particles become trapped in the flocc. This is why many people call this process entrapment. Since the density of floccs made from just polycations is very low and the density of suspended particulate is often much higher there is a new rule coming up:
RULE 3: The density of a flocc that is formed in a water containing mainly particulate is relatively high.
Lowland waters, which are often low in colour and high in particulate (turbidity) tend to result in a high-density flocc that settles readily.