Activated Charcoal/Carbon
Notes by
Newsome.
Scanning electron microscope (SEM) photo of
activated charcoal
Left:
Standard non-activated charcoal.
Those holes are the pores of the wood. Notice the smooth surface.
Right:
Activated charcoal. As you can see
the surface is heavily pitted and irregular, increasing surface area.
Activated charcoal (or activated carbon) is charcoal that has been
supercharged. Using chemicals and
heat (and sometimes pressure) regular charcoal is sort of roughed up or etched
so that it has more surface area.
Probably the simplest way to increase the surface area of charcoal is to
simply crush it up.
For example, a cylinder of charcoal, has a surface area of 2¹rh + 2¹r2. [the
circumference times the height plus the area of the top and bottom]. If our charcoal cylinder is 10cm tall and
has a diameter of 1cm, the total area is about 33cm2. If we were to slice up this cylinder (like
a carrot) into 20 pieces we'd have 40¹r2 instead of just 2¹r2.
The total surface area of this
chopped up charcoal would be about 63cm2. We nearly tripled the surface area. That
increase in surface area means that three times as much charcoal is in contact
with whatever surrounds it, such as polluted air or water. Chop up this charcoal even more and you
get even more surface area.
"Activating" charcoal goes even
further than chopping it up. It
actually pits it... it eats holes in it and roughs up the surface. [See electron micrographs above.]
Depending on a variety of factors (species of
carbonized material, added chemicals, methods, and temperatures used for both
making the charcoal and the activation processes) the measurement of the surface
area of 1 gram of activated charcoal varies significantly. Most of the data I found ranges from 500-3000
m2 per gram.
Various quantities of a 1500 m2 per gram AC
[activated charcoal]
have the following surface
areas:
|
AC in
grams |
AC in
terms of baking measurements |
Surface
area in m2 |
Equivalent
familiar areas |
|
1 |
nearly a 1/2 teaspoon |
1500 |
ca. 6 tennis courts |
|
10 |
heaping tablespoon |
15,000 |
3 soccer fields |
|
85 |
3 oz. or about 1/2 cup |
127,500 |
31.5 acres |
A micrograph of
activated charcoal (R 1) under bright field illumination on a light microscope.
Notice the fractal-like shape of the particles hinting at their enormous
surface area. Each particle in this image, despite being only around 0.1 mm
across, can have a surface area of several square centimeters. The entire image
covers a region of approximately 1.1 by 0.7 mm, and the full resolution version
is at a scale of 6.236 pixels/μm.
AC removes contaminants that range
from around 50 microns to 0.5 microns in size. The smaller the more effective. It does this by both absorption
(internalizing) and adsorption (attachment to the surface).
Active
carbon is ...
-very effective in removing at about known 80
chemicals,
-effective in another 30 and
-moderately effective for about
20.
AC doesnÕt remove everything from the tap water, but it
removes a lot.
According to the EPA activated carbon
is the only filter recommended to
remove all 32 identified organic contaminants as well as 14 listed
pesticides and 12 herbicides.
Some of the most common concerns
about tap water are the by-products from chlorination (VOCs) often referred to
as the total trihalomethanes. (TTHMs: chloroform, bromoform,
bromodichloromethane, and dibromochloromethane) Activated carbon is very effective in removing these.
The trihalomethanes are also associated with
many of the industrial processes contaminating our air and water. Chloroform (trichloromethane) is involved
in the production of teflon.
Fluoroform (Trifluoromethane) and chlorodifluoromethane (an HCFC) are
both used as refrigerants.
Tri-halo-methane: CHX3 (where X is any
halogen atom)
1 Carbon + 1 Hydrogen + 3 Halogens.
"Tri-halo" refers to 3 halogens.
"Methane" is the molecular structure of 1 carbon
and 4 hydrogens.
"Trihalo-methane" is a methane in which 3 of the 4
hydrogens of standard methane are replaced with 3 halogens ...replaced with a
"tri-halo."
Note: The
halogens are the group of elements on the periodic table [group 17, the column
headed by fluorine.] They include fluorine, chlorine, bromine, and iodine. Halo-gen means
"salt-producing" [think halo-genesis[1]]. When halogens react with metals they
produce a wide range of salts, including calcium fluoride, sodium chloride
(table salt), silver bromide, and potassium iodide.
Oddly enough, I have had trouble finding consistent
information on what exactly activated carbon filters out. In particular, it is frequently claimed
that AC filters are effective on chlorine, but then other information rates AC
poorly in filtering out chlorine.
AC is used extensively in water and air filtration. It is what is in your Britaª filter and
most cartridge respirators.
