Silica gel and adsorbed pollutants

Will Jeffers <wjeffers [at] mfa__org> writes

>I was wondering if anyone on the DistList has had any experience
>with silica gels carrying pollutants from one installation to
>another?
>
>In the case of desiccating silica gel that can be reconditioned by
>heating, I would assume that any adsorbed pollutants would be driven
>off with the moisture.  Can anyone confirm or refute this?
>
>Then there is the case of Arten and Artsorb buffering gels.  Do
>these materials readily adsorb pollutants?  If so, how can these
>pollutants be desorbed?

Silica gel does indeed absorb more than just water vapor. (For the
purposes of this email I'm not making a distinction between
absorption and adsorption. Because of silica gel's high affinity for
water vapor and the presence of significant capillary condensation,
I'm going to make several over-simplifications here and just speak
of absorption.)

Unpublished work which is now exactly ten years old included silica
gel along with Purafil, molecular sieves, and activated carbon in
determining efficacy for the removal of NO2, SO2, O3, HCHO, and H2S
from a microenvironment.  We did not include acetic or formic acids.
At the time we defined a "target performance" for the removal of the
equivalent of 20 parts per billion, of pollutant by 200 grams of
sorbent at a flow rate of 0.1 liter per minute for one year. This
was our "active filtration" design, i.e. display case air being
slowly passed through a bed of the sorbent in question. No one
thought silica gel could compete with such high performance
materials like Purafil or activated carbon (and it didn't).
Nevertheless, it absorbed some of all of these gases and even
achieved the "target performance" level for one--formaldehyde (not
surprising).  Both NO2 and SO2 absorbed fully at the inception of
the exposure but then dropped off in a manner I'd expect if this
system was acting like it contained significant free water, i.e.,
from dissolution into ever-increasing concentrations of solution
within the pour structure.  I won't speculate on sorption/desorption
of these gases if silica gel is being used to maintain an
equilibrium around some mid-RH level. However, there certainly is
enough concern  here to warrant an investigation concerning
desorption if silica gel was being used as a buffer for some time
and then tried to establish an equilibrium during a period of severe
water vapor desorption (drying period).

My next question would be, "What prompted the question in the first
place?" Is there deterioration going on which can't be tracked down
yet, or is this something to be generally anxious about?

The question of silica gel absorbing and desorbing more than just
water vapor introduces the whole issue of sorption/desorption of
any, and all, microporous materials including but not limited to,
silica gel, activated carbon, and, natural and synthetic zeolites.

A great deal is known about the physical chemistry of of these
materials, most of which does not get translated into a form which
conservators can use. As a result we have claims about what many of
these products can do which, while based upon some facts, leave open
the possibility for wildly unrealistic expectations.

As an example, the thermal decomposition of some pollutants (to
harmless byproducts) at ambient conditions due to some unique
properties of microporous materials to reduce certain energy
barriers has not been demonstrated conclusively at the
concentrations over time experienced in museum environments.  Thus,
until this can be shown more convincingly, it's hard to believe that
some microporous materials can effectively protect collections to
the extent that is often hyped. Other examples abound of
"disconnects" between performance and expectations relating to air
conditioning filtration, storage environments, and plastic
protective bags.

James Druzik
Senior Scientist
The Getty Conservation Institute

                                  ***
                  Conservation DistList Instance 12:89
                  Distributed: Thursday, May 20, 1999
                       Message Id: cdl-12-89-001
                                  ***