Allways thought it was solubility and ionic strength both of which decrease as you go down and to the right on the periodic chart (ie Row 1 NA to K, to row 2 MG to CA, to row 13 AL)

Dear dbrock:

Your explaination sounds super interesting!

But could you please possiblly further clarify the rule behind the periodic table?

I mean why the adsorption strength could have that ranking at the same ionic strengh?

Is there any easier way to understand it?

Best wishes
Wilfred

Dear Wilfred,

the phenomenon is known as 'Hofmeister Series' or lyotrophic series.
More precisely there are several phenomena: adsorption, flocculation/precipitation, hydration, which normally follow this order, not only for cations but for anions too. When you do a search on search engines or in the text books you will find out that the phenomena are still not fully understood by the experts either.
As a start you might read http://www.lsbu.ac.uk/water/hofmeist.html.
Sometimes also 'anti' hofmeister behaviour is experienced, which is explained by specific interactions - for example adsorption on silica.

Kind regards
T. Sobisch

Consider also that there are two main methods of adsorption

Electrostatic - Driven by the valance of the cation
and
Physical - Driven by Van der Walls forces which are a function of the atomic mass of the ion.

Therefore the adsorption strength is going to be some function of the valance and mass of the ion. Probably something on the order of an "atomic mass equivalent charge" (I just made that up so don't quote me).    The functionality would also be dependant on the adsorption media (hence the differing behaviour on silica that Sobisch mentioned).  

Finally It has been easier (and ultimately more profitable) to determine adsorption coefficients for various ions on various media based on empirical data.