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Cation Exchange Capacity

Hi All,

After determining the total metals of my garden soil, kitty litter, and 
Fluorite, I still had a curiosity about the CEC. I know over the past 
year, the APD has had several discussions about CEC and 
substrates. I know Steve P. gets pretty excited when it comes to 
talking about CEC, so I thought I would find some hard values to 
discuss.

CEC, for those who may not understand, is defined as the sum of 
the exchangeable cations of a soil. It's expressed as 
milliequivalents or me. per 100g of soil. Soils most often vary from 
<1.0 to >100 me./100g. It's a reversible chemical reaction. Cations 
are held on the surface of soil minerals and held within the crystal 
framework of some mineral species. Cations are also a part of 
certain organic compounds. All these cations can be reversibly 
replaced by those of salt solutions and acids.
I used EPA method 9081A CEC of soils by sodium acetate. A 
sample is mixed with an excess of sodium acetate solution, 
resulting in an exchange of the added sodium cations for matrix 
cations. Subsequently, the sample is washed with 99% isopropyl 
alcohol. An ammonium acetate solution is then added, which 
replaces the absorbed sodium with ammonium. The conc. of 
displaced sodium is then determined. From this, it's run through 
half a page of calculations to get an answer of me./100g soil. 
Here's what I got:

CEC (me./100g)              Matrix
-----------------------------------------------------
   <0.1                         Clean sand
    24.3                            Soil
    27.0                           Litter
    1.7                            Fluorite
          
As I expected (hoping for, anyway) the sand was <0.1 me./100g. 
There wasn't any Na to be found in the sand sample. There aren't 
any binding sites for cations (minute to nil) on sand grains. The soil 
had a thin layer of silt (1/3 of sample) present in the wet sample, 
so I expected it to have a good CEC. There was also some small 
pieces of organic matter mixed in, also suggesting another good 
binding site. There was ~280ppm Na leached from the sample, 
which calc. to ~24.3 me./100g. The kitty litter held the most 
promise for me. It's a small sized clay/silt and those usually have 
good CECs. The sample leached ~310ppm Na, which gave the 
highest CEC of 27.0 me./100g. The Fluorite didn't fare too bad, but 
it only leached ~19ppm of Na, leading to a low CEC value of 1.7. 
It's easy to understand, since Fluorite doesn't break down to 
expose the millions of binding sites found in clays. I would dare 
say a lot of the laterite sold for aquarium use has a CEC value 
close to the litter, probably higher for finer, more organic laterites. 
Now I'm curious what kind of soils have the >100 values. I could 
see a DI system's resin bed as being pretty high, but a natural 
source is probably pretty rare. We've tested soil from all over, 
mainly the first foot of depth, and the litter holds the record for the 
highest I've seen. Most red/orange clays (our area) have low CECs.

Jamie




Jamie Johnson
Greenwood, SC
jjohnson at davisfloyd_com
jjirons at greenwood_net  (home)