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Re: cation exchange Q

Steve asked:

> Is the number of ions which can be captured by cation binding sites of
> clay or organic matter dependent upon the valence charge of the ion?

Yes.  If the CEC amounts to 100 meq/100 grams of exchanger, then 100 grams
of exchanger will hold 100 milliequivalents of electrical charge: that's
0.1 moles of a single-charge cations, but only 0.05 moles of double-charge

> I think the answer is no because a cation with charge -2 once loosely
> bound to a site can still retain an unbalanced negative charge. But if
> the surface of this CEC site then gets a negative charge, it could
> attract and hold anions!

Remember, cations are positively charged.  The surface of a cation
exchanger always carries a negative charge.

This mechanism would probably produce some slop in the exchange reaction
but it shouldn't be a large effect.  The anion exchange effect can occur
even without the mechanism I think you're describing.  This is a "triple
layer" ion exchange model.

> In the end I suppose its an empirical number which is dependent upon the
> types of salts present in solution. The reason I'm asking this is I'm
> trying to figure out how many milligrams of nutrients could be adsorbed
> by a given weight of clay or kitty litter.

Using sodium and calcium (and E- as the exchange site) as examples, the
exchange reaction can be written as

	Ca2+ + 2NaE <--> 2Na+ + CaE2

the reaction constant for this and other exchange reactions can be called
a "selectivity coefficient".  Generally the selectivity coefficient is
sensitive to variations in the exchanger.  For detailed work the
selectivity coefficients has to be determined empirically.

There is a set of generalized selectivity coefficients in the literature.
If you want them then contact me off-list and I can give you the numbers
and some background on how to use them.

Roger Miller