Silica gel and adsorbed pollutants

The posts by Will Jeffers and Jim Druzik are interesting, and
addressed an area we have been working in for many years.

Silica gels are amorphous, not crystalline. The amount of water they
will remove is dependent upon the relative humidity, and is
typically between 10% and 40%. They can be expected to have some
affinity for some pollutants, but mainly via the same weak physical
attractions used to hold attached water (which are easily reversed
at slightly elevated temperatures (about 80 deg. C) and in some
cases when they are placed into a low humidity environment).  Any
pollutants on the silica gel will leave just as readily as water.

Most zeolites, both natural and synthetic, are hydrophilic, thus
they do remove water. Hydrophilic zeolites are better in this regard
than silica gel because they are crystalline, meaning the absorbed
material is stored inside the zeolite, in a crystalline supercage
structure, where they are tightly held via physical adsorption. They
can be expected to remove some pollutants, in considerably greater
quantity than the those weakly attracted to silica gel. However, the
hydrophilic zeolites absorb/adsorb polar compounds and will
preferentially absorb water over pollutants (and whether they
absorb/adsorb pollutants depends upon the size and the dielectric
properties of the pollutant molecule, as well as the pore aperture
of the particular zeolite). Typically most of this absorbed material
can be released upon the application of heat, 250 deg. C. Thus these
zeolites hold water and any adsorbed pollutants much more strongly
that does silica gel. Another side of the story is that at higher
humidity, the silica gel will remove a higher percentage of water
relative to its weight than the zeolite. Hydrophilic zeolites are
generally not very acid resistant, so they will eventually lose
their crystalline structure (and their ability to absorb compounds)
when placed into an acidic environment. Silica gels are even more
vulnerable to damage in acidic environments.

Hydrophobic zeolites can be formulated to be extremely acid
resistant, and they can be engineered to remove compounds such as
acids and pollutants via chemisorption inside their crystalline
cages. This method of bonding is far stronger than the weak physical
bonds via which water is held onto silica gels. Typically
temperatures of at least 250 deg. C are required to removed adsorbed
materials from within these zeolites. However, chemisorbed compounds
(covalently bonded) are never released in some hydrophobic zeolites
that are engineered for this purpose. A typical capacity of such
zeolites is approximately 40% (meaning they will internally
chemisorb 40% of their weight in pollutants, acids or whatever
molecule(s) they have been formulated to remove. These capacities do
not factor in (include) any thermal decomposition that may take
place. They are based on measurements of input and outlet
concentrations in a flow through column using sophisticated GC/Mass
spec, where breakthrough occurred.

Bill Hollinger
Conservation Resources

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                  Conservation DistList Instance 12:90
                   Distributed: Tuesday, May 25, 1999
                       Message Id: cdl-12-90-001
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