Coagulation and Flocculation
Joining them up
Polycation Chain/Network formation
So now we have polycations. What happens next? We are still assuming that we are only dealing with the flocculant aluminiumhere, we will go through what happens when there are particles in the water in the next page. What will have happened is that a large number of polycations have been formed in a very short timespan, typically 1-3 seconds. At this stage there may be some Al13 polycations, however post of the material formed is just a jumbled mess of connections between Al atoms/ions that has very little structure. Because of the very short timespan that the polycations have been formed in the polycations have had no time to organise themselves in nice, low energy structures. Once the initial reaction is over the polycations are now slowly formed through a process called ripening discussed later on.
All these polycations will be roughly the same size, which is very, very small, about 1 nm (1E-9 m). At this size the effect of stirring is not noticible (see section 5) so imagine each polycation being moved about only by brownian motion in an irregular movement. (see below)
Our polycations have a good charge (4-8+) but the anions around it mask that charge so that in effect the positive repulsive force that two polycations experience when getting close to each other is very small. So small that when two polycations come into contact with each other they can attach to each other. In fact we think that the bridging between two polycations is achieved by means of an anion (in this case it would be a sulphate-ion) This action slightly redistributes the charge at the sides of the joint up polycations to such an extend that it becomes more difficult for another polycation to approach it from the side, but easyer to approach from either ends. The effect is that we then end up with a small chain.
Lets now go through this process again but this time assume that the pH is slightly higher, or in other words there are more OH- ions to reduce the charge of our polycation. It is easy to imagine that this time it becomes possible to, on occasion, have three polycations joining together and we now end up with a structure a bit more like the chain on the right in the image below.
Increase the pH a little bit more and you will end up with a structure like the one above left. To recap: by changing the pH we change the structure of the joint up polycations form one that is "chain-like" to one that is "network-like".
Now ask yourself: Which of these structures would be stronger? The left or the right? The answer must be that a more "network-like" structure is stronger than a "chain-like" structure. This is a rule that seems to hold up in general when you are dealing with flocculation:
RULE No.1: the higher the pH the stronger the flocc
What happens next is that we quickly run out of polycations that are left in the solution and we now have a number of these networks floating about in our water. Because these networks are moving around, and some parts of the network move faster than others, they quickly take on their most economic shape: a ball with a few small chains sticking out in all directions.