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Re: Carbon properties

Dear Steve Pushak,

Upon contact with water containing soluble organic materials, granular
activated carbon selectively removes these materials by
adsorption. Adsorption is the phenomenon whereby molecules adhere to a
surface with which they come into contact, due to
forces of attraction at the surface. The use of surface energy to attract
and hold molecules is physical adsorption.

Adsorption is said to occur in three basic steps. These are film diffusion,
pore diffusion, and adhesion to the solute molecules to the
carbon surfaces. Film diffusion is the penetration of the solute molecule,
the adsorbate, through the carbon particle's "surface film".
Pore diffusion involves the migration of solute molecules through the
carbon pores to an adsorption site. Adhesion occurs when the
solute molecule adheres to the carbon pore surface.

The fact that activated carbon has an extremely large surface area per unit
weight makes it an extremely efficient adsorptive
material. The activation of carbon and its manufacture produces many pores
within the particles, and it is the vast areas of the walls
within these pores that accounts for most of the total surface area of the
carbon. In water, activated carbon has a preference for
large organic molecules and for substances which are nonpolar in nature.
The forces of attraction between the carbon and the
adsorbed molecules are greater the closer the molecules are in size to the
pores. The best adsorption takes place when the pores
are just large enough to admit the molecules.

Activated carbon, when contacted with water containing organic material,
will remove these compounds selectively by
a combination of adsorption of the less polar molecules, filtration of the
larger particles, and partial deposition of
colloidal material on the exterior surface of the activated carbon. The
extent of removal of soluble organics by
adsorption depends on the diffusion of the particle to the external surface
of the carbon and diffusion within the porous
adsorbent. For colloidal particles, internal diffusion is relatively
unimportant because of particle size. Organic
substances that pass through the column consist strongly of hydrophilic
organic molecules such as carbohydrates and
other highly oxygenated organic compounds.

Adsorption is partially the result of forces of attraction at the surface
of a particle that cause soluble organic materials
to adhere to the particle, and partially attributable to the limited water
solubility of many organic substances.
Activated carbon has a large and highly active surface area that results
from the activation process. This produces
numerous pores within the carbon particle and creates active sites on the
surface of the pores. 

Most of the commercial fertilizers contain chelated metals which are
ORGANIC compounds. These chelated metals will be binded to the carbon
pores, and as result their concentration into the water column will get
lower. As chelated compounds in aquariums slowly decade and free their
metals, the concentration of metals ions in the water will decrease
progressively too. In this way all the trace elements will be slowly but
constantly removed from thr water (if you don't add any!).

Luca Specchio