20 - Lecture notes for Clay Mineralogy

The Geochemistry of clay minerals
Suggested reading:

Drever, James I., 1997 The Geochemistry of Natural Waters: Surface and Groundwater Environments. Prentice Hall, Upper Saddle River, NJ.436 P.

Langmuir, Donald, 1997 Aqueous Environmental Geochemistry. Prentice Hall, Upper Saddle River, NJ.600 P.



Isotherms are adsorption relations between the adsorbing species and solid surfaces.

Common isotherms include the Langmuir isotherm:



where:


A special case of the Langmuir isotherm is where B>>C. This results in a simple linear isotherm.

where:


therefore,

Graphically the Langmuir isotherms appear below with different relative values of A and B. Note that the concentration of the adsorbed species can never exceed that which is possible by the number of exchange sites (e.g. the cation exchange capacity of the clay).


Another common isotherm is the Freundlich isotherm:

C-bar= A C 1/n

where:

This form handles species in non-ideal solutions or a case with heterogeneous surface sites.



Mathematical representation of ion equilibrium exchange.

A reaction can be written where:

s = adsorbed species on the surface of A or B
aq = dissolved species A or B in solution

The equilibrium activity product is therefore written as


recalling
a = C g and a = bar-Cy

then:


Ion exchange of monovalent cations: (assume y = 1)


then,


upon rearranging,

For most solutions gA = gB therefore, the above can be simplified to:



This is the most simple representation of ion exchange and shows that the ion ratio on the solid surface is the same as that in the solution regardless of concentration.

If the concentration of A is much greater than B (i.e., C A >> CB and bar-CA >> bar-CB ) then the expression becomes a simple linear isotherm.

bar-CB = K' CB
Ion exchange of mixed-valent cations: (assume y = 1)

then,


upon rearranging,



In this case, g A and g B are not equal . Even if they were equal the ratio of g2B togB ) is not equal to 1.

By re-writing the above as:


Under these conditions, it is possible to lower the ratio of adsorbed cation by lowering the concentration of dissolved A and B, even while maintaining a constant ratio of A and B in solution.

This expresses the observation that upon dilution, a greater proportion of higher valent ions will be taken up by the solid phase.

This important when trying to determine the exchange cation of clay in sea water. If you rinse or dilute the solution there will be preferential adsorption of Mg ions in the clays.