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Re: cation exchange site and cation charge [technical]

> Ok, soil science experts.

	If you want those, I don't know why you copied this one to me!  :)

> 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?
> I think the answer is no because a cation with charge -2

	That would be an _anion_.

> 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,

	That is what it starts _off_ with.

> it could
> attract and hold anions!

	It would do that if it had a _positive_ charge.  An anion goes _to_
an anode - it has a negative charge.

	If we knew what constituted a site, it would help.  I suspect the
oxygen atoms at the surface or in the open structure of these materials
are the main "source" of cation exchage sites.  This will certainly be true
of clays - I did some work a few years ago where I was looking at the
oxidation states of iron in clay-based catalysts.  The structures of some
of these materials are quite well known, and we know what goes in which site.
Some of the species (cations) are mobile, and can be replaced by others.
There must be an overall charge balance, which will apply over fairly small
regions.  If you stick in a 3+ ion where a 2+ ion is required for charge
balance, then a 1+ ion will have to go somewhere close, to compensate.
Getting a negative ion in there to balance things out is not too likely,
I think.  They are, in general, big.
> In the end I suppose its an empirical number which is dependent upon the
> types of salts present in solution.

	Not to mention the concentrations.  I suggest you read up on adsorption

> 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.

	You could do some measurements.  We have iron tests - try with 
solutions of iron salts.

Paul Sears        Ottawa, Canada