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Re: Aquatic Plants Digest V3 #193

Matthew Mason wrote:

>                                   Vermiculite has a very high CEC (cation
> exchange capacity) - 100 (quoted number) and therefore does a great job
> holding onto positively charged cations.  What we do not know is whether or
> not it actually releases the ions, and if so, is this a beneficial
> component of vermiculite.  No one has done the "experiment" to show this so
> if anyone tells you that CEC correlated to the bind and release they are
> only partially right.  It does bind cations but the release is an event
> that requires an energy input, thus it is easier to stay bound than to
> unbind (general chemistry).

According to the "classic" model for cation exchange, positively charged
ions are bound to negatively charged particles in the substrate or soil.
The binding sites are pretty much always occupied by something (if not,
then the soil or substrate will take on a net negative charge); to add
something new to a binding site it has to be exchanged for something
that's already bound there (hence the "exchange" in "cation exchange
capacity").  This is a completely reversible reaction.

Removing an ion without exchanging it for something else would take some
work (this involves exponential functions with upper case chi's and curly
F's and it's really way too messy for me to think about).  I think plants
get around that by transporting positively charged ions out through their
roots and trading them for the ions that are on the exchanger. There's
some sort of work involved in that, too.  But its easier to think about.

> ------------------------------

Ken Guin wrote:

> Could one of the chemistry experts out there help me with this problem
> (Roger, this could be up your alley)?

Hmmmm.  This is something I'd *really* rather leave to the PMDD guys.

Roger Miller