Q:What active about activated charcoal?

Curtis L. Ashendel ashendel at aclcb.purdue.edu
Fri Feb 4 12:52:04 EST 1994


On Tue, 01 Feb 1994 14:11:52 -0300, Dan Kolk wrote:

>what exactly is done to activate carbon?  How does this increase it's
>filtering qualities?

littjg at wmvx.lvs.dupo wrote:

>I don't know exactly but there are references in the Merck
>Index that you can check. Look under "Carbon, Amorphous #1815" where they
>briefly describe Activated Charcoal and give a # of references.

and Bernard Heymann (bheymann at bragg.bio.purdue.edu) wrote:

>Activated carbon is prepared from tough organic materials such as hardwood
>or the the shells of very hard nuts, by very high temperature. This creates
>a hard porous material with a large surface-to-volume ratio which is very
>much suited for the adsorption of many organic compounds. Used activated
>carbon can often be regenerated by heating it again. 

Many years ago I used DCC (dextran-coated charcoal) in an RIA and what I 
know comes from that time.  Activated charcoal is an OLD reagent, and 
seeing a BIONET question on it makes me FEEL OLD.  Alas....

I do not think the "activation" of the charcoal alters its "filtering" 
properties.  It simply makes the charcoal a better and more specific binder 
of organics and makes it more inert to your solutions.

Activated charcoal is carbon (like graphite) and I believe is made from 
crude charcoal by pulverizing (which gives it a large surface area) and 
then washing with strong acid to remove the ions (ash) (this is what is 
called "activation").  What is left after washing away the acid is a bare 
carbon matrix that is hydrophobic and non-ionic, yet is also inert to all 
solvents.  It can bind organic molecules quite well, but I think it binds 
most strongly olefins and aromatics, with planar multicyclic molecules (as 
in most dyes) the very strongest.  This is why it is used to "decolorize" 
solutions, in which it binds the colored (conjugated or quinone-like) 
molecules that often are present as small contaminants in reagents (such as 
acrylamide or cholate).  Without acid washing, the ions in it would leach 
out into your solution and would give it come ability to bind more 
hydrophilic molecules, such as macromolecules.  It is *messy* to work with 
though and has rather poor filtration characteristics (clogs filter paper, 
passes through some filters, etc.)  Preloading the filter with diatomaceous 
earth ("Celite") solves these filtering problems.

I think it is more trouble to recycle than to just use new.  It is 
inexpensive.  BTW, I would not recommend against heating activated 
charcoal in the presence of oxygen, as it is quite flammible.
Curt Ashendel
Purdue University
West Lafayette, IN
ashendel at aclcb.purdue.edu



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