I recently wrote about Prussian blue and I also explained how soil absorbs caesium and thus protects wells from caesium contamination, I want now to explain some of the important aspects of soil. The clay in the soil is normally the part of the soil which binds strongly to the caesium, one clay mineral is illite.
Illite is a layered clay which has potassium cations as the jam between slices of aluminium silicate layers (the bread) in a sandwich. Rather than a typical closed sandwich, this clay is more like a stack of open sandwiches. You could think of it like a stack of slices of cheese on toast.
What happens then cesium cations encounter the surface of the clay mineral is that they swap with the potassium cations. The metal cations associate becuase of the attractive electrostatic effect of the negatively charged aluminium silicate layers.
Here is a picture of the solid using the data from A.F. Gualtieri (Journal of Applied Crystallography, 2000, volume 33, pages 267-278). You should be able to see the blue potassium which are the jam which are between the sheets of oxygen (red), aluminium (green) and silicon (purple).
The ball and stick view is a little misleading, while it allows the viewer to look inside the solid it does not show you how the surface will look to an atom or molecule which is bobbing about in the water which is in contact with the soil. So here is a view using a space filling model. Now the atoms are scaled to be about their real sizes, it is now clear that the only potassium which you can get to is at the edges of the solid.
It so happens with clay that the crystals of the minerals are very small, which increases the surface area which is accessible to water. It is noteworthy that clay can absorb some other cationic substances. Paraquat the weed killer binds very tightly to clay, as a result while it kills the plants which are currently growing it does not leave an active residue in the soil. However it does stay in the soil bonded to the clay for a very long time. If paraquat was to be used again and again on some land then one day it might be possible to saturate the soil with this weed killer. I also suspect that if a person was to use muddy water to mix up their paraquat solution for the weed infested garden path then it might not work as well as some of the herbicide will become inactivated through binding to the clay.